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Grout injection of masonry , scientific approach and modeling Doctoraatsthesis Filip Van Rickstal promotor: Prof. dr. ir. D. Van Gemert assesoren: Prof. dr. ir. J. Berlamont Prof. dr. ir. K. Van Balen

Grout Injection

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Page 1: Grout Injection

Grout injection of masonry,

scientific approach and modeling

Doctoraatsthesis Filip Van Rickstal

promotor: Prof. dr. ir. D. Van Gemertassesoren: Prof. dr. ir. J. Berlamont

Prof. dr. ir. K. Van Balen

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Table of contents XXXIII

Table of contents

Dankwoord . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

Nederlandstalig abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III

Nederlandstalige samenvatting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

List of symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XXX

Table of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XXXIII

Englisch abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -1-

Chapter 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -3-

Chapter 2. Masonry and its need for restoration . . . . . . . . . . . . . . . . . . . . . -5-2.1. General description of masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -5-

2.1.1. Masonry components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -6-Bricks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -6-Natural stone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -8-Mortar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -8-Pointing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -11-

2.2. Causes of damage to masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -11-2.2.1. Physical and physico-chemical mechanisms . . . . . . . . . . . . . . . . . . . . . . -11-2.2.2. Mechanical damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -13-2.2.3. Biological damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -15-

2.3. The loading of masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -16-2.3.1. Probability of failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -16-2.3.2. Grout injection increases the reliability of masonry . . . . . . . . . . . . . . . -21-

2.4. Conclusions of Chapter 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -23-

Chapter 3. Injection as a consolidation technique for masonry . . . . . . . -25-3.1. Historical review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -25-3.2. Injection Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -27-

3.2.1. Modern injection installation, ideal situation . . . . . . . . . . . . . . . . . . . . . -27-3.2.2. Reality about injection equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -29-

3.3. Realization of an injection work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -30-3.3.1. General Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -30-

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XXXIV Grout injection of masonry, scientific approach and modeling

3.3.2. Diagnosis of the masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -30-Destructive techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -31-Non-destructive techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -32-

3.3.3. Preparation of the masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -36-3.3.4. Injection pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -37-3.3.5. Execution of the injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -39-3.3.6. Control of quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -40-

3.4. Types of binding agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -42-3.4.1. Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -42-3.4.2. Cement grouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -44-3.4.3. Lime based grouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -45-

3.5. Grouting improves the load bearing capacity of the masonry . . . . -46-3.6. Masonry grouting, code of practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . -49-3.7. Conclusions of chapter 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -52-

Chapter 4. Problems faced during injection - possible solutions . . . . . . -55-4.1. Wrong materials, chemical, physical and structural incompatibility

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -55-4.2. Incomplete filling of the voids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -57-

4.2.1. Rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -58-4.2.2. Stability of grouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -59-

4.3. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -61-4.3.1. Improving the injectability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -61-4.3.2. Improving the stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -63-4.3.3. Injection holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -63-4.3.4. Chemical and mechanical compatibility . . . . . . . . . . . . . . . . . . . . . . . . . -65-

4.4. Subject of research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -66-

Chapter 5. Experimental program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -69-5.1. Aim of the tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -69-5.2. Testing the grout’s properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -69-

5.2.1. Mixing procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -69-5.2.2. Rheological properties of the grout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -70-5.2.3. Dynamic viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -71-5.2.4. Thixotropy, non linear behavior and time dependent properties . . . . . . . -76-5.2.5. Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -77-5.2.6. Flow time measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -83-5.2.7. General observations about testing the grout properties . . . . . . . . . . . . . -86-

5.3. Testing the masonry’s properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -87-5.3.1. Diagnosis of the masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -87-5.3.2. Reproducible masonry samples: physical model . . . . . . . . . . . . . . . . . . -88-5.3.3. Permeability of samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -89-5.3.4. Permeability of masonry structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -92-

5.4. Laboratory injection tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -93-

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5.4.1. Description of the tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -93-5.4.2. Flow charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -94-5.4.3. Simplified mathematical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -102-5.4.4. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -106-

5.5. Important findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -111-5.5.1. Blocking mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -111-

Granularity of the cement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -111-Stability of the grout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -111-Water absorption out of the grout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -111-Pressure losses, thixotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -112-

5.5.2. Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -113-5.6. Conclusions from the experimental program . . . . . . . . . . . . . . . . . . -114-

Chapter 6. Rheology of grouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -115-6.1. Introduction to the rheology of dispersions . . . . . . . . . . . . . . . . . . . . -115-6.2. Non-Newtonian behavior of aqueous dispersions . . . . . . . . . . . . . . -118-6.3. Flow of a dispersion in a cylindrical tube . . . . . . . . . . . . . . . . . . . . . . -119-

6.3.1. General equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -119-6.3.2. Newtonian fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -121-6.3.3. Bingham fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -122-6.3.4. Discussion on the Reiner-Buckingham formula . . . . . . . . . . . . . . . . . . . -123-6.3.5. Numerical simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -124-

6.4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -126-

Chapter 7. Flow of fluids through porous media . . . . . . . . . . . . . . . . . . . . -127-7.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -127-7.2. Structure and properties of porous materials . . . . . . . . . . . . . . . . . . . -127-

7.2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -127-7.2.2. Methods for porosity measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . -128-7.2.3. Permeability, Darcy’s law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -129-

7.3. Equations governing the flow of fluid through porous materials -132-7.3.1. Differential form of Darcy’s Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -132-7.3.2. The differential equations of fluid flow through porous materials . . . . -133-7.3.3. Boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -134-7.3.4. Measurement of the permeability using compressible fluids . . . . . . . . . -136-7.3.5. Radial flow between concentric cylinders . . . . . . . . . . . . . . . . . . . . . . -137-

7.4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -140-

Chapter 8. Modeling grout flow in masonry . . . . . . . . . . . . . . . . . . . . . . . . -141-8.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -141-8.2. Discrete model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -141-

8.2.1. Justification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -141-8.2.2. A network of discrete flow channels . . . . . . . . . . . . . . . . . . . . . . . . . . -142-8.2.3. Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -144-

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XXXVI Grout injection of masonry, scientific approach and modeling

8.2.4. Dealing with water absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -144-8.2.5. Special features of the program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -146-

8.3. Structure of the program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -146-8.3.1. Menu items . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -147-8.3.2. Calculation algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -153-

Chapter 9. Validating and using the model . . . . . . . . . . . . . . . . . . . . . . . . . -159-9.1. Validation of the model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -159-

9.1.1. Flow of Newtonian fluid through one dimensional cylindrical pipe . . . -159-Cylindrical pipe with constant diameter . . . . . . . . . . . . . . . . . . . . . . . . . -159-Cylindrical pipe with varying diameter . . . . . . . . . . . . . . . . . . . . . . . . . -160-

9.2. Flow of Bingham fluid through one dimensional pipe . . . . . . . . . . -162-9.2.1. Conceptual validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -163-

9.3. Using the model for parameter study . . . . . . . . . . . . . . . . . . . . . . . . . . -164-9.3.1. Grout parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -164-

Critical shear stress t c . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -165-Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -167-

9.3.2. Process parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -169-Injection Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -169-Injection holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -169-

9.3.3. Radial network: sealing of a leakage . . . . . . . . . . . . . . . . . . . . . . . . . . -172-9.4. Using the model as an engineering tool . . . . . . . . . . . . . . . . . . . . . . . . -173-9.5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -175-

Chapter 10. General conclusions and future research . . . . . . . . . . . . . . . -177-10.1. General conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -177-10.2. Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -178-

10.2.1. Information about the masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -178-10.2.2. Final goal, online controlled consolidation injection . . . . . . . . . . . . . -179-10.2.3. Using the model for other purposes . . . . . . . . . . . . . . . . . . . . . . . . . . . -179-

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Chapter 1. Introduction 3

Chapter 1. Introduction

Imbued with a message from the past, the historic monuments of generations of peopleremain to the present day as living witnesses of their age-old traditions. People arebecoming more and more conscious of the unity of human values and regard ancientmonuments as a common heritage. The common responsibility to safeguard them forfuture generations is recognized. It is our duty to hand them on in the full richness oftheir authenticity. [The Venice Charter, May 1964]The concerns to maintain ancient valuable monuments are only to a little extend driven by thedesire to keep on using these buildings, but mainly by the desire to preserve cultural heritage asa testimony from the past for the next generations. Very often this maintenance is not possiblewithout actions of restoration. The double interest implies that two aspects have to be fulfilledduring restoration: mechanical consolidation has to be combined with a method that preservesas many original aspects as possible. These considerations are written down in theinternationally accepted Charter of Venice of may 1964 about the criteria for conservation andrestoration of monuments and sites. This chapter expresses the right attitude towardspreservation of valuable monuments. The treatment, i.e. consolidation, must guarantee thatprobable external actions produce only repairable damages and no fatal artistic damage. Theauthenticity of the historic monument, concerning both its structural and architectural values,must be safeguarded. One has to realize that without structural safety the architectural valuewill be lost for ever. Using the argument of maintaining authenticity of the building to refuseany structural consolidation could result in the final collapse of the historic monument.However, the safeguarding of the monument can be improved by using modern materials andtechniques, which preserve its authenticity.In Belgium, just as in most other European countries, bricks and stones are the common buildingmaterials. Though we often do not realize that our patrimony mainly consists of millions ofcubic meters of masonry. Without any doubt one could state that maintaining important part ofpatrimony is maintaining brickwork or masonry. Masonry, just as most other physical entities,is not for ever. Physical and mechanical actions lead to the decay of building materials. Incourse of time, masonry buildings get physically deteriorated, either by inadequate design,human intervention or natural causes. Frequently occurring natural causes for damage areweathering, freeze thaw action, erosion of the mortar by rain water flow, overload byearthquake actions. A frequent harmful human intervention is inappropriate loading which is adirect action but also the lowering of the groundwater table causes soil settlement and damageto the surrounding buildings. As these damage processes continue, repair or restoration andconsolidation are required. Among others, grout injection is a powerful consolidationtechnique to overcome structural decay. The introduction of a binding agent in liquid form intothe masonry fills the holes, voids and cracks. After the hardening of this binding agent, the

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4 Grout injection of masonry, scientific approach and modeling

masonry will regain its monolithical behavior and the overall mechanical resistance willimprove. As such, grouting restores the missing or deteriorated binding element of masonry. Many techniques for the restoration of masonry structures are available. All these techniquesimply a certain infringement on the authenticity of the monument. In the ideal situation, theintervention may not cause any damage to the structural and architectural authenticity of thebuilding and should be, as far as possible, reversible. In this complex and multi-disciplinarydomain of restoration, grout injection has found its place as a consolidation technique forancient masonry just because the technique is able to mechanically strengthen the historicmasonry monument without changing its outlook and integrity.The grout is introduced into the internal, non visible part of the masonry and, because of this,does not damage the aesthetical outlook of the building. However, grouting is not reversible,but when materials are used that are compatible with the original materials, it certainly is ajustified technique that fulfills the requirements of safeguarding the monuments authenticity. The Building Materials Division of the Civil Engineering Department of the KatholiekeUniversiteit of Leuven has been involved in several practical injection jobs as consultant anddecided to investigate restoration of masonry more thoroughly. The present thesis discusses in general grouting as a consolidation technique for masonry. Inthe first chapter the structural aspects of masonry and the physical deteriorating mechanisms arelisted and a probabilistic method is presented to judge the need for consolidation. It istheoretically shown how the uniform filling of the voids and holes overcomes the splittingforces that are present around these holes and how the reliability of the masonry structureincreases after injection. Different binding agents are listed and the advantages anddisadvantages are indicated. A technological part gives an overview of injection methods andindicates how an ideal injection installation should look like. This book also presents theexperimental program. The results of the experiments provided an enhanced physicalunderstanding of the injection process. Most important are the findings with regard to theinjectability of grouts, rheological properties of grouts, characterization of masonry forinjection purposes. For all three of these aspects new or existing tests have been developed oradapted to the peculiarities of grouting. But the originality of this work lies in the modelling ofthe flow of the grout through the masonry. The masonry is simulated by a network of discretechannels representing the big channels through which, starting from the injection hole, the groutpenetrates the masonry. The penetration of the grout into the areas with finer void structure isbuilt in by means of capacitive elements. The water absorption out of the grout by the drymasonry is also incorporated. This way, the model is a useful design tool to determine theinjection parameters: grout composition; viscosity, shear stress and hardening evolution,injection holes pattern and injection pressure.

Chapter 2. Masonry and its need for restoration

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Chapter 2. Masonry and its need for restoration 5

2.1. General description of masonryHere we intend to give an overview of the most important developments about masonry incourse of time, especially since some developments have their impact on nowadays restorationprojects. Masonry is a handmade construction element, made by assembling natural and artificialmaterials, eventually using a binding agent (Figure 2.1). The natural materials are the stones,as they were found or as they were prepared into the requested dimensions. The more reliablethe format and the size of the stones, the thinner the layer of binding mortar can be. Masonryexists for many centuries. It is a gradual evolution of placing big stones one on top of eachother to the agglomeration of fine prefabricated stones and mortar that is used today. Withregard to restoration, there exists a big difference between the massive monuments of theEgyptians and the Medieval churches of the European regions. The ancient massive monuments are very impressive, they mainly consist of big walls andpillars. However, they do not or hardly create a free space: the distance between the supportsis mostly very short. The use of arches, vaults and domes in the more recent monuments allowsto deviate the gravity forces towards the foundations [J.H. Acland, 1972][J. Fitchen, 1985].An overwhelming feeling of space is created. The latter buildings are more slender, moreelegant, but also more vulnerable to mechanical damage for which consolidation injection canbring a solution. Because of that, this chapter describes the different component of masonry, limited to theEuropean masonry types, being a agglomeration of small stones and bricks. Later in thischapter possible damage phenomena are discussed. Only part of these phenomena arerepairable using consolidation injections.

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6 Grout injection of masonry, scientific approach and modeling

Figure 2.1: The Paterskerk in Tienen, a valuable Medieval monument that wassuccesfully safegarded using grout injection

2.1.1. Masonry componentsBricks

Bricks are artificial stones made by baking clay at high temperature. Bricks were originally anersatz for the natural stones for those regions where no excavation of stone was possible Figure10.1. The baking process has been improved a lot since the first field ovens. Nowadays, thetechnology has developed and bricks having a constant composition and quality can beproduced. Though it is important to have in mind the ancient way of producing bricks in orderto be aware of some possible problems. It is important to be aware of the historical productionprocess. To give an example: the bricks that were positioned at the outer side were of a betterquality than the bricks inside. During some restorations this fact was disregarded resulting inmixing all the bricks. When a soft brick is used to rebuild the facade it will soon be eroded byrainwater and freeze thaw actions (Figure 2.2).

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Chapter 2. Masonry and its need for restoration 7

Figure 2.2: During the restoration of the Beguinage of Leuven, soft bricks were placed inthe facade leading to accelerated damage phenomena, such phenomenon can not berepaired using injections

In our countries, the brick can be seen as a religious stone. Brickwork has been the monopolyof the monks for many centuries [G. Peirs, 1979]. They have developed the technology ofmasonry and transported the knowledge all over Europe. Bricks were a necessity to buildhouses of God that would last longer in those regions where no natural stone was available. The brick has numerous appearances. The dimensions of the brick, the color and the chemicalcomposition reveal the origin of the bricks. The size, the composition and the color can help todate the building. These properties and the big aesthetical and technical difference between theold original bricks and the new bricks make it sometimes hard to replace weathered bricks bynew ones. In any case the authenticity of the building will be damaged. The technicaldifference between the modern so called hand made bricks and the old bricks is caused by thefollowing facts: C the clay, used to make the old bricks, contained more organic material than nowadays.C there were hardly any possibilities to enhance the mineral composition and the

granularity of the clay. Clay was suitable for the production of bricks if a fist of claydid not fall apart after drying.

C a higher water content to provide the necessary plasticity to the clay that had to bemixed using manpower, implies a higher porosity and a higher shrinkage.

C the limited pressure applied to the clay when introduced in the mold

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8 Grout injection of masonry, scientific approach and modeling

C a slow drying process in open air compared to the drying process in dry ovensnowadays

C uncomplete sintering because of the lower baking temperature or because of lessmaterial, present in the clay, that has a lower sintering temperature.

The higher porosity dos not necessarily mean a negative property for the brick. The highporosity can have a good impact on the moisture household, the frost resistance, the density andacoustic and thermal insulation. The actual production process provides more homogeneous bricks, better baked and containingless impurities.

Natural stoneBricks have, in course of time, reduced the use of natural stone for masonry. Though,brickwork is often combined with masonry of natural stone, especially in valuable monumentsince natural stones gave the building an image of wealth and were known to be very durable.The replacement of natural stones can be problematic if the quarries are no longer exploited.Besides, many natural stones undergo an accelerated weathering because of air pollution. Theoriginal stones and the replacing stones have a different structure and because of smalldifferences in the weathering resistance, the homogeneous outlook of the facade is disturbed.The fabric of natural stones in the old artisanal way is expensive. The knowledge, the tools andthe workers are not readily available. Most buildings were erected using the stones available in the neighborhood. Four our regionsthis means relatively soft stones such as marl, limestone, sandstone or iron sand stone. Thosenatural stones withstand poorly the effect of acid rain. When the replacement of natural stones is necessary, one should try to find a similar stone, withthe same composition, the same porosity providing similar properties with regard to watertransport and the same frost resistance.

MortarMortar consisting primarily of lime and sand, has been used as an integral part of masonrystructures for thousands of years. Until about the middle of the 19th century, lime was deliveredto construction sites, where it had to be slaked, or combined with water. Mixing with watercaused it to boil and resulted in a wet lime putty that was left to mature in a pit or wooden boxfor several weeks, up to a year. Traditional mortar was made from lime putty, or slaked lime,combined with local sand, generally in a ratio of 1 part lime putty to 3 parts sand by volume.Often other ingredients, such as crushed sea shells (another source of lime), brick dust, clay,natural cements, pigments, and even animal hair were also added to mortar, but the basicformulation for lime putty and sand mortar remained unchanged for centuries until the advent ofportland cement or its forerunner, Roman cement, a natural, hydraulic cement. In the 1930smore new mortar products, intended to accelerate and simplify masons' work, were introduced.

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Chapter 2. Masonry and its need for restoration 9

These included masonry cement, a premixed, bagged mortar which is a combination of portlandcement and ground limestone, and hydrated lime, machine-slaked lime that eliminated thenecessity of slaking quicklime into putty at the site.

CaCO3 + heat (± 900EC) Y CaO + CO2ü (Eq 2.1)

CaO + H2O Y Ca(OH)2(Eq 2.2)

Ca(OH)2 + CO2 Y CaCO3 + H2O (Eq 2.3)

The chemical reactions (Eq 2.1) and (Eq 2.2) take place during the production of hydratedlime. The calcium carbonate is provided by lime stone or by shells. This calcium carbonate isdissociated at high temperature. The Calcium oxide is then extinguished using water in theexact dosage (providing white powder) or in over dosage (providing the white putty asmentioned above). The lime provided by burning limestone has no hydraulic properties. Thelime made by burning the coquilles has, due to some clay impurities, some hydraulic features.The hydraulic properties gave these mortars a good early strength development. Reaction (Eq2.3) is using CO2 and hence requires the presence of air. For a thick wall, this might be aproblem. The transport of air towards the fresh mortar was ensured by leaving some of thepointing open. This proves the understanding of the hardening mechanisms of lime mortar.The preference for a mortar showing slightly hydraulic properties, was already mentioned. Todonate some hydraulic properties to lime mortars, mineral admixtures were used. Very wellknown in our countries is Trass. Trass has, just as the volcanic earth from the Vesuvius used bythe Romans or the Santorrini earth in Greece, hydraulic properties. Other hydraulic admixturesexisted. Actually portland cement was originally used as hydraulic admixture. Only for theapplication of making concrete, portland cement was used as a pure binding agent. Portlandcement was patented in Great Britain in 1824. It was named after the stone from Portland inDorset which it resembled when hard. This is a fast-curing, hydraulic cement which hardensunder water. Until the turn of the century, portland cement was considered primarily anadditive, or "minor ingredient" to help accelerate mortar set. By the 1930s, however, mostmasons used a mix of equal parts portland cement and lime putty. Thus, the mortar found inmasonry structures built between 1873 and 1930 can range from pure lime and sand mixes to awide variety of lime, portland cement, and sand combinations. The mixing proportions of limeand sand have changed a lot during the 19th century. In the beginning of the century no or veryfew sand was added to the lime. In 1833 to 1850 some sand is added, but still the compositionpossesses more lime than sand. A manual of 1874 mentions a ratio of 1 part of lime on one partof sand. By the end of the century a lime:sand ratio of 1:3 is usually used.

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10 Grout injection of masonry, scientific approach and modeling

Figure 2.3: Bending/ splitting caused by hard cement grout among soft lime mortar

The use of the relatively soft, air hardening lime mortar gave ancient masonry a good capacityto recover from settlements. The lime mortar has a slow strength development. For historicalbuildings that were constructed slowly, this was not a big disadvantage. On the contrary,deformations during the construction distributed and moderated the stresses. Furthermore, themortar remained less strong than the stones. Occurring cracks were located in the mortar joints,where they could easily be hidden by repointing. Generally, a lime mortar is more elastic andtougher. This provides an additional safety with regard to differential settlements. Lime mortarcontains no or hardly any sulfate or alkaline. This reduces the risk for salt efflorescence.The above implies that there is a double reason to use mortars that are compatible with theoriginal mortars. First of all there are technical reasons: using a modern mortar would resultin introducing a component that is harder than the old mortar and in most cases also harder thanthe stones that were used. Settlements become hard to follow. A hard nucleon, created by thehardening of the cement grout causes tensile forces to occur in the masonry. The new hardmortar splits the masonry just above the hard zone since the zone on the left and right hand sideof the hard nucleon are softer and more deformable. One gets a kind of bending/splitting actioncaused by the hard part of the laying mortar and the mass of the masonry above (Figure 2.3).Furthermore, a cement mortar has a different porosity causing a different action with regard towater transport.

Second valuable reason is the aim not to introduce materials that were not used in the originalbuilding as it is mentioned in the Venice Charter. General rule of thumb is to use mortars thatimitate the original mortar and that are as hard as the original one or even somewhat softer.Grout composition will be further discussed in Chapter 3 and Chapter 4.

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Chapter 2. Masonry and its need for restoration 11

PointingUntil the 18th century the pointing was done immediately after placing the stones, with the samemortar, resulting in a solid unit [G. Peirs, 1979]. Later, in the 18th century when the masonry artaimed a narrow layer between the bricks, the pointing was done afterwards. Depending on thefashion of each period and the architects vision, a flat pointing or a drawing back pointing hasbeen applied. Repointing is often an important action in the restoration of a facade. Most of the time only partof the facade is damaged. However, a global repointing is mostly done, using the argument ofaesthetical harmony. Thus, the remaining pointing are cut out. This should be done cautiouslysince the stones and the undamaged pointing form a unit that is often damaged by this action.The new pointing becomes wider than the original one, completely changing the outlook of thehistorical building. Likewise, other undesirable effects take place. The new mortar used forrepointing is, when disregarding the original composition, almost impermeable to water incomparison with the original situation. Therefore, the water concentrates just behind therepointed layer. Salt crystallization or frost can then easily push the new layer outwardscausing even more damage since the adhesion of the repointed mortar to the existing bricks isvery good. A general advise is only valuable when applicable, but one should limit therepointing to the damaged parts and use a mortar composition that corresponds as well aspossible with the mortar used for the original pointing. Very often, a consolidation injection iscombined with a partial or general repointing.

2.2. Causes of damage to masonry2.2.1. Physical and physico-chemical mechanismsPhysical mechanisms mostly need a long period of time to cause visible damage. Manyphysical mechanisms are related to the presence of water inside the structure. Porosity,capillarity and permeability regulate the transport of moisture. Masonry is a highly porousbuilding material. The rain water is absorbed by capillary action. The presence of waterinside the stones and the mortar means a real danger for weathering mechanisms. Mostimportant weathering mechanism is the frost damage. Frost damage can be recognized by thetypical fractures along the frost front. The frost resistance of building materials can be checkedeither by a direct test applying frost - thaw cycles [NBN B05-203] or by an indirect test. In theindirect test [NBN B05-201] the frost resistance is judged by analyzing the capillary waterabsorption. From this analysis, the GC-factor can be calculated. The GC-factor gives a goodindication about the frost resistance of the bricks. A low GC-factor (e.g. lower than -2,5)indicates a very good resistance to frost damage. When the GC-factor is higher, additionaldirect testing of the frost resistance is advisable.

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12 Grout injection of masonry, scientific approach and modeling

Figure 2.4: An example of typical frost damage to masonry, which can not be repaired byinjection

A hydrofobic treatment can reduce the absorption of rain water. Nevertheless, very often thewater inside the masonry is not only due to rain water absorption. Other sources exist. Thecapillary suction and rise of groundwater can not be avoided by a surface treatment with hydro-fobic agents. When groundwater is absorbed by the masonry, one should not apply ahydrofobic treatment. The outer layer of the masonry becomes impervious to water. Onlywater vapor is able to be transported. This means a much slower process of loss of water thenwhen the water is able to proceed to the surface itself. This way two phenomena take place:salts, soluble in water, are left in the transition zone were further water transport occurs byvapor. The remaining hygroscopic salts cause big internal pressure when crystallizing, resultingin the spalling off of the treated outer layer of masonry. Since the transport mechanism of vaporis slower then the transport of water, a concentration of water will be present in the transitionzone. When temperature drops, this water will freeze and the expansion at freezing will pushaway the outer layer of masonry.

Furthermore, moisture movements give rise to the dissolution and the corrosion of the binder.When the mortar is leached out, the internal cohesion decreases. Grout injection is verysuitable to repair this kind of damage. The more uniform the grout fills the voids caused by theerosion of the binder, the better the final consolidation. Moisture movement also causes the

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Chapter 2. Masonry and its need for restoration 13

transport of soluble salts [D. Van Gemert, 1988(1)]. These salts can crystallize in a differentplace and might push off the outer masonry layer.

Lime stone suffers a lot from acid rain. Sulphur dioxide enters the lime stone and dissolves theCaCO3 and forms calcium sulfate from calcium sulfate and oxygen. The calcium sulfate reactswith water and forms gypsum. This causes no problems, except for some efflorescence that caneasily be washed with water. When a cement grout is injected, C3A is imported in the masonry.In combination with gypsum and water, C3A enables the formation of ettringite (C3A . 3CaSO4 .31 H2O), a very expansive mineral due to its high water binding capacity: 31 H2O. Calcium di-nitrate, present in many fertilizers is supplied by absorbed groundwater and crystallizesexpansively. Thermic cycles cause cyclic stresses inside the material. They cause cracks, situated in theouter layer of masonry because of the large tensions that occur. The mortar may be eroded by rain water. A good maintenance of the building is the bestprevention for damage. Lack of maintenance speeds up most of the aforementioned erosionphenomena. This necessitates a repointing, but if not discovered in time, the structure can bedamaged to such an extend that a consolidation becomes necessary.

2.2.2. Mechanical damageMistakes in the original design or concept of a building can cause mechanical damage soonafter or even during the construction of the building. A poor dimensioning of structural parts oran unexpected settlement of the soil are frequently occurring reasons for early damage. Modifications by man of the original structure often cause additional damage to the structure.The construction of a higher tower on the church means a load for which the church and itsfoundations were not dimensioned. A different use of the building, for instance a libraryfunction or a dancing room often means a load much higher than the design load. Restorationactions can disturb the original distribution of forces: the placement of anchors, the placementof stiff units or enlarging the openings for the windows are just a few illustrations of possiblemistreatment by man. Lowering the groundwater table for construction works in theneighborhood changes the load bearing capacity and especially the deformational behavior ofthe soil. This can cause large settlements and subsequent fissuration to the historical buildings.These fissures mostly pass through the entire wall. They can be filled by grout injection. Vibrations induced by man, machinery, traffic or construction works may cause deformationsand smaller or larger forms of distress in buildings, their structural members and nonstructuralelements [H. Bachmann, 1987]. Forms of distress are:C cracking of walls and slabsC aggravation of existing cracking in structural members and nonstructural elementsC falling down of equipment or cladding thereby endangering occupants

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14 Grout injection of masonry, scientific approach and modeling

Figure 2.5: Maximum allowed peak values for vibrations, dependingon the category of building [DIN 4150, part 3]

Continuous vibration, however, can also lead to problems of fatigue and overstress in principalload-bearing members. The degree of damage depends upon the quality of the buildingmaterial, the type of construction, the properties of the building foundation, the main dimensionsof the principal load-bearing members, the age of the building, the duration and thecharacterization of the vibrations. The vibration velocities can be measured usingaccelerometers. Part 3 of the German standard DIN 4150 treats effects on buildings andstructural members due to an internal or external source of vibration (Figure 2.5).

The Association of Swiss Highway Engineers distinguishes in their Standard SN 640312 fourdifferent categories of buildings mainly according to the type of construction. The acceptancecriteria are again peak vibration velocities. The most severe acceptance criteria are valid forbuildings that are very vulnerable for vibration or that are worth protecting. Historicalmonuments belong to this group. The allowed peak vibration velocity depends upon the kind ofsource and the frequency of the vibrations.

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Chapter 2. Masonry and its need for restoration 15

Structuralcategory

Source M Source S

f [Hz] vmax [mm/s] f [Hz] vmax [mm/s]

IV: vulnerablebuilding/ worthprotecting

10 to 30 3 10 to 60 8

30 to 60 3 to 5 60 to 90 8 to 12

Source M: Machinery, traffic or construction works

Source S: Blasting operations

Table 2.2: Acceptance criteria of SN 640312 for historic buildings worth protecting

StructuralCategory

Definition

I reinforced-concrete and steel structures( without plaster) such as industrialbuildings, bridges, masts retaining walls, unburied pipelinesunderground structures such as caverns, tunnels galleries lined and unlined.

II buildings with concrete floors and basement walls, above-grade walls ofconcrete, brick or ashlar masonry; ashlar retaining walls, buried pipelinesuderground structures such as caverns, tunnels galleries, with masonry lining

III buildings with concrete basement floors and walls, above grade masonrywalls, trimber joist floors

IV buildings which are particularly vulnerable or worth protecting

Table 2.1: Structural categories according to SN 640312

If the occurring vibrations cause velocities that are higher than the above mentioned maximumpeak values, they might cause structural damage.Mechanical damage is not only due to human actions. Heavy wind and rain or storms can causesevere damage, as well as seismic action [M. Tomazevic, 1982]. The corrosion of steel, forinstance from anchors, is an expansive process. The corrosion spot is only an aestheticalconsequence, but the mechanical damage is often more important. Mechanical damage is very suitable to be repaired by grout injection.

2.2.3. Biological damageJust to be complete, biological damage mechanisms are mentioned. The formation of algae is amore aesthetical problem. Still they produce, in combination with organic material such asrotten leafs or pigeons excrements, acids that deteriorate limestone and mortar. The fine rootsof plants and trees can enter the openings inside the masonry, spalling it apart. Micro

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16 Grout injection of masonry, scientific approach and modeling

Figure 2.6: Plain masonry structure: a collection of heterogenities

organisms bring along bacteria that produce nitrates and sulfates as residue of their metabolism.These are the causes of biogenic erosion phenomena.

2.3. The loading of masonry2.3.1. Probability of failureThe above description of masonry reveals that masonry is a composite, heterogenous buildingmaterial. These facts make it impossible to accurately predict its behavior, by the simpleknowledge of the mechanical properties of its constituents. This is true for plain masonrystructures (Figure 2.6). For three leaf rubble core masonry the situation is even more complex(Figure 2.7).

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Chapter 2. Masonry and its need for restoration 17

Figure 2.8: The dead weight of masonry may prevent tensile stresses due to bending

Figure 2.7: Additional heterogeneities in case of double leaf masonrywith rubble core

The general design rules avoid tensile stresses inside the material. The use of arches, vaultsand domes allows to deviate the gravity the foundations [J.H. Acland, 1972][J. Fitchen, 1985].The combination of a good resistance to compression and the ability to absorb largedeformations provide a great capacity to absorb deformation energy. The dead weight ofmasonry helps to protect the masonry from tensile stresses that could be caused by bending,eccentric loading or horizontal forces such as wind load (Figure 2.8).

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18 Grout injection of masonry, scientific approach and modeling

Figure 2.9: A hole in a massive masonry part induces both higher compressive andtensile stresses. [D. Van Gemert, 1984]

Figure 2.10: The compressive loading causes tensile stresses where the brick is incontact with the mortar layer due to a different value of the Poisson ratio.

In the ideal situation, as shown in Figure 2.9, theory of elasticity learns that around circularholes, not only compressive stresses up to three times the average stress arise, but also tensilestresses up to the average compressive stress. In the non ideal case of non circular holes thesituation is worse and the stress concentration levels are even higher.

The difference in module of elasticity and the difference in the Poisson ratio also causes tensilestresses in the bricks as can be seen from Figure 2.10

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Chapter 2. Masonry and its need for restoration 19

R(t) & S(t) < 0 (Eq 2.4)

pf ' P[ R & S < 0] for the reference period tL (Eq 2.5)

pf ' P[ g(R,S) < 0] where g (R,S) is the limite state (Eq 2.6)

To characterize the behavior of masonry under vertical and horizontal loads, the twofundamental mechanical properties are its compressive strength and its shear strength. Thecompressive strength is a function of the quality of the binder and the strength of the stones orbricks. Empirical formulas found in literature enable a first estimation of this strength as afunction of the strength of binder and stone or brick and according to the quality of the masonry. Appropriate design methods are based on a reliability analysis of the building [Eurocode 1] [L.Schueremans, 1996] [L. Schueremans, 1999]. The idea behind a reliability analysis or behindthe evaluation of the reliability is relatively simple to explain using the basic reliabilityproblem. Both the load on a wall (S) and the strength of that wall (R) are stochastic variables.Since their exact value is unknown, they are represented by the probability functions, fS(s) andfR(r) respectively. The loading S and the resistance of the masonry R are both a function oftime. The load has the tendency to increase whereas the load bearing capacity of the structurehas a decreasing trend due to all kind of degradation processes. Many mechanisms ofdeterioration are discussed above. The edge of safety will be passed at a certain moment t,where

The probability that this happens is the probability of failure pf. For mathematical reasons thetime dependent probability functions are transposed to time invariant functions. The reliabilitycalculation is then made for a determined period of time, a reference period. The probabilityof failure can then be expressed by

The above expression can be generalized to

The probability of failure is calculated using a FORM-algorithm (a First Order ReliabilityMethod). This FORM-algorithm calculates the reliability index ß that is related to theprobability of failure pf by the standard cumulative probability function. The relation betweenboth parameters is given in Table 2.3. The smaller the probability of failure, the higher thereliability index. A reliability index of 3,7 corresponds to a probability of failure of 1/10.000within the design working life, which is generally seen as a minimum safety level.

