Report on Concrete Admixtures for Waterproofing Construction

Report on

Concrete Admixtures for

Waterproofing Construction

Structural Engineering Branch

Architectural Services Department

By: Raymond W. M. Chan Peter N. L. Ho & Eric P. W. Chan

December 1999

Content Page No. 1.0 Introduction 2.0 Definition of Admixtures 3.0 Requirements for Water-retaining Structures 4.0 Effects by Application of Admixtures in Water Retaining

Structures 5.0 Types of Admixtures

5.1 Retarding/Water Reducing Admixtures 5.1.1 Materials 5.1.2 Effects of Retarding/Water Reducing Admixtures

on Concrete Properties

5.2 Superplasticizers 5.2.1 Materials 5.2.2 Effects of Superplasticizers on Concrete Properties

5.3 Polymer Admixtures 5.3.1 Materials 5.3.2 Effects of Polymer Admixtures on Concrete

Properties

5.4 Mineral Admixtures 5.4.1 Materials 5.4.2 Effects of Mineral Admixtures on Concrete

Properties 5.4.3 Silica Fume 5.4.4 Rice Husk Ash 5.4.5 Pulverized Fuel Ash 5.4.6 Ground Granulated Blastfurnace Slag 5.4.7 Other Slags 5.4.8 Combined use of Mineral Admixtures

5.5 Air-Entraining Admixtures 5.5.1 Materials

5.5.2 Effects of Air-Entraining Admixtures on Concrete Properties

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Page No.

5.6 Miscellaneous Admixtures 5.6.1 Pumping Aids 5.6.2 Corrosion Inhibitors 5.6.3 Shrinkage Reducing Admixtures

6.0 Waterproofing Admixtures

6.1 Integral Waterproofing Admixtures 6.2 Hydrophobic and Pore Blocking Ingredient

7.0 Commercial Admixture Products 7.1 Retarding to BS5075: Part 1 7.2 Normal Water Reducing to BS5075: Part 1 7.3 Accelerating Water Reducing to BS5075: Part 1 7.4 Retarding Water Reducing to BS5075: Part 1 7.5 Air Entraining to BS5075: Part 2 7.6 Superplasticizers to BS5075: Part 3 7.7 Retarding Superplasticizers to BS5075: Part 3 7.8 Pumpability Aids 7.9 Integral Waterproofers 7.10 Hydrophobic Pore Blocking Ingredients 7.11 Silica Fume 7.12 Curing Compounds

8.0 Review of Current Specification

8.1 Materials 8.2 Mix Proportions 8.3 Curing 8.4 Durability Tests

9.0 Recommendations 9.1 Chemical Admixtures 9.2 Mineral Admixtures 9.3 Hydrophobic and Pore Blocking Admixtures 9.4 Performance Requirements in the Mix Design

9.5 Compliance Tests 9.6 Curing

References

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Structural Engineering Branch - Arch S. D. 1 File code: Waterproof Admixture0.doc Technical Report – Structural Materials Group By: Eric P. W. Chan & Concrete Admixtures for Waterproofing Construction Peter N. L. Ho Issue No./Revision No.: 1/A Date: December 1999

1.0 Introduction The objective of this report is to review the effects of admixtures on concrete properties and provide some guidelines for adopting appropriate concrete admixtures for waterproofing construction. Since waterproofing concrete is a broad topic, this report only highlights the admixtures to be included in concrete for watertight construction. These structures include not only water tanks and swimming pools but also the basements and roof structures in watertight construction. As the admixtures enhance concrete properties in terms of strength and durability, the end product is also suitable for structures exposed to aggressive environments. Waterproofing materials such as membrane, spray coating and rendering that are applied after hardening of concrete are not dealt with in this report. 2.0 Definition of Admixtures Admixtures are ingredients in the form of powder or liquid, added to the concrete matrix during mixing. Additives that are blended into cement during manufacturing are not discussed here. 3.0 Requirements for Water-retaining Structures The Principle The basic requirements for water-retaining structures are simply to have dense and durable concrete that has sufficient resistance against hydrostatic pressure without seepage of water through the concrete elements. Concrete normally contains voids. Cracks are also formed due to stresses under external loading conditions, internal thermal and shrinkage strains and structural movements etc. Those are passages for water seepage. To overcome the above deficiencies, measures have been developed mainly to reduce the cracks and voids so that passages for water are minimized. The cracks in concrete due to stresses induced by external forces have very little to do with concrete properties. The structural design should be done to control the stresses so that the crack widths are minimized. For cracks due to thermal and shrinkage effects, good design which limits the cement content and proper control when placing and curing the concrete are required. Dense and durable concrete can be achieved by sound mix design with a low cement/water ratio yet highly workable mix supplemented by proper concrete placing and curing. However, there are always difficulties encountered on actual site, such as difficult access for concrete transportation, heavily reinforced sections and etc. On the other hand, stringent requirements


