Published Project Report1).pdf · ground granulated blastfurnace slag (ggbs) which is sold into the readymix concrete industry as a cement replacement. However it is forecast that

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PUBLISHED PROJECT REPORT PPR012

DESIGN GUIDE FOR PAVEMENTS INCORPORATING SLAG BOUND MIXTURES (SBM) Final version

by M E Nunn and K E Hassan

Prepared for: Project Record: Viridis Research Project 32, Design Guide for

Pavements Incorporating Slag Bound Mixtures (SBM)

Client: Viridis (Mr R A Smith)

Copyright TRL Limited July 2004 This report has been prepared for Viridis is unpublished and should not be referred to in any other document or publication without the permission of Viridis. The views expressed are those of the authors and not necessarily those of Viridis.

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Quality Reviewed

This report has been produced by TRL Limited, under a Contract placed by Viridis. Any views expressed are not necessarily those of Viridis. TRL is committed to optimising energy efficiency, reducing waste and promoting recycling and re-use. In support of these environmental goals, this report has been printed on recycled paper, comprising 100% post-consumer waste, manufactured using a TCF (totally chlorine free) process.

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CONTENTS

Executive summary i

Summary i

1 Introduction 1

2 Properties and standards 2

2.1 General 2 2.2 Advantages of SBM 2 2.3 The European Standard 3

2.3.1 Classification by composition 3 2.3.2 Classification by mechanical properties 5

3 UK experience with SBM 7

3.1 Introduction 7 3.2 Cement bound materials 7 3.3 Research development on SBM 8

3.3.1 SWPE 8 3.3.2 TRL 9

3.4 Field trials 10 3.4.1 A11 Llanwern steelworks 10 3.4.2 A289 Wainscott bypass 11 3.4.3 A485 Carmarthen bypass 13

3.5 Mechanical properties of SBM 16 3.6 Summary of UK experience 16

4 European experience 18

4.1 Introduction 18 4.2 France 18

4.2.1 Historical Development 18 4.2.2 Sand slag 18 4.2.3 Material properties 19 4.2.4 Mix design 19 4.2.5 Quality control 19 4.2.6 Bond between layers 20 4.2.7 Performance of SBM 20 4.2.8 Storage and handling of the material 20 4.2.9 Similar international experience 21 4.2.10 Design criteria 21

4.3 The Netherlands 24 4.3.1 Background 24 4.3.2 Materials and Mixes 24 4.3.3 Quality control 25 4.3.4 Layer thickness and bond between layers 25 4.3.5 Reflective cracking 25 4.3.6 Storage and handling of the material 26 4.3.7 Dutch designs of SBM 26

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4.4 Spain 26 4.4.1 Mixture and properties 26 4.4.2 Application, performance and design 27

4.5 Comparison between UK and European experiences 27

5 Proposed design guidelines for SBM 30

5.1 Current UK design 30 5.2 Design criteria 31 5.3 Calibration of design criteria 31

5.3.1 Classification of hydraulically bound mixtures 31

6 Site compliance and construction guidelines 36

7 Conclusions 37

8 Acknowledgements 38

9 References 38

Appendix A. Static stiffness and direct tensile strength 40

A.1 Introduction 40 A.2 Relationship between material characteristics 40

A.2.1 Elastic modulus 40 A.2.2 Material strength 42

A.3 Design based on static modulus and tensile strength 42 A.3.1 Heavy traffic designs (> 80msa) 42 A.3.2 Traffic < 80msa 43

A.4 Summarising remarks 44 A.5 References 45

Appendix B. Symbols and abbreviations 46

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Published Project Report Final version

Executive summary Project Reference: Research Project 32, Design Guide for Pavements Incorporating SBM Project Sponsor: Mr R A Smith, Viridis. Project Manager: Khaled Hassan, Sustainability Group, Infrastructure Division, TRL Ltd.

Summary This study to develop pavement design and material specification guidance for slag bound ‘base’ mixture was carried out by TRL Ltd with contributions from Tarmac Limited ( www.tarmac.co.uk ). The work was funded by Viridis through the Landfill Tax Credit Scheme.

The use of industrial by-products in the base layer of the pavement is expected to have the two-fold benefit of reducing demand on primary sources of aggregates and binders, and reducing the amounts of by-products requiring disposal, often in unsightly tips. At present approximately three million tonnes of blast furnace slag is produced at Port Talbot, Scunthorpe and Teeside each year. Over two million tonnes per annum is granulated blastfurnace slag (gbs) which is then ground to produce ground granulated blastfurnace slag (ggbs) which is sold into the readymix concrete industry as a cement replacement. However it is forecast that there will be sufficient gbs available in future to supply an annual demand for 0.5 to 1.0 million tonnes of slag bound mixtures for the main structural layer of a pavement.

Blastfurnace slag can be processed and used as a hydraulic binder in a manner similar to that of cement. The processed materials can be granulated (or pelletised) blastfurnace slag (gbs), partially ground (pggbs) or fully ground (ggbs). It can then be mixed with aggregate and the presence of water causes hydraulic or carbonic reactions that binds the aggregate together into a slag bound mixture (SBM). The hardening reaction of granulated blast furnace slag can be accelerated by addition of an activator.

SBM has been used by several European countries for many years and is incorporated into their material specifications and pavement design procedures. France has the greatest experience with SBM and there it is known as “grave laitier”. In the UK several trials of SBM pavements have been constructed but detailed guidance on the specification of SBM, compliance in the main works and on pavement design does not exist. The overall objective of this work was to review European practice and UK experience and use this information to develop pavement design guidance. This involved:

� Reviewing information from past UK studies together with information from overseas.

� Organising study visits to key European countries to identify best practices that could be introduced into the UK.

� Analyse the information and develop design guidance and material specifications for SBM.

The development of a more versatile pavement design is based on the approach that the performance of a hydraulically bound base is dependent on a combination of the long term values of its dynamic stiffness modulus and flexural strength that are achieved after 360 days. Furthermore, there is no unique combination of these properties. A stiffer material generates higher traffic induced tensile stresses and therefore it will require a higher flexural strength to resist cracking. The division of materials properties into zones is a practical way forward, in which material in each zone will require the same pavement designs.

This work has shown that SBM can have a range of properties and that it is possible to produce material that is superior to that has been produced in previous UK trials. The zoned approach, described in Section 5 of this report, which gives the producer the incentive to improve his material. With current UK practice it is feasible to produce an SBM that is equivalent to CBM3G. However

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higher grades are possible using finer ground gbs and special activators. Generally, improving by a zone results in a reduction of base thickness of between 25 and 50 mm.

Material that falls into the lower zones could be used as subbase to improve the foundation class. The evidence collected also showed that the risk of reflection cracking using SBM base was much less than with CBM base. Therefore, there may be potential to reduce the thickness of the asphalt cover to 150 mm. The design curves can be adjusted using a safety factor (KSafety) if there are concerns about the long term viability of the material. In view of French restrictions for using SBM with lime activator in very heavily trafficked roads, the designs could be limited to cumulative traffic of up to 40 msa until more confidence is acquired in its performance. The design curves for SBM pavements for different foundation classes are given in Section 5 of this report.

The material specification and site compliance procedures to be applied will be identical to those recently published as part of the SMART (Sustainable MAintenance for cold Recycling Treatments) project for slow curing hydraulic (SH) materials.

A further important aspect that has to be considered is that the UK has traditionally specified cement bound materials in terms of compressive strength although the dynamic stiffness modulus and flexural strength are used for the purpose of pavement design. The dynamic modulus is measured at a loading frequency of several kHz and at low stress amplitude. This differs from the conditions induced by a rolling wheel load, which has an effective loading frequency of a few Hz and generally induces a much higher stress. However, the new European Standard (BS EN 14227-2, 2004) uses the static elastic modulus and direct tensile strength to characterise hydraulically bound mixtures and this will eventually be adopted in the UK. Appendix A examines the implications of harmonising with the material characteristics classification approach in the new European Standard.


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1 Introduction The use of industrial by-products in the base layer of a pavement is expected to have the two-fold benefit of reducing demand on primary sources of aggregates and binders, and reducing the amounts of by-products requiring disposal, often in unsightly tips. At present about four million tonnes of blast furnace slag is produced at Port Talbot, Scunthorpe and Teeside each year. Approximately three million tonnes per annum is granulated blastfurnace slag (gbs) then ground to produce ground granulated blastfurnace slag (ggbs) which is sold into the readymix concrete industry as a cement replacement. However it is forecast that there will be sufficient gbs available to supply an annual demand for 0.5 to 1.0 million tonnes of slag bound mixtures for the main structural layer of a pavement.

Blastfurnace slag can be processed and used as a hydraulic binder in a manner similar to that of cement. The processed materials can be granulated (or pelletised) blastfurnace slag (gbs), partially ground (pggbs) or fully ground (ggbs). It can then be mixed with aggregate and the presence of water causes hydraulic or carbonic reactions that binds the aggregate together into a slag bound mixture (SBM). The hardening reaction of granulated blast furnace slag can be accelerated by addition of an activator.

Unlike in the UK, SBM has been used in several European countries for many years and is incorporated into their materials specification and pavement design procedures. France has the greatest experience with SBM and there it is known as “grave laitier”. Grave-laitier is the most widely used hydraulically bound base material in French roads (Sherwood, 2001). SBM is also widely used in the Netherlands, Spain, Italy and Eastern Europe. Although the material forms a rigid pavement layer, its slow curing nature is claimed to result in a number of reported benefits over traditional cement bound materials and their usual method of construction. These include early trafficking; several days permissible between mixing and laying; and reduced susceptibility to reflection cracking.

Several trials of SBM pavements have been constructed in the UK but detailed guidance on the specification of SBM, compliance in the main works and on pavement design are limited to individual experiences. SBM is mentioned in Clause 809 of the Specification for Highway Works (MCHW1) (Highways Agency et al, January 2004.) but this makes reference to the Overseeing Organisation for design guidance. The Design Manual for Roads and Bridges (DMRB) makes general reference to hydraulic and pozzolanic binders without giving specific advice.

The aims of this project were to:

1. Review information from past UK studies together with information from overseas. This will cover materials properties, performance, design, specification and economic and environmental issues. This information will help to identify any gaps and weaknesses in our knowledge and the countries that have the greatest experience of using these materials.

2. Organise study visits to European countries with a view to identifying best practices that can be introduced into the UK.

3. Analyse the information collected in stages 1 and 2 and develop a design guide and material specifications for SBM. The benefits accruing from the use of SBM and any problem areas associated with its use will be identified.

The properties and standards related to slag bound mixtures are covered in Section 2. Sections 3 and 4 review the UK and International experiences with slag bound mixtures. Based on the information reviewed, proposed design guidelines for slag bound mixtures are given in Section 5. The implications of harmonising with the material characteristics classification approach in the proposed new European Standard are given in Appendix A.



2 Properties and standards

2.1 General

Blastfurnace slag is a co-product from the production of pig-iron within a blastfurnace. The slag consists of mineral components of the ore, which react with calcium that is added to the blastfurnace, usually in the form of limestone. The reactions take place at temperatures between 1300 and 1600 °C, produced by the burning of coke, which is fed into the furnace with the limestone and ore. The molten blastfurnace slag floats on top of the molten iron, and is removed. The physical appearance and chemical structure depends on the method of cooling the molten slag.

Air-cooling of blastfurnace slag (abs) produces a rock-like material with a crystalline structure, which can be used as a substitute for natural aggregate. Quenching the slag in water produces a granulated and glassy material of gbs. The quenching is conducted by the slag being poured into a ceramic chamber where the flow is then met by a jet of water. The proportion of water to slag, and hence the cooling rate, can change the nature of the slag. Pellitised blastfurnace slag is produced in a similar way. The gbs produced, comprises predominantly coarse sand sized particles. The gbs contains the same principal oxides as found in Portland cement; CaO, SiO2 and Al2O3, which results in gbs acting as a hydraulic binder. The proportions of the constituents vary from cement, giving gbs much slower curing properties, the use of lime or other similar activator initiates the reaction, and helps to increase its speed. The material can be ground to ggbs and it is then used as a replacement for cement, producing cements and concretes with different properties to that made of conventional Portland cement.

2.2 Advantages of SBM

SBMs are composed of aggregate, granulated slag (binder), an activator, and water. They are used to replace cement bound materials (CBM) in pavement construction. Whilst SBM possesses lower strength characteristics than CBM, mainly at early ages, it is cheaper and has considerable advantages over CBM. These advantages are summarised below (OECD, 1977).

� SBM contains a higher binder content than CBM, which facilitates the more homogeneous distribution of the binder within the material. Part of the slag remains available, enabling a renewed setting (self-healing) should cracking occur.

� The setting time of SBM takes a relatively longer time, allowing several days of storage without difficulty. It also allows flexible organisation of the roadworks, with better operating of individual machines and maximum output.

� The material can be trafficked with roadworks equipment immediately after laying. Secondary compaction under traffic improves the material.

� In heavy rain, excess water is simply allowed to drain off before compaction. If necessary, the material can be removed, allowed to dry out, then relaid and recompacted.

� The setting of SBM is halted under frost, but setting will continue once normal temperatures are reached.

� The strength takes a long time to develop (360 days or longer) and is not affected by the delay in initial setting.

� The low rate of setting allows the stiffness of SBM to increase progressively with the consolidation of the sub-grade and increasing traffic.