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20 Grout injection of masonry, scientific approach and modeling

Pf 10-1 10-2 10-3 10-4 10-5 10-6 10-7

ß 1,3 2,3 3,1 3,7 4,2 4,7 5,2

Table 2.3: The relation between pf and ß [Eurocode 1]

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15

stress (S) - strength (R) [N/mm2]

f

fSfR1fR2

R2

L

0

1

2

3

4

5

6

5 7 9 11 13

Mean value [N/mm2]

Rel

iab

ility

in

dex

reliability index

target value : beta = 3.7

R1R2

Figure 2.11: Influence of increasing the strength of the wall on the reliability index.

Looking at the method of reliability analysis, one can discover the parameter that influences theprobability of failure. First possibility is to increase the separation between the strengthfunction R and the loading function L. This can be done by either adapting the loading, forinstance be reducing the loading of the floors, or by adapting the strength. The latter ispresented in Figure 2.11. The original situation is represented by R1. The overlap areabetween the stresses caused by the load on the structure (L) represents the probability offailure. Increasing the average strength as is the case for situation 2 (Figure 2.11, f R2 > f R1)makes the strength probability distribution function to shift to the right. Therefore, thereliability index increases (Figure 2.11, R1 ÷ R2)

A second possibility is to reduce the variance on the strength. The average strength remains thesame, but the extreme values differ less from the average value. Therefore the uncertainty onthe strength decreases. The strength distribution function f R3 is much narrower than thedistribution f R1. As can be seen, the overlap area (Figure 2.12) decreases significantlyalthough the average strength does not increase. The reliability index for situation 3 issignificantly higher than for situation 1.

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Chapter 2. Masonry and its need for restoration 21

µ ( f wc,i ) 'Vtot& V0

Vtot

. µ( f wc, o) % (Vi

Vtot

) . µ( fi c) (Eq 2.7)

0

2

4

6

8

15 25 35 45 55Coefficient of variation [%]

Rel

iab

ilit

y in

dex reliability index

target value : beta = 3.7

R1

R3

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15

stress (S) - strength (R) [N/mm2]

f

fS

fR1

fR3

S R3

Figure 2.12: Influence of the variance on the strength of the wall on the reliability index

2.3.2. Grout injection increases the reliability of masonry The increase of the average strength of the masonry or the reduction of the variance of thestrength or both are the intention of a consolidation injection as stated by L. Schueremans [L.Schueremans, 1996]. To have an idea of the influence of a consolidation injection on thereliability, one needs an expression that indicates how the strength function R changes infunction of the injection: the used grout and the injected volume. It is supposed that the averagestrength depends upon the average strength of the original masonry and the average strength ofthe injected grout. Both materials contribute to the global strength in relation with their relativevolume. This brings us to the expression for the average strength of injected masonry [P.Tassios,1995]:

where fwc,i : the strength of the masonry after injection, which is on its turn astochastic variable fwc,i .LN ( µ(fwc,i),(s (fwc,i))2)

Fwc,o : the original strength of masonry without holes, also a stochasticvariable: fwc,i .LN ( µ(fwc,o),(s (fwc,o))2)

Fic : The compressive strength of the injected grout being a stochasticvariable: fic . LN ( µ(fic),(s (fic))2)

Vo : volume of holes in the masonryVi : injected volumeVtot : total external volume of the masonry structure

It is obvious from the above expression thatC the higher the injected volume, the more the averages strength of the injected material

will increase for constant average compressive strength of the original masonry and ofthe injection grout. This is a very important finding and it proves that a uniform filling

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22 Grout injection of masonry, scientific approach and modeling

cov ( fwc,i ) ' Vtot & d12

Vi % VL

Vtot & VL

cov (fxc,o) % d12

Vi

Vtot & VL

cov ( fic) (Eq 2.8)

of all voids is very important. This fact imposes high rheological demands on thecomposition of the grout. They will be discussed later.

C the higher the mechanical strength of the grout, the higher the resulting average strengthof the injected masonry on the condition that the injected volume remains the same. Ithas to be mentioned that the injection of a very strong grout increases the heterogeneityof the masonry.

C or only the voids can be injected.Vtot & Vo

Vtot

%Vi

Vtot

'Vtot & Vo % Vi

Vtot

< 1

It can be concluded from the above discussion that injecting a grout provides a higher averagestrength of the masonry and hence reduces the probability of failure.

In the mean time, grout injection also reduces the uncertainty of the strength of the masonry.Filling the voids reduces the variance on the masonry strength by making the material morehomogeneous. Compression tests on cylinders, cored from ancient masonry, show a varianceof about 40 %. The decrease of this variance on the compressive strength is due to:C a uniform filling of the masonry by the grout.C a better internal cohesion of the masonryThe final variance depends on the degree of filling, but also on the variance of the strength ofthe grout. It can be concluded that grout injection transforms the masonry into a more homogeneousmaterial and it provides a higher average strength and that the uncertainty about the strengthbecomes smaller, even if the average strength would remain the same. A possible expression to calculate the variance on the strength is given by

where cov(...) : the variance d12 : kronecker delta, d12 = 0 if Vi = 0

d12 = 1 if Vi Ö 0VL : volume of remaining voids after injection

Equation (Eq 2.8) is the result of the following reflections:

C the better the injection, the smaller the amount of voids that are remaining VL.. Thisleads to a lower variance on the strength

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Chapter 2. Masonry and its need for restoration 23

C the better the injection, the higher the injected volume (Vi). This increases the relativeimportance of the variance on the strength of the injected material in equation (Eq 2.8).If the variance of the injected material is smaller then the variance on the originalmasonry, this effect is enhanced.

2.4. Conclusions of Chapter 2Above, a general description of masonry as a heterogeneous building material is given.Possible causes of damage are listed and discussed. From literature study it is obvious thatpart of these damaging phenomena give rise to structural problems. In those cases, groutinjection can be a solution to retrofit the building [P. Shing, 1994] [G. Penelis, 1989]. As isproved by the presentation of the reliability method to judge the safety condition of an ancientmasonry, the increase of the average strength and the reduction of the strength variance give riseto an improved safety and a lower risk of collapse. The reduction of the variance of thestrength mainly depends upon the degree of uniform filling of the voids and cracks. Aninjection of a piece of masonry that only partially filled the masonry, might increase the averagestrength but will certainly also increase the variance on the strength. So, a good and uniformpenetration of the grout inside the masonry is crucial to achieve a successful consolidation.The study, presented in this book, aimed to investigate which conditions give rise to therequired penetration and the uniform filling of internal voids in masonry.The experimental program enabled to understand the physical phenomena that happen during aninjection. It has shown which conditions cause a poor penetration and which conditionsprovide uniform filling. The test injection provided information about the conditions leading toa good penetration of the grout. The model facilitates to find out which parameters of thegrouting process can successfully be adapted to guarantee the most uniform filling of the voids.This way the experimental results, as well as the use of the model, can help to realize betterconsolidations of monuments.

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24 Grout injection of masonry, scientific approach and modeling

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Chapter 3 - Injection as a consolidation technique for masonry 25

Figure 3.1: The grout filling the rubble core can be seen as a gravity injectionFigure 3.2: Schematical representation of gravitational injection, used nowadays in Mexico

Chapter 3. Injection as a consolidation technique formasonry

3.1. Historical reviewHumanity has always tried to maintain its patrimony. Not in the first place to continue to makeuse of it, but mainly to preserve cultural heritage as a testimony from the past for the nextgenerations. The double interest implies that two aspects have to be fulfilled during restoration:mechanical consolidation has to be coupled to a method that preserves as many original aspectsas possible. Therefore, engineers need help from art historians to know about the historicalmaterials, the historical way of building. The materials used for restoration should becompatible to the historical materials, physically as well as chemically. The intervention maynot cause any damage to the structural and architectural authenticity of the building and shouldbe, as far as possible, reversible. In this complex and multi disciplinary domain of restoration,grout injection has found its place as a consolidation technique for ancient masonry toovercome structural decay. Grouting consists of introducing a binding agent in liquid form into the masonry to fill the holes,voids and cracks. The binding agent will cure and increase the internal cohesion. Afterhardening, the masonry will regain its monolithic aspect and show an increased load bearingcapacity. The grout is introduced into the internal, non visible part of the masonry and does notdamage the aesthetical outlook of the building. However, grouting is not reversible but when materials are used that comply with the originalmaterials, it is a justified technique, complying with the Charter of Venice.

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26 Grout injection of masonry, scientific approach and modeling

One could enumerate different injection techniques depending on the driving force that isforcing the grout to penetrate inside the masonry. The filling of the rubble core masonry couldbe seen as the first injection what so ever. This technique consists in the pouring of a fluidmortar inside the space between the two leafs of the structure Figure 3.1. This fluid mortarpartly fills the openings of the rubble and provides some adhesion of the middle part to bothleafs. Since gravity was used to make the mortar flow downwards it can be seen as aninjection by gravity. Also for the first consolidation injection of a masonry building, performedin 1802 under the supervision of engineer C. Bérigny, hydrostatical pressure was used to forcethe grout to penetrate inside the walls [C. Besson, 1989]. The hydrostatical pressure is createdby a grout column. The constant height of the column keeps the injection pressure constant,which is a main advantage of gravity grouting. The pressure will be limited because it ispractically unfeasible to create a very high grout column. This procedure guarantees a constantlimited injection pressure, which is a big advantage to prevent further damage. The density of acement grout is about 1600 kg/m3. Hence, the height of the grout column to apply a pressure ofone bar equals 6.25 m. In 1871, the royal building inspector Daser injected cement grout to stabilize tunnels. He wasthe first to use a mechanical pump to inject the grout. Already in 1888 the first industrialpressure grouting machines are produced in England and Germany enabling higher injectionpressure. Nowadays, modern pumps can build up very high pressures. So the grout can bepumped to elevated injection holes up to 100 meters or higher. Control systems prevent thepressure to exceed the desired injection pressure. In case of important consolidation works,pumped grouting is actually the only option due to its superior efficiency and versatility and dueto its ability of injecting large volumes of grout.For smaller jobs, for instance the consolidation of a gate, the pedestal of a statue or thefoundations of a small building, manually pumped grout can be considered.

3.2. Injection Technology3.2.1. Modern injection installation, ideal situationA modern injection installation, suitable for important injection works, consists of a completeset of devices to achieve an optimal result. Hereafter, the necessary components of aprofessional injection equipment for mineral grouts, as represented on Figure 3.3, aredescribed with regard to their ability of making an injection work successful.

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Figure 3.3: Modern injection installation with the necessary components

C Mixing installation [Figure 3.3, A]This device mixes the materials. The binding agent, the water, admixtures and additives are puttogether and thoroughly mixed. A thorough deflocculation of the particles of the binder is atleast as important as producing an homogeneous grout. This requires a mixing procedure thatprovides high shearing. High turbulence mixing is one possibility, ultrasound mixing is evenbetter [E.E. Toumbakari, 1999(1)]. It has to be mentioned that ultrasound mixing installationsare not commercially available yet. According to Miltiadou [A. Miltiadou, 1991] ultrasoundmixing has important and observable advantages. Therefore, it is advised to obtain a suitablegrout in two steps: a first high turbulence mixing to produce an homogeneous liquid, followed

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28 Grout injection of masonry, scientific approach and modeling

by an ultrasound mixing to deflocculate. The latter increases the stability and the injectabilityof the grout significantly. After mixing the grout enters the collector.

C The collector [Figure 3.3, B]This recipient contains the grout, previously mixed in the mixing installation. To keep thecement particles in suspension, not more not less, the collector continuously stirs the grout. Ithas to be mentioned: the action of the collector is insufficient to mix the grout properly.Because of dosage, the mixing takes place in discrete batches, whereas the pumping is acontinuous process. This requires a buffer. The collector acts as a buffer to the pumpinginstallation.

C Pumping installation [Figure 3.3, C]The pumping installation is fed by the collector. The pumps can either be of volumetric ofcontinuous type. The volumetric pumps provide a pulsating pressure, just as the heart beat.One might think that this ram effect helps the injection. This seems not to be the case. Binda[L. Binda, 1993] indicates in her research that a constant injection pressure provides the bestresults. The pumping installation should therefore be able to supply a uninterrupted dischargeof grout at constant pressure. A volumetric pump, at the moment of recharging the grout stopsflowing and thixotropic mechanisms act. This phenomenon is enhanced by the water absorptionout of the grout. Both actions reduce the penetration of the grout in the masonry.Furthermore, the pumping installation needs to be sufficiently strong to overcome thehydrostatic pressure. For high buildings, such as church towers, this hydrostatic pressure canbe quite important. The density of the grout is about 1.6 kg/dm3. This means that the hydrostaticpressure reaches 8 bar to reach the injection holes at the top of the building. The installationhas to overcome the pressure losses in the conduit to maintain the injection pressure at the inletof the injection hole.For large volumes, an electric pump is recommended. It is essential that this device enables arapid control with a facility to stop the pumping in a second. An suitable admission system canprovide the same safety without halting the pump. Manual pumping provides a better control ofpressure. They are more compact and lighter, so they can be positioned next to the injectionplace.

C The conduits [Figure 3.3, D and E]Flexible tubes lead the grout from the ground installation to the infection hole. These tubes canbe considerably long, depending on de size of the building and the mobility of the groundinstallation. To limit the charge losses it is recommended to take the diameter of the tubes largeenough. To decrease the amount of grout in the tubes, and hence the time it takes for the grout toreach the injection hole, a smaller diameter is preferable. In a professional equipment a doubleconduit is used. The use of a return conduit is strongly recommended as it prevents the grout to

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stand still too long in the tube. Thus, the grout will not set inside the tube and the workers donot need to stop and restart the pump all the time. The return tube ends in the collector.

C The admission system [Figure 3.3, F, G and H]The admission system is equipped with a joining part that enables a fast and watertight couplingof the injection conduits with the injection hole. An adjustable three way gate valve limits theinjection pressure. If the counter pressure is too high the three way gate valve will direct thegrout to the return pipe. If the counter pressure is below the injection pressure the grout willenter the injection hole. The use of this three way gate valve has two major advantages. Thepressure at the inlet of the injection hole is controlled close to the injection hole. This is betterfor every control system, since it shortens the reaction time. The pump does not need to be verycomplex. The pressure will never, even not for a short period of time, exceed the allowedinjection pressure and hence, the risk for further damage due to over pressure reduces. On theother hand, the grout will never stop flowing. Whether the counter pressure is too high orwhether one has to switch from one completed injection hole to the next one, the grout will bedirected to the return pipe. By this double pipe system with three way gate valve, the grout isrenewed all the time. During injection, the three way gate valve will allow grout in bothconduits: part into the injection hole and part into the return pipe.

3.2.2. Reality about injection equipmentAbove the ideal injection installation and its components are described. It can not alwayseconomically be justified to bring into action such a complex and expensive installation. Forsmall scale projects one could prefer a manual installation or even gravity grouting could beconsidered. For electric pumping a device to limit the pressure inside the masonry should bepresent in any case. A pressure of one bar corresponds to a load of 100 kN/m2! One canimagine that for higher pressures inside the masonry structure additional damage might occur.For longer distance between pumping installation and injection holes, a return conduit shouldbe present. If there is no return conduit, the grout will stand still in the main conduit whileswitching from one injection hole to another. This will cause the time depending features of thegrout to occur: instability and thixotropy. It depends on the quality of the grout to what extentthese phenomena will arise. If the installation does not include a separate mixing installation, one has to stop the injectionafter finishing a batch of grout from the collector, produce another batch of grout, and restart theinjection. In the chapter about the experimental program, it will be shown that it is nearlyimpossible to restart an injection after a period of standstill. The experiments, as well as thesimulations show that a long stand still is one of the worst things that can happen.

3.3. Realization of an injection work

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30 Grout injection of masonry, scientific approach and modeling

In this chapter the essential steps in a consolidation are described. The general procedure forstructural grouting of uncovered massive or double leaf masonry is considered here. In case ofcovered masonry (masonry covered with frescos or plastered masonry) extra precautions arenecessary. They are not discussed in this work.

3.3.1. General RequirementsBefore discussing in detail the different steps of an injection, it is appropriate to provide aschematical overview of the requirements for the materials used and the technique. It will beobvious from the next overview that one has to compromise between different requirements.Depending on the character and the state of the masonry, the importance of some requirementscan change.

3.3.2. Diagnosis of the masonryIt seems quite logical: before there are any further steps to take, one should wonder ifconsolidation injection is necessary or can be of any help to maintain a building. Maybe thereare other techniques, more suitable for the situation at hand. If consolidation can be part of therestoration, one should determine the procedure to follow. To be able to answer this questiona thorough diagnosis is necessary. Presently, non destructive testing methods are, bythemselves, only able to provide a qualitative evaluation of the masonry. This implies that theiruse alone is not enough to justify a grouting operation and to support the design of thisoperation. Therefore, the diagnosis of the masonry is most of the time a mixture of non-destructive techniques and classical destructive techniques. The visual charts, produced by thenon destructive testing enable to locate the relevant areas where destructive tests should becarried out. Non destructive tests are very suitable to obtain a qualitative picture of themasonry structure. They are ideal to compare the initial state with the injected one.

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Relevant properties of the grout Requirement

viscosity, critical shear stress as low as possible

Stability (no bleeding, no seggregation) as high as possible

Adhesion to the injected material as high as possible

Chemical and physical compatibility with theoriginal material

this is the main criterium to choose betweenpolymers, cement grout or lime grout

Penetration and injectability As good as possible

Water retaining properties As good as to avoid excessive absorption

Mechanical properties Comparable to the original material

Execution of the injection Remarks

Preparation of the masonry surface to avoid leakage

Drilling of the injection holes closest pattern is preferrable, density canvary depending on the quality of the masonry

Prewetting of the masonry only if absolutely neccessary to improve theinjectability

Introduction of the grout either by gravity orpressure

avoid large internal pressure values

Modern diagnostic tools

Non destructive techniques for the preliminary investigation to decide about consolidationand for the quality control after injection

Numerical and probabilistic methods to quantify the need, the benefit and the efficiency of aninjection

Table 3.1: Overview of the general requirements for consolidation injection of masonry

Destructive techniquesAmong the destructive techniques, coring is probably the most frequently used. The coringenables furthermore to judge the quality of the inner masonry by visual inspection of the cores.Compressive and splitting tests can be executed on the cores to get an idea of the mechanicalproperties of the masonry. Eventually the core hole can be inspected using an endoscope. Another destructive technique is offered by the resistographical method. A hole is drilled inthe masonry and the force needed to advance is plotted versus the depth. Hard parts of solidand sound masonry need a high force to advance whereas soft and deteriorated parts only needa small force. This method causes less damage to the structure: only some small drilling holes,and provides a transverse scope of the inner masonry.

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32 Grout injection of masonry, scientific approach and modeling

Non-destructive techniquesSince coring is an intensive job but especially since coring is actually damaging the building,one has to minimize the number of cores to take. Destructive testing therefore can only serve asa calibration of the data from the non-destructive analysis. Popular non-destructive testing methods are the sonic [M. Schuller, 1995] and ultrasonictechnique, likewise called acoustic tomography, measuring the transmission speed for alongitudinal displacement wave inside the material. Sonic measurement is a low cost methodenabling a quick qualitative information about the masonry state. The basic principle is that thevelocity of the sonic wave depends upon the state of the material. The pulse will be transmittedthrough the material. Cracks and voids imply that the wave has to find another, longer way andthis increases the transmission time, or that the wave has to pass the crack through a layer ofair, which has a much lower transmission velocity than the surrounding material. Sonic testsare suitable for detecting voids and cracks in walls of great thickness due to the strength of thesignal. Ultrasonic signals are much more sensible to surface conditions and the loss of energy, that istypical for a high frequency wave, leads to a rapid attenuation of the waves. Therefore, theultrasonic measurement is only suitable for limited transmission distances. As for most other non destructive testing methods, no constant relation exists between the(ultra)sonic measurements and the mechanical parameters or porosity values. However, thesonic measurements provide an idea of the state of homogeneity of the masonry. Therefore, thetechnique is suitable to compare the initial state with the state after injection. It is then possibleto see to what extend the injection was able to overcome the heterogeneity in the masonry.

The Building Material Division has concentrated on the electrical resistivity measurementtechnique. The technique has successfully been applied to judge the condition of “HetGravensteen” in Ghent [D. Van Gemert, 1988]. It consists in measuring the electricalresistance of the masonry, based on the equation of Ohm-Pouillet:

V = R . I

The presence of cracks inside the masonry increases its electrical resistivity. Differentconfigurations are possible, but the equipment needs at least two electrodes and two probes.The electrodes introduce a current in the structure and the probes measure the potentialdifference between them. The values are a function of the masonry properties in-between them. A circle with a larger diameter corresponds to a configuration with a larger distance betweenthe electrode S1 and the measuring electrodes. Hence, the bigger the distance between theprobes, the more the measurements reflect the state of the masonry that is located deeper inside.

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Figure 3.4: Schematic presentation of the Schlumberger configuration for geo-electricalmeasurements

The information on the internal state of the structure, visualised in an electrical resistivity map,is partly masked due to the influence of the limited dimensions of the analyzed masonry. Thisobservation led to the idea of making maps in which the influence of the boundaries would beeliminated [H. Janssen, 1993] [K. Haelterman, 1993]. The resulting map can then directly becorrelated to the real properties or anomalies of the structure [D. Van Gemert, 1998]. The Arenberg Castle is surrounded by a brick masonry wall. The technique to filter the geo-electrical resistivity measurements from the geometrical information was applied to this wall.Figure 3.5, Figure 3.6 and Figure 3.7 provide an example of the filtering of the geometricalinfluences on the resistivity measurements. As can be seen from the map of relativedifferences on Figure 3.7, the only heterogeneity that is left, is a thin horizontal layer. Thishorizontal layer physically corresponds to the interface between two kind of stones with adifferent resistivity. This interface gives rise to an accumulation of iso-resistivity lines.

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34 Grout injection of masonry, scientific approach and modeling

Figure 3.5: Electrical resistivity map of part of the wall in the Arenberg Park (Horizontaldistance = 0.00, vertical edge)

Figure 3.6: Electrical resistivity map for a homogeneous wall, horizontal distance = 0corresponds to the left edge

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Chapter 3 - Injection as a consolidation technique for masonry 35

Figure 3.7: Map of relative differences, reflecting only the relevant information: the interfacebetween two kind of stones used to erect the wall

Other influences disturb the relevant information. The humidity influences to a high extend theresistivity values. The more humid the masonry, the lower the resistivity. The filtering of themoisture content from the resistivity maps and the uncertainty about the accuracy and theresolution has been studied by Kathleen Venderickx [K. Venderickx, 1996]. She worked out amethod to filter the information from these unwanted disturbing influences.Radar techniques that use electromagnetic signals for the investigation of masonry structuresprovide charts that are similar to the ones obtained from electrical resistivity measurements.They also enable to judge the homogeneity of the structure. Nevertheless, the technique requiresmore expensive equipment. The acquired information is not very detailed in the sense that theresolution is poor [C. Colla, 1995]. The presence of water attenuates the electromagneticwaves in order to reduce the maximum thickness of the wall that could me measured. Besides,this attenuation masks the relevant information.

Non destructive test data, related to physical evidence, enable to draw a map showing an imageof the transition time for acoustic testing or the electrical resistivity of the wall that wasscanned. This way the heterogeneity of the wall is displayed. This qualitative information iscalibrated using the test results of the destructive tests. This way the engineer can decide if, andin which areas, consolidation injections are necessary and what strength gain is desired withoutcoring the hole building. Probabilistic methods [L. Schueremans, 1997] as mentioned above in Chapter Chapter 2 allowto calculate the probability for the structure to fail. If the probability to fail is too high,

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36 Grout injection of masonry, scientific approach and modeling

consolidation has to be considered. Eventually the effect of the consolidation of certain parts onthe probability to fail can be studied.

3.3.3. Preparation of the masonryOnce the areas to inject are determined, the building is prepared for injection. An important partof this preparation is the sealing of the masonry to prevent the leakage of the grout [J. Ashurst,1989] [J. Ashurst, 1990]. Therefore, a general repointing is recommended. Besides, a generaldeep repointing can be seen as a very effective structural intervention. The repointing as suchalready consolidates the masonry. The pointing must be fairly porous to absorb the water of theinjected grout. This will improve the setting of the grout and the adhesion to the masonry. VanGemert [D. Van Gemert, 1988 (1)] refers to the casing of the masonry using a cement mortar ora coating based on soluble gels for outside masonry. Cellulose or clay based temporarycoatings are alternatives to these systems. After consolidation they can be washed by water.Nevertheless leakage can still occur and should then be stopped using quick-setting cement andcleaned immediately . Leakages prevent to build up pressure inside the flow channels. If thestability of the masonry is very doubtful and if hydrostatical pressure is feared to arise, anexternal reinforcement can be justified.Second part of the preparation is the drilling of the injection holes. Preferably the holes aredrilled in the joints. This way they will be less visible afterwards. The holes should inclinedownwards. Three parameters of the injection holes are important: the pattern, the density or thedistance between two adjacent holes and the depth of the holes. The parameters depend on thetype of masonry, the overall condition of the masonry, the rheological properties of the grout andincidence of cracks. A more precarious zone with many cracks will be easier to inject andhence the pattern in this zone can be somewhat less dense. On the other hand the appliedpressure in this zone should be lower. Therefore and for reasons of simplicity the density isoften kept constant for the whole structure. Existing major cracks can easily be used forinjection.The pattern can be square Figure 4.7 or staggered Figure 4.8, considering a cylindricalinjection, spacial geometry learns that a staggered pattern theoretically increases the coveredsurface with around 11 % in comparison with a square pattern. In literature nothing more than afew general guidelines are available. The density of the injection holes is expressed as holesper square m. The recommended number of holes per square meter found in literature arementioned in Figure 3.7.

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Reference number of holes / m2 Pattern

Lizzi [1982]

2 - 3

Pume [1989]

2 - 4 Oblique holes, lengte equal 2/3 of the wallthickness, diameter of 33 mm

Zagorcheva[1988]

3 - 4 Distance 0.5-0.7m in vertical direction and about0.5m in horizontal direction, diameter of 12 mm

Tomazêvic[1992]

- 0.5m to 1.0m interval between holes, at least halfthe thickness of the wall deep

Binda [1991]

2 Injectors inserted till 2/3 of the wall’s thickness

Table 3.2: Layout of the injection holes pattern found in literature

The holes should at least reach the middle of the wall. Two-third of the wall would be better.Above some general rules about injection hole density and depth are mentioned. Case studieslearn that it is hard to find rules that are generally applicable. An experimental program carriedout by Baronio [G. Baronio, 1992] about masonries with cracks and voids irregularlydistributed and of different dimensions showed them difficult to inject properly. Therefore,Baronio states that the usual 2 to 4 holes per square meter are not enough. A possibleexplanation can be formulated intuitively. During injection, when a grout reaches a large void,no pressure can be built up in the neighborhood of that void. Due to this low pressure, the groutwill enter the fine cracks only over a short distance. Thixotropy, water absorption andinstability of the grout cause the blocking for further injection in these finer cracks. When thelarge void is finally filled, the pressure can increase again, but too much water of the grout isabsorbed in the fine cracks to restart flowing. The zone hidden by the finer cracks will never beinjected through this one hole. So she advises to shorten the distance between holes. Thecovered area for one injection hole depends on the penetration of the grout inside the masonry. The calculation of this penetration of the grout starting from the properties of the grout, theinjection pressure and the permeability of the masonry is one of the main goals of this thesis.

3.3.4. Injection pressureOne of the most important parameters of injection was already mentioned above discussing theinjection holes. Of course the type of grout and it rheological properties on one hand and thequality and properties of the masonry on the other hand stay, by far, the most importantparameters. Their influence on the grouting process will be discussed thoroughly, but not in thistechnological chapter. The injection pressure is the pressure at the inlet towards the injectionholes and can be quite different from the pressure generated by the pump. The injection

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38 Grout injection of masonry, scientific approach and modeling

Figure 3.8: The hydrostatic pressure adds to the injection pressure andmight cause additional damage to the masonry

pressure is the driving force behind the penetration of the grout inside the masonry. The higherthe pressure the easier and faster the grout will pass. Because the grout flows faster, the groutwill loose less water by absorption and the particles will remain better in suspension.However, the pressure is limited to a few bars. The internal pressure of the grout blows up themasonry introducing tensile stresses that can not be taken by the masonry.

Increasing the pressure would soon cause additional damage to the structure. The pressure builtup inside the masonry is the addition of the injection pressure and the hydrostatical pressure.The hydrostatical pressure is proportional to the height of the injected grout column that is stillin fluid state, Figure 3.8. Therefore, it is recommended not to inject the injection holes in avertical order. InTable 3.3 some recommended values for the injection pressure are given.

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Reference Pressure Type of grout Site

Feilden[1982]

2.0 bar lime + fly-ash + cement Central tower, Norwisch cathedral,England, stone masonry

Vogiatzis [1989]

1.0 bar cement + fine sand + SP Church of holy Apostles in Kalamata,Greece

Tomazevic [1992]

2.0 bar cement + puzzolane Rubble stone masonry buildings,Kozjansko, Slovenia

Binda [1993]

3.5 bar Hydraulic lime +additives

Irregularly cut stone masonry,Rovereto, Italy

Table 3.3: Injection pressure values found in literature

Typical compressive stresses in ancient masonry are about 1 MPa or 10 bar. Tensile stressesare close to zero. Internal hydrostatic pressure might cause big tensile stresses or might pushout the outer leaf of the masonry structure.

3.3.5. Execution of the injectionThe injection itself starts at the lowest injection holes, in the middle of the wall. Then itprogresses sidewards before the next level of injection holes can be is injected. Thanks to thefeedback conduit, the switch between two injection holes takes place without any problem: thegrout keeps on flowing, there is no risk for setting in the conduit. A quick joint enables a fastcoupling and decoupling from the nozzle to the taps. The filling of one hole continues until C the pressure exceeds permanently the injection pressureC the grout emerges freely at adjacent injection holesC a predetermined quantity of grout is injected in that holeThe latter stop criterion is used in case the grout flows away through an invisible leakage. Aninvisible leakage can not be sealed by the normal procedures, but can be stopped by injecting afast setting grout or a grout with a high critical shear strength value (plastic threshold).Furthermore, it is advisable to register the amount of grout that is injected in every injectionhole. These data can be analyzed in order to check for a complete filling of the masonry, tocontrol if no grout ran away. In spite of a good preparation of the masonry, it can not beexcluded that leakages occur. A fast sealing of these leakages using fast hardening cement isadvised. These leakages are almost inevitable and they provide information about the flow ofthe grout. Three workers are required for a good execution of an injection job. One workerserves the mixing installation and the pump. He takes care for the grout to be available all thetime in the collector by mixing in time the successive batches of grout in the mixing installationand to continuously pump the grout towards the injection hole. If necessary, he has to stop the

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pumping activity or to adapt the pressure. At the end of the day he has an important role incleaning the installation. The other men work in the neighborhood of the injection hole. One ofthem handles the conduits and takes care of connecting the conduits to the injection hole. Healso handles the three way valve to adjust the injection pressure. The third person helps thissecond man in moving the conduits and seals the occurring leakages. If the grout that streams outof the leakage smudges the facade of the building, it should be washed away before hardeningtakes place. When the work has been completed, the injection holes need to be repointed. For this purpose,the drilling powder that was collected during drilling the injection hole, can be used to add tothe binding agent to assure the same tint.

3.3.6. Control of qualityAlthough general guidelines exist, technological parameters and the composition of the groutdepend highly on the specific situation. To insure a high quality work, the masonry buildingmust be carefully studied. The best way to prevent problems and to avoid a poor quality is agood preparation of the job. An extensive study should be carried out. This study results in thedetermination of the following items:C specification of the preparation of the masonryC the pattern of injection holesC the injection pressureC the composition of the groutC the mixing procedure of the grout: sequence and mixing timeFor every batch of grout one should check the fluidity by a flow-test type Marshall funnel orAfnor cup. Additionally one could check the stability as described further in this thesis.In the ideal situation, the quality of the work is controlled on line. This means that duringinjection, the flow of the grout is monitored. If things go wrong, one should be able to interfereas soon as possible. Two important conditions must be fulfilled for the on line system ofcontrol and correction of the injection. In first instance the on line control needs to be workedout. Until now, these on line control techniques are not operational. Though, it should bepossible to realize on line control using the electric resistivity method. The presence ofhumidity and hence of the grout, influences to a large degree the electric resistivity of themasonry. The penetration of the grout suddenly decreases the electrical resistivity of that part ofmasonry. It should be possible to detect and visualize the decrease in resistivity and hence thearrival of the grout in a certain point. So, let us suppose that it is technically possible to monitorthe grout penetration. In case of problems one has to react promptly. First possibility: thepenetration is insufficient. In that case possible reactions are: the use of a different groutcomposition with an improved rheological behavior; an increase of the injection pressure;additional injection holes etc...For this purpose a reliable model of the grout flow can help todecide what action to take. The present thesis offers such a model.

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Figure 3.9 : Electric resistivity map before and after cement grout injection in twopoints: the resistance decreased significantly [D. Van Gemert, 1988(2)]

If on line control cannot be done, as it is in the actual state, off line control is necessary. Theinjected region should be checked. One possible and reliable method is to use the same nondestructive testing method as was used for the diagnosis of the masonry structure. It is possibleto compare the maps that were obtained before the injection with those obtained during thecontrol measurements after injection. Ultrasonic measurements, electrical resistivitymeasurements or radar technique were discussed above. In case of electrical measurements oneshould take care about the influence of the humidity on the measurements. Since a large amountof water is brought in by the action of injection, the resistivity will be changed. Research isgoing on to filter the obtained information from the influence of water. In case of a successfulinjection work, the heterogeneity in the resistivity map will fade away because of the filling ofthe voids by the injection. Any non destructive testing is best completed with destructive teste.g. coring.