such as high strength concrete, low permeability and long durability for aggressive environments have to be fulfilled. To facilitate the construction work and to fulfil the requirements of the specifications, there is a need to have the aid of admixtures to produce such concrete. Pore Reducing Concrete is a composite material, which consists of cement and aggregates. The aggregates are dense materials that do not usually have permeability problem. It is the permeability of the cement paste that determines the water-tightness of the concrete. The gel pores are very small. It does not affect too much the concrete density. The capillary pores inside the cement paste together with the microcracks and macrocracks form capillaries and passages for water. The capillary pores formed during the early age of concrete can be blocked by further hydration during the curing period if there is sufficient hydration product produced later. The application of admixtures could reduce the amount of pores and make better dispersion of cement particles so that more dense concrete can be made. Water/cement Ratio Water/cement ratio is directly related to the mechanical and physical properties of concrete, and thus the durability. The lower the w/c ratio, the higher the concrete density, hence the higher concrete strength and durability. As mentioned above, hydration products formed during curing of concrete blocks the capillary pores. Concrete with high w/c ratio would have insufficient hydration products to block the capillary pores, and is therefore porous and permeable. With the use of admixtures, very low water content can be achieved and the porosity of concrete can be reduced. A figure showing the relation between water-cement ratio and permeability of concrete is shown in Figure 3.1

Figure 3.1 Relation between W/C Ratio & Permeability of Concrete


Regarding concrete durability, the degree of carbonation varies with water cement ratio. The higher the w/c ratio the deeper the carbonation depth. The relation between carbonation and w/c ratio is shown in Figure 3.2.

Figure 3.2 Relation between W/C Ratio & Carbonation

Workability Portland cement is in a form of agglomeration of particles held together by forces. During mixing process the agglomerates break down into fragments and hydration takes place at the surface of fragments. With the application of admixtures the hydrating cement on the surface of fragments deflocculates each other and reduces the inter-particle friction. This effect reduces the retention of water between particles. More workable concrete can be obtained or less water is required for a specified workability. As the inter-particle forces are reduced a more even dispersion of cement particles hence a more even hydration of cement can be achieved with an improvement to the density and strength of concrete. The relation amongst the workability, water cement ratio and strength of concrete is shown in Figure 3.3.

Figure 3.3 Relation amongst the Workability, W/C Ratio and Strength of Concrete


More workable concrete may be designed to have higher proportion of fine aggregates with low flakiness index such as river sand. With the presence of admixtures, the concrete can even be flowable and pumpable. However, bleeding of concrete may occur and require attention. Also the pressure of wet concrete on formwork is increased. Thus formwork design should acknowledge the rate of concrete pour. The workable concrete will allow complete compaction without segregation. Shrinkage Cement content should be limited so that the thermal and shrinkage effects during setting of concrete can be minimized. In order to reduce these effects, admixtures and or partial cement replacement may be used to achieve the purpose. Curing Curing of concrete is very important. A complete cement hydration process can only occur in proper curing condition so that high strength, dense and durable concrete can be achieved. It is equally important that some admixtures reacting in concrete, in particular those partial cement replacements, also require good curing conditions. Autogenous Healing Cracks in concrete with sizes not more than 0.2 mm can be healed. Further hydration in the presence of water produces hydration products to fill up the fine cracks. 4.0 Effects by Application of Admixtures in Water Retaining Structures The effects produced by admixtures for water retaining structures are listed below: I. Water reducing - To reduce water content. W/c ratio can be lower. Or higher cement

content can be used hence high strength dense concrete can be achieved. II. Plasticizing - To make flowable concrete. Workability can be increased. Or to

maintain the workability but to reduce cement content so that the thermal and shrinkage effects can be controlled.