In addition to the above advantages, SBM contributes to the reduction in energy consumption associated with replacing cement or bitumen and therefore supports the UK government policy on sustainable construction and energy supply (DETR, 2000; DTI, 2003). When compared to CBM, the use of SBM reduced the emissions of carbon dioxide associated with cement manufacture. It was estimated that with every tonne of Portland cement produced, one tonne of carbon dioxide is released to the environment (Mehta, 1998). SBM is produced without the use of bitumen or cement, which both require a large energy input in manufacture, and can be produced without heating, which is required for conventional bituminous materials. Therefore, it can be produced with a relatively low energy input. An energy audit for the production of SBM has been undertaken by TRL (Parry, 1995). The investigation revealed that energy savings of the order of 10-15% were feasible when compared with asphalt bases.

2.3 The European Standard

A harmonised European standard has been prepared to be formally approved by the Working Group 4 of the Committee European Normalisation, Technical Committee CEN/TC 227. This standard, proposed as BS EN 14227-2 (2004) for slag bound mixtures provides definitions, composition and classifications of the material. This system of classification is derived from various European countries where the material has been in use for many years.

Definitions are contained in this Standard and the definitions relevant to this project are summarised as follows:

2.3.1 Classification by composition SBM is a mixture of aggregate and one or more types of slag (e.g., air-cooled blastfurnace slag (abs), air-cooled steel slag (ass), granulated (or pelletised) blastfurnace slag (gbs), partially ground granulated blastfurnace slag (pggbs), ground granulated blastfurnace slag (ggbs)) and water that hardens by hydraulic or carbonic reactions. Granulated blastfurnace slag (gbs) is a hydraulic binder, i.e. hardening is due to the reaction with water, and hardening can be accelerated by addition of an activator.

2.3.1.1 SBM, Types A and B

SBM contains one or more of the above slags and water, it may contain other constituents, and is mainly characterised by slow hardening. The mixture behaves as an unbound material in the short term and as a bound material in the medium to long term. Based on the stiffness development, there are two types of SBM. Type A is slow setting when significant stiffness is not required and it is normally characterised by a CBR test. Type B, however, attains significant stiffness in the medium to long term and its laboratory performance is normally characterised by tensile or compressive strength. Type A materials would normally be used in the foundation layers in the UK. SBM Type B is suitable for the main structural layers of the road and this type of material is the concern of this report.

2.3.1.2 Constituents of gbs

The following constituents are the average percentage of total dry material of gbs:

� SiO2: 27 to 41 (m/m%) � Al2O3: 7 to 20% � CaO: 30 to 50% � MgO: <20%



2.3.1.3 C.A product

The hydraulic activity of gbs, pggbs and ggbs is a function of their chemical composition, fines content and the activators used. In terms of chemical composition the important factor is the C.A product; where C is the CaO content and A is the Al2O3 content. The higher the C.A product, the more reactive the slag. There are three categories for this product:

� Category CA1 > 550 � Category CA2 425 to 550 � Category CA3 < 425

2.3.1.4 Fineness

The proportion of fines in a compacted SBM is a function of the friability of the gbs. The softer the slag, the more the production of fines under the roller, and the more reactive the slag. The friability is determined by the alpha coefficient, and the lower the alpha coefficient the less friable the material. There are four categories of friability (Sherwood, 2001):

� Category α1 <20 not used in road construction � Category α2 20 to 40 the most frequently used � Category α3 40 to 60 reserved for difficult to handle materials � Category α4 >60 used only exceptionally

For pggbs, there are four categories of fines (> 0.063 mm):

� Category PG1 1 to <5% � Category PG2 ≥5 to <8% � Category PG3 ≥8 to <14% � Category PG 4≥14%

For ggbs, there are four categories of Blaine specific surface area of the fines in the slag:

� Category GG1 150 m2/kg � Category GG2 150 to <300 m2/kg � Category GG3 300 to <400 m2/kg � Category GG4 400 m2/kg

The α coefficient describes the slag particles smaller than 0.08mm and the friability, where the friability is the percentage of elements smaller than 0.08mm and obtained after grinding according to BS EN 13286-44 (2003). The α coefficient is an empirical value that measures physical properties of the slag and characterises its reactivity, and is given by the following equation:

α = S × P × 10-3 (2.1)

In this equation, S is the Blaine specific surface area, in cm2/g, of the fines in the slag (particles less than 80 microns in size). P is given by an empirical test that measures the friability of the granulate and therefore the ease with which it is ground into fines. P therefore evaluates the potential reactivity realised by the grinding of the gbs that occurs during the mixing and compaction of the SBM. The test is conducted within a standard ball mill, the fines content of the gbs being measured after a standard length of time in the mill. If the α value of the gbs is too low, it can be ground before mixing, although this extra process will of course increase the cost.



2.3.1.5 Activator

Activators increase the rate of hardening of the SBM and can be an alkaline and/or sulfatic material. These include hydrated lime, calcareous fly ash, cement, mixtures of gypsum and lime, sodium and potassium salts and similar products. Some steel slags have activation properties, in particular Basic Oxygen Steel Slag (BOS).

2.3.2 Classification by mechanical properties There are three methods of classifying SBM based on California bearing ratio (CBR), compressive strength (Rc) and combination of tensile strength (Rt) and modulus of elasticity (E). The BS EN 14227-2 (2004) states that no correlation is intended nor shall be assumed between the three methods of characterisation.

2.3.2.1 CBR

This method of classification is applicable to SBM Type A with low stiffness values. It is based on the CBR values obtained immediately on construction and after 28 days or 91 days.

2.3.2.2 Rc, Rt and E

SBM Type B is classified by Rc or by Rt and E. These properties are measured using laboratory prepared specimens. Thirteen classes are defined for compressive strength as follows C0,4/0,5, C0,8/1, C1,5/2, etc through to C27/36 and including a possibility of a declared value CDV. Where the first number applies to the compressive strength (MPa) of cylinders with a slenderness ratio of 2, and the second number applies to cylinders with a slenderness ratio of 1 or cubes. The age of classification and curing conditions shall be specified in accordance with practice at the place of use. The 360 day strength may be predicted from earlier age results (7, 28 or 91 days) at normal curing temperature of 20°C or at elevated temperature of 40°C. Rc is the mean of three tests, with no result varying by more than 20% from the mean.

The classification by tensile strength and elastic modulus (Rt and E) is made according to five classes. In BS EN 14227-2 (2004), the indirect tensile strength Rit is used to determine Rit, using the relationship Rt = 0.80Rit. Similar to Rc, the age of classification and curing conditions shall be specified in accordance with practice at the place of use. Also the variation of results should not exceed 20%.

The design of pavements incorporating SBM is mainly based on the 360-day mechanical performance of the product, which is in turn influenced by the composition and properties of the constituents. Richardson and Haynes (2001) illustrated that a higher content of granulated slag and a higher alpha coefficient increase the strength of SBM. Typical mechanical properties of SBM in the hardened state, at 360 days, have been reported within the following range:

� Compressive strength of 6-10MPa

� Tensile strength of 0.6-1.5MPa

� Elastic modulus of 15-20GPa

A research programme has been carried out on the mechanical properties of cementitious bound materials towards improving the precision of the French standards in road pavements (Voirin et al., 2003). The programme covered a wide range of materials, cement, slag, fly ash, lime and gypsum as binders with siliceous or calcareous aggregate, obtained from the north-eastern of France. The results of 156 cementitious mixtures and more than 2000 tests have indicated that the development of mechanical properties is not only dependent on the binder



type, but also the type of aggregate. Crushed calcareous aggregate resulted in high rate of strength development by achieving approximately 75% of the long term strength after 28 or 60 days, depending on the binder type. This value is higher than the figures quoted in the French standard of 60 or 65%. They attributed the improved performance to the calcareous fines acting as sites of nucleation, thereby increasing the amount of precipitated hydrates and accelerating hydration. Furthermore the portlandite, released during hydration, combines with the aluminate phases and with carbonates to form carboaluminates, which have a certain binder effect. To this extent alumina rich binders, such as slag, are particularly appropriate for calcareous mixtures in providing the most rapid increase in strength.



3 UK experience with SBM

3.1 Introduction

Currently there is no provision in the Specification for Highway Works on the use of hydraulic bound mixtures for base layers. SBM is however included in Clause 809 of the Specification for Highway Works for use in subbase applications (Highways Agency et al.). The use of SBM within the structural pavement layers is limited to individual experiences. The material properties and performance is commonly compared to cement bound materials (CBM). This section reviews the different types and properties of CBM, the development of research in SBM and field trials. Results from laboratory and field trials were used to define typical mechanical properties of SBM for design purposes.

3.2 Cement bound materials

There are seven categories of cement bound materials (CBM1 to CBM5, CBM1A and CBM2A) characterised by aggregate grading and 7-day compressive strength, Table 3.1 (MCHW1, Clauses 1036-1038). The minimum average compressive strength increases from 4.5MPa for CBM1 to 20MPa for CBM5 with the CBM1A and CBM2A having a compressive strength similar to a CBM3. Only CBM3, CBM4 and CBM5 are permitted for use in the construction of the base layer.

Table 3.1. Requirements for cement-bound subbase and base materials

CBM1 CBM1A CBM2 CBM2A CBM3 CBM4 CMB5 Grading (% mass passing) 63 mm 31.5 mm 20 mm 10 mm 4 mm 2 mm 0.500 mm 0.250 mm 0.125 mm

100 85 45 35 20 NR 6 4 0

100

85-100 45-100 35-100 20-95 12-90 7-55 5-35 0-9

D40 100

85-95 45-80

NR 20-50

NR 6-25 0-7* 0-5

D20 - 100 95-100 NR 30-55 NR 9-30 0-7* 0-5

Minimum 7-day compressive strength (MPa) Average (Note 1) Individual (Note 2)

4.5 2.5

10.0 6.5

7.0 4.5

10.0 6.5

10.0 6.5

15.0 10.0

20.0 13.0

Minimum compressive strength after 7 days air-cured + 7 days water-immersion (% average 14 days air-cured)

80 80 NR NR NR

Minimum soaked 10% fines value (kN)

NR 50 NR NR NR

NR- No requirement * 0-10 for crushed rock fines Note 1 Average strength of five cubes Note 2 Strength of any individual cube

It is important to note that the European standard BS EN 14227-1 (2004) provides eight classes of cement bound granular mixtures (CBGM) based on characteristic compressive strength at 28 days. The strength varies from class C0, C1,5/2, etc through to C20/25, where the first number applies to the compressive strength (MPa) of cylinders with a slenderness ratio of 2, and the second number applies to cylinders with a slenderness ratio of 1 or cubes.



3.3 Research development on SBM

3.3.1 SWPE

In 1992 the Highways Agency placed a research contract with Scott Wilson Pavement Engineering (SWPE) to examine all aspects of industrial by-products for possible use as a bound base layer in the road. This included evaluation of the structural properties and workability of selected by-product materials by laboratory and pilot-scale testing. This would relate material performance to the design criteria and propose and plan full-scale road trials for materials shown to be the most promising. The SWPE study initially considered nine materials and five mixtures were selected for final evaluation in full-scale road trials.

SBM was selected as one of the trial materials in the preliminary work (Elliott et al., 1994). The work was conducted using a limestone aggregate, meeting the mid point of the 20mm HDM grading, with 14% gbs, obtained from Scunthorpe steel works, 1% lime as activator and 6% water content. The mixture was tested in the Pavement Test Facility (PTF) in combination with a CBM3 control. This facility comprises a pit 5m long, 2.4m wide and 1.5m deep in which trial pavements can be constructed. The pavements can be loaded with a moving wheel of up to 15kN load travelling back and forth across the pavement at up to 8km/hr. The trial pavements comprised compacted Keuper Marl as the subgrade, Type 1 sub-base with the trial mixtures placed and compacted to the pre-determined depths as the surface exposed material.

The base layer was subjected to wheel loads of 3 and then 9 kN with a slowly increasing number of passes, due to its low early strength. At 60 days the wheel loading was increased to 12kN and loading was conducted every working day until the completion of 100,000 passes. The surface profile, subgrade stresses, and horizontal strains at the sub-base/base interface were all measured regularly. The base was subject to testing with the Falling Weight Deflectometer (FWD) after 50,000 and 100,000 passes. Companion cubes and beams were made at the time of placing a mixture in the trial, in order to determine density and strength development with time.

The PTF results for the SBM were found to be encouraging. Early rutting was noted, during the loading at 3 and 9 kN. However, after the increase to 12 kN at 60 days no further deformation was observed. This was concluded to be the effect of the material curing. The subgrade stresses recorded were also found to have dropped over the early life, indicating an increase in stiffness of the base material.

The results of the companion cube and beams specimens for the SBM and the control CBM3 are given in Tables 3.2 and 3.3. Table 3.2 gives the average density and compressive strength values up to 1 year, whereas Table 3.3 presents the mechanical properties of tensile strength, flexural strength and stiffness measured at 180 days. The results in Table 3.2 indicate the low strength development of SBM at early ages compared to CBM3. At 7 days, the compressive strength of SBM was 2.8MPa, about 30% of the CBM3. SBM however exhibited higher rate of strength development to achieve approximately 50% of the CBM3 strength at 28 and 60 days. The 360 day compressive strength for the SBM was not reported. The density of the SBM ranged from 2400 to 2460 kg/m3, compared to 2440 to 2450 kg/m3 for the CBM3, with the exception of CBM3 at 360 days.