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42 Grout injection of masonry, scientific approach and modeling

Figure 3.10: A map of the difference in resistance [Ohm.m] before and after the injection of anepoxy resin, the measurements reflect the material property to one meter inside [D. Van Gemert,1988 (2)]

Figure 3.11 displays another nice example of the use of non destructive testing for the control ofthe effectiveness of consolidation injections. The study is carried out by Atkinson-Noland &Associates, USA and provides the three dimensional surface plot of through-wall ultrasonicpulse arrival time measurements for the original, damaged and repaired condition of alaboratory masonry wallet. For on site application, the first image will not exist. However, thesecond image of the wallet in damaged condition can be analyzed in order to decide aboutinjection. To control the execution of the injection the third image indicates that the injectionprovided a uniformly filled masonry structure, with an almost constant pulse arrival time for theultrasonic waves. The pulse arrival time is the time that passes between the moment the pulsewas sent and the moment the receptor notices the arrival of the pulse. When the voids are filledwith hardened grout, the pulse arrival time will generally decrease. Furthermore, the pulsearrival time will show less scatter than before the repair.

3.4. Types of binding agents3.4.1. PolymersPolymers are the most recent binding agents used for consolidation injections. Polymers arepure liquids (no dispersions like all the other binding agents used for injection purposes) andshow a relatively low viscosity and low critical shear value. Since a polymer is a pure liquid,it will not suffer from thickening due to water loss. The wide range of viscosity values and theabsence of particles that might hinder the flow, make polymers very suitable for injection.Moreover, polymers show a very good adhesion to dry surfaces and have a high compressivestrength and, even more important, a high tensile and adhesion strength. This makes them verysuitable for particular cases were the penetration of hydraulic grouts is problematic.

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Figure 3.11: Plots of the pulse arrival time for ultra sound measurements, original condition (a),after loading (b) and after grout injection (c) [R.H. Atkinson, 1991]

Beside these advantages of polymeric grouts, one has to mention the weak adhesion to wetsurfaces. The physical properties (water permeability, thermal expansion) of these grouts arecompletely different from the original materials. This could cause problems with the moisturetransport or could cause thermal stresses.Polymers are very expensive materials when compared to other possible injection materials.When large quantities are to be injected in a historical building, an injection with polymers willsoon become economically unfeasible. Therefore, the Building Materials Division advised ondifferent occasions to inject the structure first with a mineral grout. A second injection withpolymers can than fill the remaining voids.Probably the biggest problem that polymers are facing is that they are considered not tocorrespond to the materials that were historically used. Many architects tend to favor materialsthat are more compatible: these modern chemical materials do not belong in valuable historicalbuildings [The Charter of Venice, 1964]. This is not only a philosophical statement, but can bemotivated with scientific physical arguments: the material has a completely different stress-strain behavior, it is impervious to water and has a different thermal dilatation factor. Polymeric grouts show certain disadvantages discussed above but regarding strengthening, theyare the most efficient ones. This fact is undoubtedly connected with the outstanding mechanicalproperties and the good rheological properties of these grouts that are associated with anabsence of solid particles in suspension.

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3.4.2. Cement groutsCement is the most popular binding agent of modern times. Cement is not expensive, the rawmaterials are widely available. From a more technological point of view: the fast hardening ofcement allows to proceed faster in applying new loads to the structure.Basically a cement grout consists of cement and water. The same combination of cement andwater is applied for soil consolidation. In case of soil injection very high pressures, up to 100bar, are applied. For the injection of masonry structures, the injection normally proceeds atpressures lower then 2 bar. The cracks and voids through which the grout has to penetrate canbe relatively small. Such a cement grout needs a good fluidity. To achieve this asuperplasticizer is added to the mix. The superplasticizer enables to reduces the water contentfor the grout to an acceptable level without damaging the required fluidity. Besides,superplasticizers have a deflocculating action. This prevents the flocculation of cementparticles. Superplasticizers based on melamine formaldehyde and sulfonated naphthaleneformaldehyde are the most popular. They decrease the electrostatical forces that keep theparticles together. In addition to superplasticizer, other additives or admixtures are applied. Because of the natureof a cement grout, a dispersion of cement particles in water, the bigger cement particles sink.This phenomenon creates a heterogeneous consolidation. The lower region contains morecement and will be harder and stronger, whereas the higher region will be weak due to a lack ofbinding agent. The top of the injected zone could, in case of severe bleeding, only be filled withwater. Stabilizing agents prevent the bleeding of the grout or the segregation of the cementparticles. An extended study of A. Miltiadou [A. Miltiadou, 1990], A. Miltiadou, 1991] and astudy of A.M. Paillère [A. Paillère, 1986] indicate that the addition of ultra fines improves theinjectability of a grout. These ultra fine puzzolanic admixtures correct the granularity of thecement in the finer region and influence positively the injectability and the stability of the grout.A finer dispersion of cement particles in the water will enhance the flow of the grout. The groutwill be able to progress longer inside the masonry before it will be blocked. On the other hand,the finer the particles, the more water needed for a good fluidity and hence addition ofsuperplasticizer is inevitable. Depending on the application and the concrete situation, a type ofcement with modified granularity can be necessary. This can be achieved by adding ultra finemineral admixtures.Strictly spoken, cement does not correspond to the binding agent used in most historic masonrybuildings. The nature of the historical binding agents is air hardening lime or natural hydrauliclime. Therefore, art historians and architects disapprove of the injection of cementitious groutsin historically valuable buildings. The arguments come down to the aversion of putting amaterial, strange to the original materials, into cultural patrimony. To my point of view thisargument is less valuable than in the case of polymers. Cement is a mineral binding agent, justas lime. The physical properties with regard to moisture transport, thermal expansion,temperature household etc... are much closer to those of the historical materials than in case ofpolymers. Many buildings have been injected satisfactory using cement grouts.

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Sometimes, in specific situations, it is possible to use special kinds of cement. A blast furnaceslag cement is always preferable to ordinary Portland cement because less of its C3A -content isreducing the risk for the formation of ettringite and with low alkali reducing the danger forefflorescences. Hydration is a little slower, and less danger exist for possible expansivereactions. Furthermore, blast furnace cement is somewhat finer than pure Portland cement. Ifthe masonry to consolidate is hard to inject due to fine cracks and voids, one could consider theuse of micro cement. Micro cement is a fine ground kind of cement frequently used for soilinjections. The extreme fineness, Blaine specific surface value of 8000 cm2/g and higher,against around 3000 cm2/g for OPC, provide a very good injectability. Based on our experiencemore water is required and the addition of more superplasticizer is necessary. During ourexperiments, we were not able to produce a stable micro cement grout without the addition of aseparate stabilizer. The additional amount of water will sooner or later evaporate from themasonry structure. This could cause problems for the moisture household and if soluble saltsare present, could cause crystallization effects and efflorescence. However, the results of otherresearchers mention very successful injections using micro cement grouts. Miltiadou studied thestrength increase of a traditional rubble core, through the injection of ultrafine cement basedgrouts. The results show a remarkable increase of strength and stiffness. The averagecompressive strengthening increase factor was seven, for stiffness it was five. The questionremains what would happen in case of an additional settlement when the stiffness has increasedso much. One could state that micro cement requires more professional skill than normalcement. Besides, micro cement is more expensive. The use of micro cement is more demandingfor the mixing procedure: a badly mixed micro-cement grout may be worse than a badly mixednormal cement grout. An extensive study has been done by O. Benhamou on the rheology ofgrout using microcement [O. Benhamou, 1994]. From this study the behavior of microcementgrouts seems to be complex and sometimes unpredictable.

3.4.3. Lime based groutsLime based grouts are without any doubt the grouts the most compatible with the originalmaterials for the consolidation of ancient masonry. For centuries lime of both types, airhardening and hydraulic lime, has been used for the construction of buildings. The use of limebased grouts for consolidation of masonry should be very popular. Tough there exist severalcontra-indications.The carbonation of thick layers of lime is a very slow process due to the slow diffusion of CO2

through the carbonated layer. The air hardening lime can be given hydraulic properties byblending with puzzolanic material. Several natural products are available for this purpose:Trass or Santorini Earth. Also cement provides hydraulic properties to the grout. Hydrauliclime is an even better alternative. The hydraulic properties tend to give an acceptable earlystrength whereas the lime provides the wanted ductility making the grout suitable for theapplication in seismic areas.

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Only few studies have been done on the rheological properties and efficiency of these grouts.Our own research reveals that the water content of a fluid lime based grout is very high withoutthe use of an appropriate superplasticizer, that we could not find Figure 10.1. The samples thatwe injected with the lime grout obtained very low mechanical strength, although the grout wasvery injectable. We will discuss the results in the chapter about the experimental program. Dueto its fineness (Blaine Specific Surface > 8000 cm2/ g) lime based grouts are very injectable infine cracks.

3.5. Grouting improves the load bearing capacity of the masonryMasonry is a heterogeneous material, composed of a relatively hard component (the bricks orstones) on one hand and relatively soft mortar on the other hand. By its specificity it is not easyto predict the behavior of the whole starting from the properties of the components. Thisstatement is true for the physical behavior (moisture transport, heat transfer or migration ofsalts) as well as for the mechanical behavior. The mechanical behavior includes the functioningunder loading, the attitude when a setting occurs or, for seismic regions, the interaction betweensoil and structure. Grouting mainly intends an improvement of the mechanical behavior, thoughthe impact on the physical behavior may not be disregarded.Many rules for dimensioning masonry structures are based on empirical rules. Those rules areonly approximately applicable. The reader will understand that it is nearly impossible tocalculate the efficiency of a consolidation injection. Literature provides some globalconsolidation factors, deduced from experiments, part of which are presented above. The loadbearing capacity after injection is compared to the load bearing capacity of the structure inundamaged condition (Eq 2.7). Depending on the original state, the injection and thecomposition of the grout, the strength improvement is 40 % to 150 %. Extrapolating thesevalues to real masonry is not very reliable. The factors give a notion of the possibilities ofinjection consolidation. All authors agree that the consolidation factor increases as the initialcondition of the structure is poorer.

The strengthening effect is due to:C Internal voids cause tensile stresses in the material. By filling the voids, those tensile

stresses can not give rise to extra tensile cracks.C The injection of additional binding agent into the structure will enhance the internal

cohesion. Especially the zones that show poor cohesion due to fine cracks and loosematerial will be infiltrated by the grout and be strengthened. The resistance to splittingand to shear will improve.

C Monolithical behavior after injection: this is the way masonry structures are designed.Generally, a lime grout injection provides less strength improvement than a cement based grout.But it has to be mentioned directly that strength is not the only property that is involved when itcomes to the consolidation of masonry. The strength improvement is most of the time connected

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to a loss of ductility. This loss of ductility is much higher in case of cement grouts and hence,the masonry fails to maintain its self-healing property and its ability for auto-equilibration offorces. However, it is clear that grouting is an effective method to improve the load bearingcapacity of solid or double leaf masonry. The original mechanical strength can easily beachieved. Concerning the masonry support, there is a clear difference between the efficiency ofgrouting a double leaf masonry wall with a rubble core and a solid brick or stone masonry. Theexplanation to this fact remains on the much higher level of porosity in the first case. In doubleleaf masonry, not only the voids are bigger but also the channels of communication betweenvoids are much more than in the second type. The importance of porosity is easily understoodconsidering that the main principle of grouting is to fill the inside voids. Concerning the groutitself one important factor is its injectability - it explains why ultra fine cement based grouts aremore effective than cement based grouts with SP and these more effective than pure cementgrouts. However, this parameter by itself is not enough to explain why modified lime basedgrouts are less efficient than cement based grouts or the different results between polymericgrouts and ultra fine cement based grouts. Lime based grouts modified with SP have probably asimilar rheological behavior of cement based grouts. Moreover, the injectability of epoxygrouts and ultra fine cement based grouts is similar. Thus, the explanation must lie in theintrinsic mechanical properties of the grout. Concerning lime based grouts, the fact thathydraulic lime or hydrated lime plus puzzolanas are used, enables the slow process ofcarbonation of lime by a puzzolanic reaction to be overcome. However, there is always freelime that needs carbonation to set. As CO2 has difficulties in reaching the inside of the masonry,the process is slow, explaining the low efficiency of these grouts, and suggesting that it willslightly improve in long term [K. Van Balen, 1991]. With polymeric grouts, although theadhesion properties are similar to ultrafine cement based grouts, their intrinsic mechanicalproperties are significantly superior and this factor only can explain their outstanding behavior.Nevertheless, a grout with high intrinsic resistance is not always needed, since the finalresistance of masonry is also a function of its own mechanical properties. If the results of polymeric grouting are compared with those obtained by fine cement grouts, itcan be seen that the latter can be a real alternative to polymeric grouts, when their demandingmixture procedure will be available on site. Firstly, because the actual knowledge of polymericgrout behavior clearly suggests it is too early for their unlimited use in historic buildings.Secondly, the cost of cement grout is significantly lower, which is particularly important forconsolidation of high porosity masonry, such as double leaf masonry with a rubble core. For themoment, whenever a significant mechanical improvement of a historical building is needed,cement based grouts with SP are a good choice. They are effective and the mixing procedure isnot so demanding as for ultra fine grouts. Concerning lime based grouts, they can restore theoriginal mechanical properties of a double leaf masonry. Therefore, their use is a real option,whenever only a repair operation is envisaged. However for solid brick masonry, if therelation that exists for the other grouts between the strength factors of the two types of masonryis applied, a average strength increase factor of only 1.3 is obtained, suggesting that they are not

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48 Grout injection of masonry, scientific approach and modeling

Lime basedgrout

Cement groutwith SP

Ultra fine cementgrout with SP

Polymeric grout

Double leaf masonrywith rubble core

2 2,5 3 4

Solid stone or brickmasonry

1,3 1,6 2 2,7

Table 3.4: Average mechanical strengthening factors found in literature [N. Gil, 1995]

adequate to this type of masonry Table 3.4. Finally, it must be emphasized that these values areonly reference values, once they are a function of parameters that cannot be controlled, and thusthey can never be used on a design phase. The porosity is not constant within a masonry'sglobal type, nor can the rheological and mechanical properties of the grouts be assured to be thesame. Nevertheless, they enable a first approach of the expected improvement by grouting andclarify our expectations.

The more damage happened to the structure, the more both effects, increase of average strengthand the reduction of the variance of the strength, can enhance the behavior of the building.Therefore, the strengthening factor needs always to be related to the specific situation. European historic masonry is often composed of two leafs with a very open core inside. Thiscore consists of rubble on which fluid mortar was poured. The load bearing capacity of thisinner part of the structure can be significantly improved by filling the voids. Besides, by fillingthe inner part, both leafs become interconnected. This provides a nearly monolithical unit thatwithstands very well mechanical loadings.The mechanical strength of the injected grout for sure has influence on the expectedimprovement. Nevertheless, this influence is less important than one might expect. Even if thevoids are properly filled with a grout with relatively poor mechanical properties, the structurewill regain a lot of strength because of the decrease of the variance of the strength. M.Tomazevic [M. Tomazevic, 1992] injected wallets with four different kind of grouts; thecompressive strength of the grout varied from 7 MPa to 32MPa. Still he found that themechanical strength of the hardened grout did hardly influence the final mechanical behavior ofthe injected wallets. When using a grout that is too hard or too rigid after curing, additionalsettlements might cause splitting forces just above the injected holes. More important for theeffect of a consolidation injection is the complete filling of all the voids and cracks. If this isnot the case, additional concentrations of stresses will occur that can cause additional damage tothe injected or neighborhood structure. The reasoning developed above implies that the flow properties of the grout, the rheologicalproperties, deserve more attention than the mechanical strength. We will extensively come backto this issue later in this thesis. The quality of the carrying out of the injection procedure has

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major influence on the final result of the consolidation. The presence of local stiffness in aglobal structure can cause severe damage in case of future solicitations of the monument.The problem must be studied in a probabilistic way. This is part of the research in the BuildingMaterials Division. In paragraph 2.3.1 a model for calculating the increase of the reliability ofa masonry structure by injection is presented. The increase of the average strength and thedecrease of the variance on the strength reduce the probability of failure.

3.6. Masonry grouting, code of practiceA reading of codes of practice of several countries, dating from the 1970's, showed a completeabsence of references to the retrofitting techniques of masonry structures.The early official mentions of repair techniques to masonry buildings dates of 1986, in theItalian technical standards for construction in seismic areas. These norms had their origin on theofficial requirement to guide the immense retrofitting interventions after the earthquakes ofFriuli (1976) and Campania (1980). They define two levels of intervention.The first level is the upgrading intervention, obligatory in case of vast interventions that canlead to changes in the global behavior of the building. The norms define it as a substantialintervention on the masonry building to make it resistant to the official seismic loads.The second level is an improvement level, obligatory in case of renovation or change ofmasonry elements in the structure. The norms define it as the execution of one or moreoperations on the masonry structure to improve its original mechanical properties in at least 20percent, without changing the global behavior of the building. They only mention grouting as onepossible intervention technique~ in the aim of this second level . This code clearly illustratesthe poor state of knowledge about grouting at that time.ln 1991, the city of Los Angeles developed a detailed specification for the repair of cracks withgrouting, but it only concerns the technological point of view. They give a specific volumetricgrout mix and fully describe the technical parameters, and the quality control measures [City ofL.A., 1991].A thorough approach to this matter only took place with the issue of the Part 1.4 of theEurocode-8, concerning the repair and strengthening (against seismic actions), in 1993[Eurocode 8].In this document, criteria for the assessment of the seismic performance of existing structures arementioned. The code presents a decision-making process regarding the corrective measures totake and criteria for the repair and/or strengthening of existing structures.At last, there is a specific chapter, concerning the monuments and historical buildings, where thecultural level to preserve is officially mentioned for the first time.Following, a concise reading of the code is presented, regarding its position to grouting as aretrofitting technique of existing masonry structures and in particular to monuments and historicbuildings.

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The code first emphasizes the importance of a correct structural evaluation preceding theintervention itself it presents a methodology to undertake this evaluation referring the followingitems:C a historical inquiry must be carried out to help understanding the present structural

scheme and to improve the knowledge of the past behavior, especially with reference toearthquakes;

C a detailed recording should be done to correctly evaluate the present situation.Destructive and non destructive tests must be used to estimate the integrity andmechanical properties of the masonry.

C if grouting is considered, the code specifically recommends the determination of thechemical properties of the mortars, to avoid detrimental reactions, particularly in thepresence of sulphates.

Grouting is mentioned as a possible intervention to mechanically improve masonry, in particularin the repair of cracks and on the strengthening of rubble core masonry walls.To repair cracked masonry, the code refers cement grouting, if the width of the cracks is small(less than 1 mm) and the thickness of masonry is considerable. it recommends the use ofadmixtures preventing shrinkage and proposes the use of epoxy grouting for fine cracks.Lastly, it suggests cement grouting as an efficient method for strengthening rubble core walls,with the condition of a satisfactory absorption.Concerning the quality assurance of interventions, it mentions the following control measures forgrouting interventions:C protective surface treatments must be assured and final cleaning methods. The

effectiveness of these technologies must be checked on trial areas;C inspection of certificates of filling materials and, possibly, acceptance tests

(composition, stability, conditions of use);C measurement of local strains and control of deflections produced during grouting

procedures;C visual inspection of the final work; possible extraction of cores across selected check-

-areas, to evaluate the efficiency of grouting.

The reading of the Eurocode-8 clearly shows a full but only qualitative official acceptance ofgrouting as a retrofitting technique. However, the present knowledge about the technique enablesa deeper approach instead of only general considerations.In future, the code could more clearly present the differences between the grouts available,namely between the cementitious grouts and polymeric grouts. It should present the several typesof cementitious grouts available and the levels of injectability associated. The code simplyignores the existing (micro) cement grouts with SP, when it only makes reference to the epoxygrouts for injection of fine cracks.

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The remark concerning the efficiency of grouting to strengthen masonry walls with rubble coreis correct. However, the absorption remark is very vague and it may lead to misunderstandings.The recommendations about the methodology to follow in an intervention, although veryconcise, are pertinent and point the main factors to take into consideration.The chemical incompatibility of normal cement grouts with a sulphated environment ismentioned, but the risk to form the expansive ettringite or to create efflorescence should beexplained. Also, for quality assurance, the code concisely points the most important aspects totake into consideration.The lack of quantitative data over the effectiveness of the masonry strengthening reflects theprudence philosophy subjacent to the whole code. This attitude is also reflected on the solemention of core extraction to evaluate the efficiency of grouting. This prudence can be explainedas masonry grouting works with a parameter not yet well understood - the masonry. Therefore, itcannot be limited to a design in the office, but it utterly demands calibration of the results on siteand a tight quality control to ensure the final efficiency.However, a future code should emphasize the existence of non destructive methods to controlthe global efficiency of masonry grouting, when associated with core removal. This way, itwould help to change a still common attitude of suspicion towards grouting, mostly supported onignorance of its present state of knowledge.Concerning the historical buildings and monuments, Eurocode 8 mentions the cultural level topreserve, beyond safeguarding the human lives involved, defining a new safety concept.Thus, a "Monument's safety level" corresponds to a situation where the maximum probableearthquake B is only expected to produce repairable and not fatal artistic damages.The intervention techniques proposed for a monument should fulfill the following requirements:C Effectiveness

Shown by qualitative or numerical proofs.C Compatibility

From the mechanical, chemical, technological and architectural point of views.C Durability

Comparable to that of the other materials of the building, unless a periodicreplacement is foreseen.

C ReversibilityAs far as possible, to allow for different future decisions.

Concerning the effectiveness of masonry grouting, it can be clearly stated that this is presentlyguaranteed, as long as the correct methodology in design is followed and the quality control onsite is assured.As shown in Table 3.4, the average masonry increase strengthening factors are known,according to the general masonry type and the type of grout. Moreover, beyond core sampleremoval, other non destructive methods enable the efficiency evaluation. The results of researchprograms carried out worldwide during the last twenty years assure the mechanical andchemical compatibility and durability of cementitious grouts within time, since a correct

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52 Grout injection of masonry, scientific approach and modeling

designing of the grout's formulation is undertaken. However, the present lack of knowledgeabout these main questions concerning polymeric grouts, definitely limits their use on historicalbuildings.From the technological viewpoint, grouting technology is presently mastered to avoid any kindof damage to a historical building.In terms of architectural and structural compatibility, the fact that a binding element isintroduced where in the past another cementitious or lime mortar existed, enables grouting topreserve the authenticity of the building.Finally, the fact that grouting is not a reversible technique, does not hinder its use on historicalbuildings. However, it demands from the technicians responsible for a grouting operation, acomplete diagnosis and a careful evaluation of all the parameters involved, as the only way toachieve a qualified restoration project. Moreover, a permanent supervision of the works is avital condition to ensure a demanding level of quality.

3.7. Conclusions of chapter 3This technological study on grouting shows that the technical requirements are presentlyunderstood. There are solutions to overcome the difficulties inherent to interventions on historicbuildings. The fact that grouting involves working with masonry - a media far from being completelyunderstood - makes it difficult for the present to achieve an analytical modeling. It stronglydemands that a grouting design must be fully calibrated on site. Moreover, a continuoussupervision of the works is the only way to ensure an effective quality control. Concerningcementitious grouts, they are compatible with the traditional masonry fabric, if the predefinedrequirements are followed.Lime based grouts are poorly studied in rheological terms and provide a weaker mechanicalimprovement. In particular, this urges a further study over the effectiveness of adding aplasticizer. Nevertheless, whenever a given repair operation of a double leaf masonry isenvisaged, they can be an alternative.Cement grouts with superplasticizer added, are better studied and they offer a satisfyinginjectability capacity, with a simultaneous not demanding mixing procedure. Moreover, theprovided mechanical improvement enables to already use them on strengthening operations.Ultrafine cement grouts, obtained by grading correction, are undoubtedly the most effective .However, their use is rather postponed for the moment, as high speed mixers to use on site arenot easily available, and mixing procedure is complex, even in laboratory conditions.Concerning micro cement grouts, the state of the present knowledge is not yet adequate to enabletheir use in masonry grouting. They are associated with a high W/C ratio, which leads tounanswered questions. What mechanical improvement and shrinkage can be expected?Polymeric grouts offer the highest effectiveness but their compatibility with masonry and thebehavior in long time is not completely known. These facts should strongly limit their use inhistorical buildings.

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The mechanical effectiveness of grouting is mainly a function of two parameters:the injectability of the grout and the type of masonry. Two main types can be distinguished, dueto the significant different porosity levels associated: double leaf masonry with a rubble coreand plain stone or brick masonry. Moreover, sole grouting should not be used in buildingswhose first mode of collapse concerns the rotation of the walls out of their plane and astrengthening operation to horizontal loads must be performed.As the grouting principle is to reinforce the mechanical properties, it poorly strengthens thebuilding behavior to that first mode. In these cases, grouting must be carried out in parallel withother strengthening techniques, such as tie-rods or ring beams. Concerning the control of effectiveness, it presently demands destructive tests to be carried out,although minimized by non destructive techniques. Coring is the most effective destructivemethod, while either electrical resistivity or sonic measurements are effective non destructivetechniques. The fact that destructive tests must be carried out, strongly points to the need ofassuring a close interdisciplinary work. Grouting answers to the demanding conditions ofinterventions in historic buildings. Presently, its effectiveness is guaranteed as well as thedurability of cementitious grouts. it is compatible in the physical, structural and architecturalviewpoints, preserving the authenticity of the building. The fact grouting is not reversibleimplies that it should only be performed, whenever the means to ensure a complete and qualifiedoperation are present. Grouting is still unpopular and in particular cement based grouting is seenas undesirable to intervene in historic buildings. On the basis of this common suspicious ideas,there is lack of knowledge over the matter. It is hoped that this work helps to clarify techniciansin charge over the powerful resources of this technique. Thus, the porosity of the media and therheological behavior of the grouts are main parameters to be correlated with the mechanicalimprovement. The rheological behavior and the injectability are studied in detail by the LCPC in France. As itis shown in Chapter 4, most of the problems that consolidation injections are meting are relatedto a poor penetration of the grout in the masonry. This provides a non uniform filling of thevoids resulting in a poor or uncertain strengthening. This research program has focused on themodeling of the flow of the grout inside the masonry. The experimental program helped tounderstand most of the physical mechanisms taking place when the grout penetrates the masonry.The model is built, using this information, to simulate the penetration of the grout. This helps tojudge which combination of process parameters, such as the injection pressure, the bore holediameter, the pattern and density of the bore holes on one hand and the rheological parameters ofthe grout determined by the composition of the grout on the other hand can lead to a satisfyingand uniform filling of the internal voids in the masonry. One could resume that the effectiveness of grouting of historic masonry implies a structuralimprovement and an improved reliability. The strength increases because of the better internalcohesion and the filling of the voids make the structure. However, the structural improvementdoes not only depend on the strength. The ductility of ancient masonry might be endangered ifthe grouts composition is chosen without competence. The creation of a solid stiff part in a hole

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that has certain ductility may lead to additional damage during the following solicitation. Themore the injection leads to an even (re)distribution for stresses the more efficient it will be.

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Chapter 4. Problems faced during injection - possiblesolutions

4.1. Wrong materials, chemical, physical and structural incompatibilityA consolidation injection is always an important operation. As clarified above it is a multidisciplinary task to accomplish. During injection many problems can occur. Sometimes, theseproblems reveal clearly during the injection itself, for instance when the injection blocks verysoon after it started. The grout does not penetrate inside the masonry. On other occasions, theproblems appear only many years later. A known example of the latter is efflorescence or saltcrystallization because of the wrong injection material. Similarly, the use of high strength groutscan cause severe damage during the next mechanical solicitation (eg. Italy). This chapter treatssome known mistakes and tries to formulate some guidelines to avoid them. The possible injection materials, all binding agent in liquid form, are generally discussed inparagraph 3.4. They all have their peculiarities, their advantages and disadvantages. Acautious selection of the binding agent is of major importance for a successful execution of theinjection. The injection material has to be selected on three criteria. In the order of importancethese criteria are: chemical and historical compatibility, rheological properties and mechanicalproperties.

To my opinion the chemical compatibility is the most important requirement. If the injectedmaterials are not chemically compatible, sooner or later the injection will turn out to be a bigproblem for the building. We already mentioned the possible formation of ettringite when injecting cementitious grouts.The CA3 in the cement reacts with the gypsum in ancient masonry. This is way blast furnace slagcement is preferable to ordinary Portland cement. The alkali in cement might causeefflorescence. This is why E. Toumbakari [1991(1)] recommends low alkali (marked LA on thebags) cement for grout injection. For similar reasons the injection of cement grouts inmonuments that are erected using gypsum is to be a avoided. However, chemical incompatibility is the most important form, other possible incompatibilitiesmust be considered. The structural and physical compatibility justifies a lot of attention.Ancient masonry is astonishingly capable of taking settlements due to its very ductile behavior(Figure 4.1). In part 3.4 it was indicated that cementitious grouts provide very goodstrengthening, but also mean a loss of ductility to the structure. This loss of ductilityjeopardizes the self healing properties of masonry. Although the injected structure becomesstronger and is able to carry higher load, it is possible that foundation settlements cause anearlier damage than in case of a more ductile binding agent providing a less strong injectedbuilding.

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56 Grout injection of masonry, scientific approach and modeling

Figure 4.1: A nice demonstration of the ductile behavior of masonry: the settlement of thecentral part is supported without major cracking [Van Balen, 1991]

Injecting cement grout in an ancient building that was erected using lime mortar, introduceslocally stiff parts between relatively soft surrounding material. These hard parts can causesplitting forces as was shown above on Figure 2.3. The soft mortar is more compressible thanthe hard cement grout. In case of additional loading, bending occurs.

The above problem of introducing some hard material must be a concern, but one has to realizethat the bricks are also stiff material. Replacing the relatively soft mortar by stiff grout mightcause problems. Physical compatibility is important for the temperature and moisture household of the building.The physical compatibility becomes a bigger concern when polymers are used. Polymers havea completely different physical nature than mineral binding agents. The thermal expansion isdifferent from the thermal dilatation of the mineral components in masonry. The facade ofbuildings is subjected to rather severe temperature fluctuations. They cause thermal stressesand by the dynamic nature of the fluctuation they can damage the consolidated material. The moisture household can be changed dramatically by injecting polymers. Polymers areimpervious to water. This means that they form a barrier for rising damp and that in specificsituations, the water can not escape any longer from the masonry structure. The water will, bynecessity, find another way to evaporate. Through this new way it is harder to expel the sameamount of water. Therefore, the overall moisture content of the building will increase. This cancause additional damage, for instance to decorations that are sensitive to humidity or can causea different salt migration problem. Similar problems can happen in case of mineral mortars.

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Chapter 4 - Problems faced during injection - possible solutions 57

Figure 4.2: Loss of bond between repointing mortar and the brick due to freezing water behindrepointing. The poor evacuation of water is related to the different pore structure ofthe new repointing mortar. [Van Balen, 1999]

Mostly, the moisture transport properties are less different than in case of polymers, but it has tobe mentioned that the pore structure of a cement mortar differs a lot from the pore structure of alime mortar or an hydraulic lime mortar. Figure 4.2 shows an example of a repointing, donewithout considering the problem of moisture evacuation through the pointing layers. The wateris stopped right behind the repointing and the freezing water has soon pushed away the newlyplaced mortar. Thermal incompatibility between laying mortar and pointing mortar can causethe same damage phenomenon.

4.2. Incomplete filling of the voidsIt has already been mentioned that the reduction of the risk of failure depends highly on thedegree of homogeneity of the masonry after injection. At a uniform filling the variance on thestrength decreases and this way the reliability is improved. Similarly, the increase of theaverage strength reduces the risk of failure. Indications exists that the uniform filling of themasonry is more important than the mechanical properties of the injection grout. Tomazevicperformed an experimental program where four different cement based grouts were used[Tomazevic, 1992(1)]. The quantities of cement were varied and in some grouts a hydrophobicadditive was added. Although the compressive strength of the different mixtures varied from 7to 32 MPa, there was no significant difference in the final mechanical strength of the injectedmasonry samples. This proves again that a complete filling and a good penetration areessential. Filling all the voids inside the deteriorated masonry is not an easy challenge. Several

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58 Grout injection of masonry, scientific approach and modeling

Q 'p D 4

128 µ? PL

[1 &43

(4t c

DL

? P)%1

3(

4t c

DL

? P)4] (Eq 4.1)

parameters are involved. The distance between the bore holes, the injection pressure, therheological properties of the grout and the water absorbing properties of the masonry.Furthermore, the overall condition of the masonry is important, the amount of cracks and theirwidth. The model, described later in this thesis, is able to estimate the influence of theseparameters.

4.2.1. RheologyThere exist different causes for incomplete filling of the voids inside the masonry. The mostobvious one is a lack of fluidity of the grout. Considering the final aim of this research programof predicting the flow of the grout inside the masonry, some rheological considerations aboutdispersions are more than justified. This topic is treated extensively in Chapter 6. The mostimportant findings are listed here. Many of the expected problems involve the nature of grout: itis a dispersion of cement particles in water. C If the flow channels are too narrow, the particles will get stuck in these channels.

Hereby one has to realize that flocculated grains act as big grains. The concentration ofgrains is relatively high and hence they hinder each other during motion.

C The water absorption causes cement particles to stick to the wall of the flow channel.Therefore it can be stated that the flow channels narrow by the suction of water out of thegrout and that it is not true that there is an overall increase of concentration of grains thatwould cause a dramatic increase of the rheological parameters. The grout that keeps onflowing however, has the same properties as the grout that is injected. Experimentsindicate no changes in the rheological properties between the grout that leaves the testsamples and the grout that was injected. Of course, the result will be the same: in bothhypotheses the grout will stop flowing. In the first assumption, the grout will stopflowing because the flow channel becomes too small, whereas in the second assumptionthe grout flow will halt because of differences in rheological properties. Bothphenomena bring the correction factor for Bingham fluids in the Buckingham formula (Eq4.1) closer to zero.

Where Q = discharge [m3/s]? P = pressure difference [Pa]D = diameter of the flow channel [m]µ = dynamic viscosity of the grout [Pa.s]L = length of the flow channel [m]t c = shear stress of the grout [Pa]

Figure 4.3 shows a slice of the injected cylinders, filled with crushed bricks. In 1, thetest configuration will be explained as well as how the test injections were done and

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Chapter 4 - Problems faced during injection - possible solutions 59

Figure 4.3: The dark layer around each fragment of crushed brick indicates azone with a higher cement concentration. This is caused by the waterabsorption out of the grout forcing the cement grains to stick to the walls ofthe flow channels.

what the results were. As can be seen, there is a dark layer around each fragment ofcrushed brick indicating a zone with a higher cement concentration. This is caused bythe water absorption out of the grout forcing the cement grains to stick to the walls of theflow channels. The water absorption is beneficial for a good bond to the existingmasonry. It creates an interfacial layer with high cement content. Besides, since thewater is absorbed inside the capillary pores of the bricks, some very fine cement grainsalso move into these pores.