III. Pore filling - Finely divided minerals to fill the voids in concrete to produce denser concrete and to replace part of the cement as they are mostly cementitious materials with pozzolanic effect. The thermal effect due to hydration can be reduced hence a reduction of shrinkage cracking. IV. Waterproofing – A combination of mainly water reducing, pore reducing and other ingredients to give greater plasticity and workability to fresh concrete. Some act as water repellent. 5.0 Types of Admixtures Concrete admixtures with related effects on watertightness of concrete are listed below: I. Retarding/water-reducing II. Superplasticizers III. Polymer IV. Mineral V. Air-entraining agents VI. Miscellaneous – Pumping aids, corrosion inhibiting, shrinkage reducing and fibre

admixtures VII. Waterproofers – Permeability reducing and water repelling 5.1 Retarding/Water Reducing Admixtures Retarding admixture delays the setting time of concrete. This is due to the lower hydration rate during the first hours. The retarding effect is appropriate for cases when extended time for concreting is required and longer time for difficult pours. It is because most of the retarding admixtures are water reducing; therefore retarding and water-reducing admixtures are often in the same category. Reduction of water in concrete results in an increase of strength and improvement on permeability. It is most widely used for water retaining structures.


5.1.1 Materials The materials in this group are: Lignosulphonates - may be visualized as a polymer of a substituted phenyl propane unit with hydroxyl (OH), methoxyl (OCH), phenyl ring (C6H5) and sulphonic acid (SO3H) groups. This group is the most popular material in commercial use. Hydroxycarboxylic acid – it has several hydroxyl (OH) groups and either one or two terminal carboxylic acids (COOH) groups attached to a relatively short carbon chain. This group is less popular than lignosulphonate. Carbohydrates – they include natural compounds such as glucose and sucrose or hydroxylated polymers. Other compounds – organic compounds such as glycerol, polyvinyl alcohol, sodium aluminomethylsiliconate, sulphanilic acid etc. 5.1.2 Effects of Retarding/Water Reducing Admixtures on Concrete Properties Mechanism When Portland cement is mixed with water, the cement agglomerates stick to each other due to insufficient mutual electrostatic repulsion. With the presence of retarding/water-reducing admixtures, the attraction between particles is reduced and the cement particles disperse, hence a more cohesive and consistent mix is achieved. The mechanisms on this effect are: I. Reduction of interfacial tension – The liquid-solid interface is thermodynamically

unstable compared to a flocculated state, and the adsorbed molecules make the transition from solid phase to the aqueous phase less abrupt.

II. Multi-layer adsorption of organic molecules – The adsorbed layer thickness

corresponding to several thousand of molecular layers changes the interparticle interaction energy.

III. Increase in electro-kinetic potential – Cement particles that do not migrate in an

electric field has been changed to carry negative charge. IV. Protective adherent sheath of water molecules – The negative charge on the cement

surface orients the water dipoles forming a hydrated sheath that prevents cement particles from coalescing.


V. Release of water trapped among cement particle clumps – This frees to add to the fluidity of fresh concrete.

The mechanisms on retarding effect are as follow: I. Retarding effect on cement hydration – The slower rate of ettringite formation

responsible for the reduction on water demand. II. Change in morphology of hydrated cement – The interlocking effect of the ettingite

bridges connecting solid particles results in improvement on rheological behaviour. Plastic Concrete To achieve a specified workability, this group of admixtures reduces the water requirement in concrete. The effectiveness of water reducing is in the order of gluconate > glucose > lignosulphonate. For a given slump, water requirement varies with temperature for concrete without admixture. However, for concrete with admixtures the variation of water requirement is small. Bleeding of fresh concrete is affected by the admixtures. For a given slump, lignosulphonate and particularly glucose reduce the rate and capacity of bleeding whereas sodium gluconate increases the capacity for bleeding even though the mixing water is reduced. This group of admixtures can alter the initial and final setting times. By changing the dosage, the vibration time limit can be delayed. Figure 5.1.1 shows the vibration time limit against penetration.