The equivalent compressive strength results in Table 3.3 fall within the trend of strength development given in Table 3.2, with the SBM exhibiting 50% of the strength of the CBM3 control. However, lower values were obtained for the tensile and flexural strengths, less than one third of CBM3. In fact the SBM mixture was made with limestone aggregate, which is known to enhance the tensile and flexural properties of cementitious materials, but the tensile strength was only 0.33MPa at 180 days. This value is much below the typical range of tensile strength for hardened SBM, 0.6-1.5MPa, reported by Richardson and Haynes (2001).



Table 3.2. SWPE density and strength results of SBM and CBM3 (Elliott et al., 1994)

Mix Age (days) 7 14 28 60 360

SBM Cube compressive strength (MPa)

Density (kg/m3)

2.8

2405

4.3

2440

5.8

2425

6.8

2460

-

CBM3 Cube compressive strength (MPa)

Density (kg/m3)

9.5

2445

10.8

2450

12.5

2450

13.8

2440

21.8

2350

Table 3.3. Mechanical properties of SBM and CBM3 at 180 days (Elliott et al., 1994)

Mix Equivalent cube strength (MPa)

Flexural strength (MPa)

Tensile splitting (MPa)

Stiffness (GPa)

SBM* 7.3 0.6 0.33 7.9

CBM3** 14.5 2.0 1.43 10.9

* Beams were used for testing ** Beams and cores were used for testing

3.3.2 TRL Following the initial investigations by SWPE, the Highways Agency placed a contract to further evaluate the selected materials in full-scale road trials with TRL in 1995. Trials were carried out on two sites A289, Wainscott by-pass in Kent; and A485 link road on the A40 Eastern Carmarthen by-pass, Carmarthenshire. The project investigated both the buildability and long term performance of SBM as a base in a live road situation. Before carrying out a full-scale trial of SBM, it was deemed prudent to carry out a detailed mix design on the slag materials. This was mainly due to uncertainties about some of the laboratory test results and sources of materials used for the work carried out by SWPE.

SWPE’s original research used limestone as the aggregate source to be bound with gbs and hydrated lime. However, for the Carmarthen trial site it was agreed to use abs as aggregate. Tarmac supplied materials for the full-scale road trial and provided details of research work they had carried out on the use of SBM, based on grave-laitier mixture and French mixture design. The constituents were an abs aggregate bound with gbs, gypsum and hydrated lime. Due to the volumetric problems that gypsum can cause, it was decided that gypsum should not be used in conjunction with the SBM being investigated. Therefore it was important to conduct a detailed material test programme to establish the characteristics of the mixture and the design mixture proportions to be used.

The properties of SBM mixtures, made with varying contents of gbs binder and lime activator, were investigated. Analysis of the TRL mixture design testing indicated that varying the proportions of gbs between 15 and 25 % and the proportions of lime between 1 and 2 % had no significant effect on the measured parameters (Milton et al., 1997). Table 3.4 presents the mean results of the six mixtures tested, with each value representing the average of 12-18 tested specimens.

The results in Table 3.4 indicate the superior mechanical properties of the amended TRL trial mixtures to that used by SWPE. At 180 days, the TRL compressive strength was about 20% higher; the flexural strength was 35% higher, with almost twice the value of tensile strength to those reported in Table 3.2.

The mechanical properties of the SBM at 360 days, the age at which the material is characterised for design in France, are 10.8MPa, 0.81MPa and 17GPa for the compressive strength, indirect tensile and the dynamic modulus of elasticity (dynamic stiffness),



respectively. These values fall within the range of typical mechanical properties of SBM discussed earlier.

Table 3.4. Summary of mean SBM laboratory results (Milton et al., 1997).

Age (Days ±±±±1) 32 90 180 360

Bulk Density (kg/m3) 2124 2159 2094 2094

Dry Density (kg/m3) 2094 2128 2073 2079

Dynamic modulus of elasticity (GPa) 17.0 18.9 17.2 16.9

Flexural strength (MPa) 0.61 0.73 0.81 0.80

Indirect tensile strength (MPa) 0.42 0.56 0.66 0.81

Compressive strength (MPa) 6.4 8.1 8.7 10.8

Static modulus of elasticity (GPa) 2.8 5.0 6.4 7.4

The 360-day compressive strength, 10.8MPa, is greater than the average 7-day cube strength required for CBM3, indicating that the 7-day compressive strength for CBM3 could be achieved in 1 year by the SBM. It should be noted that the strength gain is not linear over that period of time. At 32 days, over half the 360-day compressive strength was gained. The compressive strength of the SBM at 90 days was 75% of the 360 day strength. This approximately equates to French experience, where the 90-day tensile strength is estimated to be 70% of the 360-day strength (LCPC/SETRA, 1998). The dynamic modulus of elasticity (stiffness) values, which have been traditionally used in the UK for design purposes, ranged between 17 and 19GPa with little variations within the 360 days testing programme. In contrast, the static modulus of elasticity (static stiffness) gave lower values but increased with time as the material cured.

The results in Table 3.4 also indicate that the ratio of flexural/compressive strength for SBM at 360 days is 0.074, which is lower than that quoted for the design of CBM in LR1132 for flexible composite pavements (Powell et al., 1984). The high flexural/compressive strength ratio is mainly dependent on the types and properties of binders and aggregates, and the interface between them. Finer gbs binder would improve the bond with aggregates and consequently increases the flexural and tensile properties of SBM.

3.4 Field trials

A number of SBM field trials have been constructed in the UK. These trials helped to confirm the benefits of SBM over traditional CBM, due to its slow curing and lower rate of strength and stiffness developments with reduced risk of reflection cracks. Also the ability to remain workable for several days after mixing; it can be stockpiled and it can be removed and re-laid, if necessary, within a few days of mixing. The field trials covered in the report are Llanwern steelworks constructed in 1995, Wainscott bypass in 1998 and Carmarthen in 1999.

3.4.1 A11 Llanwern steelworks

Tarmac Quarry Products Limited constructed a pilot-scale trial in September 1995 of SBM base and sub-base at Llanwern steelworks, south Glamorgan. The road carries heavy quarry trucks, up to 70 tonnes, and was constructed of four sections as shown in Table 3.5. The SBM layers, sections S2, S3 and S4, were overlaid with a 15mm ethylene vinyl acetate (EVA) modified sand carpet, as a regulator layer to minimise reflection cracking (Carswell and Megan 1997).



Table 3.5. Construction of Llanwern steelworks (Carswell and Megan, 1997)

Layer/section S1 S2 S3 S4 Surface course 30mm DBM 30mm DBM 30mm DBM 30mm DBM Binder course 60mm DBM 60mm DBM 60mm DBM 60mm DBM Base 180mm HDM 180mm SBM

(limestone agg.) 180mm SBM

(abs agg.) Sub-base 165mm Type1 150mm Type1 150mm Type1

330mm SBM (abs) directly on

the sub-grade

The SBM consisted of 20% gbs binder, 1.5% activator and 78.5% aggregate. The activator used was in the weight ratio of 80% lime and 20% gypsum. The aggregate was of 28mm maximum size of either limestone or abs. The water content was 6% for the limestone and 7% for the abs mixtures.

SBM testing undertaken by Tarmac Quarry Products on various trial mixtures gave a 90-day equivalent compressive strength of 7.3MPa for the abs slag mixtures and 7.1MPa for the limestone slag mixtures.

The stiffness of the SBM test sections, as determined by the FWD technique, indicated a steady increase up to 7 months in service followed by a rapid increase up to 15 months. The FWD deflections on the SBM at 15 months were comparable to the asphalt control section. However, conventional back analysis techniques to calculate individual layer stiffness from FWD measurements were unreliable due to the strong sub-grade of abs used. Also, no differences in performance were observed between the two aggregate types used with SBM (Carswell and Megan, 1997).

3.4.2 A289 Wainscott bypass The Wainscott bypass was a Kent County Council scheme that utilised a Dutch slag base, containing a granulated blastfurnace slag binder (10%), activated with steel slag (2.5%), and phosphoric slag aggregate (87.5%). As the steel slag used had a high lime content, no extra lime was added to the mixture. The slow curing nature of the mixture allowed several days between mixing the raw materials in the Netherlands, importing to site and laying. The SBM was constructed in two layers, and the second layer was generally laid one day after the first layer. The road is a dual, two-lane construction and was designed for 45 million standard axles (msa) and was completed in August 1998. Table 3.6 gives the nominal construction used throughout the scheme on the main carriageways.

Table 3.6. Construction of A289 Wainscott bypass

Layer Thickness Material Surface course 30 mm Porous asphalt Binder course/upper base 160 mm DBM50 Lower base/ subbase 295 mm gbs bound phosphoric slag Subgrade 350 mm Cement stabilised clay or untreated chalk

The SBM was easily laid, in a similar manner to CBM, with no extra effort or precautions. Satisfactory compaction and a good surface finish was readily achieved using a Bomag BW-161-AD dual drum vibratory roller. The roller has a static linear pressure of 2710 kg/m, and was used with vibration. The construction was controlled by Nuclear Density Gauge (NDG) measurements and monitoring of moisture content.

It was noticed that the first layer of SBM showed no signs of distress when trafficked by the lorries delivering the SBM for the second layer. The SBM was overlaid by the bituminous



surfacing several weeks later, in some areas months later, which allowed FWD measurements to be made directly on the SBM base at different ages. It was also noted that the surface of the cured material was friable and the fine material in the top few millimetres could be brushed loose by hand. However, the FWD results indicated that this was only a surface defect and good stiffness results were obtained for the cured material.

A series of laboratory and site testing was carried out to assess the strength development in SBM and presented for the whole scheme by Walsh (1999). Accelerated curing at 40°C was used to provide earlier indications of the material strength. Cylinders cured at 40°C were quoted to give an average compressive strength of 13.2MPa at 56 days, equivalent to a cube strength of 12.4MPa, and that 99% of the strength was achieved in 28 days. This indicates that at this high curing temperature of 40°C little change takes place in the strength development after 28 days. It was also noted that there was significant difference in strength between the initial laboratory and routine site specimens. Walsh (1999) attributed this to the large difference in density to which the materials were compacted. During the scheme samples were compacted for 1 minute per layer, with a vibrating hammer, while the initial laboratory tests samples were compacted for 2 minutes per layer. The difference between the results was approximately a factor of 2, indicating the sensitivity of the material to the density obtained.

The density in the field was specified at 95% of the site cube density, a value of 2205 kg/m3. NDG measurements indicated that none of the tested locations were below the specified value. However, the average moisture content was 6.4%, on the high side of the specified range (4.5 to 6.5%). According to Walsh (1999), the high moisture contents did not lead to significant loss of strength due to the improved compaction of the relatively free draining material.

The FWD measurements were made on top of the wearing course and the average stiffness values are presented in Table 3.7. The first two surveys in Table 3.7 were conducted before the road had been opened. The bound layer is the combined thickness of all the asphalt layers plus the SBM layers. The results are not temperature corrected due to the presence of the SBM layer.

Table 3.7. Average FWD stiffness results, Wainscott bypass

Age 1.5 Months

(30°C)

6 Months

(6°C)

15 Months

(25°C)

27 Months

(21°C)

Bound Layer stiffness (MPa) 2100 10000 9000 11400

Foundation stiffness (MPa) 224 286 263 276

A substantial increase in the stiffness of the SBM could be observed between the measurement taken at 1.5 and 6 months, as the bound layer stiffness increased from 2.1 to 10GPa. There was little difference between the 6-month and 15-month surveys, and the 27-month results were slightly higher than the 15-month results. The foundation results showed a slightly lower stiffness at 1.5 month followed by almost steady values, as the sub-base and sub-grade reach equilibrium following construction.

At 27 months, attempts were made to extract cores from the pavement. It was found that the material broke up under the action of the coring rig, possibly made worse where layers de-bonded and ground against each other. The cores were located in the edge strip and so the weakness was not due to traffic (Megan, 2001). This was surprising because of the relatively high stiffness obtained from the FWD results. However, successful attempts to obtain cores were made at about 42 and 55 months using air flush coring, instead of water-cooling, and the results of tested cores are given in Table 3.8.



Table 3.8. Range and average* SBM core results, Wainscott bypass

Age Density (kg/m3)

Estimated cube strength (MPa)

Tensile split (MPa)

42 months Lower layer 2240 – 2360

(2280)

5.8 – 7.6

(6.4)

-

Upper layer 2240 – 2350

(2310)

- 0.84 – 0.85

(0.84) 55 months

Lower layer 2270 – 2360

(2310)

5.0 – 7.4

(6.1)

0.45 – 0.69

(0.60)

*Average values in given between brackets.

The density ranged between 2240 and 2330 kg/m3 with little variations between the determined values at the two tested ages. At 55 months the density of the upper SBM layer was similar to that of the lower layer. The compressive strength ranged between 5.0 and 7.4MPa with a slightly higher average value at 42 months. The average compressive strength values of 6.4 and 6.1MPa were about 50% of the value reported earlier for the accelerated cured laboratory specimens at 56 days. These average values are also lower than that obtained from the laboratory mixtures, Table 3.4, indicating the effect of gbs binder content and activator type on the strength gained. For the indirect tensile strength, the average value for the upper SBM layer was 40% higher than the lower layer, in spite of the similar density values. It is important to highlight that visual monitoring of the SBM trial up to the age of 55 months has indicated no signs of reflection cracks.