C When the grout penetrates further inside the masonry, the pressure gradient decreases.At a certain moment, in case of a Bingham fluid, the critical shear stress will not bereached any longer. Then the grout stops flowing. Firstly in the finer channels, butfinally also in big flow channels. This is a pure rheological phenomenon, the samewould happen in a plastic tube. This proves again that it is not correct to treat the groutas a Newtonian fluid since a Newtonian fluid has no critical shear stress.

4.2.2. Stability of groutsAnother phenomenon can give rise to zones that are insufficiently strengthened. Those zoneswere filled during injection, but due to instability of the grout, the cement particles sink. Thiscauses a strength gradient in the injected zone: a stronger zone downwards, because of a highercement concentration and a weaker zone on top of this. This strength gradient was clearlynoticed when analyzing the strength results from the test injection that will be discussed in 5.4.4.

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60 Grout injection of masonry, scientific approach and modeling

Figure 4.4: A very unstable grout, 30 % bleeding, was injected resulting in an upper zone withhardly any cement left [Chandra, 1993][Van Rickstal 1999]

The finding about strength, stiffness and density gradient are discussed there in detail. Theconsequences can even be worse in case of a very unstable grout. The segregation of the cementparticles occurs in the collector and so a grout is injected with a much higher water content aswas meant. Or the sinking happens in the injected masonry. In the worst case the upper layer ofthe injected zone contains no cement at all. This situation is shown in Figure 4.4. Similarly,properly injected regions are emptied by invisible leakages. The grout flows away and leavesthe injected zone partly empty.

Stability is thus a very important property of the grout. Another argument to try to compose astable grout is the injectability. When flow slows down, the cement particles in an unstablegrout sink to the bottom of the flow channel. This narrows the channel and finally blocks furtherinjection. Addition of stabilizing admixtures (bentonite, ultra fines) significantly improves the stability andthe injectability of the grout [Miltiadou, 1991][Paillère, 1986]. For this reason it is important tocheck the stability of the grout and the evolution in time of the stability [Van Rickstal, 1995]. The newly developed test (see paragraph 5.2.5 for more information about the test method)enables a detailed analysis of the stability of the grout. The data that are recorded are plottedagainst time. As an example, Figure 4.5 shows how powerful the technique is to quantify theimpact of the dosage of a stabilizer on the stability. It has to be mentioned that for neither of thedosages, except for the one without stabilizer, a visual difference could be observed. Accordingto the classical existing test method, which consist of simply pouring some grout in a measuring

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Chapter 4 - Problems faced during injection - possible solutions 61

Influence of % Bentonite on Stability

82

84

86

88

90

92

94

96

98

100

0 200 400 600 800 1000 1200 1400 1600 1800

Time[sec]

% o

f Ini

tial

Den

sity

[%

]

0 % bentonite1 % bentonite1,5 % bentonite2 % bentonite3 % bentonite

Figure 4.5: The stability test is used to check the influence of bentonite as astabilizing admixture.

jug and observing if any color difference is occurring between the toplayer and the rest of thegrout, they would all be classified being stable since the segregation is not visible.

4.3. Solutions4.3.1. Improving the injectabilityObviously, the composition has a major impact on the injectability of the grout. A researchproject for the composition of a consolidation grout for the consolidation of the Basilica of OurLady in Tongeren was set up [Chandra, 1993][Van Gemert, 1989][Van Gemert, 1990]. It turnedout to be nearly impossible to formulate a well injectable grout without the use ofsuperplasticizer. All the grouts that are considered contain a certain dosage of superplasticizer.The PhD study of A. Miltiadou [1990] deals with a great number of grout compositions. Mostimportant findings are related to the selection of the type of cement, the fineness of the cement,the water content and the mixing procedure.She studied the relation between the diameter of the bigger particles of the cement and thedimension of the cracks to insure the injectability of the mixture. If necessary, the cement has tobe sieved to eliminate the coarse fraction. The study of the cement grading enabled to define theabsolute condition for a pure cement grout to be injectable in a sand column. The dimensions ofthe sand grains in the sand column can be used to calculate the approximate dimensions of thewidth of the cracks. Although grading demands are normally not possible in practice, thegranularity can be adapted by adding ultrafines. Ultrafine cement is another possibility. Thegrading conditions are listed in Table 4.1.

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62 Grout injection of masonry, scientific approach and modeling

Dimensions of thecracks [mm]

Granularity of the cement: percentage of refusal

160 µm 80 µm 64 µm 32 µm

0,1 - 0 2 0 0 # 1 # 12

0,25 - 0,4 0 # 7 # 8 # 23

Table 4.1: Conditions of injectability for pure cement grouts [Miltiadou, 1990]

Rheology of grouts

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

W/C ratio

Vis

cosi

ty [P

a s]

0

20

40

60

80

100

120

Shea

r st

ress

[Pa]

Viscosity µ (literature)Viscosity µ (own experiments)Shear stress

Figure 4.6: Rheological properties of grouts in function of the water content [Gil, 1995]

The water content has a large impact on the rheological behavior, as will be discussed inChapter 6. The rheological behavior of grouts has extensively been studied by O. Benhamou[1994]. A higher water content improves the rheological features of a cementitious grout. Theviscosity and the yield stress decrease more than linear as can be seen on Figure 4.6. For otherkinds of cement, with other admixtures or other type of superplasticizer, the figures will differfrom this case, but the trend will remain similar. Intuitively one might find the solution to abetter injectability by increasing the W/C ratio. But doing so has two negative consequences:firstly, the mechanical strength of a grout with a high water content is poor and secondly the highwater content has detrimental effects on the stability of the grout. A better option is to useadditional superplasticizer.

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Chapter 4 - Problems faced during injection - possible solutions 63

Ultrasonic mixing [Miltiadou, 1990][Toumbakari, 1999(1)] improves dispersion, especiallywhen ultra fines are added to the grout. It permits the use of a water content lower than in caseof high turbulence mixing, still providing the same penetrability of the grout. Penetrabilityperformance does not depend only on the maximum diameter of the particles, present in thegrout. It is known that fine materials in suspension coagulate very easily due to interparticleinteractions. The use of superplasticizer permits the development of repulsive forces due to theadsorption of the ionic polymers on the surface of the grains. However, their action might notbe sufficient when the grout contains very fine materials such as silica fume and lime. Thoseparticles tend to coalesce in flakes of different size. The penetration capacity of such grouts issignificantly decreased. Thus, for a required penetrability performance, either the water contentof the grout must increase, with detrimental effects on the stability of the suspension and themechanical properties of the hardened grout, or the mixing procedure must be able todeflocculate the particle clusters formed in the suspension. It had been demonstrated earlier that an ultrasonic treatment can easily disperse fine substancesin water. More recently, the ultrasonic dispersion technique has been applied to thedevelopment of cement grouts for the repair of masonry structures. In the study high turbulencemixing was compared to ultrasound mixing. The conclusions of those studies [Toumbakari,1999(1)] can be formulated as follows. The high turbulence mixing procedure was found to beunable to ensure a constant penetrability of grouts composed of cement and fine materials. It isnot capable to deflocculate all the formed flakes. This does not necessarily hinder the injection,because the heavy flakes settle rather soon after mixing in the recipients. However, obstructionof pumping cannot be excluded and the water content of the injected grout will be higher thanplanned. Furthermore, if the suspension contains silica fume, the high turbulence mixingprocedure is not able to produce an injectable grout unless the water content is increased or thedosage of superplasticizer is increased. The ultrasonic mixing procedure on the contrarypermits to produce a high penetrability grout with a limited water content, even if silica fume isused. This is due to the high dispersion capacity, which permits to deflocculate even very smallparticle clusters.

4.3.2. Improving the stabilityThe stability can be corrected by changing the W/C ratio, by adding stabilizing agent such asbentonite, by adding ultafine admixtures. The testing method that was developed, has been usedto measure the effect of the above parameters on the stability of the grout. The results of thesetest are discussed in paragraph 5.2.5.

4.3.3. Injection holesAn intelligent layout of the injection holes can reduce the uncovered zone. Actually there arethree parameters involved about injection holes. The injection hole diameter, the injection hole

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64 Grout injection of masonry, scientific approach and modeling

depth and the pattern of injection holes. The depth of the injection hole should at least be half ofthe wall thickness. If the cracks are narrow, or if the cracks are rather rare, the drilling holeshould reach at least two third of the walls thickness. The diameter of the hole should besufficiently large. Especially if the wall is thick, the discharge through the injection hole canreach significant values. The smaller the diameter, the more important the pressure loss will be.If the pressure loss is large, the injection time will increase and the grout will stick more to thewalls of the flow channels as will be demonstrated in paragraph 9.3. During injection, when thegrout reaches a large void, the time to fill that void will be much higher in case of a smallinjection hole. During that time the grout will settle in the fine neighbourhood channels andwhen the void is finally filled, the grout in the fine cracks lost too much water to restart flowing.The zone, hidden by the fine cracks will not any longer be injectable through that particularinjection hole. The injection holes are the only flow channels that one has under control, so theyshould be as perfect as possible.In the hypothesis that the action radius is equal for all injection holes, the closest pattern (Figure4.8) provides and injected area that is more than 90 % of the total area. The square patterncovers only 78.5% of the global area. Making the pattern denser until no area is uncovered, onegets some overlap. This gives the situation as drawn in Figure 4.9 and Figure 4.10. In case ofthe closest pattern this overlap is 20.9 %, in case of the square pattern the overlap is 57.1 %.From the figures and from the calculated sections of overlap it will be clear that the closestpattern will be more economical than any other pattern. In other words the closest patternrequires less drilling holes for a complete covering of the area than any other configuration.

Figure 4.7:Square pattern, 21.5 % notcovered

Figure 4.8 : Closest pattern, 9.3 % notcovered

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Chapter 4 - Problems faced during injection - possible solutions 65

Figure 4.9: Square pattern withoutuncovered zone, 57.1 % overlap

Figure 4.10: Closest pattern withoutuncovered zone, 20.9 % overlap

From theoretical point of view, the denser the injection hole pattern, the more homogeneouslythe masonry will be injected, the lower the injection pressure can be kept and the better the finalresult. But there are economical constraints that reduce the possible number of injection holes.First of all, those holes need to be drilled. Secondly, a switch from one hole to another meansextra work, extra time needed to complete the job. Apart from economical reasons, alsotechnical reasons limit the density of the injection holes. If not sealed during the injection of ahole, the neighboring holes may act as leakages that prevent the pressure to build up. Inparticular cases a denser pattern can be used for zones that are problematic to inject. Locallyadditional holes can be drilled.

4.3.4. Chemical and mechanical compatibilityThe use of polymers for consolidation should be limited for those cases where the structuralchallenges are such that cementitious grouts can not achieve them. In those cases polymerscould mean the answer to the problem. The discussion about chemical compatibility is limitedto cementitious grouts.A justified method is trying to imitate the original mortar. Therefore, the original mortar isanalyzed with regard to the binding agents’ nature and eventually the ratio of binding agent oninert material. When the analysis is done, the results can be used to see if it is possible tocompose a similar mixture that can provide the required strengthening. By using an imitationmortar, the mechanical compatibility should not induce further problems. A very liquid form ofthis mortar, consisting of pure binding agent, modified using an suitable superplasticizer, is thensuitable for injection purposes. Eventually, some cementitious material, that will not be presentin the original mortar, could replace the hydraulic portion of the binding agent. It will providethe initial strength, needed for immediate consolidation and if used cautiously and in limiteddosage, it will provide a stronger material without compromising the ductility. Anyway,because of the chemical compatibility blast furnace slag cement is preferred above OPC.

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66 Grout injection of masonry, scientific approach and modeling

Natural puzzolanic materials, such as Santorini Earth or Trass are alternatives. Experimentswere carried out by E. Toumbakari [1997], that showed the requested possibilities of theseblended materials. Anyhow, the building should be checked on soluble salts. The formation of ettringite or anyother expansive crystallization caused by using chemically incompatible cementitious groutshould be prevented It is also worth looking at possible efflorescence, although efflorescencewill not endanger the stability of the building. Aesthetically however, it should be avoided.

4.4. Subject of researchChapter 2 illustrates that some massive masonry structures require consolidation due to variouscauses and deterioration mechanisms. In many cases a consolidation injection is able toprovide the wanted strengthening. From the probabilistic evaluation method [Schueremans1997, 1997(2),1998], it is clear that both, the increase of the average strength and the decreaseof the variance of the strength, can improve the reliability of masonry structures. Paragraph2.3.2 indicates how a uniform filling gives rise to a limited variance on the strength of themasonry. Chapter 3 gives the state of knowledge about injection as a consolidation techniquefor masonry. The technological possibilities are described, the correct way of analyzing amasonry building with regard to consolidation is given. In paragraph 3.4 gives a closer look tothe possible grout compositions. Chapter 4 mentions problems that are regularly encounteredduring consolidation injections. From this list of problems and from the description of thebenefit of an injection for the building, it is obvious that it would be useful to possess a modelthat could simulate the penetration of the grout inside the masonry. Knowing if the grout is ableto uniformly fill the voids in the masonry, could mean a breakthrough in increasing the efficiencyof grouting. The above introduction and positioning of the injection technique is the result of aliterature study, the first pillar of this research program. The following chapters contains thetwo other pillars that lead to the model used for simulating the penetration of grout in masonry. Chapter 5 describes the experimental methods, newly developed for or adapted to thepeculiarities of grouting, lists the results of the experiments and draws conclusions for modelingthe grouting process. The third pillar is a study of the relevant theories for the grouting process. In Chapter 6 therheology of dispersions is discussed. It is explained how the fact that a hydraulic grout is adispersion has an impact on the grouting process, more specifically the penetration speed andpenetration depth of the grout in the masonry. The grout flows mainly through the big cracks ofthe masonry. For that reason attention is paid to the flow of dispersions through cylindricalchannels. The material that is surrounding the main cracks is porous masonry. Although, thetransport of the grout through the masonry is not treated using a continuum approach, someaspects of the broad theory about flow of fluids through porous materials are selected anddiscussed in Chapter 7. Darcy’s law, already mentioned in paragraph 5.4.3, is analyzed and themathematical finite element formulation is given. It is used for incorporating the waterabsorption [paragraph 8.2.4] in the model.

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Chapter 4 - Problems faced during injection - possible solutions 67

The input from these three pillars - literature study, theoretical study and the experimantalprogram - will lead to the discrete modeling of the penetration of the grout inside the masonry. Chapter 8 explains the options that were taken in building the model. And finally the use of themodel is demonstrated for some relevant applications in paragraph 9.3.

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68 Grout injection of masonry, scientific approach and modeling

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Chapter 5 - Experimental program 69

Chapter 5. Experimental program

5.1. Aim of the tests

The experimental program splits up in three parts. One part is dedicated to appropriate testingmethods for the relevant properties of the grout. A second part discusses methods to investigatethe masonry structure, especially with regard to injectability. Of major impact is the sizedistribution of cracks and voids. Current state of non destructive testing is hardly able to mapthe major cracks. A method will be presented that results in a global permeability value foreach injection hole. The last and most important part treats a large amount of test injections. Inthis last part masonry properties and grout properties are combined to come to an understandingof the interaction of both components on the complex injection process. As was explained before, a homogeneous filling of all gaps and voids is the most important goalfor achieving a successful consolidation of the masonry structure. The injectability of the groutwill therefore obtain all the attention that is needed in this chapter. When talking aboutinjectability, two kind of grout properties can be seen: the rheological properties in strict senseand the additional properties of the grout related to the fact that a grout is a dispersion of cementparticles in water. The experimental program points out the basic aspects of grouting, and forms the basis forfurther mathematical modeling.

5.2. Testing the grout’s properties5.2.1. Mixing procedureThe characteristics of the materials used for the grouts’ composition are given in Annex 1 of thisbook. Since mixing the composition to obtain a stable, injectable and well dispersed grout is acomplex procedure in which every step is important for the final performance of the grout, thesame mixing procedure is used throughout all the experiments.The bentonite, Bentonile CV 15, supplied by Denys N.V., was mixed in advance with water,taking the proportion of 1 weight unit of bentonite to 9 weight units of water. (This amount ofwater is taken into account for the final W/C ratio.) After the swelling of the bentonite for morethan 24 hours, this mixture obtains the outlook of a putty. Then, the bentonite water mixture isable to provide the required stability enhancing action. The mixing procedure contains thefollowing steps:

C mixing at low speed the bentonite putty with half of the waterC adding the cement (and the mineral admixtures such as silica fume)C mixing at low speed the cement, half of the water and the bentoniteC 2 minutes of waiting time

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70 Grout injection of masonry, scientific approach and modeling

t ' t c % µ 0? (Eq 5.1)

C adding half of the superplasticizer and mixing at high speed during 2 minutesC 2 minutes of waiting timeC adding the other half of the superplasticizer and mixing at high speed during 2 minutes

5.2.2. Rheological properties of the groutAs extensively described in 1, a grout, being a dispersion of cement particles in water, has acomplex rheological behavior. It is justified to treat a grout as a fluid obeying the Bingham lawof flow. This means that there exists a critical yield stress t 0 . If the shear stress does notexceed this critical value, no shearing will take place. Apart from this critical shear strength,the relation between the shear stress and the shear rate is assumed to be linear as expressed bythe Bingham formula:

where µ dynamic viscosity [Pa s]t c critical shear strength [Pa]

shear rate0?

Relatively simple testing methods exist to determine both rheological values:Viscosity was measured using the Brookfield viscometer. The reading on the Brookfieldviscometer is actually the torque, necessary to realize a certain rotation speed. Depending onthe rotation speed, the torque needs to be multiplied by a factor providing the dynamic viscosityin cP (1 centiPoise = 0.001 Pa s). The viscosity at different shear rates was estimated. Byextrapolation to shear rate being zero, the critical shear strength was computed. This way ofworking can be criticized, since the behavior of a dispersion when approaching a shear rate ofzero seems to be very uncertain [Toorman, 1992]. One might find an sudden increase ordecrease of the relation shear rate vs shear stress. Generally, the obtained results correspondedfairly well with values found in literature. Figure 5.1 displays the possible error made byextrapolation of the value near to zero. This way the critical shear stress could beunderestimated.

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Chapter 5 - Experimental program 71

Figure 5.1: Possible mistakes made by linear extrapolation of the measurements [Toorman, 1995]

5.2.3. Dynamic viscosityThe dynamic viscosity measurements were performed on four kinds of grouts. These four kindsof grout cover the complete gamut of mineral grouts that can be used for injection: the cementgrout using normal blast furnace slag cement (CEM III/A 42.5, LA from Obourg), hydraulic limegrout Lime B-fluid (a very fine hydraulic lime meant for repairing the adhesion of frescos to thesupporting wall), micro cement grout (using spinor, an ultra fine cement that was commerciallyavailable) and fine grout using normal blast furnace slag cement with an adapted grading byadding condensed silica fume. The results are listed and discussed in this paragraph.

1° Influence of water contentGrout 1binding agent CEM III/A 42.5, LAsuperplasticizer 1.5 % of Rheobuild 716, sulfonated naphthalene formaldehydestabilizer 2 % of bentonite, Bentonile CV 15

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72 Grout injection of masonry, scientific approach and modeling

W/C factor Rotation speed Viscosity(opm) (cP)

40 3 520 0.50 30 93

60 82 0.60 30 78

60 72 0.67 30 42

60 39 0.7 30 40

60 28 0.8 30 30

60 19 0.90 30 27

60 20 1.00 30 25

60 20

Table 5.1: Viscosity measurements of grout 1

W/L factor Rotation speed Viscosity(opm) (cP)

1.2 12 56.25 30 35.5 60 27.5

1.5 12 16.25 30 9 60 6.875

Table 5.2: Viscosity measurements of grout 2

Grout 2binding agent Hydraulic lime, Lime B-fluid 0, from Unilit superplasticizer nonestabilizer none

Grout 3binding agent Microcement, superplasticizer 1.5 % of Rheobuild 716, sulfonated naphthalene formaldehydestabilizer 2 % of bentonite, Bentonile CV 15

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Chapter 5 - Experimental program 73

W/C factor Rotation speed Viscosity(opm) (cP)

0.67 to viscous to measure1.00 30 22.5

60 15 1.20 30 5.25

60 4.5

Table 5.3: Viscosity measurements of grout 3

W/C factor Rotation speed Viscosity(opm) (cP)

0.75 12 137.5 30 62 60 37

0.85 12 91.25 30 45 60 26

Table 5.4: Viscosity measurements of grout 4

Grout 4binding agent 80 % CEM III/A 42.5, LA + 20 % condensed silica fumesuperplasticizer 1.5 % of Rheobuild 716, sulfonated naphthalene formaldehydestabilizer Silica fume has stabilizing action

Table 5.4 presents the viscosity in function of the W/C ratio for the first grout. The viscositydecreases remarkably when increasing the W/C ratio from 0.47 to 0.7. A further increase of thewater content of the grout has no significant influence on the viscosity. Besides, the stability ofthe grout gets worse. This is why a W/C ratio of 0.67 is chosen for the basic composition of thegrout. This W/C is used to check the influence of the other parameters of the composition.Einstein and Brinkman propose a numerical relation between de volumetric concentration ofparticles and the viscosity. For Einstein, who assumes that the particles doe not interfere, theincrease of the viscosity related to the viscosity of the pure liquid is linear, Brinkman has anexponentially increasing relation. Both relations are represented in Figure 5.2. Our results tendto confirm the exponential relation. Besides, the above theoretical relations suppose nointeraction of the particles. In case of cement grouts this condition is not fulfilled. Cementparticles interact by their electro statical charge. Experimental results prove that the viscosityis increasing faster than the theoretical relations predict.

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74 Grout injection of masonry, scientific approach and modeling

W/C Concentration Viscosity [cP]of solids Theoretical relations Experimental data

Einstein Brinkman Ish-Shalom Vom Berg Van Rickstal0,4 0,442 21 96,8 39 300 5200,5 0,388 19,7 63,9 16 120 820,6 0,346 18,6 48,2 12 45 720,7 0,312 17,8 39,2 8,5 20 280,8 0,284 17,1 33,4 5,8 9 20

Table 5.5: Theoretical and experimental values for the viscosity in function of the W/C ratiowith a dosage of 1,5 % of superplasticizer

T h e o r e t i c a l a n d e x p e r i m e n t a l r e l a t i o n s b e t w e e n W / C a n d V i s c o s i t y

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

0.3 0.4 0.5 0.6 0.7 0.8 0.9

W/C ra t io

Vis

cosi

ty [

mP

a s]

0

20

40

60

80

100

120

Vis

cosi

ty [

mP

a s]

I sh-Sha loV o m B e r gVan R icks t a lE ins t e in (2nd ax i s )Br inkman (2nd ax i s )

Figure 5.2: Two theoretical relations (Einstein and Brinkman) and three experimentalrelations between viscosity and W/C ratio

Table 5.5 compares the theoretical values for Einstein and Brinkman with experimental findingsof Ish-Shalom, Vom Berg and our own experimental results. Generally, the viscosity isdecreasing more than linearly if the dosage of superplasticizer increases. The injectability willdepend highly on the rheological properties. Therefore, it can be stated that superplasticizersare highly efficient in improving the injectability of the grout. Besides, Petrie has stated that fora cement suspension with a high content of superplasticizer, the viscosity in function of theconcentration obeys an exponential law of type Brinkman. Since the general theoritical valuesare much lower than the experimental ones, these values are referencing to the second axis.

2° Influence of stabilizer

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Chapter 5 - Experimental program 75

Viscosity measurementsinfluence of stabilizer

020

406080

100

120140

160

0 0.5 1 1.5 2 2.5 3 3.5bentonite [%]

visc

osity

[cP]

Figure 5.3: Influence of dosage of stabilizer on the viscosity of cementgrout

bentonite rotation speed viscosity% of cement mass cP

0 60 17,51 60 23,2

1,5 60 28,82 60 43,33 60 135,5

Table 5.6: Viscosity measurements, influence of stabiliser

It is obvious that the stabilizer will affect the viscosity and critical shear strength. Thestabilizer thickens the liquid to keep the particles in suspension. One has to compromisebetween stability and viscosity. Bentonite CV 150 has been used as a stabilizer.

The composition for testing the influence of the dosage of bentonite on the viscosity is thefollowing:

binding agent CEM III/A 42.5, LAsuperplasticizer 1.5 % of Rheobuild 716, sulfonated naphthalene formaldehydewater content W/C = 0.67stabilizer parameter, Bentonile CV 15 supplied by Denys N.V.

3° Influence of ultra fines

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76 Grout injection of masonry, scientific approach and modeling

ultra fines rotation speed viscosity% of Cem + SF cP

0 60 17.5 5 60 39.2

7.5 60 58.3 10 60 151.7

12.5 60 223.3

Table 5.7: Viscosity measurement, influence of ultra fines

Viscosity measurementsinfluence of ultra fines

0

50

100

150

200

250

0 2 4 6 8 10 12 14

silica fume [%]

visc

osity

[cP

]

Figure 5.4: Influence of dosage of silica fume on the viscosity of cement grout

A. Miltiadou [1990] states that the addition of ultra fines improves the injectability of a grout.The assumption is that this improvement is due to a better stability of the grout and waterretaining properties. On the other hand, the extreme fineness of these admixtures increases thewater demand. Since ultra fines have stabilizing action, no additional bentonite is added to thegrout. The composition for testing the influence of the dosage of ultra fines on the viscosity isthe following:

binding agent CEM III/A 42.5, LAsuperplasticizer 1.5 % of Rheobuild 716, sulfonated naphthalene formaldehydewater content W/C = 0.67ultra fines parameter, dosage is percentage of cement + ultra fines

5.2.4. Thixotropy, non linear behavior and time dependent properties

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Chapter 5 - Experimental program 77

Figure 5.5: The classical stability test provides only little information

Most compositions were tested for two different values of the rotation speed: 30 rpm and 60rpm. The results reveal that viscosity at lower speed is slightly higher than viscosity at higherspeed. This shows that grouts have a pseudo plastic behavior. A pseudo plastic fluid showslower viscosity values for higher shear rate. This phenomenon enhances the blocking processwhen an injection slows down. This might explain the practical findings that a constantinjection pressure is preferable to a discontinuous injection with a periodical increase anddecrease of pressure. The addition of bentonite provides some thixotropical aspects to the grout. Thixotropy meansthat the viscosity decreases at sustained shear deformation, which occurs when the grout ismixed during a longer period of time. Also, the initial shear strength increases after a momentof rest. These aspects are not visible from the above measurements, since the grout wasthoroughly mixed after each period of stand still. Thixotropy increase the problems to restart aninjection that was halted.

5.2.5. StabilityStability is a first requirement for a grout to be injectable. When the flow slows down, thecement particles in an unstable grout will sink to the bottom of the flow channel. This narrowsthe channel and finally blocks further injection. Stability also means that the grout is able toretain the water. When the grout passes through the relatively dry masonry, some water will beabsorbed from the grout. The more water is absorbed the less fluid the grout will become andhence injection will slow down and finally stop. These two blocking mechanisms show theimportance of the stability of the grout.

Stability of a grout used to be checked by putting some grout in a measuring glass (Figure 5.5).After a period of time, it was visually checked if any segregation or bleeding appeared. This

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78 Grout injection of masonry, scientific approach and modeling

way only very poor information is obtained about the evolution in time of the stability of thegrout. An other disadvantage is that segregation must be visible before it can be recognized. Theclassical tests are thus not very suitable for detailed analysis of the stability. For those reasonsa more powerful test was developed. Instability of the grout means that the heavy particles of the dispersion sink to the bottom of therecipient due to gravity. This means that the density of the grout in the top region decreasesbecause of this loss of heavy particles. The more unstable the grout, the more the density willdecrease. The developed experiment makes it possible to measure the evolution of the densityat any height in the recipient. An object, hanging in a liquid undergoes an buoyance force according to Archimedes’ law. Thisforce can be expressed by the following formula:

F = ? . g .V ? = density of the fluidg = gravity accelerationV = volume of the object

When the fluid around the object becomes less heavy due to the settlement of the cementparticles, the density will decrease and hence the force decreases in the same proportion. Byrecording the upward force one gets information about the evolution in time of the density andthe stability of the grout. This buoyance force is recorded using the setup in Figure 5.6 andFigure 5.7. The balance is indicating the upward force, which is recorded every 5 seconds by acomputer. This method is analogue to the Andreasen Method in soil mechanics, used to measurethe content of very fine parts in a soil sample. The balance is put to zero when the recipientwith the grout is put on the balance. Once the object hangs in the grout the reading of the balancegives the mass corresponding to the buyance force. Since the density ? varies when theparticles sink due to instability, the recording of the buoyance force implies the recording of thestability.

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Chapter 5 - Experimental program 79

Figure 5.6: Principle of the newly developed stability test for grouts

Figure 5.7: Configuration for the stability test

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80 Grout injection of masonry, scientific approach and modeling

Stabilityinfluence of W/C ratio

94

95

96

97

98

99

100

0 200 400 600 800 1000 1200Time [sec]

% o

f ini

tial d

ensi

ty [%

]

W/C = 0,5

W/C = 0,6W/C = 0,7W/C = 0,9

W/C = 0,67W/C = 0,8

Figure 5.8: Stability of cement grout in function of W/C ratio

1° Influence of water contentThe composition used to test the influence of the water content on the stability has a normal blastfurnace slag cement. Blast furnace slag cement is preferable to Portland cement because of thelower CA3 content. This reduces the risk for the formation of ettringite. The low alkali content(LA) reduces the risk for efflorescence [Toumbakari, 1997]. The dosage of 1.5% of Rheobuildis chosen because it provides a relatively low viscosity. Two percent of bentonite turned out toprovide good viscosity values.

binding agent CEM III/A 42.5, LAsuperplasticizer 1.5 % of Rheobuild 716, sulfonated naphthalene formaldehydestabilizer 2 % of bentonite, Bentonile CV 15Water content parameter

As can be seen from Figure 5.8, the stability of all grouts is good. Bentonite performs verywell as a stabilizing admixture. On first sight the behavior of W/C = 0.8 grout is surprising.This composition was tested a few months later then the other series. The cement was partlyhydrated through moisture uptake from the air. This way the normal mixing procedure could notdeflocculate the cement clusters as intense as before. Those heavy clusters sink quickly to thebottom of the recipient, resulting in a decreased stability. This result proves how delicate the

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Chapter 5 - Experimental program 81

Stabilityinfluence of stabilizer

8 2

8 4

8 6

8 8

9 0

9 2

9 4

9 6

9 8

1 0 0

0 200 400 600 800 1000 1200 1400

Time [sec]

% o

f Ini

tial

Den

sity

[%

]

0 % bentonite1 % bentonite1,5 % bentonite

2 % bentonite3 % bentonite

Figure 5.9: Stability of cement grout as a function of stabilizer addition

formation of a well injectable, stable grout is. The cement has to be of good quality. Thesmallest lack of deflocculation causes a complete change in stability.

2° Influence of stabilizerThe composition for testing the influence of stabilizer on the stability is the following:

binding agent CEM III/A 42.5, LAsuperplasticizer 1.5 % of Rheobuild 716, sulfonated naphthalene formaldehydestabilizer parameter, % of cement mass, Bentonile CV 15Water content W/C = 0.67

One would expect the grout to become more stable when the dosage of bentonite is increased.This seems to be the case until 2 % of bentonite. Adding more stabilizer does not improve thestability behavior any longer. As can be seen from Figure 5.9, some stabilizer is absolutelynecessary to provide a grout that is sufficiently stable. A grout without stabilizer loses 15% (!)of its initial density after half an hour.

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82 Grout injection of masonry, scientific approach and modeling

Stabilityinfluence of ultra fines

82

84

86

88

90

92

94

96

98

100

0 200 400 600 800 1000 1200 1400

Time [sec]

% o

f Ini

tial D

ensi

ty [%

]

5 % SF7,5 % SF10 % SF12,5 % SF0 % SF, bent0 % SF

Figure 5.10: Stability of cement grout in function of dosage of ultra fines

3° Influence of ultra fines (silica fume)The composition for testing the influence of ultra fines on the stability is the following:

binding agent CEM III/A 42.5, LAsuperplasticizer 1.5 % of Rheobuild 716, sulfonated naphthalene formaldehydeWater content W/C = 0.67ultra fines parameter, % of the sum of cement and ultra fines

Again we notice that a grout without any ultra fines is not stable. The stabilizing features ofsilica fume are obvious from Figure 5.10. After all, bentonite is still a better stabilizer thensilica fume. As we will see later in this text, silica fume corrects the granularity of the cementin the finer region and therefore, it improves the penetration of the grout in zones with finecracks. Another remarkable fact appears when too much ultra fines are added. The stability ofa grout with 12.5 % of silica fume is less stable then a grout without stabilizing agent. This issimilar to what was noticed when testing the influence of the dosage of bentonite on the stability.A possible explanation is the presence of many very fine particles. The distance between thoseparticles inevitably becomes very small. Therefore, flocculation might happen although adeflocculating superplasticizer is used. The different efficiency of both stabilizing admixtures can be explained by their different shape.Silica fume consists of almost perfect spheres whereas bentonite has a lamellar shape. Besides,

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Chapter 5 - Experimental program 83

Figure 5.12: SEM picture of condensed silicafume

Figure 5.13: Comparison between a flow cup (on the left) and a capillary viscometer (onthe right)

Figure 5.11: SEM picture of bentonite

bentonite has electrostatic charges providing a better stabilizing action than silica fume [VanDen Berghe, 1994].

5.2.6. Flow time measurementsTo determine the rheological parameters of a fluid, one disposes of different families ofviscometer [Blom, 1988]. For the determination of the viscosity we used the Brookfieldviscometer from the family of coaxial viscometers. Other type of instruments exist, all havetheir own advantages and disadvantages. Regarding our research field, it would be logical toconsider the family of capillary viscometers Figure 5.13. The instrument is basically arecipient for a calibrated volume of liquid equipped with an opening to let the liquid flow out ofthe recipient.