Figure 5.1.1 Vibration Time Limit for Setting of Mortar against Penetration

The vibration time varies with temperature. For any initial temperature of the concrete, the dosage can be adjusted to maintain the time for the vibration approximately constant. When used with other cementitious materials, such as pozzolanic or blast furnace cement, the water reducing effect is similar to Portland cement. The initial setting time is slightly increased but delayed in final setting effectively.


Over dosage of retarder and water reducing admixture would result in concrete not setting and/or a reduction in concrete strength. The compatibility of some cement type with a combination of water reducer and high-range water reducer admixtures should also be established in the trial mix. It has been reported that when high dosage of lignosulphonate water reducing admixture and a high-range sulphonated-naphthalene water reducing admixtures were added to cement with a low C3A content (<1.0%), it resulted severe set retardation with suppressed strength development. Hardened Concrete The reduction of water content leads to improvements on concrete mechanical properties such as compressive, flexural, tensile, shear and bond strengths, and abrasion resistance. This group of admixtures reduces the porosity of cement paste hence give a reduction to the permeability of the concrete. This is because of the higher degree of hydration of cement as a result of water reducing/retarding actions. The improvement also results in denser concrete thus durability and the resistance to sulphate and chloride attack is improved. Drying shrinkage at early ages is increased but decreases with time and possibly reversed after several months. 5.2 Superplasticizers Superplasticizers are high range water reducers with different chemical to normal water reducers. With superplasticizers, water reducing can reach about 30%, whilst the normal water reducers can reach 10 – 15% only. Very highly workable even flowing concrete can be produced and is used in tremie concrete. However, superplasticized concrete is associated with slump loss, the problem with segregation requires attention to the mix design. Considerations on formwork and concrete placing on sloped structure are also required. The good characteristics of water reduction and high workability are very useful in high performance concrete and superplasticizers are a suitable admixture for water retaining structures.


5.2.1 Materials The superplasticizers are broadly classified into four groups, viz., sulphonated melamine-formaldehyde condensate (SMF), sulphonated nahthalene-for-maldehyde condensate (SNF), modified lignosulphonates (MlS) and others including sulphonic-acid esters, carbohydrate esters, etc. 5.2.2 Effects of Superplasticizers on Concrete Properties Mechanism Suspensions of cement in water are in large irregular agglomerates of cement particles. In the presence of superplasticizers the material disperses into finer particles. The absorption of superplasticizer is high, some even in a few seconds. This relates to the workability of concrete.

Workability Superplasticizers influence the rheological behaviour. The viscosity and yield value of the fresh concrete are reduced in certain concrete mix proportions. In the presence of superplasticizers, the slump value can be greatly increased and the concrete is even flowable. Figure 5.2.1 shows the effect of dosage of superplasticizer on slump of concrete. Figure 5.2.1 Effect of Superplasticizer on

Concrete Slump Superplasticizers greatly improve the pumpability. However, for high slump concrete, in particular the flowing concrete, adjustment to the mix proportion is required to have sufficient sand in order to prevent segregation.


Normally the higher workability provided by superplasticizers could only be maintained for 30 – 60 minutes. Loss of slump occurs after that. Adding of superplasticizers has to be done at the point of discharge. The slump loss depends on the type of superplasticizer, the chemical and mineralogical composition of cement and the temperature. It may be restored by repeated dosages at certain time intervals. Figure 5.2.2 shows the effect of repeated dosages of superplasticizer on slump of concrete.

Figure 5.2.2 Repeat Dosages of Superplasticizer on Slump

Other method to retard the loss of slump is by combination of retarding admixture. Figure 5.2.3 shows the effect of a combination of superplasticizer and retarding admixture against the slump loss. Generally superplasticizers retard the setting of concrete. They can be used in combination of retarding admixture to produce a better effect than the retarding admixture alone. Normally, superplasticizers are found to be compatible with retarders, water reducers, accelerators and air-entraining agents. They are also found to be compatible with mineral admixtures such as silica fume and fly ashes. However, this has to be checked with manufacturers.