3.4.3 A485 Carmarthen bypass Carmarthen bypass was a Welsh Office scheme, of conventional construction, which allowed trial sites with control sections to be set up. The A485 Carmarthen site was constructed in March 1999, and comprised two sections of SBM, one with SBM base and the other with SBM base and subbase. The two trial sections were placed on opposite carriageways, each with an adjacent CBM control section and a section of the standard fully flexible construction. A summary of the pavement design is given in Table 3.9.

Table 3.9. Construction of Carmarthen bypass A485 link road

Layer SBM/CBM base sections Asphalt base section

Surface course 40/50mm Masterpave 40/50mm Masterpave

Binder course 150 mm HDM 50 mm HDM

Base 200 mm SBM/CBM 170 mm HDM

Subbase 150 mm Type 1 (150 mm SBM in one section)

150 mm Type 1

The SBM mixture design was based on the laboratory trials given in Table 3.4. It was decided not to use the higher level of lime (2%), as it represented the most expensive constituent of the mixture. The selected mixture design was within the TRL proposed limits and consisted of 16% gbs, 1.5% lime, 77.5% abs and 5% limestone filler. A water content of 7.5% of the dry mass was targeted. The control CBM3 was a standard mixture supplied by Tarmac and



consisted of 73.2% limestone aggregate, 21.4% sand and 5.4% cement. The target water content was 5.9% of the dry mass.

Both SBM and CBM materials were mixed in a Tarmac Topmix concrete batching plant near Llanwern. The specified period for the CBM between batching and laying was 2 hours. No problems were reported with the batching of SBM using the concrete plant and the material was treated and laid in the same manner as the CBM control, with the exception of the 2 hours time limit between mixing and laying. A Bomag BW-151-AD2 dual drum vibratory roller was used for compaction. A load of 2158 kg/m is applied across the width of the roller, and was used with vibration. Generally, the surface finish of the SBM was coarser than the CBM. The binder course was laid 2 weeks after laying the SBM base.

The FWD bound-layer stiffness values for the Carmarthen trials, up to an age of 2 years, are presented in Table 3.10. The asphalt control measurement was adjusted to a reference temperature of 20°C, whereas other sections of CBM3 control, SBM base and SBM base+subbase were not corrected for temperature, and the temperature varied between 10 and 28°C.

Table 3.10. Average FWD stiffness results (MPa) at Carmarthen

Age Asphalt control CBM3 control SBM base SBM base+subbase

14 4897 10528 2487 1857

90 5286 7050 4039 7294

180 6276 10725 9000 12214

360 5888 14203 9675 13864

730 5916 13927 9861 13520

From this data a number of trends are apparent. Both SBM with and without subbase showed significant increase in the combined bound layer stiffness over time, with higher stiffness values for the SBM base+subbase. The stiffness attained at early life, mainly due to aggregate interlock, allows the material to be trafficked immediately after construction until the strength and stiffness have increased. The stiffness of both SBM sections exceeded that of the asphalt control section in the long term, after 180 days. On the 180-day result, the temperature of the survey was approximately 20°C and therefore temperature had a minimal effect on the measured stiffness.

The comparison of the SBM with the CBM3 control is more complicated by the inconsistent results of the CBM control section. With the exception of the 14-day results, both CBM3 control and SBM base+subbase exhibited almost similar mean stiffness values. It is however more appropriate to compare the SBM base results to the CBM3 control due to the same nominal construction thickness. The comparison shows that the SBM is not as stiff as the CBM3, with only a small improvement of the SBM stiffness after 180 days.

At the time of construction, the SBM mixture used was sampled on site with material being taken from the paver’s screws. Beam and cylinder specimens produced from this material were compacted and cured in sealed condition at 20°C. Full details of the results are given by Megan (2001). Table 3.11, however, summarises the results of the beam specimens.

The relatively low density, compared to conventional concrete and bituminous mixtures, could be attributed to the lower density of the abs aggregate compared to primary aggregates. The dynamic stiffness is gained very early with a slight variation with time, whereas the static stiffness and strength build more slowly over time. It is interesting to note that despite the



slow curing, over 50% of the 460-day compressive strength had been obtained at 33 days and over 75% at 90 days. In fact the 90-day compressive strength exceeds the requirements for CBM3 of 10MPa at 7 days. Similar trends of strength development can be seen from the flexural and indirect tensile strength results.

Table 3.11. Results from beam specimens produced at Carmarthen

Age (Days ±1) 33 90 460

Density (kg/m3) 2380 2300 2260

Dry Density (kg/m3) - 2250 2250

Dynamic modulus of elasticity (GPa) 18.0 17.4 17.1

Flexural strength (MPa) 0.54 0.71 0.99

Tensile strength (MPa) 0.34 0.63 0.77

Compressive strength (MPa) 7.4 10.4 13.3

Static modulus of elasticity (GPa) 4.3 4.7 7.0

Cores were successfully obtained from the SBM as early as 110 days (under 4 months). A number of cores exhibited voids and loss of material, but to a lesser degree than at Wainscott, where the weakness of the material did not allow coring at an age of 27 months. The Camarthen cores were cut and tested for density and strength at different ages, up to 3 years, and the SBM results are presented in Table 3.12, together with the results of CBM3 control.

Table 3.12. Mean results of cores taken from CBM and SBM bases, Carmarthen

CBM3 SBM

Age (nearest month) 4 23 36 4 23 36

Bulk density (kg/m3) 2348 2343 2372 2223 2269 2276

Equivalent cube strength (MPa) 18.1 18.5 20.3 5.4 9.0 9.2

Tensile strength (MPa) - - 2.15 - - 1.12

The compressive strength results for the SBM core specimens are different to that for the prepared beam specimens cured in the laboratory. In fact the average equivalent cube strength of the cores at 3 years is lower than that measured for the laboratory beams at 90 days. This confirms the great influence of curing conditions on the strength development of SBM.

The specified average 7-day compressive cube strength for CBM3 is 10MPa, Table 3.1, although not specified this equates to a 28-day strength of approximately 12MPa (Nunn, 2004). Trial mixes have indicated that the lowest 7-day cube strength gained was 21.5MPa. High early strength of CBM increases the risk of wide cracks generated at wide spacing, with potential to generate reflection cracks in the overlay. Table 3.12 shows the equivalent cube strength of CBM3 ranged from 18 to 20MPa, with marginal strength development after 4 months. In contrast the SBM gave an average strength of 5.4MPa at 4 months, which increased to 9MPa at 2 years. The tensile strength test was only carried out at the age of 36 months. The SBM core tensile results ranged between 0.93 and 1.35MPa with a mean value of 1.12MPa, which is 52% of the mean strength of the CBM3 control. This tensile strength of 1.12MPa is equivalent to a flexural strength value of 1.5MPa, based on a conversion factor of 0.75 (Raphael, 1984).



3.5 Mechanical properties of SBM

The design criteria of pavements require parameters relating to the structural properties of the material. In the UK designs, the parameters used for the design of CBM in flexible composite pavements are flexural strength and dynamic modulus of elasticity. This section defines the range of these properties for SBM based on laboratory and field trials data obtained from reviewing UK experience. Due to the slow hardening nature of the material, the results discussed here are limited to 360 days, which is commonly used for design purposes.

The most comprehensive data at 360 days are those obtained from the TRL laboratory trial mixtures. The mean compressive strength, flexural strength and dynamic modulus of elasticity (stiffness) results of these mixtures together with the field trial data at Wainscott and Carmarthen are given in Table 3.13. For all mixtures, the flexural strength results ranged between 0.7 and 1.5MPa and the dynamic stiffness between 14 and 20GPa.

Table 3.13. Mean mechanical properties of SBM at 360 days

Source Compressive strength (MPa)


Dynamic stiffness (GPa)

TRL trial mix 1 8.8 0.8 17.6

TRL trial mix 2 9.8 0.8 16.9

TRL trial mix 3 17.4 1.3 19.9

TRL trial mix 4 11.7 0.8 17.0

TRL trial mix 5 10.4 0.8 16.8

TRL trial mix 6 6.8 0.7 13.5

Wainscott cores (42-54 months) 6.1-6.4 0.8-1.1 -

Carmarthen beam specimens (460 days) 13.3 1.0 17.1

Carmarthen cores (36 months) 9.2 1.5

3.6 Summary of UK experience

Review of the UK experience has helped to confirm the benefits of SBM over traditional CBM, due to its slow curing and lower rate of strength and stiffness developments with reduced risk of reflection cracks. Also the ability to remain workable for typically 7 – 14 days after mixing; it can be stockpiled and it can be removed and re-laid, if necessary, within a few days of mixing. Another key advantage of SBM is that it can be trafficked immediately; relying on aggregate interlock and good compaction during early life, until its strength and stiffness has increased. SBM technology is also based on using a ‘secondary’ binder in keeping with government sustainability policy. The review has indicated the following findings:

� At a normal curing temperature of 20°C, SBM could achieve the 7-day compressive strength of CBM3 in 360 days. The strength development of SBM in not linear with time, as over 50% of the 360-day compressive strength could be gained in 28 days and 75% at 90 days.

� After accelerated curing at 40°C little improvement in the compressive strength is achieved after 28 days, indicating that the material has almost achieved its ultimate strength.



� Compaction has a great influence on the mechanical properties of SBM. Laboratory testing indicated that increasing the compaction time from 1 to 2 minute/layer doubles the strength.

� Based on the data reviewed in this section, the typical mechanical properties of SBM at 360 days have been found to be in the range of 0.7 to 1.5MPa for flexural strength and 14 to 20GPa for the dynamic stiffness.



4 European experience

4.1 Introduction

A review of European experience was sought before introducing a design guide on the use of SBM in the UK. A questionnaire was prepared and sent to many European countries, based on the responses study visits were made to those countries with the most experience. Some countries were found to share similar experience. This section reviews the general practices in France, the Netherlands and Spain with SBM in terms of material characteristics, handling, compaction, site compliance and design criteria.

4.2 France

A study visit meeting took place with representatives from Laboratoire Central des Ponts et Chaussées (LCPC), TRL, and various French organisations promoting the use of SBM. The meeting was to discuss the French experience on the use of SBM (grave laitier), with the aim of introducing SBM into UK pavements as a base material. SBM uses either granulated or partially ground granulated (or pelletised) blastfurnace slag as a hydraulic binder in a similar manner to cement in CBM.

4.2.1 Historical Development Mechanical stabilisation began in France after the Second World War. Firstly, unbound aggregate was used and then cement was added, however, the severe winter of 1963 destroyed many roads and exposed the shortcomings of CBM (grave ciment). Following this experience, slag bound mixture without an activator was used initially, which hardened slowly. The addition of an activator produced a material that was similar to grave ciment (after 1 year). An organisation, the National Commission of Slag, was formed to organise the use of the material.

The first SBM was made using 15-20% of granulated slag with a lime activator of 1%. By the end of the 1970’s and through the 1980’s partially ground granulated blastfurnace slag (pggbs) was introduced to the French market. The particle sizes of pggbs, between 0-3mm and 12% passing 80 microns, are finer than that of gbs. The fineness of pggbs contributes to the reactivity of the material and therefore allows for the reduction in binder content. The pggbs content ranged between 12-15% with a lime activator, the top end of the range being used with fine aggregate of sand and the lower end for coarse aggregates. Pelletised slag was also used in France, but the material is no longer produced.

More recently, the French introduced Gypsonat, a proprietary activator, to produce a stronger material and allow the minimum content of pgs to be as low as 8%. Gypsonat is a combination of lime and gypsum with the weight ratio of 90% and 10%, respectively. Lime as an activator is no longer favoured and it is only used near to the slag production plants. Steel slag and ggbs are not used in the production of SBM (grave laitier).

4.2.2 Sand slag

The required properties of the aggregate for use in SBM are dependent on the position of the SBM layer in the pavement structure and the traffic to be carried. The technique of sand and gravel treated with bfs has existed in France for several years. Sand slag, known in France as sable laitier, could have economic and environmental advantages of utilising numerous local materials in areas lacking quality coarse aggregates, and has been also used in Tanzania, Italy, Luxembourg, Gabon and Tunisia. The dosage of the binder in sand slag is relatively high compared to that in gravel slag, due to the increased aggregate surface area. The French



design catalogue (LCPC/SETRA, 1998) only allows sand laitier to be used in the lower base layer of heavily trafficked roads. There is no UK experience of using sand slag to date.

4.2.3 Material properties

4.2.3.1 Slag

An α Coefficient between 20 and 40 is used to control fineness and hardening properties of the material. French experience indicated that the variations of chemical composition are much less with the use of imported raw materials.

4.2.3.2 Aggregate

The nominal maximum aggregate size used in France is 20 mm. Three aggregate gradings are specified 0/10 mm, 0/14 mm and 0/20 mm.

French experience on the aggregate requirements indicated no strong correlation between SBM strength and the strength of aggregate, unless the aggregate used is very weak. The requirements of aggregates, however, are given in terms of grading, Los Angeles (LA) test and Micro Deval (MDE). For the highest traffic, a LA value of 30 and a MDE value of 25 are preferred, generally, LA + MDE values should be less than 55. These requirements are easily achieved with different types of limestone aggregate. Recycled aggregates such as recycled concrete aggregate, blastfurnace slag are commonly used. There is also a potential to use reclaimed aggregate from pavements.

4.2.3.3 Activator

As indicated earlier, lime was used initially in France but has been currently replaced with Gypsonat for improved strength development.