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84 Grout injection of masonry, scientific approach and modeling

Correlation Viscosity - Flow time

R2 = 0,1804

R2 = 0,9893

R2 = 0,9407

R2 = 0,8906

30

35

40

45

50

55

60

65

70

75

80

0 50 100 150 200 250

Viscosity [cP]

Flow

tim

e [s

ec]

W/C ratio

Stabilizer

Ultrafines

Figure 5.14: The correlation between the flow time and the viscosity depends on theinvestigated parameter

An ideal capillary viscometer has a long capillary cylindrical conduit with a small diameter.This way the discharge is mainly determined by the flow resistance of this conduit throughwhich a laminar flow takes place. The available viscometers do not have this long conduit tolet the liquid flow out of the recipient. A capillary viscometer has a ratio length over diameterof at least 15, whereas the flow cups have a ratio L/D of only 1,5. The Afnor cup, viscometeraccording to the Franch code NF T 30-014, has only an opening. The Marchall cone (ASTM D1084) has a small cylindrical outlet, but far too short to force the flow to be laminar or to limitthe discharge according to known rheological laws such as Haegen-Poisseuill’s law orBuckingham formula [Hinch, 1975]. The charge losses due to the the transition from therecipient to the conduit master the flow completely. As a consequence of this it is not possibleto calculate the rheological parameters out of the flow time measurements and to use thesevalues in the model. For that purpose, a capillary viscometer would be perfect. Though themeasurements enable to judge the fluidity of the grouts. Indeed, a comparison between allmeasurements of flow time and Brookfield viscosity for the basic grout with varying W/C ratio,shows that there is only a poor correlation between both (Figure 5.17).

The overall correlation is only 0.18 whereas the correlation for varying W/C ratio, bentonitedosage and ultra fine dosage is 0,97; 0,98 and 0,89 respectively. When only one parameter of

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Chapter 5 - Experimental program 85

0

50

100

150

200

250

Vis

cosi

ty [c

P]

30

40

50

60

70

80

Flow

tim

e [s

]

0.5 0.6 0.7 0.8 0.9 1 W/C ratio

Viscosity Flow time

Flow time measurementsinfluence of water content

Figure 5.15: Influence of W/C ratio on the flow time and the viscosity

0

50

100

150

200

250

Vis

cosi

ty [c

P]

30

40

50

60

70

80

Flow

tim

e [s

]

0 0.5 1 1.5 2 2.5 3 dosage [%]

Viscosity Flow time

Flow time measurementsinfluence of stabilizer

Figure 5.16: Influence of stabilizer on the flow time and the viscosity

the basic grout composition ( CEM III A, 42.5 LA, 1.5 % of SP, 2% of bentonite, W/C=0.67) ischanged, the correlation is good as can be seen from Figure 5.15, Figure 5.16 and Figure 5.17.One could state that the flow time measurements do not allow to derive the exact value for therheological parameters. They only give an indication about the fluidity. For on site controlmeasurements they are very suitable: a chronometer and the recipient are the only devices thatare required for the measurement.

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86 Grout injection of masonry, scientific approach and modeling

0

50

100

150

200

250 V

isco

sity

[cP

]

30

40

50

60

70

80

Flow

tim

e [s

]

0 2 4 6 8 10 12 14 dosage [%]

Viscosity Flow time

Flow time measurementsinfluence of ultrafines

Figure 5.17: Influence of ultra fines on the flow time and the viscosity

It is remarkable that the influence of the water content on the flow time is proportionally as bigas the influence on the viscosity, whereas the influence of the dosage of bentonite on the flowtime is only half of the influence of the stabilizer on the viscosity. For the influence of ultrafines, this phenomenon is even worse: the influence on the flow time is almost none, theinfluence on the viscosity, on the contrary, is extremely big. This proves that different physicaleffects are playing an important role in both types of measurements. It is remarkable that theflow time of a grout with ultra fines remains low although the viscosity increases. This could bean explanation for the good injectability of a grout with ultra fine admixtures. The physicalphenomenon that takes place inside the flow channels of deteriorated masonry is closer to whathappens during the flow time measurement than it is to what happens during the coaxialviscometer test. Apparently the addition of fine admixtures has a completely different effect onthe viscosity than on the flow time.

5.2.7. General observations about testing the grout propertiesThe first observation about testing the grout properties is a rather critical one: standard methodsgenerally used in practice to find out about rheological properties of fluids are not very suitable.In case of the stability measurement, the existing method is rather simplistic and not veryefficient providing only very limited information. A completely different method wasdeveloped providing the evolution in time of the density and hence the stability. The methodenables to judge admixtures with regard to their stabilizing capacities, the efficiency of mixingprocedures etc... For the measurements of the rheological properties, the standard equipment isin fact not very suitable for dispersions, nor for the viscosity nor for the critical shear strength ofthe dispersion. The flow time measurements enable to judge the fluidity of the grout, but as was

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Chapter 5 - Experimental program 87

proved by the experiments, the relation with viscosity is poor. Therefore it is recommended toadapt the existing flow cups. Using a longer capillary conduit would probably solve theproblem about the correlation flow time to viscosity. Further research in this is beyond thescope of this thesis.Second observation is the significant increase of flow time and viscosity when the water contentdecreases. This observation is a confirmation of theoretical and experimental findings inliterature. This fact makes the water retaining capacities of a grout composition very importantfor deep penetration.

5.3. Testing the masonry’s properties5.3.1. Diagnosis of the masonryThe penetration of the grout inside the masonry structure is determined by the interaction ofgrout and masonry. Tests to check the quality of the grout were described above. Although someproblems were mentioned and some adaptation of existing testing equipment seems to benecessary to obtain reliable results, grout properties can be analyzed in the laboratory.Different compositions can be compared. The test results can be used to quantify the parametersin the model that will be explained later. The internal structure of the masonry, the permeability, the crack size distribution and themoisture content are the most important properties of the masonry with regard to injectability.The overall condition of the masonry used to be determined by coring and analyzing the coreswith regard to mechanical properties, cracks, condition of the bricks and the mortar. Theseresults are often used to judge the possibilities of consolidation. Less destructive is thecombination of a non-destructive testing method, i.e. (ultra) sonic measurements or electricresistivity measurements, with coring. This combination enables to quantify the qualitative dataobtained from the non-destructive testing. These data help to make the diagnosis: does thebuilding need consolidation and can grouting be a part of this consolidation? For the groutingtwo questions are important: first, what strength improvement is required and what grout canprovide it; and secondly how will the grout progress in the masonry, to what extend will thevoids be filled?The required strength improvement is relatively easy to judge. Fact is that filling the cracks andthe voids will cause the masonry to behave monolithically. Research indicated [Tomazevic,1992] that the intrinsic mechanical properties of the grout do not or hardly influence the finalstrength of a deteriorated masonry wall in case of comparable rheological properties of thegrout. This actually reduces the first question to the second one, provided the mechanicalproperties of the grout are not too low. The flow of the grout through the masonry will dependon the permeability of the masonry and even more on the crack size distribution. For the modelthat was developed, a map of cracks would be the ideal information to make correctsimulations. The existing non destructive techniques however, are not able to provide that

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88 Grout injection of masonry, scientific approach and modeling

Figure 5.18: The reproducible masonry sample: crusher bricks inside a plexiglass tube

information. None of the techniques has a resolution that is accurate enough to find out aboutindividual pores and cracks, even to give an idea about the crack size distribution. If one wants to simulate the transport of grout by a continuum description of the masonry,average permeability values are the required information.

5.3.2. Reproducible masonry samples: physical modelFirst problem is to compose a reproducible “masonry sample” suitable for test injections. Realmasonry structures have two disadvantages. Firstly they can not be remade or copied. Everytest zone, after injection is lost for further tests with other grouts or other parameters. Secondly, it is hard to control or visualize what is happening inside the masonry structure. It isnot possible to see if the grout has penetrated the complete injection zone. Eventually cores canreveal some of the desired information. To improve the knowledge about the physical mechanisms that take place during injection andthat determine the penetration of the grout in the masonry, plexiglass cylinders, diameter 100 mmand height 450 mm, are filled with a fraction of crushed bricks and are injected unidirectionallyfrom bottom to top. At the bottom, a layer of fine stones ensures a good distribution of the grout.Crushed bricks are preferred over sand grains since crushed bricks have, just as masonry, awater absorbing action. The crushed bricks were bought from a company constructing tenniscourts. Then they were sieved to obtain different grain size distributions to enable thesimulation of different permeability for the masonry Figure 5.18. The cylinders are made oftransparent plexiglass to make observation and thorough analysis possible.

5.3.3. Permeability of samplesPermeability of the masonry determines the practical injection process. The injection holepattern, the necessary fluidity and hence the composition of the grout depend upon the

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Chapter 5 - Experimental program 89

Figure 5.19: Setup for permeability measurements using air flow.

Re 'v d?

'? q dµ A f

' 8,46 (Eq 5.2)

permeability of the masonry. The more permeable the masonry the coarser the injection holepattern may be that guarantees a successful injection. In constructing a model to describe the injection process, the permeability will be one of themain parameters of the masonry that influence the evolution and distribution of the grout in themasonry mass. Once again, before being able to determine the influence of the permeabilityparameter, it must be possible to determine it as exactly as possible. The test setup to obtain thepermeability of the reproducible masonry samples will be explained first. Afterwards it isindicated how this test could be adapted to obtain permeability values on real walls.The samples used to perform the test injections are plexiglass cylinders, filled with differentfractions of crushed bricks. These fractions are densified by vibrating the cylinder after fillingone third and two third of it. That way a reproducible compaction is realized.The setup on Figure 5.19 is used to determine the permeability of these test samples. Differentdischarges are established and the pressure difference over the tube is measured. The formula (Eq 5.3), based upon Darcy’s law for laminar air flow through porous media, provides thepermeability of the tested tube.

Where Re = Reynolds numberv = velocityd = diameter of channel

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90 Grout injection of masonry, scientific approach and modeling

porous medium

area A

discharge q

Figure 20: Geometry and pressures for the calculation of the permeability

? = kinematic viscosity [m2/s]µ = dynamic viscosity [Pa.s]A = cross section of sampleq = air discharged = average grain sizef = porosity

Darcy’s law can be used here because the Reynolds number for the most turbulent configurationequals only 8.46, which is in the beginning of the transition region from laminar to turbulentflow (1 < Re < 10) [Collins, 1965] and hence air flow through the crushed bricks can beconsidered being laminar.

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Chapter 5 - Experimental program 91

q 'K .Aµ.L

( ? p ) (Eq 5.3)

Permeability using air flow, fraction 2-4 mm

00,5

11,5

22,5

33,5

44,5

0 10 20 30 40 50 60 70 80

Discharge [l/h]

Pre

ssur

e [P

a]

measurementsregression

Figure 5.21: Permeability measurements for the fraction 2-4 mm, using air flow

Where q = discharge [m3/s]µ = Dynamic viscosity of air

= 17,1 x 10-6 Pa.sK = Permeability [m2]A = Area of the tube [m2]L = Length of the tube? p = pb - pa

For each discharge (10, 20, ..., 80 l/h) the pressure difference over the plexiglass cylinder ismeasured taking into account the pressure loss over the tubes. In this way 8 data points areobtained. Linear regression is applied and from this curve the K-value is calculated. Figure5.21 shows the relation between discharge and pressure difference over the cylinder. Thecharge losses in the conduits are taken into account by doing a measurement with an emptyplexiglass cylinder. The permeability of the samples is considered to be constant. However, inprecise terms, as the bottom layer of crushed bricks is more vibrated than the top layer, thepermeability at the bottom is probably slightly lower than at the top, for the same brick fraction.In the contact zone of a finer fraction with a coarser fraction, a locally lower permeability willbe present because of the finer particles entering and filling the voids of the coarser zone.

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92 Grout injection of masonry, scientific approach and modeling

Fraction Permeability K K ? g / µ

[mm] [ m2 ] [1 darcy = 10-12 m2 ] [m/s]

1-2 7.28 E-10 0.728 E+03 7.11 E-03

2-4 68.7 E-10 6.87 E+03 67.1 E-03

4-10 150 E-10 15.0 E+03 147 E-03

1-4 61.9 E-10 6.19 E+03 60.5 E-03

2-4 / 1-2 / 2-4 29.5 E-10 2.95 E+03 28.8 E-03

Table 5.8: Listing of the permeability values in several units

This test provided the permeability values of the different fractions that were used to determinethe influence of the permeability on the injection process and on the evolution of the grout insidethe masonry. Table 5.8 lists the obtained values.

5.3.4. Permeability of masonry structureThe above setup enables us to determine the permeability of the test samples in the laboratory. Itwould be interesting to adapt the setup for in situ measurements of the permeability of themasonry to inject. Of course, the laboratory situation is completely different from reality.Nevertheless it is possible to use the above principle to blow air into the masonry and tomeasure the pressure needed to realize a certain discharge to obtain indications about thepermeability.Figure 5.22 shows a possible setup for this purpose. Through a drilled hole, a certain dischargeof air is blown into the masonry. For each discharge the air pressure is measured just beforeentering the masonry. Inside a homogeneous masonry a radial flow will be generated. UsingDarcy’s law in differential form and in cylindrical coordinates it is possible to get a goodestimation of the permeability by integration between the hole radius and infinity [Collins,1965][Dullien, 1979].

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Chapter 5 - Experimental program 93

Figure 5.22:Possible setup for in situ permeability measurement

Precaution is needed due to possible leakage of air trough big holes. The masonry will never behomogeneous, so it is possible that the main flow will occur through one big hole instead ofthrough the cracks and voids of the masonry and this way the measured permeability would behigher than the average permeability of the masonry.

5.4. Laboratory injection tests5.4.1. Description of the testsThe reproducible masonry samples that are characterized in paragraph 5.3.2, are used toimprove the knowledge about the physical phenomena that take place during the penetration ofthe grout. During the first series of test injections, the progress of the grout in the plexiglasssample is recorded on video tape. During the second series, the indication of the balance onwhich the reservoir was put is also taped on video. This provides data about the discharge thatcould be realized. Both recordings enable to analyze the evolution of the injection in time. Thefirst recordings provide the progress of the grout whereas the second series enables to quantifythe amount of injected grout in function of time. In addition to this analysis, a thoroughobservation is possible. The complete injection of a sample of 45 cm heigh, takes only 30seconds to one minute, so the time to observe in detail is too short. The recorded images can belooked at more exhaustively. At first the layout of the samples was straightforward: the cylinders were filled with only onefraction of crushed brick. In a later stage, the samples’ layout was more complex. Severalfractions of the crushed bricks were combined to study what happens when a high permeablefraction is in parallel with a low permeable fraction. The first series of injections were stopped

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94 Grout injection of masonry, scientific approach and modeling

Height 1/2 1/4 2/4 4/10 cm CEM III A CEM III A CEM III A CEM III A0 0 0 0 05 2,7 3,6 2,9 2,8

10 6,1 7,5 5,8 6,215 9,1 10,2 9,2 9,420 12,7 15,5 12,3 1325 16,2 19,1 15,4 16,330 19,7 23,7 18,2 20,135 25,1 28,8 22 24,140 29,5 33,2 25,2 27,545 33,6 37,6 27,9 31

Table 5.9: Progress of the grout in the samples filled with only one fraction

when the grout comes out of the sample. No further information can be obtained. The secondseries however, are continued to see how the discharge changes with time. Most of the injectedsamples are emptied right after recording the injection to save the plexiglass tubes. Only insome particular cases, the mechanical strength of the hardened sample is tested. The soundvelocity is checked in order to calculate the dynamic Young’s modulus of the hardened sample.The injection pressure for all these experiments was 1 bar = 100.000 Pa. If not mentioneddifferently, the injections are done using the grout mix that came out best from the rheologicalstudy, with proportions:

binding agent CEM III/A 42.5, LAsuperplasticizer 1.5 % of Rheobuild 716, sulfonated naphthalene formaldehydestabilizer 2 % of bentonitewater content W/C = 0.67

5.4.2. Flow chartsThe flow charts are the graphical presentation of the results from analysis of the video images ofthe test injections. During the analysis, the time is noted to reach a certain height. The resultsfor the samples filled with one fraction of crushed brick are listed in Table 5.9. The sameinformation is given by the flowchart on Figure 5.23. Looking at the permeability of eachfraction as listed in Table 5.8, there seems to be some correlation between the permeability andthe progress of the grout in the porous medium. However, the ranking is not the same forpermeability and penetration speed.

The 1-2 mm fraction, that has by far the smallest permeability, is only the second slowest toreach the top of the sample. So, other parameters are involved besides the permeability of theporous medium. One of these parameters is the total volume to inject. Before a grout cancontinue to progress, the voids needs to be filled. The fraction 1-4 mm contains a biggervolume of voids than the fraction 1-2 mm. Other possibility might be that it is easier to fill the

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Chapter 5 - Experimental program 95

F l o w c h a r t s o f t h e G r o u t I n j e c t i o n sI n j e c t i o n p r e s s u r e = 1 0 0 . 0 0 0 P a

0

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

0 1 0 2 0 3 0 4 0 5 0

H e i g h t [ c m ]

Tim

e [s

ec]

4/102/41/21/4

Figure 5.23: Flow charts of the samples filled with only one fraction of the crushed bricks

voids of the fraction 1-4 mm. This makes further progress impossible because no pressure canbe build up before the voids are filled. When the grout arrives at the top, the complete cylinderis filled. If it is difficult to fill the voids, some pressure can be build up. The grout is able toproceed, leaving part of the voids empty because the resistance to fill these voids is big. Hencethe denser and less permeable fraction requires less time to reach the top because a smalleramount of grout is to be injected.

The flow curves are almost linear. Because of the increasing resistance of that part of thesample that is already injected, one expects an evolution of the penetration depth that is everslowing down. This seems not to be the case. There are two possible explanations for thisbehavior. First possibility: the resistance for the grout to penetrate is located at the injectionfront. The rest of the resistance can almost be neglected with regard to this front resistance.Some simple simulations at the end of this chapter show that by incorporating a front resistancevalue, the simulations fit well with the experimental data.

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96 Grout injection of masonry, scientific approach and modeling

Figure 5.24: Layout of horizontally split sample (a) and vertically split sample (b)

A second series of samples contains three layers of fractions of crushed bricks. The middlelayer is finer than the first and the last layer. The layout of such samples is shown in Figure5.24 a Aim of this layout is to analyze what happens when a grout meets a zone with lowerpermeability that is in series with a zone with higher permeability. Figure 5.25 clearly showshow the grout slows down when arriving in the denser zone. The penetration speed decreasessignificantly. It is strange however, to see that the penetration speed increases again to theinitial value ones the low permeable zone is passed. The front resistance theory is able toexplain also this observation.

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Chapter 5 - Experimental program 97

Flowchart 2-4 1-2 2-4 mmInjection pressure = 100.000 Pa

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40 45

Height [cm]

Tim

e [s

ec]

Figure 5.25: Grout penetration in a horizontally split cylinder, 2-4 1-2 2-4 layout

A similar effect happens when the low permeable zone is put at the bottom and at the top. Thenthe penetration speed increases somewhat when the more permeable zone is reached.Afterwards the penetration slows down again (Figure 5.26). In order to analyze the behavior of the grout when a less permeable zone is in parallel whit ahigh permeable zone, some samples were split vertically. The layout is then as represented inFigure 5.24 b. Although the 1-2 mm fraction is on its own well injectable as shown in theprevious flow charts of the horizontally split cylinders and of the uniform cylinders, invertically split layout this fraction could not at all be injected at a first attempt. Therefore, itwas decided to make two more tests of the same layout. In one other sample the zone parallel tothe 2-4 fraction was only partially injected. In a third attempt, the zone was injectedcompletely, but somewhat later than the 2-4 mm zone. This is of course not obvious from theflow chart (Figure 5.27), but is shown in Figure 5.28. The overall penetration is much slowerthan in all previous cases. The slowing down is much more explicit. The penetration speeds upsomewhat after the split zone, but does not regain its original speed.

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98 Grout injection of masonry, scientific approach and modeling

Flow chart 1-2 2-4 1-2 mmInjection pressure = 100.000 Pa

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35 40 45

Height [cm]

Tim

e [s

ec]

High speed

Low speed

Low speed

Figure 5.26: Grout penetration in a horizontally split cylinder, 1-2 2-4 1-2 layout

Flow char t ver t i ca l ly sp l i t , 2 /4 1 /2 2 /4I n j e c t i o n p r e s s u r e = 1 0 0 . 0 0 0 P a

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

0 1 0 2 0 3 0 4 0 5 0

H e i g h t [ c m ]

Tim

e [s

ec]

Figure 5.27: Grout penetration for the vertically split cylinder

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Chapter 5 - Experimental program 99

Figure 5.28: Picture of the partly injected, vertically split cylinder

Lime grout v s CementgroutInjec t ion pressure = 100 .000 Pa

0

10

20

30

40

50

60

0 10 20 30 40 50

Height [cm]

Tim

e [s

ec]

2/4 1/2 2/4 CEM III A

2/4 1/2 2/4 Lime - B

Water

2/4 1/2 2/4 Lime - B

blocked

Figure 5.29: Comparison between the injection of cement grout, limegrout and pure water of the horizontally split cylinder

Due to its fineness, the lime grout, consisting of hydraulic lime, B-fluid 0 from Unilit and awater/lime ratio of 1.5 without any admixtures nor superplasiticizer seems to have less hinderfrom the finer fraction. No slowing down can be noticed. The total time to reach the top of thecylinder however, is smaller for the cement grout. The lime grout shows a more Newtonianbehavior: the penetration speed continuously decreases resulting in a parabolic evolutionFigure 5.29. A second attempt was not successful in the sense that the grout blocked at 15 cmfrom the top. Again, the parabolic behavior is clearly visible.

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100 Grout injection of masonry, scientific approach and modeling

Figure 5.30: The diffusivity for water of porous material is related to the moisturecontent. Curve 1: experimental data, curve 2: zone with constant relation, curve 3:exponential fitting Figure 10.1

Since it was expected that the dry masonry would absorb water out of the grout, the crushedbricks were wetted in advance by simple injection with water. Then, the valve at the bottom ofthe cylinder was opened and the water flew out of the sample. After 30 minutes the samesample was injected with the cement grout. The injection of the prewetted sample behavedcompletely linear. The time needed to reach the top was half of the time for the dry masonrysample. Compared to water, the grout injection was only 3 seconds slower. Obviously, theresistance to flow has been reduced significantly by the water injection. It is possible that thisreduction has to do with the front resistance that has disappeared. Besides, the conductivity of aporous medium is related to the water content as is displayed in Figure 5.30. This means thatthe conductivity to water of a dry porous medium is smaller than the conductivity to water of awet sample. The same reasoning goes for the conductivity to airflow: a porous medium with ahigh moisture content will provide a greater resistance to flow of air than a dry sample [Groot,1993].

At the water injection, the voids are already filled with water. Then the water is let out of thesample. After half an hour, not all the injected water has flown away. Additionally, the waterinjection has washed out and cleared the major flow channels.

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Chapter 5 - Experimental program 101

I n j e c t i o n o f D r y a n d P r e - w e t t e d C y l i n d e rI n j e c t i o n p r e s s u r e = 1 0 0 . 0 0 0 P a

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

0 1 0 2 0 3 0 4 0 5 0

H e i g h t [ c m ]

Tim

e [s

ec]

2 / 4 1 / 2 2 / 4 C E M I I I A s p l i t2 / 4 1 / 2 2 / 4 C E M I I I A s p l i t / w e t2 / 4 1 / 2 2 / 4 W a t e r

Figure 5.31: Behavior of the injection of the prewetted, horizontally split cylinder

From this experiment it is clear that prewetting can solve penetrability problems. But, sincethere is no water absorption out of the grout, the mechanical strength of these particular samplesis very poor. Therefore prewetting has to be used with precaution.

As mentioned before, during some experiments the mass flow has been recorded. This enablesto filter the influence of unfilled voids. Besides, when numerical or analytical simulations aredone, the mass flow is calculated and not the injection height. Furthermore, since there is anassumption that the main resistance to flow is situated at the injection front, once the sample isfilled, this resistance should disappear and the discharge should increase suddenly. Figure 5.32indicates such a discharge increase. The vertical line indicates the moment when the cylinder iscompletely filled. The increase however, is too small to be the consequence of the disappearingof a major resistance to flow. Though, the discharge does increase and hence the totalresistance to flow decreases somewhat. The discharge gradually decreases after the smallincrease when the cylinder is completely filled. This can be explained by two phenomena thatamplify each other. The flow channels are narrowed by the water absorption out of the grout.Cement particles stick to the walls of the flow channels. By this, the overall resistance to flowincreases. Second phenomenon is the clogging up of the fine channels by the big or flocculatedcement grains that pass. Once a few particles are caught, the constriction acts as a filter for eversmaller particles and finally the channel is completely blocked. Again, the overall resistance toflow increases and hence the discharge decreases.

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102 Grout injection of masonry, scientific approach and modeling

Mass flow through fraction 1-4 mmInjection pressure = 100.000 Pa

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300 400 500 600

Time [sec]

Tot

al M

ass

[kg]

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Mas

s F

low

[kg/

s]

Total Mass Mass Flow

sample is filled

increasing discharge

Figure 5.32: Evolution of mass flow for the injection of the 1-4 mm fraction, basic grout

Fraction [mm] Reynolds number

1 - 2 2,7

1 - 4 4,2

2 - 4 6,7

4 - 10 13,6

Table 5.10: Reynolds number at the test injections for the used fraction

5.4.3. Simplified mathematical model The experimental observations are used to build a first mathematical model. The experimentsalso improved the understanding of the physical mechanisms taking place during the penetrationof the grout through the masonry. At first we tried to model the injections using Darcy’s law,governing the laminar flow of fluids through porous materials. The flow is considered to belaminar since the Reynolds numbers, that characterize the nature of flow for all used fractions ofcrushed brick (Table 5.10) are far below the range that indicates turbulent flow.

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Chapter 5 - Experimental program 103

Leq [m] ' 0,27 .E09 K [m 2] & 0,21 (Eq 5.4)

Darcy's law, uniform cylinders Injection pressure = 100.000 Pa

0

5

10

15

20

25

30

35

40

45

50

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Height [m]

Tim

e [s

ec]

Experiment (1-2 mm)Darcy (1-2mm)Experiment (1-4mm)Darcy (1-4mm)Experiment (2-4mm)Darcy (2-4mm)Experiment (4-10mm)Darcy (4-10mm)

Figure 5.33: Inadequacy of Darcy’s law to simulate the experiments

Figure 5.33 shows the inadequacy of Darcy’s law to model the injection tests. The averagevelocities of the injections calculated using Darcy’s law are about three times higher than theexperimental ones. The qualitative difference in shape between the experimental and thetheoretical flow chart (the theoretical flow is not as linear as the experimental one) proves thatthere is also a physical dissimilarity.

The observations mentioned above enabled us to develop the front resistance theory, based uponthe hypothesis that beyond the overall media resistance, there is an additional resistance to theflow at the grout front. This front resistance is characterized by an equivalent height (Leq) ofsaturated crushed bricks, having the same resistance as the thin layer of dry masonry. It is calculated by curve fitting of numerical simulation of the progress and the experimentaldata. The optimized magnitude of this front resistance is not a constant but depends on thefraction of the crushed bricks. The relation between the permeability of the crushed bricks andthe magnitude of the front resistance is found to be almost linear and can be expressed as:

where K = permeability of crushed bricks

The higher the permeability, the more important and the more dominating is the front resistance(Table 5.11).

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104 Grout injection of masonry, scientific approach and modeling

R 'µ LK A

% RFront 'µ LK A

%µ Leq

K A(Eq 5.5)

Front ResistanceTheoryInjection pressure = 100.000 Pa

0

5

10

15

20

25

30

35

40

45

50

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5Height [ m ]

Tim

e [

sec

]

Experiment (1-2 mm)Fr. Res.(1-2mm), Leq=0,13 mExperiment (1-4mm)Fr. Res. (1-4mm), Leq=1.52 mExperiment (2-4mm)Fr. Res. (2-4mm), Leq=1.44 mExperiment (4-10mm)Fr. Res. (4-10mm), Leq=4.14 m

Figure 5.34: The combination of Darcy’s law and the front resistance theory fits very wellwith the experiments

Although it is often used, curve fitting is not a very scientific method. In order to verify the frontresistance values, obtained by optimization for the short uniform cylinders, and to see if thetheory is able to reproduce and predict injections in longer cylinders and other configurations,an injection was carried out on a split cylinder with a height of 0.9 m. The cylinder ishorizontally split in three parts of 30 cm each, filled with the 2-4 mm, 1-2 mm and 2-4 mmfraction of the crushed bricks. The front resistance is incorporated in the model. Pressure at the bottom of the cylinder issupposed to remain constant. The grouts viscosity is supposed to stay constant according to thefinding that the grout coming out of the samples has the same rheological properties as theinjected grout. Further, the hydrostatic pressure is taken into account. The total resistance forone dimensional flow through dry crushed bricks can thus be expressed as the Darcy resistance(first term) and the front resistance (second term):

with L = length of injected zoneLeq = equivalent length

and the corresponding mass flow equals:

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Chapter 5 - Experimental program 105

q [ kgs

] ' ? ? pR

(Eq 5.6)

RFront 'Leq

K. µ

A(Eq 5.7)

Fraction [ mm ] 1 - 2 1 - 4 2 - 4 4 - 10

Permeability [ m 2 ] 0.73 E-09 5.8 E-09 6.5 E-09 14.0 E-09

Leq [ m ] 0,12 1,52 1,44 4,14

Leq / K [ m-1 ] (x 109) 0,168 0,262 0,222 0,296

Rfront [Pa . s / m3] 1,07e+08 1,67e+08 1,41e+08 1,88e+08

Table 5.11: Relative magnitude of the front resistance

The integration is done numerically by means of finite differences.

The combination of (Eq 5.4) and (Eq 5.5) indicates that the front resistance is proportional toLeq / K (Eq 5.7). For the Leq values found from the injections of the short cylinders (Eq 5.4),and the permeability values listed in Table 5.8, this factor can be calculated. Table 5.11 liststhese values.

In (Eq 5.7), µ/A is independent from the permeability or the granularity of the media. Therelation between the front resistance and the permeability and the linear regression line areshown in ?. Figure 5.35 shows that the calculations according to the front resistance theory, with the Leq

obtained from the optimization for the injections on the short cylinders, fit very well with theexperimental data for the long cylinder.

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106 Grout injection of masonry, scientific approach and modeling

One Dimensional Injection, Time vs MassWith Front Resistance

Injection pressure = 100.000 Pa

0

50

100

150

200

250

0 1 2 3 4 5 6 7

Total Mass Flow [kg]

Tim

e [ s

ec ]

Front ResistanceDarcy's LawExperimental Data

Figure 5.35: Incorporating the front resistance theory in Darcy’s law makes the calculateddata to fit well with results from theinjection of the basic grout in the longcylinder

5.4.4. Mechanical propertiesIn the first part of this thesis, it is mentioned that the mechanical properties are not as importantas one might think. Nevertheless, it is interesting to get an idea of the obtained mechanicalstrength of the hardened grout. Since pure grout will not be present in the injected masonry, wepreferred to test the mechanical strength of the injected samples. For this purpose some of thesamples were not emptied. They were kept in normal laboratory conditions and tested after 28days of curing. First, the plexiglass cylinder was removed and then the cylinder was cut inslices. For these slices the sound velocity was measured to calculate the modulus of elasticity,for some specimen the compressive strength was determined, for others the tensile splittingstrength. The crushed brick particles are completely embedded in the grout. The crushed bricks play therole of the aggregates in concrete. These particles absorb the water out of the grout. This way

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Chapter 5 - Experimental program 107

Description of the specimen Compressive strength [MPa]

Cement grout, H = 20 cm, fraction 1 - 2 mm 41,1

Cement grout, H = 20 cm, fraction 1 - 4 mm 33,5

Cement grout, H = 20 cm, fraction 4 - 10 mm 22,4

Cement grout, H = 35 cm, fraction 1 - 4 mm 50,8

Lime grout, H = 20 cm, fraction 1 - 2 mm 11,8

Table 5.12: Compressive strength of the injected cylinders

the interfacial zone between the cement grout and the particles is not a weak zone as it is inconcrete. The compressive strength of most samples is very high. The strength is probablymuch higher than the strength of a real masonry structure that is injected with the same cementgrout. Because of the partly different curing nature of natural hydraulic lime based grouts, themechanical strength of the samples that were injected with hydraulic lime grout, was tested onlyafter sixty days. Only five specimen were tested in compression according to the Belgianstandard NBN B15 220. The results are listed in Table 5.12.

The position of the samples is indicated by the height where they were cut.

The tensile strength of the injected samples is more important and hence most of the specimenwere tested in tension. The Brazilian splitting test according to NBN B 15-218 was applied.The height of the samples is only 50 mm, which is lower than prescribed in the standard in orderto be able to test as many samples as possible. For each cylinder the tensile strength was testedat different heights. The results are shown graphically in Figure 5.36. The modulus of elasticity is an important property. The compressive strength is directly relatedto the modulus of elasticity. When a massive material is loaded, that part that has the highestmodulus of elasticity carries the biggest part of the load. Besides, the modulus can be obtainedfrom the non destructive testing of the sound velocity in the sample.

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108 Grout injection of masonry, scientific approach and modeling

? [m/s] 'E?

. f (v) (Eq 5.8)

Tensile strength of grouted cylinders

0

1

2

3

4

5

6

7

8

0 5 1 0 1 5 20 2 5 3 0 3 5 40 4 5

Height [cm]

Ten

sile

Str

engt

h M

Pa

previously wet

Fr 4 - 10

Fr 2 - 4

Fr 1 - 4

Fr 1 - 2

Linear (Fr 1 - 2)

Linear (Fr 4 - 10)

Linear (Fr 2 - 4)

Linear (previously wet)

Linear (Fr 1 - 4)

Figure 5.36: Results of the splitting test for the various specimen in function of theheight location in the test cylinders

The measurement of the sound velocity in a material is a relatively simple and fast test. Thetransmission time is measured for a sound wave to traverse the specimen. The relation betweenvelocity, modulus of elasticity and density is given by equation (Eq 5.8).

Where v = velocity of the soundE = Young modulus? = density? = Poisson’s ratio

The velocity is measured on each sample.