4.2.4 Mix design

The SBM mixture design is optimised with the selection of well-graded materials, including the slag binder. Compaction of the mixture is assessed using a modified Proctor test. The maximum dry density of the mixture should not contain more than 20% voids. The strength and modulus of the mixture are important and the site compaction should achieve greater than 97% of the maximum dry density of the mixture. The contractor can propose a higher value to reduce the proportion of gbs if the strength requirements are achieved.

4.2.5 Quality control

The quality control of the mixture is based on mixture design, appropriate selection and proportion of the ingredients, and compaction.

For laboratory specimens, compaction is carried out using a vibrating table and curing in sealed conditions at 20°C. The strength and modulus properties are measured at 90 and 360 days. The 90-day strength is used as an indicator for the 360-day strength. Fatigue properties are not generally measured, and typical values are assumed for design.

A comparison between laboratory curing and site curing has been carried out in France and the conclusion was that the in situ cured material is normally superior provided that the composition requirements are respected. This work will be published in the Revue Generale des Routes (LPC) bulletin in the near future.



For site mixtures, mixing is only carried out at mixing plants. It is the responsibility of both the client and the contractor to check compliance in terms of mixture composition, compaction and strength properties. Compaction on site is normally carried out using a vibrating roller with a static load higher than 3000 kg/m across the width of the roller. A pneumatic roller, 35 tonnes, is mainly used to produce a “closer” surface with more seal against water ingress or applied first for materials that are difficult to compact. When two layers of SBM are used, the density at the bottom of the lower layer should be greater than 95% of the maximum dry density.

At the beginning of a contract, trials are made to decide the number of roller passes, speed and pattern of rolling. For each roller there is a chart for thickness and compaction level.

4.2.6 Bond between layers The minimum thickness of SBM is 150 mm whereas the maximum thickness is 350 mm, depending on the traffic loads. When two layers of SBM are used, the binder type mainly influences the bond between them. Full bond is assumed with slow reacting materials such as gbs with any activator and pggs with lime. However, partial- or semi-bond is assumed with pggs when activators other than lime are used.

Full bond is also considered between SBM and the upper asphalt layer, provided a bituminous asphalt bond coat (500 g/m2) is used.

4.2.7 Performance of SBM

The main problem associated with SBM is cracking. However, the cracking is less severe than that with CBM. Previously SBM was not pre-cracked and resulted in problems with reflective cracks. In the last 10 years, pre-cracking was introduced to France using the Craft process or a similar method at every 2-3 m. In this process 60% of the layer depth is grooved and filled with bitumen emulsion. Pre-cracking is compulsory for SBM base of highly trafficked roads. It is important to mention that the standard reference axle load in France is 13 tonnes, which is not favourable for the comportment of cracks. For other cases, pre-cracking is not compulsory but recommended to reduce the cost of maintenance. French experience also indicated the benefits of using limestone aggregates in reducing reflection cracking and associated problems.

Another problem highlighted with the use of SBM is surface disruption. This mainly occurs on the surface of a SBM layer with the use of relatively thin asphalt layers and appears on the edge of the road. In France, the minimum top asphalt layer thickness is 60mm for low traffic and 80mm for high traffic levels, with a maximum thickness of 140mm.

4.2.8 Storage and handling of the material

As indicated earlier, mixing of SBM is always conducted at mixing plants and the mixture is transported to site, to ensure good quality control. There is no specification on the workability requirement (handling time) but SBM allows more handling time than CBM and is generally laid within one day of mixing.

In frosty conditions, there is a risk of slow hardening material that causes surface disruption. It is therefore recommended to re-compact the material after frost. When raining the material gets wet and it can have a low stability. To overcome this problem, it is possible to wait until the conditions improve, or the materials can be removed to drain water and then re-laid and compacted.



4.2.9 Similar international experience Italy uses similar formulations and technology to that of France. However, it is believed that in Belgium most of the slag is used by the cement industry as ggbs.

4.2.10 Design criteria

Hydraulic binders, other than cement, are used on a routine basis in France. Indicative values for some of the material properties, taken from the French design catalogue for new pavements (LCPC/SETRA, 1998), are given in Table 4.1. It should be pointed out that the stiffness modulus used in France is the static value, whereas dynamic stiffness is used in the UK. These measurements should be defined to avoid confusion. These can be used for design, provided that the material has been mixed, laid and compacted in compliance with the standards.

Table 4.1. Indicative properties used in French design method

Properties at 360 days Relative to 360 days Binder Min.

stiffness modulus

(GPa)

Max. stiffness modulus

(GPa)

Min. tensile

strength (MPa)

Age in

days

Tensile strength

Stiffness modulus

Bou

nd g

rade

d ag

greg

ates

Cement (CG3)

Cement (CG4)

Slag, GLp + lime

Slag, GLg + lime

Slag, GLp + sulphate

23

25

15

15

20

40

40

25

20

25

See NF

See NF

0.65

0.65

0.9

28

28

90

90

90

0.60

0.60

0.70

0.70

0.70

0.65

0.65

0.70

0.70

0.70

Com

pact

ed

conc

rete

s Cement and special binders

Activated slags

28 - 2.8

GLg is grave laitier granulé, and is granulated blast furnace slag. GLp is grave laitier préboyé, and is ground or partially crushed granulated blast furnace slag NF refers to the French Standard (Norm Français) for the particular material concerned.

Results from a complete laboratory study of the reference mix can also be used. In view of the differences generally found, at the same age, between the results from samples prepared in the laboratory and those extracted from the pavement, values that are lower than the mean values obtained for the reference mix are used. This is to allow for variations in mix composition on site compared with the reference mix used in the laboratory. The calculation of the critical stress (σr) is based on 70% of the tensile strength and 90% of the elastic modulus measured after 360 days in the laboratory. The cores are cured under standard conditions; i.e., 20°C under sealed conditions to prevent evaporation of water.

It is interesting to note that although it is recognised that some hydraulic materials cure more slowly than others, no allowance is made for this in the French design method. Design is carried out solely on the basis of the properties achieved after 360 days. The question of material compliance in the permanent works was raised in the study visit to LCPC. In France different hydraulic binders have been used routinely for over 30 years. Therefore, the French have built up confidence in these materials and so when it came to formalising procedures



they were based on developing standards for materials and methods that had been proven by experience. This differs from the case in the UK, where we are trying to modify our current procedure to allow materials to be used for which only a very limited amount of knowledge has been accumulated. Naturally, this fosters a feeling of uncertainty.

In France, the material producers have long term experience with their materials and they can easily demonstrate, using their database of knowledge, that materials can be designed to meet the required standards. There is no test that can be used soon after the material is laid to give assurance that the material will achieve its design properties after 360 days. The method of ensuring that the properties of the material laid on site comply with those specified is as follows:

� Prior to the contract a laboratory study will be carried out on the reference mix. The aggregate will be well graded and will conform to a grading envelope. Materials will be compacted in a mould using a proctor hammer and the density measured. The compaction of materials on site will be required to be 97% of this.

� Materials will be cured under standard conditions and the mechanical properties will be measured after various curing times. The properties that are measured are the stiffness modulus and tensile strength. The indirect tensile strength is easier to measure and is less variabile compared to the direct tensile strength. Therefore, this test is generally used and the direct tensile strength is taken as 80% of this value.

Site compliance involves ensuring that the mixture has the correct proportion of binder, the aggregate grading conforms to the reference mix and the compacted density on site meets the specification.

In the French design method for hydraulically bound material, the traffic induced critical stress at the underside of the hydraulically bound layer is limited to a permissible value for a given level of traffic loading. This critical stress is the stress induced by a 13 tonne axle load and it is calculated using a linear elastic pavement model. Adjustment factors are used to take into account various design issues. These criteria are:

σr = σ6 x (NE/106)b x kckdkrks for hydraulically bound layer (4.1)

εZ = A x NE-0.22 for subgrade (4.2)

Where:

� εZ - Vertical strain at the top of the subgrade; � σr - Horizontal stress at the underside of the base; � σ6- Stress for a life of 1 million equivalent standard axles; � b- The fatigue exponent; � NE- Number of 130 KN standard equivalent axles; � A- Calibration coefficient for subgrade deformation criterion.

With flexible composite pavements, the subgrade strain is rarely the controlling design criteria.

The adjustment factors (k), take the following into account:

� kc- is a materials specific calibration factor. � kd- accounts for the effects of transverse cracks and thermal gradients. � kr- accounts for variations in pavement thickness and fatigue behaviour and includes a

risk factor associated with the level of traffic (it is a function of b, SN and Sh – see LCPC/SETRA design guide (1997). SN and Sh are coefficients that allow for variability in fatigue results and in layer thickness, respectively.

� ks- accounts for variations in bearing capacity of the construction platform;



The design parameters are given in Table 4.2. In design calculations, semi-slip is assumed for the interface condition between two GLp layers. Full bond is assumed for the slower curing materials, GLg and GLp with lime. GLp or GLg with a proprietary activator is faster curing and can achieve superior structural properties.

The IQE (indice qualitaire elastique) is used to classify materials into different categories for design. It is a function of the stiffness and strength of a material and it is best for a material to have a lower stiffness for a given tensile strength. Using this grading system, GLg and GLp + Lime are classified as Grade 1 (G1) materials and GLp with sulphate as a G2 material. In the French system it is preferable to use material in the G2 class and above, for traffic flows of greater than 150 commercial vehicles (> 50 kN payload or 90 kN gross weight) per day at opening.

The minimum moduli values, given in Table 4.1, correspond to the bottom end of the range of observable characteristics from a broad survey of materials that comply with the standards. Maximum values are specified to prevent excessive shrinkage cracking.

The designs given in Table 4.2 for these materials are published by LCPC/SETRA (1998).

Table 4.2. French design for the primary road network (30 year life)

Construction platform

PF 2 PF3 PF4

Cumulative traffic over 30 years

(Traffic Category, TC: 13 t ESALs x 106) Grave Latier Préboyé (GLp)

TC8: 112 94 x106 CV

- 145 mm asphalt 380 mm GLp

145 mm asphalt 340 mm GLp

TC7: 49 38 x106 CV



TC6: 18.4 14 x106 CV



TC5: 7.3 6 x106 CV




TC3: 3.6 3 x106 CV




Grave Latier Granulé (GLg)

TC6: 18.4 14 x106 CV

- 105 mm asphalt 410 mm GLg

105 mm asphalt 380 mm GLg

TC5: 7.3 6 x106 CV




TC4: 3.6 3 x106 CV




For example in Table 4.2, the traffic category TC8 represents 112 x 106 13 tonne standard axles (NE) or 94 x106 commercial vehicles (CV) greater than 35 kN gross weight.

Designs are given for other roads, not primary road network, where traffic is less aggressive. These are designed for a 20 year period and the SBM layers are between 20 and 30 mm thinner. Commercial vehicles are now classified in France as vehicles of over 35 kN. For traffic levels higher than or equal to 14 million 13t axle loads, pre-cracking of the base is compulsory. For all other traffic levels it is advisory.



4.3 The Netherlands

4.3.1 Background

The first application of unbound material (asphalt granulate) as a base course in road pavements was in the 1970’s, and this showed poor performance after two years in service. The Dutch therefore moved from unbound to cement bound material with a cement content of between 6 and 10%. High cement content resulted in high early strength/stiffness base and was more susceptible to cracking. This led to an introduction of a low cement content in bases such as 3 to 6% in 1995, and 2 to 4% in 1998, and the introduction of SBM. The Dutch standard specifications currently specify three road bases with slags.

The production of slag in the Netherlands amounts to 1.85 million tonnes/year (Mt/a) and is derived from two sources. 1.3Mt/a of blastfurnace slag is produced and 1.1Mt/a of this is used in ggbs cement and the remainder in SBM. Also, 0.55Mt/a of steel slag is produced and used as aggregate and as an activator in SBM.

4.3.2 Materials and Mixes

4.3.2.1 Material definition

Stone mix of natural stone: crushed natural stone;

Furnace slag: slag obtained at the production of rough iron in a furnace;

Phosphoric slag: slag obtained from the thermal extraction of phosphor from phosphate ore;

Steel slag: slag obtained from the production of steel;

LD-slag: slag obtained at the production of steel according to the Linz-Donawitz process;

Electro-oven slag: slag obtained at the production of unalloyed carbon steel by the electro-oven process.

4.3.2.2 Types of road bases using slag

The Dutch standard specifies three road bases containing, partially or totally, slags:

Hydraulic mixed granulate road bases: Aggregate consisting of recycled aggregates bound with 5 to 10% of hydraulic slag. The slag could be blastfurnace slag, electro-oven slag, LD slag, or a mixture of these.

Furnace slag road bases: Aggregate consisting of broken furnace slag, granulated furnace slag and optionally up to 25% of steel slag.

Phosphoric slag road bases: Mix of broken phosphoric slag, granulated furnace slag and optionally up to 25% of steel slag.

4.3.2.3 Aggregate

The nominal maximum aggregate size used is 0/40mm. Two aggregate gradings could be used (0/20 and 0/40mm). No requirements of the Los Angeles (LA) and Micro Deval (MDE) tests are used.



4.3.2.4 Activator

Steel slag (0-4mm) is the only activator used for all slag types at 5-10% of the slag content.

4.3.3 Quality control The quality control of the SBM mixture is based on optimised mix design, by appropriate selection and proportion of the ingredients, and compaction.

4.3.3.1 Laboratory testing

Specimens are prepared and cured under standard conditions, 20°C and 90% relative humidity, until testing at 7 and 28 days. The specimens are tested for particle size distribution, percentage of fines (maximum 8% passing 0.063mm), maximum Proctor density and slag-content.

4.3.3.2 Site testing:

Degree of compaction: Every individual value must be at least 98 % of the maximum Proctor density. The average degree of compaction (minimum 5 samples) has to be at least 101% from the maximum Proctor density.

The initial compaction is with a vibratory roller and the compaction is completed using a static roller. Compaction is monitored by the Nuclear Density Gauge method (Troxler) and gravel sand replacement method (BS 1377-9, 1990).

CBR: The test is carried out on the fraction of the material passing 22.4mm and the requirements are:

Immediately after preparation: at least 50%

28-day: at least 125% of the value immediately after preparation.

Surface level tolerance: the unevenness in longitudinal and transverse direction of the stone mix pavement layer should not exceed 15mm, measured with a 3 metre straight edge.

4.3.4 Layer thickness and bond between layers

The thickness of SBM is usually 250mm and could be used up to 400mm. For the 400mm, two layers of 250mm and 150mm are commonly used. After compacting the first layer, the surface is gently ripped (grooved) by the tooth of a shovel to provide a good contact and full bond with the second layer.

4.3.5 Reflective cracking

SBM is not pre-cracked in any application in the Netherlands. When overlaid with a rigid pavement, a 50mm asphalt regulating layer is constructed between the SBM and the concrete pavement. No special measures are taken when SBM is overlaid with asphalt.

The Dutch have expressed that reflective cracking is not a problem due to the low strength requirement of SBM, and the use of thin flexible pavement between rigid layers.



4.3.6 Storage and handling of the material There is no specification on the workability requirement (handling time) but generally SBM allows more handling time than CBM but it is generally laid within one day of mixing. The material is not to be laid in a frosty environment.

4.3.7 Dutch designs of SBM The Dutch designs are empirically based and assume that the structural equivalence between asphalt base and SBM base is 1:2 at lower traffic levels (1msa) and 1:3 at higher traffic levels (10 and 100msa), see Table 4.3. This in turn equates to a relatively low stiffness modulus of about 1,000 MPa in their design method. For hydraulic mixed granulate, 600 MPa is assumed which equates to a structural equivalence of approximately 1:4 and 1:5 at the lower and higher traffic levels, respectively. The function of the SBM base could be regarded as a foundation enhancement to enable the thickness of the more expensive asphalt layers to be reduced.

Table 4.3. Dutch designs* of SBM

Base type Bituminous Furnace slag Phosphoric slag Hydraulic mixed granulate

Asphalt overlay (mm) 186 85 85 124

SBM thickness (mm) - 200 200 250

1 m

sa

Foundation at least 1m of sand capping layer



10 m

sa




100

msa


*Stated weight of equivalent standard axle is 100kN dual wheel.

4.4 Spain

4.4.1 Mixture and properties SBM has been used in Spain for more than 20 years. The mixture comprises ggbs as a binder, lime as an activator and limestone aggregate, with a binder content of 15% and moisture content range of 5 to 7%. About 3,000 tonnes/month of ggbs is used as a binder in SBM base. The material is expected to have the following compressive strength:

� 7 days: 1.5 - 2.0MPa

� 28 days: 4.0MPa

� 90 days: >5.0MPa



4.4.2 Application, performance and design A laboratory material design is undertaken prior to the main works of a contract and site compliance of plant mixed material. The properties of the material are assessed in terms of the aggregate grading of samples during laying, density determined using a nuclear density gauge and strength.

The performance of SBM is considered to be similar to that of CBM with a compressive strength of 6.0MPa and it has a similar modulus. In Spain CBM is commonly used on a bound subbase, however, experience with the use of SBM has shown that reflection cracking is not a problem and therefore it is laid on an unbound granular subbase.

In Spain a catalogue method of pavement design is used based on empirical and analytical considerations. The design life is 20 years for pavements with bituminous surfacing. SBM is not used on very lightly trafficked roads and is used principally on roads expected to carry over 2,000 commercial vehicles per day. A commercial vehicle is defined as a vehicle of over 5 tonnes and the maximum axle load in Spain is 13 tonnes, similar to that of France and Greece. The design is typically 120 to 150mm of asphalt over 200 to 250mm of SBM base.

The Spanish designs have an analytical basis in which a stiffness modulus 15GPa is assumed for the SBM. The designs are given in Table 4.4. The foundation layer is considered to have a California Bearing Ratio (CBR) of greater than 20%. Relative to French design, the Spanish design is thin. For 10 million 130kN ESALs the French would require about 320mm of SBM.

Table 4.4. Spanish designs for SBM

Design traffic

Asphalt (mm)

SBM (mm)

1 million 10 million

100 150

200 250

*Stated weight of equivalent standard axle is 13 tonne.

4.5 Comparison between UK and European experiences

Table 4.5 summaries the European experiences reviewed in this section. It can be seen that France has the most notable experience with the use of SBM. The earliest UK experience of SBM type mixtures dates back to 1987 when the A228 Pembury bypass, Kent, was constructed using imported Flushing SBM (FSBM) as a combined roadbase and subbase. Subsequent monitoring by Kent CC has demonstrated very good performance, confirming a structural life in excess of 40msa. Also, 12 years after laying, there was no evidence of reflective cracking through the 130mm asphalt overlay.

UK materials currently available are mainly into the GLg material class, however, this material is not normally used for very heavily trafficked roads in France. It should be possible to improve SBM performance by using pggbs or ggbs as the hydraulic binder rather than gbs, but UK has limited experience of this to date. The cost differential between ggbs and gbs is also likely to restrict the use of ggbs in such mixtures. The design catalogue gives designs for GLg for up to 14 million commercial vehicles. The blastfurnace slag used in GLp is ground finer and it uses a proprietary activator containing sulphate. These factors ensure that it cures faster and has superior structural properties. This enables it to be used as a base in the most heavily trafficked roads and the design thicknesses are almost identical to those of the Class 3 CBM used in France (GC3).

The difficulty is in establishing the link between the structural properties of the French materials and the material proposed for the UK. The design curves cannot easily be transformed into UK equivalents. The transformation would involve:



1. Converting French 130 kN ESALs into UK 80 kN ESALs. This would involve a large shift that is complicated by the French using a 5th power damage law whilst a 4th power law is used in the UK. With the 4th power law 75 million French standard axles would equate to about 500 million standard axles in the UK. Hence, the use of the 4th power law in this way is questionable.

2. In France the elastic modulus used for design is the static modulus. Whereas in the UK, the dynamic modulus has been used traditionally for all hydraulically bound mixtures.

3. Much of French experience involves the use of GLp and proprietary binders. These are unlikely to be used in the UK.

4. The effects of different construction practices, climatic conditions, traffic characteristics, levels of risk, etc would need to be calibrated into the design curve.

The tensile strength values used in French design and given in Table 4.1 are the values determined in direct tension. The relationship between the direct tensile strength (Rt) and indirect tensile splitting strength (Rit) is:

Rt = 0.8 Rit (4.3)

This relationship is confirmed by Voirin et al. (2003). The information that is available on the strength of slag bound mixture laid in the UK is in terms of the indirect tensile strength and this indicates that the properties of UK SBM material are much lower than those traditionally used in France. This suggests that design curves need to be developed using classes of SBM with lower structural properties than those used in France.



Table 4.5. Summary of European experience with the use of SBM

Properties France Netherlands Spain Experience with SBM (years) >30 >30 >20 Slag binder content (%) 8 to 20 5 to 10 15

Activator used Hydrated lime 1% Gypsonat 0.8 - 1% Cement 1 – 1.2%

Steel slag

Hydrated lime

SBM requirements Aggregate grading Density/compaction

Moisture content Curing

Tensile strength Elastic modulus

Aggregate grading Density/compaction

Moisture content Curing CBR

Aggregate grading Density/compaction

Moisture content

Laboratory mix design procedure Yes No Yes

Mat

eria

ls

Material stockpiled before laying No Yes Yes

Layer thickness Maximum 350mm 250-400mm 150-250mm

Moisture content Optimum ±1% 6 – 8 % 5 – 7 %

Compaction procedure Vibrating rollers and pneumatic roller

(surface)

Dynamic (3 times) and static (once)

rollers

Static + vibratory + static

Monitoring compaction Roller working and density control

Nuclear method and BS 1377

Density measurements

Surface level tolerance Subbase: ±3% Base: ±2%

Max 15mm in all directions

Max 15mm Com

pact

ion

Uniform compaction through full layer

Density: 97% and 95% (sections 4.2.4, 4.2.5)

Yes

On pavement design traffic levels No Yes No

How soon the layer is trafficked Immediately Immediately Immediately

On types of aggregate used Yes Yes No, but only limestone is used

Use of recycled aggregate Crushed concrete, blastfurnace slag

Crushed concrete and masonry, blastfurnace

and phosphor slags

No

Environmental restriction (leaching) No No No

Res

tric

tion

on u

se

Seasonal restriction on laying Except heavy rain and frost

Not in frost Special consideration in

summer

Use of bond coats Yes, 500g/m2 No Yes, 300g/m2

Is it mandatory? Yes No Yes

Bond

Test to assess bond strength No No Yes, not mandatory

Basis for design method Analytical Empirical and analytical

Catalogue with analytical basis

Pave

men

t D

esig

n

Material properties used in design E: 20GPa, Rt: 0.9MPa σ6: 0.7MPa

CBR: 75% E: 1GPa

Thickness: 250mm

E: 15GPa



5 Proposed design guidelines for SBM

5.1 Current UK design

The current pavement designs are concerned mainly with the use of asphalt and cement bound material in a restricted range of design options. A new guide to analytical pavement design has recently been published (Nunn, 2004), which increases the versatility of the design method to enable a wider range of materials to be used in the pavement structure. The new approach can be used to produce design curves for a SBM base layer.

The new approach enables design criterion to be calibrated empirically against a design in which there is confidence of it being able to achieve its expected life. The criterion can then be used more generally. The new approach also allows advantage to be taken of stronger road foundations and four categories of foundation are proposed.

The design criterion will ensure that the hydraulically bound layer does not crack prematurely as the result of traffic and thermally induced stresses. The design criterion is based on a permissible level of stress to achieve the required design life. The calculation procedure and the design criterion utilise the values of tensile strength and elastic modulus attained 360 days after the layer is placed.

The UK design method has never included a fatigue criterion for hydraulically bound base material. The fatigue relationship for hydraulically bound mixtures (fatigue exponent of 12 to 16) is very sensitive to the traffic induced stress compared with that of asphalt (fatigue exponent of approximately 4). A high exponent will produce a design curve in which a relatively small increase in thickness will result in a large increase in life. Because of this, UK design has required that the combined traffic and thermally induced stress should be somewhat below the fatigue criterion. If this condition is met the pavement is considered to have a long but indeterminate life.

The current UK method only considers CBMs. However, recently a more versatile approach has been developed (Nunn, 2004) to allow other hydraulically bound mixtures to be used. This approach requires that either an empirical link between pavement thickness and performance be first established to calibrate the design criterion, or the calibration factor can be determined from the structural properties of the hydraulically bound mixture. The former method is preferred and the latter should be used with judgement and caution.

The basic design criterion for cumulative traffic of over 80msa (long but indeterminate life) designs is that the predicted critical tensile stress σr at the underside of the hydraulically bound base will have to satisfy the following relationship:

σr ≤ Flexural strength. KHyd.KSafety (5.1)

The 360 day values for the structural properties are used to calculate the critical stress, σr. The factor, KHyd, is a materials specific calibration factor that includes temperature effects, curing behaviour and transverse cracking characteristics. KSafety, is a factor that can be used to control the inherent risk in pavement design. The KSafety factor could be adjusted for very heavily trafficked roads or roads constructed in sensitive areas or to give added conservatism to designs using new materials or construction practices.

The following design criterion was developed for pavements carrying less than 80msa:

Log(N) = 1.23 x (SR*KHyd.KSafety + 0.1626)2 + 0.2675 (5.2)

Where, N is the number of equivalent 80kN standard axles and SR is the strength ratio between flexural strength and σr determined using linear elastic theory.

It should be noted that the thickness of asphalt should be as given in the DMRB. That is the asphalt thickness (HAsphalt) considered to give adequate resistance to reflection cracking, and it can be determined using the following empirical equation:



Hasphalt = -16.05 x (Log (N))2 + 101 x Log(N) + 45.8 …… (5.3)

The asphalt layer consists of 35 mm of thin surfacing with the remaining thickness consisting of DBM incorporating 100 penetration grade binder. The design stiffnesses of these layers are 2.0GPa and 3.1GPa respectively. The thickness of asphalt is required to reduce the risk of reflection cracking to an acceptable level. The thicknesses determined using Equation 5.3 should be adhered to even if stiffer grades of asphalt are used. Advantage can be taken of the improved load spreading ability of stiffer grades by appropriate reductions in the thickness of the hydraulically bound layer.

5.2 Design criteria

It is proposed that the design criterion given in section 5.1 be adopted for SBM. The criterion for cumulative traffic of less than 80msa will be:

Log(N) = 1.23 x (SR*KSBM.KSafety + 0.1626)2 + 0.2675 (5.4)

And, for greater than 80msa:

σr ≤ Flexural strength. KSBM.KSafety (5.5)

The KSBM factor requires calibration. This requires knowledge of the stiffness and strength of the material after 360 days and, ideally, performance data from in-service pavements. If some measure of conservatism is justified, with a new material, it could be dealt with by setting an appropriate value for the safety factor (KSafety).