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Chapter 5 - Experimental program 109

Young's modulus' variation in grouted cylinders

15000

20000

25000

30000

35000

0 10 20 30 40 50

Height [cm]

E [M

Pa]

Fr 4 - 10Fr 2 - 4Fr 1 - 4Fr 1 - 2Linear (Fr 2 - 4)Linear (Fr 1 - 2)Linear (Fr 1 - 4)Linear (Fr 4 - 10)

Figure 5.37: Modulus of elasticity for the different fraction in function of the height

?grout '1 % W/C1

?cem

%W/C?wat

(Eq 5.9)

It is obvious that the mechanical results highly depend on the position of the specimen in theoriginal sample. The strengths of the specimen taken at the base of the cylinder are similar.Then, for some samples the relation between mechanical strength or modulus of elasticity showa positive gradient whereas for other there is a negative gradient. When this was noticed, thedensity variation was also investigated. The same relation was found. The density variation infunction of the height is given in Figure 5.38. Two phenomena can cause a density gradient.While the grout is going up through the cylinder filled with crushed bricks, its W/C ratiodecreases because the bricks absorb some of the water. This water absorption leads to anincrease of the density of the grout. The relation between water content of the grout and density of the grout can be specifiedanalytically. Let us assume that the density of cement ?cement = 3050 kg/m3. Let us furtherassume that the grout is only composed of cement and water without air. The density of themixture of water and cement equals:

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110 Grout injection of masonry, scientific approach and modeling

Density variation in grouted cylinders

1600

1700

1800

1900

2000

2100

0 10 20 30 40 50Height [cm]

E [M

Pa]

Fr 4 - 10Fr 2 - 4Fr 1 - 4Fr 1 - 2Linear (Fr 1 - 2)Linear (Fr 1 - 4)Linear (Fr 4 - 10)Linear (Fr 2 - 4)

Figure 5.38: Density variation in function of the height for the various specimen

The grout that penetrates gradually loses water and becomes denser. This creates a positivedensity gradient.As the crushed bricks are finer, the specific surface is bigger and consequently the waterabsorption is higher. This explains why the density gradient increases as the pore size getssmaller. If the pores are greater, this phenomenon is masked by another process. Thegravitational forces that are acting upon the cement particles in the grout are superior to theelectro statical and viscous forces that keep the particles in suspension. When the injectionstops, the particles start falling towards the bottom of the cylinder. This causes a negativedensity gradient. In this case, when the pores are greater, the water absorption is also smaller.

It is logic that when the grout is heavier, because richer in cement, the mechanical results arebetter. It is normal that these phenomena are more significant if the grout is not very stable andif the water retaining properties of the grout are poor. This observation has importantconsequences for practical jobs. The upper zone of an injected piece of masonry might lose toomuch cement and therefore become weaker than the bottom zone. The mechanical strength andthe stiffness are negatively affected. This observation shows that the maximum injection heightis not only limited by the hydrostatic pressure, but also by the strength gradient that arises.The mechanical strength of the pre-wetted samples is very poor. The specimen taken at the topcan be crushed by hand. Apparently they lost almost all the binding cement. Also the specimen,taken at the bottom of the cylinder are significantly weaker than those that were not prewetted.

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Chapter 5 - Experimental program 111

5.5. Important findings 5.5.1. Blocking mechanisms

Granularity of the cementThe first mechanism depends on the granularity of the cement and the crack size distribution ofthe brickwork to inject. In literature, there are different empirical formulas expressing therequirements for the granularity of the cement with regard to the smallest crack diameter of thebrickwork. An incompatible grain size distribution of the used binding agent results in a suddenobstruction of the injection. The biggest grains should at least be smaller then 0,1 to 0,15 timesthe smallest crack width [Benhamou, 1994][Miltiadou, 1990]. Flocculated cement grains act asbig cement grains. Therefore, a proper mixing procedure is necessary. For the same reason itseems to be impossible to produce an injectable grout without superplasticizers. They bring theW/C ratio to acceptable values and more important, they have a deflocculating action.

Stability of the groutThe second mechanism depends on the stability of the grout. When flow slows down, thecement particles in an unstable grout sink to the bottom of the flow channel. This narrows thechannel and finally blocks further injection. Addition of stabilizing admixtures (i.e. bentonite orultra fines) significantly improves the stability and the injectability of the grout [Miltiadou,1990][Paillère, 1993]. For this reason it is important to check the stability of the grout and theevolution in time of the stability [Van Rickstal, 1995(1)].

Water absorption out of the groutThe third possible blocking mechanism is the result of water absorption out of the grout into themasonry Figure 5.39. When meeting a dry and absorptive masonry it is important to use a groutwith excellent water retaining properties. Here again, a stable grout will be more able to retainthe water.

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112 Grout injection of masonry, scientific approach and modeling

Figure 5.39: The dry bricks absorb water out of the grout

A prewetting of the brickwork by water injection is not recommended. First of all it can bedangerous when the stability of the construction is doubtful. The water will decrease theinternal cohesion and friction and could cause a collapse of the building. By injecting water themasonry will be saturated and, although the grout will pass easier, the W/C ratio of the groutremains very high and produces a weak binding material. The little absorption needed for agood adhesion between grout and masonry does not occur and the mechanical improvement willbe poor. For the test injection on the plexiglass cylinders the tensile strength of the pre-wettedsamples was only 1 N/mm2 or lower, whereas the tensile strength of the dry samples variedbetween 3 N/mm2 and 7.5 N/mm2 (Figure 5.36). This third blocking mechanism produces a slow obstruction of the injection. When the groutloses too much water the viscosity and the yield stress become too high to make any furtherpenetration possible. A stand still of the flow allows locally a big amount of water to beabsorbed and this can be crucial for further penetration. Therefore it is recommended to injectcontinuously keeping the pressure as constant as possible.

Pressure losses, thixotropyThe resistance of the fine cracks and voids to the flow increases at greater distance form theinjection hole. Due to this increasing resistance finally the yield stress will not be reached atthe front of the injection. Only gravity could compensate this pressure losses for downwardflow and make the grout to keep on flowing. Due to the limited discharge of the injection pumpand the radial injection, the flow velocity will drop and thixotropic behavior of the grout willenhance the above phenomenon. Also this mechanism causes a gradual stop of the injection.Decreasing the yield stress by an appropriate composition and mixing procedure of the grout can

Page 116: Grout Injection

Chapter 5 - Experimental program 113

Figure 5.40: Heterogeneous consolidation of the reproducible samplesbecause of instability of the grout

retard this last blocking mechanism. This is one more reason why it is not possible to formulatea well injectable grout without superplasticizer.

5.5.2. ObservationsMany test injections were carried out, varying the parameters of the grout, the injection pressureand the fraction of the crushed bricks. The following observations were made:

C when an injection stops, it is not possible to restart the flow by increasing the injectionpressure;

C there are two different blocking mechanisms: a sudden obstruction of the flow or agradual obstruction Figure 5.28;

C when a permeable zone is in parallel with a less permeable zone, the latter will not beinjected, although this medium is, on itself, perfectly injectable;

C when injecting an unstable grout, it can be noticed that cement particles are sinking to thebottom of the cylinder, causing a strength gradient and heterogeneities in theconsolidated masonry;

C when the sample is prewetted by water injection, the grout injection is executed manytimes faster than through a dry sample. However, the strength after curing and theadhesion of the grout to the original material is very poor. Therefore, prewetting themasonry structure can only be made after cautious preliminary research.

5.6. Conclusions from the experimental program

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114 Grout injection of masonry, scientific approach and modeling

This chapter about the experimental program explains a number of test methods that are newlydeveloped or adapted to the peculiarities of the grouting process. The methods that are testingthe rheological properties of the grout, in the laboratory and on the site should be part of ascientific approach of the consolidation injection. An injection grout basically consists of abinding agent, water and admixtures eg. a superplasticizer and a stabilizer. The influence ofevery component on the viscosity and the stability is analyzed. Furthermore, the experimental program, especially the numerous test injections that whereperformed on the reproducible masonry samples, provide a better understanding of blockingmechanisms that occur. The influence of a prewetting procedure is investigated and althoughprewetting could improve the penetration of the grout inside the masonry, it has dangerouseffects on the mechanical strength, the strength gradient and the stability of the injected grout.The water that is absorbed out of the grout provides a better adhesion to the flow channels.The analysis of the results of the mechanical tests, reveals exciting findings about the strengthgradient and the density gradient that originates from the sinking of the cement grains.Finally, the experiments provide data about the penetration speed and depth of the grout insidethe reproducible masonry sample. These data are used to calibrate and check the numericalsimulations.

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Chapter 6. Rheology of grouts 115

µµ0

' 1 % a ? (Eq 6.1)

µµ0

' 1 % a ? % ß? 2(Eq 6.2)

Chapter 6. Rheology of grouts

6.1. Introduction to the rheology of dispersionsConsidering the final aim of this research program of predicting the flow of the grout inside themasonry, some rheological considerations about dispersions are justified. Rheology ofdispersions is an important research field on its own. The complete characterization of therheological behavior of grouts: the rheological parameters in function of the concentration, theinfluence of mixing procedure or more generally the history of the grout, the problems tomeasure both the viscosity and the shear strength of grouts in function of the shear rate is beyondthe objectives of the thesis. Many books are available about the rheology of dispersions. Mostresearchers in the field of grouting or in the field of grout compositions, spend a larger orsmaller part of their work on the measurements of the rheological parameters of grouts and try todiscover some trends in the influence of the composition on the rheology of the grouts. Theirqualitative results will be used in the present study. Some assumptions will simplify thecomplex rheological behavior of cement grouts. If necessary, the implications of theseassumption will be mentioned.

A liquid with small particles scattered all over is called a dispersion. If the magnitude of theparticles is very small in comparison with the other dimensions in the given situation and if theparticle concentration is small, one can characterize the behavior of the dispersion in the sameway as the dispersing fluid. The particles present in the dispersing fluid do not affect the flowin any way. This is certainly not true for injection grouts. If the concentration increases fromzero % the particles will initially not hinder each other. The viscosity for such a dispersion canapproximately be expressed by the relation, formulated by Einstein (Eq 6.1) [Wilkinson, 1960].

If the concentration increases further, generally a second order term appears in the descriptionof the rheological behavior (Eq 6.2) [Wilkinson, 1960].

where ? volumetric concentration of particlesµ0 the viscosity for the pure fluid without any particles [Pa.s]µ viscosity at concentration ? [Pa.s]a, ß constant factors, depending on the type of fluid and particles

The importance of particle size is linked to the presence of Brownian motion. The influence ofthe particle size on the rheological behavior is incorporated in the parameters a and ß. This is

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116 Grout injection of masonry, scientific approach and modeling

Name formula number of parameters

Newtonian 1t ' µ 0?

Power law 2t ' ? 0? n

Bingham 2t ' t 0 % µ 0?

Herschel/Bulkley 3t ' t 0 % ? 0? n

Crosst ' µ4 0? %

µ0 &µ4

1 % ? 0? n

4

Table 6.1: Listing of frequently used rheological models [Wilkinson, 1960]

the omni-present randomizing process which will always seek to generate a statisticaldistribution of the positions and orientations of particles, while any impose of motion (shear orextension) seeks to impose some organizing effect on the particles. Thus when speaking of sizewe must realize that it is the ratio of viscous to Brownian forces that matters most. Also theshape of particles influences the viscosity in two main ways: first it gives the particles a definedorientation vector. Both, the Brownian motion and flow will compete to influence thisorientation vector. Secondly, irregular shape implies that the effect will be felt in the liquid to agreater distance than with a sphere of equal volume. Therefore, one instinctively expects alarger a-factor in (Eq 6.1) for an irregular shape than for a spherical shape. If the particlescarry a charge, the electro-viscous effect plays. Each particle is surrounded by a charge cloudof counter-ions and any deformation of the suspension tends to deform this cloud. Thermal andelectrical forces try to counter this distortion by moving ions relative to the fluid, therebydissipating extra energy and therefore increasing the viscosity.A dispersion can be stabilized by surfactant stabilization. Surfactants not only play a large rolein the stabilization of formed dispersions but also in their original formation by breaking downof the solid to reduce the primary particle size. This is achieved by both the action of thesuperplasticizers and by the ultrasonic mixing procedure. The surfactant leads to a stronglyadsorbed water of fluid layer which can cause stabilization of the particle distribution in thedispersion. If the adsorbed surfactant is of the nonionic type then it comes into the class ofstearic stabilization. If the adsorbed polymer is ionic, then irreversible flocculation isprevented by the repulsive forces generated from the presence of an electrical double layer inbetween the particles and the particle solution interface. In this way, (super)plasticizers areactive.Whereas the approach, described above, gives an insight in the motion of dispersions, most usedfor calculations are empirical relationships between viscosity and shear rate. In growing orderof complexity one can list these relationships in Table 6.1.

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Chapter 6. Rheology of grouts 117

Figure 6.1: The continuous phase moves through the dispersed phase, a filter cake is formed andfinally the flow stops

In the search for a useful expression one works up from the simplest. Many mistakes are madein the choice of a model, not because the model was to simple but because the range of shearrates over which the data were fitted to determine the parameters was inappropriate to thesituation it was used for. Frequently occurring mistakes are: calculating the shear stresses byextrapolating from only a few relatively high shear rates to obtain a simple Bingham equation[Toorman, 1995]! Another inaccuracy arises by considering the dispersion as a continuum,disregarding the particles inside. These particles give rise to three phenomena. Thesephenomena create a gradient in concentration. Therefore, the dispersion can not be consideredbeing a homogeneous liquid. The phenomena are:C In many situations the continuous fluid phase in dispersions moves relative to the

suspended phase. In case of instability, particles move down and liquid moves up whenno three dimensional structure of particles or yield stress of the continuous phase canprevent the particle from sedimenting under gravity.

C When a water based dispersion is flowing through a medium with capillary action, thismedium sucks out part of the continuous phase. The concentration increases and henceviscosity increases too. The experimental observations indicate that this is whathappens when water is absorbed out of the grout.

C And finally, it is possible that it is easier for a system to appear to flow with thecontinuous phase moving through the dispersed phase. The remaining particles form afilter cake through which no flow is possible. This narrows the flow channel becausethe filter cake sticks to the wall of the flow channel (Figure 6.1).

As in a grout the particles are slightly charged, there is a repulsive force. The attractive forcesare the London dispersion forces arising from the coupling of the fluctuations in charge

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118 Grout injection of masonry, scientific approach and modeling

t ' µ dudy

' µ 0? where µ is constant (Eq 6.3)

distribution of the electron clouds of the atoms making up the particle. The repulsive forcesgrow very fast when the particles’ distance decreases. A system for which the repulsive forceswin is called a dispersive structure. The dispersion can be concentrated to such an extent that the average inter-particle separation isof a similar magnitude as the working range of the inter-particle forces. As there is often apreferred state for these systems: the state of minimum energy with the particles being as farfrom their neighbors as possible, there is the possibility of elasticity. Because the forcesholding the particles in these positions are not very large, we can still cause flow once weovercome the critical shear stress. So we can expect a classical Bingham type of fluid.

6.2. Non-Newtonian behavior of aqueous dispersionsA liquid is called to be Newtonian if it obeys Newtons Law for fluid motion:

Almost every aqueous dispersion shows non-Newtonian behavior in simple shear flow, whichmay originate from any of the following:C forces of attraction or repulsion existing between the particles, which are modified by

the applied flow. Therefore, the material exhibits a yield stress in order to overcome theforces of attraction present between the particles in that state.

C the adsorption of the suspending liquid onto the particle surface. Therefore, such adispersion shows shear thinning behavior. It probably originates in the progressiveshearing off of layers of adsorbed water from the particles, reducing the apparentvolume concentration with a consequent decrease in viscosity.

C mechanical interference between particles. Such interference will increase withincreasing shear rate, with increasing concentration and with decreasing particleisometry. A shear thickening behavior will be the consequence.

C mechanical fracture of particles. This results in the removal of irregularities that arehindering the motion. Again a shear tinning effect will occur.

C time dependent behavior. Flocculation arising from attractive forces normally takes afinite time to re-establish itself when broken down by shear.

The complexity of rheology of dispersions can be underlined if we add that it is possible for adispersion to exhibit a yield stress, shear thinning, shear thickening time dependence andelastic-viscous behavior in one and the same dispersion depending on the level of shear rate atwhich the measurement is made.

Forces of attraction and repulsion will invariably exist between particles in suspension, sinceLondon-van der Waals dispersion forces will always be present, though only significant whenparticles are close together. Repulsive forces generally mean that the particles can move asindividuals and the suspension is completely dispersed, while attractive forces mean that the

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Chapter 6. Rheology of grouts 119

2pL (r t&(r%dr) (t %dt )) % (p1&p2)2p rdr ' 0 (Eq 6.4)

Figure 6.2: Dynamic state of equilibrium for an annulus between radiusr and r + dr

particles are less likely to be able to move as individuals, and that an linking structure is presentthroughout the suspension.On top of this, some dispersions show a time dependent behavior. This behavior is due to thefact that temporary bindings are formed as soon as the fluid motion stops. These bindings needto be broken if the fluid starts moving again. The longer the fluid rests, the stronger or the morenumerous these bindings are. The following derivations do not take into account this timedependent behavior, called thixotropy. Thixotropy will only occur after a period of stand still.Because of the permanent stirring in the collector, thixotropy will not happen there. Besides,when an injection is properly executed, the grout flow will never stop until the final blocking.

6.3. Flow of a dispersion in a cylindrical tube6.3.1. General equationThe following derivations are obtained by considering the dispersion as a continuumdisregarding the influence of the particles. We also suppose that the next conditions arefulfilled:C We deal with laminar flow, all fluid parts are moving parallel to the axis of the tubeC No shear movement occurs at the walls, the speed of the fluid part at the wall is zeroC The shear rate in one point is only function of the shear in that point

We now consider an elementary volume being a annulus between the cylinders with radius r andr + dr. We suppose that all transient phenomena are over and that the system is in stationarymotion. This dynamic equilibrium state is expressed by equation (Eq 6.4).

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120 Grout injection of masonry, scientific approach and modeling

0? ' &dudr

' f (t ) ' f (t wrR

) (Eq 6.8)

QpR 3

'1

t 3wmtw

0

t 2 f ( t ) dt (Eq 6.9)

tt w

'rR

and t w 'R2

? pL

'D4

? pL (Eq 6.7)

With t % dt ' t %dtdr

dr

one gets d (rt )dr

'p1 & p2

Lr (Eq 6.5)

After integration t 'p1 & p2

Lr2

(Eq 6.6)

Taking into consideration the boundary conditions, (Eq 6.6) can be reformulated as

Where t = shear stresst w = the shear stress at the wall of the tube.? p = p1 - p2 = pressure differenceL = length of elementary volumeD = diameter of the tubef(t ) = rheological behavior expressing the relation between the shear

rate and t

The discharge can be calculated by the integral Q ' mR

0

2pr u dr

providing the relation of Rabinowitsch (Eq 6.9) [Midoux, 1985]

By specifying the relevant rheological behavior f(t ) one finds an expression for the relationbetween pressure losses and the discharge. The Rabinowitsch equation is only valid in case oflaminar flow: the forces of inertia must be weak in comparison with the viscous forces.

6.3.2. Newtonian fluid

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Chapter 6. Rheology of grouts 121

Q 'pR 4

8µ. ? p

L'

pD 4

128µ. ? p

L(Eq 6.11)

0? ' f(t ) 'tµ

(Eq 6.10)

As an of example one can derive the well know formula of Hagen-Poisseuille for the laminarflow of a Newtonian fluid in a cylindrical pipe. For a Newtonian fluid, relation (Eq 6.10)expresses the relation between share rate and viscosity.

After integration we find . When we take into consideration relation (EqQp R 3

't w

4 µ

6.7), we find the Hagen-Poisseuille relation (Eq 6.11) between discharge and pressure gradient.

From (Eq 6.11) it follows that for a Newtonian fluid, the discharge will never be zero as longas there is any finite pressure gradient and as long as the pipe’s diameter is not zero. Theobservations learn that in case of injection the flow can drop to zero. So we conclude that theflow of a grout through masonry can not be described correctly by using the above equation (Eq6.11).

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122 Grout injection of masonry, scientific approach and modeling

t ' t c % µ 0? (Eq 6.12)

0? ' f (t ) 't & t c

µwhen t > t c

0? ' f (t ) ' 0 when t < t c

(Eq 6.13)

Vc

tc

Figure 6.3: Velocity profile for a Bingham fluid

Qp R 3

'1

µ t 3wmtw

t c

t 2 (t & t c) dt (Eq 6.14)

6.3.3. Bingham fluidGenerally, dispersions are considered to be Bingham fluids. The basic equation of the Binghamfluid is (Eq 6.12) as was already listed in Table 6.1.

Or written in a different way according to eq. (Eq 6.8)

When the yield stress is less then the critical yield value t c, the fluid is not sheared any longer.

The fluid then moves as a block or rigid body: for that part of the fluid. In ad udy

' 0? ' 0

cylindrical pipe this gives approximately the velocity profile as shown in Figure 6.3. When theconfiguration is such that the critical yield stress is bigger then the yield stress at the wall of thepipe, the fluid will not move at all, since for the complete cross section of the flow channel,there is not one location where the shear stress exceeds the critical shear stress.

Introducing the rheological model from eq. (Eq 6.13) in the equation of Rabinowitsch (Eq 6.9)gives:

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Chapter 6. Rheology of grouts 123

Q 'p D 4

128 µ? PL

[1 &43

(4t c

DL

? P)%1

3(

4t c

DL

? P)4] (Eq 6.15)

t c $ D4

? PL

(Eq 6.16)

4 t c L

D ? p' 1 (Eq 6.17)

The integration of eq. (Eq 6.14) provides the relation of Reiner-Buckingham [Midoux, 1985][Schowalter, 1978].

When the critical yield strength t c equals zero, the second factor in (Eq 6.15) vanishes and thelaw of Hagen-Poisseuille (Eq 6.11) comes out again.

6.3.4. Discussion on the Reiner-Buckingham formulaEquation (Eq 6.7) expresses the magnitude of the shear stress at the wall of a cylindrical flowchannel. At that place, the shear stress is the highest. The layer that is making contact with thewall does not move, this is one of the assumptions. Assuming a constant injection pressure, theshear stress at the wall will decrease when the grout penetrates the channel. This correspondsto an increasing L value in equation (Eq 6.15). If the shear stress t p decreases to the criticalshear strength of the fluid, no further shearing off will take place. A shear stress at the wall ofthe flow channel which is lower than the critical shear strength will result in a complete standstill of the fluid. This happens when:

When the injection pressure is too low in relation to the length of the channel, the shear strengthwill not be reached any longer, even not at the wall of the flow channel. This causes the flow tostop. The discharge drop to zero if the correction factor from equation (Eq 6.15), taking intoaccount the Bingham behavior of a fluid, vanishes. This happens when:

and hence, there is a linear relation between the penetration depth (L), the yield strength (t c), thepressure difference over the channel (? p) and the diameter of the channel (D). Combining thiswith the problem of water absorption by the dry masonry out of the grout, it can be stated that thepenetration depth of the grout will always be limited. The explanation that is generallymentioned is thickening of the grout because of the water absorption. Thickening magnifies theshear strength. There are reasons to believe that the limited injection depth is not due tothickening in the first place but that the water absorption cause a reduction of the diameter of theflow channel by forcing the cement particles to stick to the wall. Experiments show indeed thatthe grout that exits the test samples, has the same properties as the original grout.

6.3.5. Numerical simulation

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124 Grout injection of masonry, scientific approach and modeling

Q(x) 'pR 4

8µ. ? p

x'

pD 4

128µ. ? p

x(Eq 6.18)

dxd t

'p D 4

128µ pD 2

4

. ? px

'D 2 ? p32 µ x (Eq 6.19)

Figure 6.4: Configuration to simulate the flow of a Newtonian and Bingham fluidthrough a one dimensional tube

mt1

0

dt ' mX

0

32µxD 2? p

dx (Eq 6.20)

To clarify the influence of the yield strength on the progress of the grout in a cylindrical pipe(Figure 6.4), a numerical example has been constructed. For viscosity, yield strength, pressureand pipe dimensions realistic values for the grouting case are used taken from own experimentaldata or literature [Benhamou, 1994]. The simulations, plotted in Figure 6.5 shows how aNewtonian fluid and a Bingham fluid progress in cylindrical tube. For the Newtonian fluid thedifferential equation can be solved analytically. Let x be the coordinate of the position along theaxis of the pipe, x = 0 at the entrance of the pipe.

Taking into account that with S(x) being the cross section of the pipe at?(x) 'dxdt

'Q(x)S (x)

position x, this can be transformed to

Integration of (Eq 6.19) for the case of one pipe with constant diameter yields

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Chapter 6. Rheology of grouts 125

Name Symbol Value

Pipe diameter D 2 mm

Inlet pressure ? P 10.000 Pa

dynamic viscosity µ 0.02 Pa.s

Yield strength t c 1 Pa to 5 Pa

Position at beginning x0 0.2 m

Time step ? t 0.2 s

Table 6.2: Numerical values for the worked out example

xn ' xn& 1 %D 2

32 µ? Pxn& 1

1 &43

4t c xn&1

D ? P%

13

4t c xn&1

D ? P

4

? t (Eq 6.22)

t1 '16µ X 2

D 2 ? p(Eq 6.21)

where t1 is the time when position X is reached by the fluid. The solution of this equation isgiven by

The implicit relation between discharge and pressure difference of equation (Eq 6.15) is not soeasy to solve analytically. Therefore we choose the numerical solution by the finite differencemethod. The differential equation can be converted into a finite difference equation. Forcalculating the progress we need a non-zero x0-value. In this example we arbitrarily took 0.2 mas x0.

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126 Grout injection of masonry, scientific approach and modeling

Flow through a cylindrical pipe

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60

Time [s]

Dis

tanc

e in

pip

e [m

] Newtonian fluidBingham fluids

Increasing yield strength

Figure 6.5: Influence of yield stress on penetration in a narrow pipe

Figure 6.5 plots the progress of both fluids in the cylindrical pipe. As can be seen the Binghamfluid slows down much faster. Besides the Bingham fluid with yield stress of 5 Pa (Figure 6.5,lowest curve) would never be able, under the given conditions, to penetrate further than 1 m. Atthat moment the shear stress at the wall of the 2 mm channel would not exceed the critical yieldstress of 5 Pa and the movement would stop as expressed by equation (Eq 6.16).

6.4. ConclusionsIn this chapter a theoretical base is given for the rheological behavior of dispersions. Thepresence of many small particles influences the behavior of the fluid. Generally, a dispersion isseen as a Bingham fluid. The equations that relate the discharge and the pressure gradient arebuild. Still some aspects of a dispersion are not present in the description of a Bingham liquid:time dependent features and the influence of the particle size. The equation that expresses therelation between the discharge and the pressure gradient will be used in the model.

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Chapter 7. Flow of fluids through porous media 127

Chapter 7. Flow of fluids through porous media

7.1. IntroductionStudying flow of fluids through porous materials is of importance in many fields of engineering,e.g. petroleum engineering, soil mechanics and ground water hydrology. Also for manyapplications in transport phenomena in the research field of building physics, masonry is dealtwith as a continuous porous medium, eventually with varying pore structure properties andpermeability. This continuum approach is used in the calculation of moisture transport,combined moisture and air transport, soluble salt transport etc... For these purposes thecontinuum approach is valid and leads to very good simulations of experiments and to goodpredictions for particular situations. The driving force for al these transport phenomena iscapillary action, the transport phenomenon is also called imbibition. In case of injections themain driving force is the injection pressure. In literature a penetration caused by an externalpressure is called invasion. In this chapter, attention is payed to the description of a porousmedium, Darcy’s law , the formulation and solution methods of the differential equations of fluidflow through porous materials. However, the main transport for an injection occurs throughrelatively big flow channels. The capillary forces in these channels can be neglected incomparison with the driving force of external pressure.

7.2. Structure and properties of porous materials7.2.1. DefinitionsIn the most general sense, a porous material is a material containing holes. This definition is toogeneral: a metal part containing a bore hole is definitely not a porous material. Therefore, westate that a porous material is a solid containing holes or voids, either connected or non-connected, dispersed within it in either a regular or random manner. It is obvious that greatvariations in the size and structure of pores exist. A fluid can flow through a porous material only if the pores are interconnected. Theinterconnected pore space is termed the effective porosity, whereas the whole of the pore spaceis termed the total porosity. The total pore space is affecting the density and strength of amaterial, but only the effective pore space affects the permeability and related properties. Thesmallest pores are termed interstices, the large holes are termed caverns. The intermediaterange of voids are termed the pores. Porous materials can be classified according to thepacking of the pores: ordered porous material and random porous material. Most naturalporous materials contain randomly distributed pores. On the microscopic level their porestructure must be described in terms of statistics. Even so, it is possible to treat the flow offluids through such materials on a macroscopic basis in precise terms. At first stage we willdescribe the flow on a macroscopic scale. Many theories have been formulated which attemptto relate in a detailed manner the macroscopic properties of porous materials to the statistical

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128 Grout injection of masonry, scientific approach and modeling

f 'VP

VB

'Volume of pores

Bulk Volume(Eq 7.1)

1 & f 'Vs

VB

'Volume of solids

Bulk volume(Eq 7.2)

f eff '

VB & Va & Vb

P2

P2 & P1

VB

(Eq 7.3)

properties of their microscopic structure, to relate pore size distribution to the macroscopicproperties of the material. While such theories contribute greatly to our understanding of basicphysical processes within porous media, they do not, in general, contribute to the solution ofproblems on a macroscopic scale. Probably, the increased computer power will soon enableto deal with the microscopic level in order to calculate macroscopic behavior.The porosity of a porous material is the fraction of the bulk volume of the material occupiedwith voids.

The volume fraction not occupied with voids, but with material can be expressed by

As defined above distinction has to be made between effective porosity and total porosity.

7.2.2. Methods for porosity measurementSeveral methods can be used for measuring the effective or global porosity. To measure thetotal porosity the direct method is the only method. The method consists in determining the bulkvolume, crushing the specimen to remove all pores and then measuring the remaining volume.The use of a pyknometer provides the necessary accuracy to the measurement of the volume ofthe crushed material.The gas expansion method provides a percentage of effective porosity. This percentage can becalculated using equation

Where f eff = effective porosityVB = bulk volume of sampleVa = volume of sample chamberVb = volume of second chamberP1 = initial pressureP2 = final pressure

The Mercury intrusion method is based on the fact that, due to the surface tension and the non-wetting properties of mercury, external pressure is necessary to enter mercury in a sample. The

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Chapter 7. Flow of fluids through porous media 129

K 'qµ

A ? PL

(Eq 7.5)

Vp 'Msat & Mdry

?w

(Eq 7.4)

finer the diameters of the pores the higher the pressure needed to fill the pore with mercury.Therefore, this method provides a pore size distribution. This distribution is in favor of thesmall pores since the volume of a bigger pore that is connected to the outside by smaller poreswill be counted as small pore’s volume. This volume will only be mercury filled when thepressure corresponding to the filling of the small pores is reached.The Imbibition method yields the effective porosity. The technique consist of two weightmeasurements. One in dry condition and one after vacuum saturation of the material. Aftervacuum saturation all the pores are considered to be filled with water. The effective porevolume can be calculated with equation (Eq 7.4).

Since weighting can be very accurate this is probably the best method for effective porositymeasurement despite the long time needed for vacuum saturation. More recently, with the aid of image analysis, microscopical methods are developed.Advanced image analysis of an enlarged part of the sample can even provide a pore sizedistribution. This method gives, although more complicated and expensive, much more reliableresults then the mercury intrusion method. This method does not affect the pore size distributionin favor of the smaller pore size as is the case of the intrusion method because of the bottle neckphenomenon. For weak materials, the intrusion method results in the collapse of the pore walls.

7.2.3. Permeability, Darcy’s lawPermeability is that property of a porous material which characterizes the ease with which afluid may be made to flow through the material by an applied pressure gradient. Permeability isthe fluid conductivity of the porous material. The equation which defines permeability in termsof measurable quantities is called Darcy’s law. If horizontal linear flow of an incompressiblefluid is established through a sample of porous material of length L in the direction of the flow,and cross-sectional area A, the permeability K, of the material is defined as:

In this equation q is the flow rate in volume per unit time and µ is the viscosity of the fluid. ? Pis the applied pressure difference across the length op the specimen. Dimensions of thepermeability are m2. The permeability is mostly determined using a setup similar to the one inFigure 7.1.

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130 Grout injection of masonry, scientific approach and modeling

area A

porous medium

Pb

Pa

Figure 7.1: Possible setup for measuring the permeability

F ' µ dvdz solid

(Eq 7.6)

Fµ ' BµqL (Eq 7.7)

For a plate, the shear force per unit area between the solid surface and a fluid tangent to it isgiven by Newton’s equation:

where v velocity of the fluidin the direction of the flowz the distance perpendicular to the direction of the flow

It is reasonable to suppose that, for a steady laminar flow, the lateral forces, perpendicular tothe main macroscopic flow direction, associated with the microscopic random variations invelocity average to zero over any macroscopic volume. However, the inertial forces in the flowdirection will not average to zero and hence will only be negligible for low flow rates. Theonly non-zero macroscopic force exerted on the fluid by the solid is that associated with theviscous resistance to flow. For steady flow this force must be in equilibrium with the externaland body forces on the fluid. The fact that inertial forces can be neglected in comparison withthe viscous forces results in a low Reynolds number (Eq 7.12). Darcy’s law is only valid forthose situations that show a low Reynolds number. The viscous resistance to the flow aspresented in Figure 7.1 is directed opposite to the flow direction and can be obtained byintegrating equation (Eq 7.6) over the microscopical surfaces in the complete volume. Thevelocity v and hence dv/dz must be proportional to the flow rate divided by the cross-section.Since the total surface involved must be proportional to the volume we get:

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Chapter 7. Flow of fluids through porous media 131

BµqL % ? f AL g ' Pb & Pa f A (Eq 7.10)

Fp ' Pb & Pa f A (Eq 7.8)

Re 'q? dµAf

(Eq 7.12)

Fg ' ? f AL g (Eq 7.9)

The external resultant force is proportional to the pressure difference and the cross-section onwhich the pressure is exerted. This cross-section is the overall cross-section multiplied by theporosity. We obtain

The gravity is the only body force that is acting on the fluid and can be expressed as

Combining (Eq 7.7), (Eq 7.8) and (Eq 7.9) we can write the equilibrium of forces as

or q ' &KAµL

Pa & Pb % ? g L (Eq 7.11)

with , as a constant characteristic of the porous material.K 'fB

The above derivation of Darcy’s law could lead one to think that it is only applicable for steadyflow. However, the viscous forces involved in laminar flow through porous media are thatmuch greater than any inertial forces, that also the inertial forces in the flow direction can beneglected. For practical purposes, Darcy’s law is thus also valid for variable rate q. This isimportant for the injection of grouts since a steady flow will never happen there.The laminar flow regime breaks down for sufficiently high flow rates. For high flow ratesDarcy’s law is not valid. The onset of inertial effects occurs rather gradually in the range ofReynolds number from one to ten. The Reynolds number definition was already mentioned inChapter 5, (Eq 5.2) and is defined as:

where d is a measure for the pore diameter. Since a pore diameter is difficult to measure, the

grain diameter can be used instead or, as an alternative for d, one can use [Collins, 1965],Kf

which can more easily be determined.