5.3 Calibration of design criteria

Generally, there will be insufficient long term performance data from pavements constructed with non-standard, hydraulically bound mixtures to calibrate the design criterion empirically. However, KHyd factors are related to the structural properties of the hydraulic base material, and a good approximation of this relationship can be determined using the data for CBM materials. A regression analysis using the CBM data gives the following relationship between dynamic modulus (E in GPa) and flexural strength (Rf in MPa) and the adjustment factor (KHyd) with a correlation coefficient (R) of 0.98.

KHyd = 0.368 + 5.27 x 10-5E – 0.0351Rf (5.6)

Long term performance data from a representative sample of in-service slow or quick curing hydraulically bound pavements would be preferred to calibrate the adjustment factor KHyd. However, a representative sample is rarely available, and generally data will only be available from a few sites that are part way through their life.

Therefore, it is proposed to use the relationship given in Equation 5.6 to generate indicative values for KHyd and then use the limited performance data available to help corroborate this choice.

5.3.1 Classification of hydraulically bound mixtures

The combination of stiffness and strength is of crucial importance for design of a hydraulically bound base. Two different hydraulically bound mixtures can have the same base thicknesses for a given level of traffic, provided their flexural strengths compensate for any differences in their stiffness. If stiffness is increased, the traffic induced tensile stresses in the base, that influences performance, also increases. Therefore, the strength would need to be higher to achieve the same performance. Relationships between elastic stiffness modulus and flexural strength required for equal performance are defined in Figure 5.1.



0.1

1.0

10.0

1 10 100

Dynamic elastic modulus at 360 days (GPa)

Flex

ural

stre

ngth

at 3

60 d

ays

(MP

a)

CBM3G

CBM4G

CBM5G

CBM3R

CBM4R

CBM5R

TRL Specimens

Carmarthen trial

Wainscott (estimatedmodulus)

Zone

9

7

123456

8

Figure 5.1. Relationship between stiffness and strength for equal performance

The number and division of the zones is arbitrary, and more or less zones could be specified. However, nine zones are shown in Figure 5.1, in which each zone is bounded by combinations of dynamic modulus of elasticity (stiffness) and strength that will produce the same design thicknesses. Values corresponding to the lower bound curve would be used for the pavement design calculations, for materials whose properties lie between this curve and the one above. The structural properties corresponding to any point on this lower curve will result in the same pavement designs. It is suggested that the flexural strength corresponding to a dynamic stiffness of 20GPa be used as standard design values for materials lying in each Zone. This would result in the standardised design inputs for materials in each zone given in Table 5.1.

Table 5.1. Standardised design inputs for hydraulic materials in each zone

Zone Dynamic stiffness modulus (GPa)


9 2.96

8 2.27

7 1.85

6 1.51

5 1.25

4 1.03

3 0.85

2 0.69

1

20

0.55

The values for standard CBMs are also plotted on this graph. With this classification, CBM5R would be a Zone 9 material, CBM5G and CBM4R a Zone 8 material, CBM3R and CBM4G a Zone 7 material and CBM3G a Zone 5 material. The bounding curves for the zones have been



selected to give a reasonable change in design thickness. The resulting designs are given in Figures 5.2 to 5.5 for pavements laid on different classes of foundation. The standard UK foundation (equivalent to 225mm of Type 1 subbase on a subgrade with a CBR of 5%) will correspond to a Class 2 foundation. The Class 1 construction platform is applicable to construction on a capping layer, Class 3 and 4 foundations incorporate bound subbases. More information on foundation classes can be found in TRL Report 615 (Nunn, 2004).

50

100

150

200

Asp

halt

Laye

r (m

m)

100

150

200

250

300

350

400

450

500

1 10 100Cummulative Traffic (msa)

Hyd

raul

ical

ly b

ound

bas

e th

ickn

ess

(mm

) 1

9

4

5

6

78

Zone

2

3

Figure 5.2. Design thicknesses for materials with a Class 1 foundation

50

100

150

200

Asph

alt l

ayer

(mm

)

100

150

200

250

300

350

400

450

500

1 10 100

Cumulative Traffic (msa)

Hyd

raul

ical

ly b

ound

bas

e (m

m)

1

9

4

5

6

78

Zone

2

3




50

100

150

200

Asp

halt

Laye

r (m

m)

100

150

200

250

300

350

400

450

500


Hyd

raul

ical

ly b

ound

bas

e th

ickn

ess

(mm

) 1

4

5

6

7

8, 9

Zone

2

3


50

100

150

200

Asp

halt

Laye

r (m

m)

100

150

200

250

300

350

400

450

500


Hyd

raul

ical

ly b

ound

bas

e th

ickn

ess

(mm

)

7, 8, 9

1

4

5

6

Zone

2

3


Figures 5.1 to 5.5 can be used to design pavements incorporating SBM base material. Figure 5.1 can be used to determine the zone of the SBM and Figures 5.2 to 5.5 can be used to determine the pavement layer thicknesses for a given foundation class. For example a pavement incorporating a SBM material in zone 4, laid on a Class 2 foundation will require 290mm of SBM overlaid with 166mm of asphalt to achieve a design life of 40msa.

Figure 5.1 also shows the stiffness modulus and flexural strength of SBM measured in UK trials. This shows that SBM materials fall into zones 2, 3 4 and 5. French experience shows that superior SBM can be produced using more finely ground gbs and proprietary activators.



The design curves shown in Figures 5.2 to 5.5 assume that the design for indeterminate life requires 180mm of asphalt surfacing. This would be the case if a pavement with an SBM base carried the same risk of reflection cracking as a CBM pavement. The information obtained from Spain and the Netherlands suggested that reflection cracking was not an issue with SBM pavements. In France, SBM pavements are pre-cracked in the same manner as CBM pavements. However, it was recognised that the risk was less. The French also use finer ground gbs (GLp) with a proprietary activator. This causes the SBM to cure faster and attain a higher stiffness and strength. These practices increase the risk of reflection cracking in French pavements.

The European Community 4th Framework Project, PARIS - Performance analysis of road infrastructure (Transport Research Fourth Framework Programme, 1999), compiled performance information on a number of different road construction types throughout Europe. Empirical models were developed to predict the progression of different types of pavement deterioration, including reflection cracking. SBM material was present in 42 sections of road, including original construction, overlaid and resurfaced pavements. Although the database was too small to produce a good empirical relationship for predicting the onset and development of reflection cracking for SBM pavements, the study concluded that reflection cracks formed earlier in pavements with conventional CBM base.

In the limited trials carried out in the UK, it has been noted that reflection cracks have not appeared until at least 8 years of trafficking. Compared with CBM base, the evidence points to the risk of reflection cracking being considerably lower for pavements constructed using slower curing SBM base. For this reason, it would be possible to reduce the thickness of the asphalt cover. It has been suggested (Nunn, 2004) that the thickness of the asphalt surfacing for the indeterminate design could be reduced to 150mm, if the hydraulically bound base is unlikely to cause a reflection cracking problem.



6 Site compliance and construction guidelines Guidance on material specification and testing for site compliance for slow curing materials has been recently published as part of the SMART (Sustainable MAintenance for of roads using cold Recycling Treatments) project (Merrill et al., 2004). The specification covers four generic families of materials from Quick-curing Hydraulic material (QH) to Slow-curing Visco-Elastic material (SVE).

In the material classification proposed, it is anticipated that SBM would have to conform to the requirements for a Slow-curing Hydraulic (SH) material. The provision of SBM can be made using the ex situ specifications contained within the guidance (Merrill et al., 2004).

The underlying concept of the specification centres on the declaration by the Contractor of a Material Quality Plan. This plan describes the results of the work that has been undertaken by the Contractor prior to commencement of the recycling scheme, such as Mix Design, as well as method statements and process controls that are employed during the recycling process. The specification is designed to be flexible with the emphasis on the appropriate declaration of the elements in the Material Quality Plan; detailed Notes for Guidance have been provided to ensure that a suitable method for each element is adequately described, however, the actual method undertaken will be accepted at the discretion of the Overseeing Organisation.

In the pre-construction phase, the emphasis is on Mix Design and Pavement Design as well as the Method statements and Process Control procedures. The Contractor will declare the properties of the material as determined in the laboratory or other recognised source and justify the pavement design. During the later construction phase, the Contractor will be required to show that the construction of the recycled pavement layer is in accordance with the process declared in the Material Quality Plan. Furthermore, the Contractor must also show that the material is achieving the expected performance as defined in the Material Quality Plan.

The specification is a generic approach that recognises the value of treating recycled materials in a similar fashion whilst allowing flexibility in order to accommodate the wide range of materials that are covered within the SMART project. It is also a pragmatic approach that is not reliant on extensive in situ testing nor on delayed compliance to account for slow curing materials. It is envisaged that the in-service performance of materials produced under the SMART specification will be monitored in order to provide feedback on the effectiveness of the specification and to identify improvements.

For SBM in particular, for which modulus and flexural strength are the primary material parameters used for design, the common tests associated with this material can be easily accommodated in the specification framework. The specification is suitable for SBM because it was specifically drafted to deal with slow curing materials.



7 Conclusions 1. This study contributes to sustainable construction by supporting the wider and more

efficient use of industrial by-products in the form of SBM within the structural layers of pavements.

2. Pavement design curves have been developed for SBM base material based on their structural properties. Design depends on the combination of dynamic stiffness modulus and flexural strength. This will determine which zone the material falls into for design purposes. Several zones are possible and this research has shown that it is possible to produce material that is comparable to CBM3G. The zonal classification system provides an incentive to improve the quality of SBM and moves it into a superior zone.

3. The review showed that the risk of reflection cracking in SBM flexible composite pavements is less than that for comparable CBM pavements. This introduces the possibility of reducing the thickness of asphalt cover from 180 to 150mm for heavily trafficked roads.

4. A review of UK laboratory studies and site trials has indicated that the typical range of values for flexural strength and stiffness modulus is 0.7 to 1.5MPa and 14 to 20GPa, respectively, at 360 days.

5. In Europe, France has the most experience with SBM pavements, where they have been in widespread use for more than 30 years. The SBM used in France involves finer graded gbs and faster acting proprietary activators resulting in high quality materials that can be used in the most heavily trafficked roads.

6. In France, the use of SBM with a lime activator is restricted to lighter trafficked roads. With this in mind when using lime activator only, the use of the proposed UK design curves for traffic levels of up to 40msa should be considered, until more experience and confidence in the material is accumulated.

7. The material specification and site compliance procedures to be applied will be identical to those published in the SMART (Sustainable MAintenance for cold Recycling Treatments) Project for slow curing hydraulic (SH) materials.



8 Acknowledgements The work described in this report was carried out in the Sustainability and Network Groups of the Infrastructure Division at TRL Limited. John Chandler was the Quality Audit and Review Officer. The kind contribution of Tarmac Limited, and the assistance of Bernard Roussel (LRPC), Arthur van Dommelen (DWW) and Aurelio Ruiz (CEDEX) with arranging the study visits to France, the Netherlands and Spain, respectively, are gratefully acknowledged.

9 References BS 1377-9 (1990). Methods of test for soils for civil engineering purposes. In-situ tests. British Standards Institution, London.

BS EN 13286-44 (2003). Unbound and hydraulically bound mixtures. Test method for the determination of the alpha coefficient of vitrified blast furnace slag. British Standards Institution, London.

BS EN 14227-1 (2004). Unbound and hydraulically bound mixtures. Specifications. Part 1: Cement bound mixtures for road bases and subbases. British Standards Institution, London.

BS EN 14227-2 (2004). Unbound and hydraulically bound mixtures. Specifications. Part 2: Slag bound mixtures. Definitions, composition, classification. British Standards Institution, London.

Carswell I and Megan M (1997). Structural assessment of grave-laitier pilot-scale trial. Unpublished TRL Project Report (PR/CE/35/97). TRL, Crowthorne.

DETR (2000). Building a better quality of life-. A strategy for more sustainable construction. Department of the Environment, Transport and the Regions. HMSO, London.

DTI (2003). Energy white paper.r- Our energy future – creating a low carbon economy. The Department of Trade and Industry. TSO, Norwich.

Elliott RC, Brodrick BV, Rowe GM, Kennedy J and Dawson AR (1994). Assessment of industrial by-products for use in roadbases. Stage 2 report: laboratory and pilot scale testing. Volume 2. SWK (PE), Nottingham (unpublished).

Highways Agency, the Scottish Executive Development Department, the Walsh Office (Y Swyddfa Gymreig) and the Department of the Environment for Northern Ireland.

Manual of Contract Documents for Highway Works. London. Stationary Office. Volume 1. Specifications for Highway Works (MCHW1)

Highways Agency, the Scottish Executive Development Department, the Walsh Office (Y Swyddfa Gymreig) and the Department of the Environment for Northern Ireland.

Design Manual for Roads and Bridges. Stationery Office, London. Volume 7. Pavement Design and Maintenance HD 26 Pavement Design

LCPC/SETRA (1997). French design manual for pavement structures. Paris.

LCPC/SETRA (1998). Catalogue des structures types des chaussées neuves (catalogue of structures for new roads). Paris.

Megan MA (2001). Industrial by-products in roadbases - Performance monitoring 2000. Unpublished Project Report PR/IP/10/01. TRL, Crowthorne.

Mehta PK (1998). Role of pozzolanic and cementitious material in sustainable development in the concrete industry. Proceedings of the 6th CANMET/ACI International Conference. Volume 1, Bangkok-Thailand, pp. 1-20.