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132 Grout injection of masonry, scientific approach and modeling

vi ' &?µ

Ki1MUMx1

% Ki2MUMx2

% Ki3MUMx3

i ' 1, 2, 3 (Eq 7.16)

& LP % µ BfPv % ? g Pi 3 f dPsdA ' 0 (Eq 7.13)

Pv ' &Kµ

(LP % Pi 3? g ) ' &Kµ

Pi 1MpMx1

% Pi 2MpMx2

% Pi 3MpMx3

% ?g (Eq 7.14)

Pv ' &Kµ

LU (Eq 7.15)

7.3. Equations governing the flow of fluid through porous materials7.3.1. Differential form of Darcy’s LawAbove Darcy’s Law was interpreted as resulting from equilibrium of the forces acting on thefluid flowing within a macroscopic sample of porous material. The same law can be written foran element of volume of length ds and plane cross-section area dA. The equilibrium equationfor the fluid in the elementary volume can then be written as[Collins, 1965]:

where unit vector in the first horizontal directionPi 1

unit vector in the second horizontal directionPi 2

unit vector in the vertical direction, the direction of gravityPi 3

or with K = f / B defined as before

where , and are unit vectors parallel to the respective orthogonal Cartesian axes x1, x2Pi 1Pi 2

Pi 3

and x3. This equation is the logical generalization of the linear form given by the Darcy equation(Eq 7.10). The integrals of this differential law generally agree well with observations during experiments.The differential law of flow for incompressible fluids can be expressed in very compact formby defining a flow potential as: U ' p % ?gx3

Then the law of flow becomes:

In the above derivations of the different forms of Darcy’s law we assumed that permeability isindependent of the direction of fluid flow within the medium. This is not generally true for allporous media. The most general linear relationship between vi and the components of M? /Mxi

that can be postulated takes the form:

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Chapter 7. Flow of fluids through porous media 133

v1

v2

v3

' &?µ

K11 K12 K13

K21 K22 K23

K31 K32 K33

MUMx1

MUMx2

MUMx3

(Eq 7.17)

& P? @ PO % G 'MGMt

(Eq 7.19)

vi ' &Ki?µ

MUMx i

i ' 1, 2,3 (Eq 7.18)

The nine Kij form the elements of a tensor. The three equations from (Eq 7.16) can be written inmatrix form:

The latter equation will be useful for the finite element formulation of the flow throughanisotropic porous media. In most cases, the K-matrix is a symmetric matrix. The rotation of theoriginal coordinate system to the so called principal axes of the porous medium produce adiagonal matrix. (Eq 7.16) simplifies to

7.3.2. The differential equations of fluid flow through porous materialsIn flow phenomena of any kind, one of the most useful mathematical tools is that obtained from aconservation principle, that no physical quantity can be destroyed or created. Generally thislaw of conservation for any physical quantity can be written as

where G is the quantity that is released per unit volume and per unit time, G is the concentrationof the considered physical quantity. For the single phase and incompressible fluid, the volume of an element of fluid is not altered bychanges in pressure. Hence, fluid volume is conserved and in the general equation of continuity

becomes the volumetric flux density . For a fully saturated volume, the concentration GPO Pvbecomes the concentration of fluid volume which is just the porosity f .

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134 Grout injection of masonry, scientific approach and modeling

M2p

Mx 21

%M2p

Mx 22

%M2p

Mx 23

' 0 (Eq 7.23)

P? @ Pv 'Mv1

Mx1

%Mv2

Mx2

%Mv3

Mx3

' G(x1,x2,x3, t) (Eq 7.20)

MMx1

MUMx1

%MMx2

KµMUMx2

%MMx3

KµMUMx3

% Q ' n MfMt

(Eq 7.21)

L2 U 'M2U

Mx 21

%M2U

Mx 22

%M2 U

M x 23

' 0 (Eq 7.22)

Therefore, the equation of continuity becomes

If there are no sources or sinks, G / 0. For an isotropic porous medium the components of theflux density are to be expressed in terms of the components of the potential gradient of equation(Eq 7.15). Depending on the particular situation, (Eq 7.20) results in other several differentequations. The resulting differential equation for an isotropic porous medium yields

If the medium is homogeneous and µ is constant, the above equation reduces for the steady stateto the Laplace equation:

Since , and since g and ? are both constant, this Laplace equation can be furtherU ' p % ?gx3

simplified to

For the more general case of a homogeneous anisotropic porous medium, a particularmodification of the coordinate system permits to formulate (Eq 7.20) in a comparable way[Collins, 1965]. For the particular case of grout injection the masonry is considered to beinhomogeneous but isotropic. Therefore, the latter case is not worked out in this text.

7.3.3. Boundary conditionsThe equations governing the flow of fluids through porous materials are second-order partialdifferential equations. It will be necessary to specify the boundary conditions for the dependentfunctions or its derivatives.

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Chapter 7. Flow of fluids through porous media 135

K1

µ

M? 1

Mln

'K2

µ

M? 2

Mln

(Eq 7.28)

vn ' Pv @ Pn ' &KµL? @ Pn ' &

M?Mln

' 0 (Eq 7.24)

P x1,x2 , x3 , t ' constant or U x1, x2,x3, t ' constant (Eq 7.25)

p1 ' p2 (Eq 7.27)

vn ' &Kµ

MUMln

(Eq 7.26)

C Closed boundary conditionsAt a closed boundary, the fluid velocity normal to the boundary equals zero. Darcy’s Law gives

or simply MUMln

' 0

In (Eq 7.24), is a unit vector normal to the boundary and ln is the distance measured parallelPnto .Pn

C Fluid entry or exitOn any section of boundary through which fluid enters or leaves the porous medium differentconditions may be obtained. For a homogeneous fluid entering from a reservoir at constantpressure or more generally constant potential, the boundary conditions can be expressed as:

The boundary conditions often applying to boundaries across which flow occurs is thespecification of the velocity normal to the boundary. Thus

C Discontinuity in the porous mediumVery frequently, flow occurs in material in which a discontinuity in the permeability exists. Theproper boundary conditions in this case are double. First condition is a consequence of thepressure. Obviously, since only one value of pressure may exist at any point we can state that

where p1 is the pressure at the boundary location for the first medium and p2 in the secondmedium.Since what enters the boundary from one side must come out the boundary on the other side.Therefore, the velocities normal to the boundary must be equal on both sides.

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136 Grout injection of masonry, scientific approach and modeling

MMx

p MpMx

' f MpM t

(Eq 7.29)

K ' K4 1 %bp

(Eq 7.30)

dd x

K4p 1 %bp

dpd x

' 0 (Eq 7.31)

K4 p 1 %b

p

? pL

'qµp

A(Eq 7.32)

1

K4

'1L m

L

0

dxK4 (x)

(Eq 7.33)

7.3.4. Measurement of the permeability using compressible fluidsIn paragraph 7.2.3 it is explained how the permeability of a porous sample can be measuredusing an incompressible fluid, or liquid. One could use water for this purpose, but the poroussample, or in our case, the masonry would be saturated with water. This moisture needs to beevaporated from the masonry, which takes a very long time. The saturated masonry wouldprovide a completely different resistance to the grout than the dry masonry. Therefore, we useair to determine the permeability of the masonry. For air, being a compressible gas, equation(Eq 7.5) is not valid. The compressibility must be taken into account. For linear flow of anideal gas the flow equation becomes

In the steady state the pressures are stationary or independent of time and hence .M pM t

' 0

Klinkenberg [1994] investigated the fact that gasses do not stick to the pores’ walls as isrequired for Darcy’s law. Slip occurs and this gives rise to an apparent dependence ofpermeability on pressure. This dependence can be expressed by the Klinkenberg relation:

where K4 is the permeability as observed for incompressible liquids: the permeability we wantto know, p is the mean pressure and b is a constant characteristic of both the gas and the poreusmedium.

After integration, rearranging and again integration between 0 and length L we get

Where is the mean pressure, the mean discharge and the mean permeability given by:p q K4

For this reason, the pressure that is applied in the experimental setup to determine thepermeability of the reproducible masonry samples is kept very small. Furthermore, adepression is applied. This reduces possible errors for the permeability value.

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Chapter 7. Flow of fluids through porous media 137

Steady state, radial flow

0

2000

4000

6000

8000

10000

12000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Radius [m]

Pre

ssu

re [

Pa]

Pw

Pe

Rw Re

Figure 2: Pressure distribution for a steady radial flow from inner radius Rw = 0,1 m toexternal radius Re= 0,6 m

u ' ln (x 2 % y 2)1/2

v ' tan&1(y /x)(Eq 7.34)

M2UMu 2

%M2UMv 2

' 0 (Eq 7.35)

7.3.5. Radial flow between concentric cylinders

The steady flow of a fluid into a well or out of an injection hole is often represented as planeradial flow between concentric circular boundaries. In first stage the medium is consideredisotropic. The interior circle represents the wall of the bore hole and the outer circle representsa boundary of constant potential called action radius. To solve this problem the technique ofconformal mapping is made use of. We use the following transformation of the variables fromthe x, y plane, being the original coordinates, to the u, v plane.

This conformal mapping preserves the form of Laplace’s equation. So the Laplace equation forthe potential function remains of the form:

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138 Grout injection of masonry, scientific approach and modeling

Q 'K A(r)

µ? P? r

. K A(r)µ

dP(r)dr

(Eq 7.40)

Q 'K 2 p h

µ

Pw & Pe

ln(Re/Rw)(Eq 7.41)

U ' A % B u (Eq 7.36)

U ' Uw on u ' ln Rw

U ' Ue on u ' ln Rre(Eq 7.37)

U ' Uw % (Ue & Uw)u & ln Rw

ln re & ln Rw

(Eq 7.38)

U ' Uw %Ue & Uw

ln Re /Rw

ln x 2 % y 2

Rw

(Eq 7.39)

The boundary conditions are U(u = ln Rw ) = Uw and U(u = ln Re ) = Ue . The solution to theabove equation yields:

with the boundary conditions corresponding to the u, v plane:

The boundary conditions enable to solve for A en B the above solution in equation (Eq 7.36).

Or after back transformation we get

The above solution for the pressure gives rise to a discharge that can be calculated as

Replacing the function P(r) by the solution found in equation (Eq 7.39) gives a dischargeindependent from r:

Although equation (Eq 7.41) provides the solution for the steady state situation, it can be readthat the discharge will decrease with growing penetration depth (increasing Re -value) of thefluid into the porous media. Nevertheless, the discharge will never drop to zero for a limitedexternal radius for a fluid with constant viscosity, without critical yield strength as explained inparagraph 6.3.4. According to equation (Eq 7.39), the discharge Q is a function of the bore hole radius Rw . Forinjection purposes this is one of the technological parameters influencing the quality of theconsolidation. The discharge increases almost linearly with increasing bore hole radius(Figure 7.3). The dimensions of the injection hole will of course be limited, but precautionsshould be taken not to use too small bore hole diameter.

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Chapter 7. Flow of fluids through porous media 139

Theoretical discharge vs bore hole radius

0

0,05

0,1

0,15

0,2

0,25

0,3

0 0,01 0,02 0,03 0,04 0,05 0,06bore hole radius [m]

Dis

char

ge [d

m3 /s]

Values for calculation

K = 1e-10 m2

Re = 0,6 mUe = 0 PaUw = 10 000 Pamu = 0.01 Pa sh = 1 m

Figure 7.3: The discharge increases with increasing bore hole radius

The results in for Figure 7.3 are calculated for an isotropic porous medium, injected with aNewtonian liquid without critical yield strength. As mentioned above, the rheological behaviorof a grout can not correctly be modeled by a Newtonian fluid. Furthermore, the masonry is farfrom being an isotropic porous medium. Still the remark about the bore hole diameter holds.Actually, the effect will even be amplified by the non-Newtonian nature of a grout. The slowerthe grout will flow through the masonry, the more water can be extracted from the grout, themore both viscosity and yield strength will increase. Besides, the low velocity of the grout flowwill cause the thixotropic behavior to take place sooner. As a consequence, the penetrationdepth will decrease with a smaller injection hole.

7.4. ConclusionsTogether with the previous chapter about rheology of grouts, this chapter forms the theoreticalbase that is used to build the final model. The experimental set up for measuring thepermeability using a compressible fluid is worked out theoretically in paragraph 7.3.4. The

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140 Grout injection of masonry, scientific approach and modeling

theory about fluid flow through porous media is used to illustrate the influence of the bore holediameter on the penetration depth of the grout inside the porous medium.However, the formulas that are derived in this chapter describe the laminar flow of a Newtonianfluid through a saturated porous medium. This is not exactly what is happening when a groutenters the flow channels inside the masonry structure. First of all, grouts are no Newtonianliquids. Grouts rather show a Bingham behavior. Besides, grouts are a dispersion of theparticles of the binding agent in water. Therefore, they show a more complex behavior whenflowing through the porous medium. The particles might block part of the pores. Sedimentationphenomena and thixotropy imply that the application of Darcy’s law for describing thepenetration of the grout inside the masonry will lead to inaccurate results.

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Chapter 8. Modeling grout flow in masonry 141

Chapter 8. Modeling grout flow in masonry

8.1. IntroductionThis chapter describes the basic features of the model that was developed to simulate thepenetration of a grout inside the masonry. Firstly, the choice of a discrete model is justified andit is explained why the continuum approach is not used. After that, the model itself is discussed:a network of channels, connecting the nodes. The theory governing the flow of fluids isimplemented to define the transient penetration in the masonry. In the next chapter the modelwill be used to judge the importance of some parameters of the grout and of the masonry withregard to the degree of filling that can be achieved.

8.2. Discrete model8.2.1. JustificationTransport phenomena in porous material are usually described using the continuum approach. Arepresentative volume of material is considered to be homogeneous and to have the averagevalues for the relevant properties, e.g. porosity, permeability and density. This approachdisregards the heterogeneity of the materials on the microscopic level with the pores, smallcracks and capillary pores through which the transport is happening on one hand and the solidmaterial on the other hand. The method uses average values on the macroscopic level defining arepresentative volume. When the scale of the representative volume is corresponding to thephenomena at hand, convergence is reached. This is why capillary absorption and moisturetransport are fairly well modeled using the continuum approach. The problem of grout penetration in masonry has a different nature. The representative volumehas to become very large. Furthermore, it would take a lot of effort to characterize theproperties of this representative volume with regard to the penetration of the grout through thisvolume. The flow of grout is concentrated in the big cracks and openings of the masonry. Fromthese main vessels, the grout spreads out over the finer fraction of the masonry. The latter isnecessary to obtain a proper consolidation: if only the main cracks are filled, the masonry willnever regain its internal cohesion and its monolithical behavior. Disregarding the physicalreality that the main flow is situated in the big flow channels, will never lead to correctsimulations.The above reasoning demands a discrete modeling of the injection process. A discrete modelstarts from the real flow channels present in masonry: the main cracks with a diameter biggerthan an arbitrarily chosen value (order of magnitude 1 mm). Also, a discrete model will be abetter approximation of reality. Unfortunately, it will be unfeasible to find out about the sizeand the position of all these flow channels. The state of knowledge offers no technique that isable to provide these data.

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142 Grout injection of masonry, scientific approach and modeling

1C

' R Pa . sm 3

'? PQ (Eq 8.1)

Vol ' 2700 x length x p d 2

4' 2700 x 628.3 mm 3 ' 1696460 mm 3 (Eq 8.2)

8.2.2. A network of discrete flow channelsThe masonry is modeled by a three dimensional combination of one dimensional flow channels.A volume of porous materials is represented by a number of nodes. These nodes areinterconnected by one dimensional channels to form a network. The transition from a onedimensional pipe to a two or three dimensional network does not require any specialmodification. Strictly spoken, any two nodes can be interconnected, but in most cases only neighborhoodconnections make sense. The nodes are idealized nodes: they do not possess any volume andthey do not mean any resistance to the flow. Default shape of the flow channels is a cylinder, but when the relation between pressuregradient and discharge is known or can be determined analytically or experimentally, any othershape can be used. The conductivity of the channel is the required information. Theconductivity is the inverse of the resistance to flow, which is on its turn given by:

As can be derived, the dimension of the conductivity is m3/(Pa.s). The conductivity iscalculated using the equations that were derived in paragraph 6.3.1. In a similar way therelation can be derived for the flow between parallel plates, which is suitable for the simulationof flow through a wide crack. A volume of porous material that is modeled by nodes and the interconnecting channels has onlya very limited percentage of empty space. The portion of the volume that can be filled by thegrout is much smaller than what is observed in reality. Assume a cube with an edge of 500 mm.In order not to increase the density too much, the distance between two neighborhood nodes is50 mm. Between each node and his non-diagonal neighbors (two out of three coordinates arethe same), an interconnection is present. This interconnection is a cylindrical channel, diameter4 mm. This network consists of 2700 channels. All of them have the same length: 50 mm andthe same diameter. The total volume of the channels equals:

Related to the original volume of the cube this is only 1,4 %. In practice, as a rule of thumb, thevolume to be injected with grout is estimated to be around 20% of the outer volume. Thisconclusion proves that the capacity, provided by the volume of the channels only, is far toosmall. Increasing the diameter of the flow channels would indeed increase the capacity of thenetwork, but would provide a conductivity that is too high. Making the network denser byraising the number of nodes or by raising the number of channels is to be rejected. More nodesmeans more intensive calculations. Increasing the number of interconnections is not valid sinceonly neighborhood connections make sense. To solve the problem of the limited capacity of thenetwork itself, capacitive elements are added in (some of) the nodes. These capacitive

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Chapter 8. Modeling grout flow in masonry 143

Figure 8.1: Conceptual representation of the capacitive elements

elements do not participate in the transport, but take a large amount of grout. When the capacityis completely filled, it has no further influence on the grout flow, since it does not appear in themass balance of that node any longer. Anyhow, there is never a discharge through the capacityto any of the neighborhood nodes.

The capacitive elements can physically be justified. Weathered masonry eventually containslarge internal holes. When the grout arrives in such an internal hole, this hole is filled beforeany further penetration can be realized. Such a hole can be simulated by a capacity with a largevolume and a high conductivity. If the large hole leads to other cracks, a capacitive element cannot be used, in that case the hole has to be simulated by a large channel, providing the sameeffect since capacitive elements do not allow any transport to neighborhood nodes.Finely cracked zones are present in the vicinity of the wide cracks through which te main flowoccurs. These zones take also part of the grout. The penetration of these zones is a slowprocess and the grout penetrates further before those zones are filled with grout. Besides, itmight happen that these zones are only partially filled at the end of the injection. Such a zonedoes not participate in the global transport and is perfectly simulated by a capacity with a smallvolume and a rather small, decreasing conductivity. Coming back to the numerical example of the masonry cube with edge of 500 mm we can easilyshow that a capacity of 23.250 mm3 in each node would bring the total injectable volume to20 % of the outer volume.

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8.2.3. Boundary ConditionsAfter the network has been created, the program requires the boundary conditions. A node canbe subject to three different boundary conditions. For a dead end node, no explicit boundarycondition needs to be specified. The program is built in such a way that the incoming dischargewill automatically drop to zero. The two other possible boundary conditions consist essentiallyof imposing a constant pressure at that node. This boundary condition is most probable to occurfor nodes situated at the border of the represented volume. A node connected to the injectionpipe will be at constant injection pressure, a node representing a leakage will be at constantzero pressure since the grout can flow freely out of that node.It would have been possible to incorporate the boundary condition of constant discharge throughone channel, e.g. the injection tube. This exercise, however, is quite useless since the situationof constant discharge is never valid in case of grout injection. Only in the beginning of theinjection, it might happen that the capacity of the pump is not able to fulfill the constant pressureboundary condition. However, since the penetration of the grout inside the masonry is our goalof study, this short moment of time is only of little interest to us. The injection pressure has tobe limited to prevent that hydrostatic forces split the masonry. For this reason a moderninjection installation is equipped with a three way valve, providing an adjustable constantinjection pressure. This corresponds exactly with the used boundary conditions.

As explained above, the injection pressure has to be limited to avoid that the masonry suffersfrom additional damage due to the hydrostatical pressure. Modern injection installations,discussed in detail in ?, posses an adjustable three way gate valve and give rise to boundaryconditions that are pretty close to the constant pressure condition. There is only a limitation forthe discharge. In the beginning, when the discharge is high, the pressure might be a little lowerthan the maximum injection pressure due to insufficient capacity of the pump and pressure lossesin the conduits. Distinction can be made between experiments where one side is at constantinjection pressure and the other side is impenetrable and experiments where some nodes are atconstant ambient pressure. The latter situation arises when leakage occurs. The sealing of aleakage comes down to changing the boundary conditions for that node into the standardboundary conditions of impenetrability.

8.2.4. Dealing with water absorptionWhen the grout enters the relatively dry masonry, the masonry that is surrounding the flowchannels will absorb water from the grout. One would expect that the grout loses water and,because of that water loss, that the rheological properties of the grout get worse. So, the waterabsorption could be incorporated by changing the rheological properties depending on the timethat the grout has spent inside the masonry. However, there are some contra indications for theabove assumption. Translating the fact that the rheological properties of the grout depend on thetime that the grout spend inside the masonry would require a huge book keeping. Besides, atcertain moments, the grout that arrives at a node, comes from different flow channels. The grout

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Chapter 8. Modeling grout flow in masonry 145

Figure 8.2: The water absorption out of the grout results in a narrowing of the flowchannel

would then be a mixture of two liquids with different rheological properties since the grout fromone channel probably spent more time inside the masonry than the grout from the other channel.This would result in a very complex situation. This argument is not a physical one. It justmeans that it would be difficult to master that possibility. Fortunately, there is also a goodphysical argument not to deal with absorption like that. During the experiments, the grout thatcame out of the samples was caught in a measuring jug to check the rheological properties. Thisgrout has then penetrated the complete sample and it was expected that the rheologicalproperties would differ from the properties of the injected grout. This seemed not to be thecase. The rheological properties of the grout that leaves the sample do not or hardly differ fromthe grout that enters the sample. The above assumption appeared not to be in accordance withthe experimental findings. Therefore, we believe that the water absorption causes cementparticles to stick to the wall of the flow channel as presented in Figure 6.1. The grout thatkeeps on flowing however, has the same properties as the grout that is injected. Therefore, it canbe stated that the flow channels narrow by the absorption of water out of the grout and that it isnot true that the rheological properties of the grout change by the water absorption. Thisphenomenon is incorporated in the model by narrowing gradually the flow channel once thegrout has entered the channel (Figure 8.2). The narrowing is faster in the beginning than after awhile since the cement particles that stick to the wall form a barrier for further water to beabsorbed.

8.2.5. Special features of the program

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146 Grout injection of masonry, scientific approach and modeling

The program allows to build the network in a manual way by defining each node’s coordinates(x, y and z) and consequently by defining the interconnections and the size of the interconnection.The standard procedure to place nodes however, is an automated procedure that generates aregular pattern of nodes with a fix distance in each direction. The possibility exists to alter theposition of a node or to manually add nodes if the regular pattern does not fulfill therequirements. Not all the interconnections or flow channels have the same diameter. Theprogram allows a completely manual definition of the flow channels. The input requires thebeginning node, the end node and the diameter of the channel. A second way to define theinterconnection is by specifying up to 9 standard diameter values for each possibleinterconnection. Every interconnection is proposed by the program. The users specifies thepredefined diameter by pressing number 1 to 9. Pressing 0 means that the proposedinterconnection will not be created. Finally there is a possibility of random generation of thediameters. The diameter range can be specified by the user. It is also possible to specifylocally a smaller diameter range to simulate a region that is less permeable than the globalstructure. All the parameters that have to be specified by the user, have a default value. If no value isdetermined, the default value will be assigned.

8.3. Structure of the programAim of this chapter is to show the global structure of the program and to explain some specialfeatures. This way of clarifying the program is preferred above a complete listing in anappendix. The program is, just as most desktop applications nowadays, menu controlled. This makes theprogram user friendly. Since the number of users will be limited, this is not a goal as such.These features make it easier to adapt the parameter’s value, to save an intermediate situation.The structure of the menu also provides a good overview of the global structure of the programshowing the different steps that the user has to do in order to obtain a simulation. References to menu items in this chapter will be written in italic font. Reference to actionbuttons, button that call for one or a series of procedures are typed between {action}.

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Chapter 8. Modeling grout flow in masonry 147

Figure 8.3: Main menu of the program

Figure 8.4: Submenu “Network”

8.3.1. Menu items

NetworkFirst menu item deals with the definition of the network of pipes. The sub menu items enables,just as in a classical finite element program, to define the nodes and the elements in differentways from manually to fully automated as explained before. The Submenu “nodes” (Figure 8.5)contains five different procedures to create, add or remove nodes. Again, this can be donemanually or more or less automated. The subitem “elements” (Figure 8.6) allows to define theinterconnecting channels of the network. This item is only available if the nodes were alreadydefined. The elements can be generated randomly, only if a regular pattern of nodes wasdefined, semi-automatic or manually. The manual method is the only one that is available whenthe network has a non-standard pattern. The properties of the channels of an existing networkcan be adapted. Channels can be added or deleted. The routine {display} visualizes thenetwork on the screen, showing the diameter by varying the line thickness of the channel. Toenable the use of a predefined network or to use the same network for several simulations, it ispossible to save {“write”} the network to disk or to read it from a file {“read”}. All relevantproperties of the network are then saved: the nodes with their three coordinates and theinterconnections with the two end nodes, and the diameter of the channel. The capacities are notsaved since they are seen as a parameter rather then as a property of the network.

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Figure 8.5: Submenu “nodes”

Figure 8.6: Submenu “elements”

Network - nodesThe definition of the coordinates of the nodes can be done manually {“manual”}: the programwill prompt for the number of nodes and ask for their three coordinates one by one. This is along lasting procedure if the network counts many nodes. If the desired pattern of nodes is of aregular kind, one can use the option {“standard”}. For each direction x, y and z the distancebetween the nodes can be specified. A specific node can be added or replaced {edit}. Theconnections to a replaced node remain. If they have to disappear, this can be done in de subitem“elements”.

Network - elements

To define the channels that are connecting the nodes, there are four options in an increasingdegree of automation given by Figure 8.6. A manual definition, the only method in case of a nonstandard configuration of the nodes, requires the beginning node, the ending node and thediameter. The other input methods enable only connections between neighborhood nodes wherethe diameter is given by:- keyboard usage {Semi-automatic}

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Chapter 8. Modeling grout flow in masonry 149

Figure 8.7: Submenu Rekenparameters

- a random choice between several predefined diameters {Automatic}- random choice by the program between predefined limits {random}

Second menu item enables to change the default calculation parameters of the program. Thefollowing list explains the meaning of the parameters and provides the default value of theparameters.{Injection pressure}

The boundary condition of constant injection pressure can be specified and the nodesthat are subject to the injection pressure. Default value for the pressure is 1 bar or100.000 Pa and applied to the first node. Any other non-zero value can be given: oneinjection pressure value and then up to ten nodes that are connected to the injectionpressure level.

{Capacity}Two parameters define the capacity: its volume and the conductivity towards thecapacity. Default there is no capacity present. Choice can be made between constantcapacity volume and conductivity in every node, or variable ones. These capacityparameters can be generated randomly or manually. The physical meaning of thecapacity is commented before.

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{Leakage}Just as the injection nodes that are subject to the constant injection pressure, the leakagenodes are subject to a constant pressure boundary condition. In this case the pressure iszero, since a leakage is supposed to remain at ambient pressure. The number of leakagenodes has to be specified and then the node number.

{Viscosity}The first grout parameter to specify is the viscosity. This rheological parameter can bedetermined by laboratory test. Testing the viscosity by either a coaxial viscometer or acapillary viscometer should be a standard test when adjusting the grout composition.Default value for the viscosity is 0.005 Pa.s.

{Shear strength}When shear strength is present in the rheology of the grout, this implies that there existsnon-Newtonian behavior. The shear strength for a mixture with a superplasticizershould be rather low. It is advised to check the shear strength of a new developed grout.Default value for the shear strength is 0 Pa. Typical value for the shear strength is about5 to 10 Pa.

{Density}The density of the grout is a necessary parameter because the program includes gravityduring the calculation of the penetration. The density is only of little influence since thegravitational forces are rather small compared to the injection pressure.

{Time step}In the following paragraph, the different steps in the algorithm will be clarified. Thetime step determines the accuracy and the speed. The time step can not be too big foraccuracy reasons and not too small in order to avoid exploding the calculation time.Default value of the time step is 0,001 second.

{Acceleration}The small time step is especially necessary for the beginning of the injection. The initialdischarge is very large, but slows down after a few time steps. Therefore, it is possibleto increase gradually the time step from an initial value. The time step will bemultiplied by a factor, somewhat larger then 1, after a chosen number of time steps, e.g.50 or 100 steps. Without undermining the initial accuracy this feature reduces thecalculation time needed to accomplish a simulation.

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Chapter 8. Modeling grout flow in masonry 151

Figure 8.8: Submenu Rekenparameters

Figure 8.9: Submenu Results

Calculate

The next item enables to initialize {initialize}, to start the calculations {start} and to continue{continue} the calculation if they were stopped by the user for instance to change the time stepor to check for intermediate results. After initializing, all the positions, the pressure values, thesimulated time etc.. are reset to their initial value.

Results

Especially for analyzing the results, the fourth menu item is added. It enables to have a look atsome interesting intermediate or final results. The results are grouped into time related results:total simulated time, time step at that moment, number of iterations done, and volume relatedresults: total injected volume, volume of the voids of the network, volume of the channels only,volume of the capacities only, volume filled with grout. The total injected volume will differfrom the volume filled with grout if there is a leakage. The difference of both provides theamount of grout that is lost.

The next menu item brings us to the calculations itself. Initialization is required. This actionimposes the defined boundary conditions, initializes the positions of those channels that are incontact with injection nodes and brings the position in all other channels to zero. Finally itresets the time step to the initial one, it puts the capacities to zero after a former simulation andalso the factors that take into account the Bingham behavior of the grout. After initializing, thecalculation can start. During the calculation it is possible to halt the calculations for saving, orviewing intermediate results, to adapt the time step etc... After the adaption the calculations can

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152 Grout injection of masonry, scientific approach and modeling

Figure 8.10: Submenu output to screen

be continued. Finally, it is possible to reset the position in any channel to zero or to fill thecomplete network. The latter feature can be useful to calculate the total maximum dischargethrough a network where some nodes are at injection pressure and some of the nodes areleakage nodes.

Output items Screen and Disk

Fifth and sixth menu items enable output of results or parameters to the screen (Screen) or todisk (Disk) for analysis in any other program such as a spreadsheet program or a statisticalprogram. Among the results or parameters are : properties of each channel: the diameter, length, ending nodes discharge through each channelposition of the grout in each channelCorrection factor for a Bingham fluid in each channelPressure values for each nodeCapacity data for each nodeThe discharge towards the capacity for each node

Furthermore, it is possible to save the complete result file to disk. The calculations can proceedafterwards or the situation can be analyzed.

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Chapter 8. Modeling grout flow in masonry 153

QAi ' (Pi &PA ) CAi met CAi 'p D 4

128 µ1

PostnAi

( BAi (Eq 8.3)

QACap ' &PA CACap where CACap ' the given value (Eq 8.4)

8.3.2. Calculation algorithmThe global structure clarifies how the requested data input happens. It gives a short overviewof the different menu items and gives an idea of the possibilities of the program. The mostimportant and critical part of the program is the calculation algorithm. There, the physical rulesare translated into mathematical formulas. There these mathematical formulas are translatedinto the Turbo Pascal source. This part discusses the basic features of the program, algorithmand the mathematical techniques used to achieve the simulations. Some of the mathematicalprocedures were found in classical standard books. Tough the author realizes that the mainalgorithm is probably not the most powerful one. For sure, it is possible to enhance thealgorithm with regard to calculation speed and memory usage. This enhancement however, isbeyond the goal of the present study. The calculations are formed by a series of steps, repeatedat every time step. Each of these steps is explained.

C Building the set of equationsThe pressure values in every node are the unknowns, except for these nodes where aboundary condition of constant pressure exists such as the inlet nodes or leakage nodes.Each equation expresses that the net discharge towards the node is zero. This can bedone for every node that is reached by the grout. The number of unknown pressurevalues always equals the number of reached nodes and hence the number of equations.These nodes are firstly eliminated from the set of equations. Notice that the dimensionsof the set of equations are growing as the grout penetrates further inside the masonry.For node A, surrounded by four nodes i = 1 to 4, the incoming discharge for eachchannel can be expressed as:

where , expressing the influence ofBAi ' 1 &43

(4t c

D

PostnAi

Ptni & P

tnA

)%13

(4t c

D

PostnAi

Ptni & P

tnA

)4

the shear stress in case of a Bingham fluid 6.3.3. If the fluid has no critical shear stress,BAi equals 1.

The discharge towards the capacity, if present, can be expressed as:

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154 Grout injection of masonry, scientific approach and modeling

j4

i'1QAi % QACap ' 0 (Eq 8.5)

QA1 ' (P1&PA% ? g? h) CA1 where CA1 'pD 4

128µ1

PostnA1

(Eq 8.6)

Stating that the nett discharge towards node A results in:

In this summation, only the channels that really feed node A are mentioned. This meansthat a channel that is not completely filled, will only appear in the summation if thatchannel is filled from node A. This way, a channel that is being filled up by aneighborhood node will not contribute for the equation expressing that the net dischargefor node A equals zero. Channels that are fully filled will always contribute. A node at constant pressure provides no additional unknown. This constant pressurevalue will firstly be eliminated from the equations and appear in the right hand sight ofthe set of equations. The set of equations results in a sparse matrix, which bandwidthequals the degree of connectivity: this is the maximum number of surrounding nodes.

The procedure that builds the set of equations, checks all the nodes that were defined inthe model. If a node is reached by the grout it gets a place in the matrix. Then, theprogram expresses for all the nodes that were reached that the nett discharge must bezero. The conductivity factors C that are related to the unknown pressure values areplaced on the right place in the matrix. By only taking into account the reached nodes,and hence, by considering only those pressure values related to the reached nodes asunknowns, all other pressure values will remain zero. This is physically justified if oneaccepts that the air in the masonry structure is free to escape from the masonry withoutimposing a significant counter pressure. Having in mind the structure of ancientweathered masonry, this assumption is valid. In a similar way it is assumed that thecapacitive elements are not able to generate a counter pressure. The background forboth these assumptions is the relatively open character of masonry with regard to air.The transport phenomena that are considered here, take place in relatively big channels,but the air is able to flow away through the fine capillary pores. In the aboveexplanation, the gravity was not incorporated. However, gravity can not be neglected,especially since the height of an injected masonry mass can be relativily high. Whentaking into account the gravitational forces, the pressure difference changes. Thedischarge through the channel that connects node 1 with node A can be expressed as:

In equation (Eq 8.6), ? h is the difference in height [m] between the reached node and theposition of the grout front in the channel A-1. For a channel that is completely filled this

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Chapter 8. Modeling grout flow in masonry 155

xn %1 ' xn %discharge ( ? t

S(Eq 8.7)

corresponds to the relative difference in height between both nodes. Each equation,expressing the discharge between two nodes that are at different height, contains aconstant term. This term will appear in the right hand side when building the matrix ofequations.

C Solving the set of equationsOnce the matrix is build, the set of equations is solved using the Gauss eliminationmethod with full pivoting. This method is not the best one available, especially not forspare matrices, but offers the advantage of flexibility. No limitation have to be maderegarding the interconnections between nodes. Adding a node does not require acomplete reordering of the existing nodes. So, for reasons of general applicability, thismethod was chosen. Certainly for the manually build networks with a non-standard nodepattern, this is an important feature. Disadvantage of this method is that it demands asquare matrix. When using a method that uses the spareness of the set of equations, theamount of memory and the CPU is used more efficiently. The solution of the set ofequations returns the pressure values for all the reached nodes that have not a constantpressure boundary condition.

C Calculating the positions at time tn+1

The positions after the nth time step are known from previous calculations or from theinitialization procedure if n = 0. This initialization procedure sets all positions to zero,except for the positions in channels that are connected to the nodes at constant injectionpressure. The position of these channels get a small non-zero value. When, in course ofthe calculations, a new node is reached, that node will also be “lighted”. Therefore, theposition of the grout, in case the position is still zero, is increased with a very smallvalue. This value is calculated by taking the amount of grout that would flow out of thechannel through which the node was reached. This amount is proportionally dividedover the empty channels connected to that node according to their conductivity. Leavingthis small portion of grout out of consideration, small mistakes would be accumulated.Furthermore, problems would occur due to division by a very small number or divisionby zero. Indeed the conductivity values are depending upon the position that results fromthe previous calculation step (equations ? to ?. After that, the new position isdetermined out of the obtained pressure values, the conductivity and the previousposition as follows. The discharge is calculated using the equation of Hagen-Poisseuille. Remember, these discharges are such that the nett discharge towards thenode is zero. One could calculate the position at time tn+1 under the assumption that thedischarge does not change during the time step. Then, the position at time tn+1 is given by

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156 Grout injection of masonry, scientific approach and modeling

d xdt

'D 2

32µ? Px

(Eq 8.8)

mxn%1

xn

32 µ xD 2

dx ' mtn%1

tn

? p dt (Eq 8.9)

? t 'x 2

n %1 & x 2n 16 µ

D 2 ? P(Eq 8.10)

x n%1 ' x 2n % ? t D 2 ( ? p

16 µ(Eq 8.11)

S is the cross section of the channel. For small time steps, (Eq 8.7) can be applicable,but actually the progress of the grout is overestimated each time step, since the dischargewill decrease somewhat because of an increased resistance to flow as the groutprogresses. These small mistakes can finally accumulate to an unacceptable total error.Therefore, the integration is done analytically and not numerically. Let us assume thatthe position tn is xn. The differential formulation of the equation of Hagen-Poisseuille is:

Integration between the time limits tn and tn+1 and the limits xn and the unknown value xn+1

provides:

When all parameters are constant in time and independent from x the integration gives:

(Eq 8.10) can be rearranged as:

In the program xn+1 is the new position. For a Bingham fluid a similar expression isused.

C Visualization of the new situationAfter the calculation of the new positions, the screen is refreshed. When empty, thechannels are displayed in white. Once the channel is reached by the grout, the filled partis colored red. The coloring happens from the node whit the highest pressure value as itfills. Together with the coloring of the positions, the pressure is visualized. The virtualmanometers indicate the ratio of the obtained pressure and the injection pressure. Thisfacilitates the interpretation of the progress of the grout. When no discharge is realized,the pressure drop over a channel is only the hydrostatical pressure difference. Thecapacity is represented by a grey node when not completely filled. Once the capacity isfilled the node colors red. When the complete network is filled, it shows up completelyred.

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158 Grout injection of masonry, scientific approach and modeling

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3Time [sec]

Pos

itio

n [m

]

AnalyticModel 1 pipeModel 50 pipes

Figure 9.1: First validation: laminar flow of a Newtonian fluid through acylindrical pipe

t '16µ x 2

D 2 ? p(Eq 9.1)

Chapter 9. Validating and using the model

9.1. Validation of the modelThe mathematical model being completely new developed, it requires a thorough testing andvalidation of the model before using it for the purpose it is built for. This validation will bedone by simulating situations that can be solved analytically.

9.1.1. Flow of Newtonian fluid through one dimensional cylindrical pipeCylindrical pipe with constant diameter

The transient flow of a Newtonian fluid through a one-dimensional pipe with constant diametercan easily be calculated analytically. The relation time to penetration depth in the pipe wascalculated before and resulted in equation (Eq 6.21) on page 125.

The model is used to simulate the above case. Two different models were used. The firstmodel consists of one pipe, diameter 0.005 m and length 1 m. The second model consists of 50pipes, 51 nodes with diameter 0.005 m and length 0.02 m. In both cases gravity is neglected andthe inlet pressure is maintained constant at 1000 Pa. For the model with one pipe only, there isno difference between the analytically calculated results and the results obtained by the model:there is a perfect match. When the pipe of 1 m is built up with 50 small pipes, 0.02 m long, thedifference between the analytical figures and the model mounts up to 1 %. This difference iscaused by the numerical inaccuracy and by round off errors when initializing a new pipe.

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Chapter 9. Validating and using the model 159

Pin ' ? P '32 µ xm

D 2m

% jm&1

i' 1

32 µ vi Li

D 2i

(Eq 9.2)

? P '32 µ vm

D 2m

jm&1

i' 1

D 4m Li

D 4i

% xm (Eq 9.4)

p D 2i

4vi '

p D 2m

4vm and thus v i '

D 2m

D 2i

vm (Eq 9.3)

Figure 9.2: Configuration of a pipe with varying diameter, used for validation

Cylindrical pipe with varying diameterThe analytical solution for the transient flow of a Newtonian fluid through a one dimensionalpipe with varying diameter must be found part by part. The situation is shown in Figure 9.2.The charge losses due to the sudden change of the diameter are not taken into account. Each parthas a length and diameter specified by Li and Di.

Let us suppose that the fluid has reached the mth pipe. The pressure is maintained constant at theinlet of the one dimensional pipe. This means that the pressure difference between the inlet andthe position of the fluid is constant and equals Pin. Therefore the next equation is valid:

Furthermore, the discharge must be the same in each pipe, since there are no sources or sinks inthe closed pipeline. This requirement can be expressed by specifying the relation of thevelocities.

Combination of (Eq 9.2) and (Eq 9.3) provides

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160 Grout injection of masonry, scientific approach and modeling

mtxm

tm&1

dt ' mxm

0

32µ jm&1

i' 1

D 4m Li

D 4i

% x

D 2m ? P

dx(Eq 9.6)

txm& tm '

32µ

D 2m ? P

jm& 1

i '1

D 4m

D 4i

xm %x 2

m

2(Eq 9.7)

or vm 'dxdt

'D 2

m ? P

32 µ jm&1

i' 1

D 4m Li

D 4i

% xm

(Eq 9.5)

Integration of equation (Eq 9.5) yields

where tm-1 is the time when the m-1th pipe is completely filled. This time can be calculatedanalogously to the above formulas. For the validation of the model a pipe with a diameter alternating between 0.005m and0.0025 m is considered. The length of each part equals 0.2 m. The pipe starts with a piece of0.0025 m. Just as in the case discussed above Figure 9.3 shows the perfect match for theinvasion of the pipe when calculated analytically and the figures obtained by the model.

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Chapter 9. Validating and using the model 161

Q 'p D 4

128 µ? PL

[1 &43

(4t c

DL

? P)%1

3(

4t c

DL

? P)4] (Eq 9.8)

4 t c L

D ? p' 1 (Eq 9.9)

0

0,2

0,4

0,6

0,8

1

1,2

0 2 4 6 8 10 12 14 16 18Time [sec]

Posi

tion

[m]

Analytic

Mode l

Figure 9.3: Second validation: flow of a Newtonian fluid through cylindrical pipe with varyingdiameter

9.2. Flow of Bingham fluid through one dimensional pipeIt has already been mentioned in the previous chapters: a Bingham fluid flows through acylindrical pipe according to the Reiner-Buckingham formula [paragraph 6.3.3]:

The above equation indicates that there is no longer a linear relationship between the dischargeand the pressure gradient. The flow stops when the condition of equation (Eq 9.9) is fulfilled.Physically, equation (Eq 9.9) means that there is no shearing for the fluid. The yield value doesnot exceed the critical value tc.

The third validation for the model checks if the Bingham fluid stops penetrating at the position

where . For the calculation we used the values given in Table 9.1? P D4 L

' t c

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162 Grout injection of masonry, scientific approach and modeling

Property Value Property Value

? P 10.000 Pa D 0.002 m

t c 4 Pa µ 0.003 Pa s

Total length 2 m

Table 9.1: Parameter values for the calculation of the penetration depth of a Bingham fluid

For the above mentioned situation, the penetration depth equals 1.25 m. At that position theyield value is reached. No further penetration is possible unless the pressure is increased. Themodel was used to simulate the same situation using two different configurations. The firstconfiguration consists of one pipe as specified in Table 9.1. In the second case the pipe ismodeled using 10 pipes with the same diameter and length of 0.2 m. In first case the groutstocks at 1.25 m exactly, in the second case the final position is 1.254 m. As can be read fromthe model, a pressure gradient is present although there is no discharge. When the final positionis reached the pressure difference over each of the ten pipes equals 1600 Pa. This is exactly thepressure gradient that is bringing expression (Eq 9.9) to 1. So in case of a Bingham fluid, apressure gradient is possible even when there is no discharge. Note that the same situationhappens in reality. Although the pressure is applied, there is no discharge when injection hasstopped. This phenomenon has partly the same cause as the jamming of the fluid in thecylindrical pipe.

9.2.1. Conceptual validationThe above examples are relatively simple situations for which the progress of the grout can becalculated analytically. For the second series of examples the penetration depth followsdirectly from the correction factor in equation (Eq 9.8) as expressed by equation (Eq 9.9). Sothere is no analytical difficulty to calculate the penetration depth. The above simulations onlyshow that the mathematical formulae are correctly implemented, that the translation to the finiteelement code is correct. Still the conceptual approach needs to be checked. Therefore, theexperimental data are used. The experimental program is described in Chapter 5.

There are three parameters that influence the properties of the network, defined in the model.First property is the variation of the diameter of the flow channels. This kind of networks aregenerated randomly by the program. The diameter range can be specified by the user. Secondparameter is the volume of the capacitive elements in the model. For these situations, the voidsare normally spread over the complete volume and hence the capacitive elements are all thesame. For the first series of simulations, their total volume equals the total volume of the groutinjected during the experiments. This means that in each node, there are 90 nodes, a capacitive

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Chapter 9. Validating and using the model 163

Fraction diameter range volume of capacity Conductivity towardscapacity

1 - 2 mm 0 - 3 mm 0.01 liter 1

2 - 4 mm 0 - 5 mm 0.01 liter 5

Table 9.2: Parameter values for the simulations

volume of 0.01 liter is added. The total volume, including capacitive elements and channels,equals 0.907 liter as in the experiments. This volume is somewhat bigger than the 20 % that isnormally encountered in real masonry. The reason for this is that in real masonry completezones of healthy and uncracked masonry are present providing a very low porosity. Thirdparameter is the conductivity towards the capacitive elements. Its physical meaning is alreadydiscussed before. Two experiments are simulated using the model. Both injection are done onthe low plexiglass cylinders. One of them is filled with the crushed brick fraction between1 mm and 2 mm. The other is filled with the fraction between 2 mm and 4 mm. The capacitiveelements in both simulations have the same volume, but the conductivity is somewhat smaller incase of fraction 1-2 mm. Of course also the diameter range for the fraction 1-2 mm is somewhatsmaller than for the fraction 2-4 mm.

As can be seen from Table 9.2, the simulation fits well with the experimental data. By adaptingthe three parameters of the network displayed in Table 9.1 it is possible to obtain a good fittingbetween experimental data and the simulations. These results (Figure 9.4) prove that theconceptual approach is valid, but that the model needs some calibration. This calibrationimplies the optimization of the three parameters that are defining the model in order to obtain agood matching of experimental data and simulation of the model.

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164 Grout injection of masonry, scientific approach and modeling

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40 45 50

Fraction 2-4 simulationFraction 2-4 experimentFraction 1-2 simulationFraction 1-2 experiment

Figure 9.4: The good fitting of the experimental data with the simulation for two fractions ofcrushed bricks proves that the conceptual approach is valid

9.3. Using the model for parameter studyGenerally a model is used for two different purposes: a parameter study and the simulation of onsite situations or experiments. The first application provides a better understanding of thephysical process. One can determine the critical situations, and find the limits of both the modeland the real injection. The simulation of ‘theoretical’ experiments using the model is cheep,does not require any resources and can be done without any risk for the masonry structure.Basically it requires only some computer time and the time to analyze the results. When themodel is reliable, those simulations are an excellent alternative for experiments [Van Rickstal,1999].

9.3.1. Grout parametersBefore mentioning the parameters that will be discussed, two phenomena are commented thatare of major interest in the case of grout injection: thixotropy and water absorption. Thixotropicbehavior as such is not incorporated in the model. Though the effect of thixotropy can bestudied from this study of the critical shear value. Thixotropy means that both viscosity andyield value will increase after a period of stand still. A longer period of stand still willenhance the difficulties to restart the flow. A similar reasoning goes for the water absorptionout of the grout. When the grout thickens due to water absorption, the viscosity and the yieldvalue will increase dramatically. The relation between the viscosity and the concentration of a

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Chapter 9. Validating and using the model 165

Viscosity vs W/C ratio

0

20

40

60

80

100

120

140

160

0.5 0.6 0.7 0.8 0.9 1

W/C ratio

Vis

cosi

tiy [

mP

as]

Own experimental data

Figure 9.5: The viscosity is increasing dramatically with decreasing W/C-ratio, withincreasing concentration

dispersion is known to be of exponential order. Figure 9.5 represents the measurements carriedout during this research program for the standard grout composition with modified water contentused for the experiments.

Critical shear stress t c

The viscosity can be considered being the most critical for the velocity of flow and for the timeit takes to reach a certain penetration depth. But among all rheological parameters, the thresholdshear value t 0 is probably the most critical with regard to the penetration depth. However, theyield value is very often not known. Besides, it is quite complicated to measure the yield valuefor dispersions with the classical rheological testing devices. Before discussing the influence of the yield value on the penetration depth of a grout inside amasonry structure, it is suitable to give some remarks on the difficulties to determine this yieldvalue. The rheological study of grouts is probably worth spending a complete research projecton it. Rheology of dispersions is a complex matter. The development of a guaranteed correctway of working falls out of the scope of this research. However, an indication is given of howacceptable values for the yield stress can be obtained. One might determine the shear stress fordifferent shear rates with a coaxial viscometer, type Brookfield. This provides a number ofpoints on the diagram shear stress versus shear rate. By extrapolation one could calculate thecrossing of this curve with the Y-axis. This provides the shear value for the grout.Dr. E. Toorman showed in his PhD thesis [Toorman, 1992] that this way of working is veryoften not valid. Especially in the lower shear rate zone, the shear stress - shear rate curves fordispersions show a significant non-linear behavior. This way the yield value can be heavilyunderestimated using the value obtained by extrapolation.

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166 Grout injection of masonry, scientific approach and modeling

Property Value

Viscosity 5 mPa.s

Density 1600 kg/m3

Shear stress = parameter

Injection pressure 100 000 Pa

Table 9.3: Rheological properties for studying the yield strength asparameter

Figure 9.6: Radial network to quantify the influence of the shear stress of thegrout on the injected volume

Let us suppose that it is possible to determine the correct yield value. Since this parameterstudy is a comparative study the same network is used throughout this study. It consists of a bigentrance pipe connected to a variety of channels representing a masonry cylindrical zone with adiameter of 2m. The injection pressure is constant and equals 0.5 bar at the entrance of the firstpipe. Other specifications of the grout are listed in Table 9.3. For a specific configuration ofthe network, the obtained results are listed.

Taking into account the water absorption as described above by decreasing the channel’sdiameter, the flow does stop at a certain moment. The model is used to check the influence ofthe injection pressure, the critical shear strength and the viscosity on the injected volume.

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Chapter 9. Validating and using the model 167

Pressure t c = 0 Pa t c = 5 Pa t c = 10 Pa t c = 15 Pa25000 Pa 30,54% 14,51% 11,38% 8,50%50000 Pa 37,17% 24,66% 21,75% 18,88%

100000 Pa 50,30% 39,95% 37,31% 34,82%

Table 9.4: Influence of the shear stress and the injection pressure on the injected volume, takinginto account the effects of water absorption

Parameter Value

Viscosity = parameter

Density 1600 kg/m3

Shear Strength 5 Pa

Injection Pressure 50.000 Pa

Table 9.5: Rheological properties for studying the viscosity as parameter

Table 9.4 shows that a high shear stress has an important influence on the total volume of groutthat is injected. For the given conditions it is possible to fill 30% of the network with a groutwithout a shear stress, a pure Newtonian fluid, applying only 25.000 Pa. If the critical shearstress increases to the normal value found in literature, e.g. 5 Pa, only 14,5 % of the network canbe filled. Increasing the injection pressure solves that problem partly. But since a highinjection pressure might cause additional damage to the masonry, it is preferable to adapt thegrout composition in order to bring the shear stress to an acceptable value. A grout with a shearstress of 15 Pa requires an injection pressure of 100.000 Pa to inject 1/3 of the volume, whereasa grout without yield stress requires only one fourth of it: 25.000 Pa.

ViscosityFor one cylindrical channel the influence of the viscosity on the penetration can theoretically bederived from (Eq 6.11) or (Eq 6.15). The discharge is inversely proportional to the viscosity.For the model the same is true. Table 9.6 lists the time needed to fill the network displayed inFigure 9.6.

An increasing viscosity will slow down the flow through a flow channel. But de flow can neverbe blocked by an increasing velocity without the presence of a yield value. Mostly a grout willshow a (small) yield value and hence the viscosity will influence the pressure gradient insidethe flow channel. Therefore, when the viscosity increases, the critical shear value will soonerlead to the blocking of further penetration. Table 9.5 lists the process parameters for this study.

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168 Grout injection of masonry, scientific approach and modeling

Grout progress in function of the viscosity

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0 50 100 150 200 250 300 350 400 450

Time [sec]

Inje

cted

vol

ume

[m3 ]

µ = 0.005µ = 0.01µ = 0.02µ = 0.04

Figure 9.7: Influence of the viscosity on the progress of the injection of a radial network,taking into account water absorption

Time to fill the network Inlet pressure = 50.000 Pat c = 0 Pa

Viscosity [Pa s] Time [s]

0,005 112

0,01 225

0,02 452

0,04 905

Table 9.6: Influence of the viscosity for the injection of the 2D network. Thewater absorption effect is not incorporated in this simulation.

The pressure loss along a pipe depends strongly on the viscosity.

9.3.2. Process parametersInjection Pressure

The injection pressure is the pressure that is applied at the inlet of the injection hole. For anideal injection installation a pressure valve is mounted at the end of the feeding tubes. Thisideal situation is applied for modeling the grout flow inside the network. If in reality thedischarge is very big, the applied pressure might drop due to pressure loss in the feeding tubes.The latter phenomenon in not taken into account. For the determination of the penetration depth

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Chapter 9. Validating and using the model 169

this is less important since the penetration will drop at a moment of very low discharge andhence very low pressure loss in the feeding tubes. Within certain limits the injection pressurecan be changed. The injection pressure, this is not the pressure at the pump, should neverexceed 1.5 bar. If the pressure is too high, the grout, under hydrostatical pressure, might causesevere damage to the structure. A simple calculation proves this. Imagine a double leafmasonry structure that is injected. At a certain moment the discharge drops and pressure of thegrout between the outer leaf and the inside climbs to the injection pressure. In this situation, thehydrostatic pressure applies a load of 150 kN (!) on each square meter. Fortunately the pressureis applied by a liquid and hence the smallest leakage causes the pressure to drop. But still, if anelectrical pump is applied, caution is needed to prevent further damage. Apart from the above discussion, the injection pressure is an important parameter to reduce theinjection time, to prevent too much water absorption out of the grout or to achieve a goodpenetration in the less permeable zones.

Injection holesActually there are three parameters involved about injection holes. The injection hole diameter,the injection hole depth and the pattern of injection holes. In case of the injection hole pattern,the discussion will take place without the use of the model. For the depth and the diameter ofthe injection hole the model is suitable for giving some indications. The layout of the injection holes has been discussed in paragraph 4.3.3. From this discussion itis clear that the closest pattern should be used.From theoretical point of view, the denser the injection hole pattern, the more homogeneouslythe masonry will be injected, the lower the injection pressure can be and the better the finalresult. But there are economical constraints that reduce the number of injection holes. First ofall, those holes need to be drilled. Secondly, a switch from one hole to another means extrawork, extra time needed to complete the job. Apart from economical reasons, also technicalreason limit the density of the injection holes. If not sealed during the injection of a hole, theneighborhood holes act as leakages that prevent the pressure to build up. The diameter of the injection hole has impact on the injected volume and hence on the fillingrate. Intuitively, this can be understood as follows. If the diameter of the injection hole is large,the pressure loss will be small, also in the beginning when the discharge is big. This way thegrout is present at the entrance of the cracks at high pressure. If the diameter is small, thepressure loss will be significant when the discharge is big, as expressed by equation (Eq 6.11)or (Eq 6.15). A simulation confirmed indeed that the time to inject a fixed amount of groutincreases as the injection hole diameter decreases. The total amount of grout that was injecteddropped from 77,3 liters to 54,6 liters. Figure 9.8, Figure 9.9 and Figure 9.10 compare thefinal situation for an injection hole with a diameter of respectively 10, 15 and 30 mm. Figure9.11 displays the evolution of the injected volume of grout for the different bore hole diameters.

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170 Grout injection of masonry, scientific approach and modeling

Figure 9.9 : Injection hole diameter = 15 mmFigure 9.8: Injection hole diameter = 10 mm

Figure 9.10: Injection hole diameter = 30 mm

The depth of the injection hole should enable to reach all the major cracks. Since the positionof the major cracks are unknown, it is advisable to drill the holes deep enough. At least to halfof the walls thickness, but preferable to 3/4 of the thickness. The deeper the hole, the more riskfor leakages at the backside of the wall. Nevertheless, these leakages can be avoided bypreparing the back side as is done with the injection side.

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Chapter 9. Validating and using the model 171

Grout progress influence of the injection hole diameter

0,000

0,010

0,020

0,030

0,040

0,050

0,060

0,070

0,080

0,090

0 100 200 300 400 500

Time [sec]

Inje

cted

vol

ume

[m3 ]

10 mm15 mm20 mm30 mm

Figure 9.11: Evolution of the injected volume depending on theinjection hole diameter

Figure 9.12: Injection hole depth is 2/4 Figure 9.13: Injection hole depth is 3/4

It is obvious that a hole that reaches only half of the walls thickness, will complicate the fillingof the second half of the wall. Of course this will depend upon the situation such as thepresence of large cracks that extend to the injection hole. On the average the second half of thewall will be filled less completely. One could carry out a second injection from the secondside, but most of the time that side is not so easy to reach. Besides, the execution of a secondinjection campaign means almost doubling the cost and the work. The simulations, where themajor cracks are connected to the injection hole, indicate that, the second half of the wall isincompletely filled if the injection hole does not enters this zone. Similarly, the total injectedvolume is smaller. In case the major cracks do not reach the injection hole, the difference willeven be bigger.

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172 Grout injection of masonry, scientific approach and modeling

Grout progress influence of the injection hole depth

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0 100 200 300 400 500

Time [sec]

Inje

cted

vol

ume

[m3 ]

2/4 of thickness

3/4 of thickness

Figure 9.14: The evolution of the injected volume depends on the depth of the injectionhole

9.3.3. Radial network: sealing of a leakageA last demonstration of the use of the model is the sealing of a leakage. Sealing a leakage intime is important, not only to prevent the loss of too much grout, but also to enhance thepenetration in the neighborhood zone of the leakage. Since a leakage means that the pressure atthat place drops to zero, no pressure is built up in the neighborhood zone. This implies that thisneighborhood zone will not or only partly be injected. The grout penetrates only slowly throughthe channels. The absorption of water will stick the cement particles to the wall, reducing theflow diameter. If the sealing of the leakage takes place too late, the flow channels that are partlyfilled with grout will be so narrow as to prevent complete filling of that zone. Therefore, a fastsealing is important. The model can be used to prove the above intuitive reasoning. Fivesituations are compared: a leakage without sealing, a fast sealing after 30 seconds and a sealingafter 1 minute and after 2 minutes. Also the result in case no sealing is done is displayed. Theevolution of the injection of these five possible attitudes are represented in Figure 9.15. As canbe seen, the immediate sealing provides the best result. Sealing after a longer period of timecauses the loss of some grout and reduces the filled part of the masonry. The longer the timebefore sealing the leakage the worse the situation gets. In this case there is almost no differencebetween sealing after two minutes and no sealing at all. Table 9.7 provides other numerical data

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Chapter 9. Validating and using the model 173

Sealing of a leakage

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0 50 100 150 200 250 300 350 400

Time [sec]

Inje

cted

vo

lum

e [m

3 ]

no sealing

sealing after 2'

sealing after 1'

Sealing after 30 "

Immediate sealing

Figure 9.15: Effect of sealing of a leakage, influence of waiting time on the injected volume

Time before sealing injected volume lost grout[sec] [liter] [liter]0,0 52,6 0,030,0 51,1 3,160,0 49,8 5,6120,0 47,9 8,3

- 46,5 9,7

Table 9.7: Numerical results of the “sealing a leakage” simulations

about sealing of leakage simulations: the amount of grout that flew away through the leakage andthe injected volume.

9.4. Using the model as an engineering toolLet us assume that reliable information of the inner state of the masonry is available from nondestructive methods. Beside, the inner configuration of the main cracks in the masonry areunchangable data. Therefore, the have to be considered as the boundary conditions for theconsolidation. The more reliable these data, the better the model will correspond to reality.Other important input data are the grout properties. The first simulation can be done using thestandard grout with the standard rheological properties µ and t . The value of the absorptionparameter of the model depends on the water absorption capacity of the masonry and the water

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174 Grout injection of masonry, scientific approach and modeling

Figure 9.16: The model: tool for engineering a consolidation injection

retaining capacity and the stability of the grout. The granularity of the cement determines thesmallest channel that is still injectable.

Once the network is defined, engineering can start. Initial value for the diameter and the depthof the injection holes are defined, a certain injection pressure is used. The simulation willprovide the necessary information to decide about the possibilities of the combination ofmasonry properties, grout properties and process parameters. If the final result of the injectionis satisfying, the same combination can be used for the injection. If the results are not satisfying,three actions can be taken: using a different (more expensive) grout with better rheologicalproperties, lowering the water absorption of the masonry by a careful prewetting procedure,changing the process parameters: applying a denser injection hole pattern or a higher injectionpressure. The influence of these parameters on the injected volume are qualitatively evaluated

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Chapter 9. Validating and using the model 175

in this chapter. Similar evaluation can be done for the given project. The combination that turnsout to provide the best simulation results, can be applied. The above engineering approach is schematically presented in Figure 9.16

9.5. ConclusionsThe four validation exercises presented in this chapter, prove that the model correctly simulatesthe flow of both Newtonian and Bingham fluids through cylindrical flow channels. Theconfrontation with the experimental data indicates that, after calibration of the model, theexperiments can be simulated within an acceptable degree of accuracy. The model incorporates the gradual blocking mechanism caused by the narrowing of the flowchannel due to the sticking of the binding agent particles to the wall of the channel. The modelcan be helpful to decide about process parameters. The impact of the injection hole depth, theinjection hole diameter, the injection pressure can be analyzed qualitatively. If the necessaryinformation to define more accurately the network of flow channels and the capacitive elementsis available, the obtained results can replace test injections. The model becomes a powerfultool for the complete design of the grouting of the building: composition of the grout, injectionpressure and the injection hole configuration, that can vary locally, can be determined using themodel.

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Chapter 10. General conclusions and future research 177

Chapter 10. General conclusions and future research

10.1. General conclusionsThe safeguarding of historical monument is considered to be a duty of mankind. By far thebiggest part of human patrimony is built of masonry. Masonry structures suffer from a widerange of deterioration mechanisms. For structural and mechanical damage, grout injection ispart of the restoration. In the broad field of monument care, grout injection is situated as aconsolidation technique for masonry structures. In this thesis, an overview is given of possibledamaging phenomena. It is indicated in which case grout injection can repair the damage. Theinjection technique is portrayed by the description of an ideal injection installation. It issketched how grouting can enhance the reliability of the masonry and how should be dealt withgrout injection in a scientific way: from the diagnosis of the masonry over the implementation ofthe injection to the quality control after completion of the job is done. Although grouting hasbeen applied successfully for many times, problems still occur frequently because groutinjection is a complex and multi disciplinary task. The use of wrong materials often causesproblems some time after the restoration. Uniform filling of the voids is an important request,but very often this seems too hard to achieve. The experimental program discusses a series of tests, newly developed or adapted to thepeculiarities of the grouting problem, that can be used for a scientific approach of groutinjection, for the diagnosis of the masonry and for the composition of the most suitable grout.The major achievement of this research however, is the model that enables the simulation of thegrout penetration inside the masonry. This model is based on three pillars: the study ofliterature about grout injection of masonry, the theory about the rheology of dispersion and aboutflow of fluids through porous materials and finally the experimental program.Numerical methods are used for the mathematical description of the flow of Newtonian andBingham fluids inside channels. It is shown that the model parameters can be adjusted so that the simulations correspond to theexperiments in the reproducible masonry samples. Several examples are given to show thepossible applications of the model. For successful consolidation injections of masonry, a goodpenetration of the grout inside the masonry and a uniform filling of voids is essential. Inpractice, expensive test injections are done to see wether or not a specific grout could fulfill therequirements. The model can reduce the number of test injections and can show the influence ofa different composition of the grout, different rheological parameters, a higher injectionpressure, a more water absorbing masonry. This has economical impact on consolidationprojects and can improve the efficiency of these actions.

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The most important achievements can be summarized:C The importance of some technological options on the effectiveness of the consolidation

is given: the three way valve, the use of a separate mixing installation in addition to thecollector, the use of an admission system, practical to use.

C A new test method is developed for testing the stability of the grout. Compared to theclassical methods, the newly developed method provides more detailed informationabout the evolution of the stability of the grout in function of time. The test enables tojudge the effect of the composition, for instance the W/C ratio or the amount ofstabilizing admixtures on the stability. The mixing procedure can be adapted in order toobtain a stable grout. The stability test is relatively easy and economically to do andshould therefore be part of the scientific approach of a grouting project.

C The development of a reproducible masonry sample creates conditions that are nearer tothe reality of masonry than the sand column test. This way, the injectability test executedwith the reproducible masonry sample, provides information about the water retainingproperties of the grout.

C The experiments provided a better understanding of the penetration of the grout insidethe masonry. From the observations, we were able to conclude several blockingmechanisms.

C The model simulates qualitatively the penetration of the grout inside the masonry. Theinfluence of the grout properties and the injection pressure on the degree of filling andthe rate of filling is analyzed. General rules can be formulated from the parameter study:C Decreasing the critical shear stress of the grout is recommended instead of

increasing the injection pressureC Sealing a leakage has to be done as soon as possible. A good preparation of the

masonry reduces the risk for leakages.C The injection hole should be in contact with the major cracks.

10.2. Future Research 10.2.1. Information about the masonryAs is was indicated on several occasions in this book, the available non destructive testingmethods are not powerful enough to provide all the information that is required as input data forthe model. Information about the size and position of the major cracks would increase thereliability of the simulations. Geo-electrical resistivity measurements were successfully usedfor the diagnosis of masonry structures. The same goes for ultrasonic non destructivetesting. Nevertheless, these measuring techniques are not sufficiently accurate to provideinformation about the geometry of the major cracks and voids. Therefore, the further development and a further progress of these techniques or thedevelopment of other non destructive testing methods is a future field of research that couldenhance the possibilities of the model.

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Chapter 10. General conclusions and future research 179

Figure 10.1: On line control of the injection: a combination of non destructivemonitoring, simulation and adaptation of injection process parameters

10.2.2. Final goal, online controlled consolidation injectionAfter the injection is done, the quality of execution can be controlled using non destructivetechniques. If this quality control proves that the injection is poorly done, it can not be undone.Eventually a second injection campaign can be considered. It is obvious that it would beinteresting to be able to evaluate the degree of filling when the injection is going on. On linecontrol of the injection makes an immediate reaction possible. If, by any cause, a certain zone isnot injectable through any of the existing drilling holes, on line control enables to observe thisand to drill additional holes in this zone.By using the simulations of the model, the possible actions can be tried in order to find out aboutthe best adaptation of process parameters such as injection pressure, grout composition etc...

10.2.3. Using the model for other purposesThe concept of the model can be used for other fields of research. It is the authors’ convictionthat it is possible to incorporate capillary forces in the model. This way, the model wouldenable to simulate capillary water uptake or the progress of hydrofobic agents in the capillarypores of building materials. The capillary forces could be incorporated using the sametechnique as for incorporating gravity.A well known example of laminar flow through cylindrical tubes is the flow of blood throughthe blood vessels of human being. The contacts with medical researchers in this field, revealedthat there exists no model to simulate the bloodstream in the human body, for instance to quantifythe influence of a constriction of the blood vessels. It is worth to check how the model can beadapted to this problem.

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180 Grout injection of masonry, scientific approach and modeling

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