Merill D, Nunn M and Carswell I (2004). A guide to the use and specification of cold recycled materials for the maintenance of road pavements. TRL Report TRL 611. TRL, Crowthorne.

Milton LJ, Megan MA and Ellis SJ (1997). Construction of full-scale trials to evaluate the performance of industrial by-products in roadbases. TRL Unpublished Project Report PR/CE/31/97. TRL, Crowthorne.

Nunn M E (2004). Development of a more versatile approach to flexible and flexible composite pavement design. TRL Report TRL 615. TRL, Crowthorne.

OECD (1977). Use of waste materials and by-products in road construction. Organisation for Economic Co-operation and Development, Paris.

Parry AR (1995). Energy audit of grave-laitier pilot scale trial. Unpublished TRL Project Report (PR/CE/158/95). TRL, Crowthorne.

Powell WD, Potter PF, Mayhew HC and Nunn ME (1984). Design of bituminous roads. TRL Laboratory Research LR1132. TRL, Crowthorne.

Raphael JM (1984). Tensile strength of concrete. ACI Journal, Vol. 80, No. 2, pp. 158-165.

Richardson JTG and Haynes DJ (2001). Slag bound materials in composite roads. http://www.soci.org/SCI/publications/2001/html/pb49.jsp

Sherwood P (2001). Alternative materials in road construction. Thomas Telford Ltd, UK.

Transport Research Fourth Framework Programme (1999). PARIS: Performance analysis of road infrastructure. Road Transport DG-109, ISBN 92-828-7827-9, Luxembourg.

Voirin J, Desmoulin D and A Lecomte (2003). Predicting the long term strength of road mixtures treated with hydraulic binders. Bulletin des Labatoire des Ponts et Chaussées.

Walsh ID (1999). The evaluation and use of slag bound sub-base/roadbase with performance related tests. Proceedings of the 3rd European Symposium “Performance and durability of bituminous materials and hydraulic stabilised composites. Leeds-UK. pp. 327-342.



Appendix A. Static stiffness and direct tensile strength

A.1 Introduction

For the purposes of pavement design, the UK has traditionally characterised cement bound materials in terms of dynamic modulus of elasticity and flexural strength. The dynamic modulus is measured at a loading frequency of several kHz and at low stress amplitude. This differs from the conditions induced by a rolling wheel load, which has an effective loading frequency of a few Hz and generally induces a much higher stress. However, the new European Standard (BS EN 14227-2, 2004) uses the static modulus of elasticity and direct tensile strength to characterise hydraulically bound mixtures and this characterisation will eventually be adopted in the UK.

This Appendix examines the implications of harmonising with the material characteristics used to classify materials in the proposed new European Standard. However, there are few measurements available for the static modulus and direct tensile strength of standard cement bound materials used in the UK and there is even less information on other hydraulically bound mixtures. It is also recognised that there is not a unique relationship between these parameters and those that have been used traditionally in the UK. Therefore, a programme of material testing would be required to establish more authoritative values. The uncertainty is compounded in that the values available are generally measurements after 7, 28, 60 or 90 days of curing and not after 360 days as required for design purposes. Therefore the calculations and values contained in this Appendix should be regarded with caution and the purpose of the calculations is to explore the design implications of moving to the European characteristics.

A.2 Relationship between material characteristics

Existing data was reviewed to obtain a provisional relationship between dynamic and static elastic modulus and also flexural and direct tensile strength. Further testing may be required to establish more robust static modulus and direct tensile strength values for cement bound and other hydraulically bound mixtures.

A.2.1 Elastic modulus

The static modulus of elasticity is defined as the secant modulus at 30% of the load to failure, whereas, the dynamic modulus is measured at the resonant frequency of a prismatic specimen (BS 1881, 1990). This frequency is typically between 2 and 5kHz. Since during the vibration negligible stress is applied, the dynamic modulus refers to almost purely elastic effects and is unaffected by creep. For this reason the dynamic modulus is approximately equal to the initial tangent modulus determined in the static test and is higher than the static (secant) modulus.

The following relationship between static (Es) and dynamic modulus (Ed) was derived from the measurements on cement bound materials reported by Kolias and Williams (1978) and is shown in Figure A1.

Es = 1.08 Ed – 9.07 A1 (R2 = 0.97)



y = 1.08x - 9.07R2 = 0.97

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60

Dynamic elastic modulus, Ed (GPa)

Sta

tic e

last

ic m

odul

us, E

s (G

Pa)

Figure A1: Relationship between dynamic and static modulus for CBM

The relationship between static and dynamic modulus is material dependent and at low modulus values the difference between the two measurements becomes greater. For example, Figure A2 shows the relationship between compressive strength and dynamic and static modulus of slag bound mixture (SBM) determined by Megan and Earland (1999). This illustrates that the two curves diverge for lower modulus values.

0

5

10

15

20

25

0 2 4 6 8 10 12 14 16 18 20

Compressive strength (MPa)

Mod

ulus

(GPa

)

Dynamic modulus (Ed)Static modulus (Es)

Figure A2. Relationship between compressive strength and modulus for SBM



A.2.2 Material strength In the French design guide (LCPC/SETRA, 1997) the direct tensile strength of hydraulically bound material is taken to be 80% of the indirect tensile strength. Also the indirect tensile strength is approximately 75% of the flexural strength (Raphael, 1984). These two relationships imply that the direct tensile strength is approximately 60% of the flexural strength.

A.3 Design based on static modulus and tensile strength

Equation A1 and the relationship between the strength parameters were used to estimate the 360 day structural properties for static modulus and direct tensile strength of standard CBMs. These are given in Table A1 together with the corresponding values for the dynamic modulus and flexural strength.

Table A1. Design values

CBM base Dynamic modulus (GPa)

Static modulus (GPa)


Direct tensile strength (MPa)

CBM3G 32.9 26.5 1.65 0.99 CBM4G 38.8 32.8 2.48 1.49 CBM5G 42.9 37.3 3.30 1.98 CBM3R 34.5 28.2 2.4 1.44 CBM4R 40.4 34.6 3.6 2.16 CBM5R 44.7 39.2 4.8 2.88

A.3.1 Heavy traffic designs (> 80msa) The design methodology using static modulus and direct tensile strength remains essentially unchanged. The basic design criterion for indeterminate life (> 80msa) is that the predicted critical tensile stress σr at the underside of the hydraulically bound base will have to satisfy the following relationship:

σr ≤ Direct tensile strength. K’Hyd.KSafety A2 The 360 day values for the structural properties given in Table A1 have been used to calculate the critical stress (σr), and new adjustment factors (K’Hyd) were determined to give agreement with the designs in the Highways Agency’s Design Manual for Roads and Bridges (DMRB, Volume 7). These are given in Table A2. The standard case assumes pre-cracking at 3 m intervals and that 180 mm of asphalt is required for this crack spacing to resist reflection cracking.

Table A2. Adjustment factors

Material K’Hyd KSafety CBM3G CBM4G CBM5G CBM3R CBM4R CBM5R

0.509 0.474 0.436 0.469 0.414 0.354

1.0

Other hydraulically bound mixtures

To be determined using design of known performance



A.3.2 Traffic < 80msa The design curve given by Nunn (2004) was used to develop the following design criterion for pavements carrying less than 80 msa based on the static modulus and direct tensile strength values given in Table A2:

Log(N) = 1.25 x (SR x K’Hyd . KSafety + 0.15)2 + 0.25 A3

Where, the strength ratio (SR) is now the ratio between direct tensile strength and σr determined using linear elastic theory.

The base thicknesses using the criterion given in Equation A3 are in excellent agreement with LR1132 designs and this criterion can be applied to other hydraulically bound mixtures using the K’Hyd factors applicable to those materials.

Generally there will be insufficient long term performance data from pavements constructed with non-standard, hydraulically bound mixtures to calibrate the design criterion empirically. However, K’Hyd factors can be related to the structural properties of the hydraulic base material. The data given in Table A1 have been used to carry out a regression analysis to relate static modulus (E in GPa) and direct tensile strength (Rt in MPa) to the adjustment factor (K’Hyd). The following relationship was established:

KHyd = 0.55 + 2.0 x 10-3Es – 0.095Rt A4

(R2 = 0.96)

The quality of this prediction is illustrated in Figure A3 and predicts the values of K’Hyd given in Table A2.

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

Empirical values

Pred

icte

d va

lues

Figure A3: Comparison of predicted and empirical values of K’Hyd

It is proposed to use this relationship to generate indicative values for K’Hyd and then use the limited performance data available to help justify the reasonableness of this choice.

The combination of stiffness and strength is of crucial importance for design of a hydraulically bound base. Two different hydraulically bound mixtures can have the same base thicknesses for a given level of traffic, provided their strength compensates for any differences in their levels of stiffness. For example, if stiffness is increased, the traffic induced tensile stresses in the base, that influence performance, also increases. Therefore, the



strength would need to be higher to achieve the same performance. Relationships between elastic stiffness modulus and flexural strength required for equal performance are defined in Figure A4.

0.1

1

10

1.00 10.00 100.00

Static elastic modulus (GPa)

Dire

ct te

nsile

str

egth

(MPa

)

CBM3G

CBM4G

CBM5G

CBM3R

CBM4R

CBM5R

SBM

H9

H2

H3

H4

H5H6

H7

H8

H1

Zone

Figure A4. Relationship between static modulus and tensile strength for equal performance

The SBM points plotted in Figure A4 used the static modulus implied by the relationship illustrated in Figure A1 and the assumption that the direct tensile strength is 60% of the flexural strength.

The static modulus is always less than the dynamic modulus and for less stiff hydraulically bound mixtures the difference between the static modulus increases. On the other hand, the relationship between the strength parameters is more constant. Also the material classification system illustrated in Figure A4 shows that a disproportionate reduction in stiffness compared with strength can move hydraulically bound material into a higher classification. With the assumption used, Figure A4 shows that SBM is now comparable with CBM3G from the design point of view. Whereas it was 2 or 3 grades lower when characterised using dynamic modulus and flexural strength.

A.4 Summarising remarks

� Initially materials can be introduced based on the current UK material characterisation parameters of dynamic modulus and flexural strength with a longer term view to adopting the parameters defined in the proposed new European Standards.

� A move to material characterisation based on static modulus and direct tensile strength will require the design method to be recalibrated in the manner illustrated in this Appendix.

� At present there is only limited data available on 360 day values of dynamic modulus and flexural strength of SBM and hydraulically bound mixtures, and even less on static modulus and direct tensile strength. It is therefore recommended that further testing be carried out to develop more robust material characterisation values.



A.5 References

BS 1881: Part 209 (1990). Concrete testing: Recommendations for the measurement of dynamic modulus of elasticity. British Standards Institution, London.

BS EN 14227-2 (2004). Unbound and hydraulically bound mixtures. Specifications. Part 2: Slag bound mixtures. Definitions, composition, classification. British Standards Institution, London.

Kolias S and R Williams (1978). Cement bound road materials: Strength and elastic properties measured in the laboratory. TRL Supplementary Report SR344.

LCPC/SETRA (1997). French pavement design manual. Translation of the December 1994 French version of the technical guide. Published by Laboratoire Central des Ponts et Chaussees (LCPC) and Service d,Etudes Techniques Routes des Autoroutes (SETRA).

Megan MA and Earland MG (1999). Construction of full-scale road trials to evaluate the performance of industrial by-product in roadbases. Volume 2.4 – Slag bound material. Unpublished Project Report (PR/CE/145/99). TRL, Crowthorne.

Nunn M (2004). Development of a more versatile approach to flexible and flexible composite pavement design. TRL Report TRL 615. TRL, Crowthorne.

Raphael JM (1984). Tensile strength of concrete. ACI Journal, Vol. 80, No. 2, pp. 158-165.



Appendix B. Symbols and abbreviations α alpha coefficient of granulated or palletised slag

σr critical tensile stress

A Al2O3 content in gbs, expressed in percentage of mass

abs air cooled blastfurnace slag

C CaO content in gbs, expressed in percentage of mass

CBM cement bound material

CBR California bearing ratio, expressed in percent (%)

CEN Committee European Normalisation

CV commercial vehicles

DMRB Design Manual for Roads and Bridges

E modulus of elasticity, expressed in Giga Pascal (GPa)

ESAL equivalent standard axle load

FWD falling weight deflectometer

gbs granulated blastfurnace slag

GC grave ciment (French term for CBM)

ggbs ground granulated blastfurnace slag

GLg grave-laitier granulé (French term for gbs)

GLp grave-laitier prébroyé (French term for ggbs)

IPI immediate bearing index, expressed in percent (%)

IQE index of elastic quality, expressed in percent (%)

KHyd a material specific calibration factor that includes temperature, curing behaviour and transverse cracking characteristics.

K’Hyd modified KHyd derived using the HA design curves

KSafety a factor for controlling the inherent risk in pavement design

LA Los Angeles (test)

MCHW Manual of Contract Documents for Highway Works

MDE micro Deval

msa million standard axel

pggbs partially ground granulated blastfurnace slag

Rc compressive strength, expressed in Mega Pascal (MPa)

Rf flexural strength, expressed in Mega Pascal (MPa)

Rt direct tensile strength, expressed in Mega Pascal (MPa)

Rit indirect tensile splitting strength, expressed in Mega Pascal (MPa)

SBM slag bound mixture

SMART Sustainable MAintenance of Roads using cold Recycling Techniques

SR the ratio between direct tensile strength and σr determined using linear elastic theory.