Figure 5.2.3 Effect of a Combination of Superplasticizer & Retarder against Slump Loss


Physical Properties The ability for water reduction is unique. Water content can be reduced by about 30%. The mechanical properties of concrete, the modulus of elasticity and the durability are increased. A figure 5.2.4 shows the strength development of a superplasticized concrete with various water cement ratio. The resistance of superplasticized concrete to sulphate attack and salt scaling is no different to normal concrete. However, the use of superplasticizer leads to a reduction of water content and an improvement on workability of concrete. This results in more dense concrete and improvement on durability. Figure 5.2.4 Strength Development of

Superplasticizered Concrete When used with silica fume, the extreme fineness of this material requires higher demand of water. Superplasticizer is the solution to this problem. The combination of these two materials together with other cementitious materials can produce very high strength concrete in the order of 100-150 MPa. 5.3 Polymer Admixtures Concrete made by the modification of ordinary concrete with polymer admixture has been developed for tensile strength requirement and resistance to certain chemicals. The polymer-modified concrete has a monolithic co-matrix in which the organic polymer matrix and the cement gel matrix are homogenized. 5.3.1 Materials Polymer admixtures include latexes, powdered emulsions, water-soluble polymers, liquid resins and monomers. Most polymer admixtures in dispersed form are added to concrete during mixing.


5.3.2 Effects of Polymer Admixtures on Concrete Properties The polymer modification in concrete is in three steps. Step 1 : When polymer latexes are mixed into concrete and dispersed into the water, at the

same time the cement gel forms as cement hydration takes place. The polymer particles are then partially dispersed onto the cement gel - unhydrated cement particle mixtures. The calcium hydroxide in the water phase reacts with the silica surface of aggregates to form a calcium silicate layer.

Step 2 : As the cement gel continues to developed, the polymer particles are confined to

capillary pores and flocculate to form a continuous closely packed layer on the cement gel - unhydrated cement particle mixtures and the silicate layer on the aggregate surfaces. The large pores are then filled by the polymer particles.

Step 3 : Further hydration of the closely packed polymer particles on cement hydrates

coalesce into continuous films that bind the cement hydrates together to form a monolithic network. The polymer phase interpenetrates throughout the cement hydrate phase. This mixture binds the aggregates and forms the polymer-modified concrete.

The polymer films in the cement hydrate phase bridge the microcracks in the cement paste and prevent crack propagation. It has high bond between cement hydrate and aggregate surface. The tensile and flexural strength and the fracture toughness are increased. The degree of improvement increases with the polymer content. However, excess polymer and air-entrainment cause discontinuities of the monolithic network, would not be beneficial. The sealing effect provided by the films improves the concrete properties on watertightness, chemical resistance and durability. 5.4 Mineral Admixtures Mineral admixtures are finely divided materials, which are added to concrete by large amount to replace part of the Portland cement. The combination of both pozzolanic and pozzolanic/cementitious materials with Portland cement has been used widely.


5.4.1 Materials Some mineral admixtures are only pozzolanic and others are pozzolanic & cementitious. Pozzolan is defined as siliceous or siliceous/aluminous materials with little or no cementitious values. But in finely divided form and in the presence of moisture, chemically reacts with the free lime in the Portland cement at ordinary temperature to form compounds possessing cementitious properties. All natural pozzolans and some industrial by-products such as low-calcium fly ash are in this category. Many industrial fly ashes and slags contain calcium for pozzolanic reaction. They are self-cementing but need more calcium to develop full cementitious potential and are classified as pozzolanic & cementitious. Four classes of mineral admixtures with an order of the level of reactivity with cement are listed below:

I. Cementitious and Pozzolanic – Granulated blast furnace slag and high-calcium fly

ash

II. Highly Active Pozzolans – Condensed silica fume and rice husk ash III. Normal Pozzolans – Low-calcium fly ash and natural materials

IV. Weak Pozzolans – Slowly cooled blast furnace slag, bottom ash, boiler slag, field-

burnt rice husk ash. With the exception of diatomaceous earth, natural pozzolans are derived from volcanic rocks and minerals, which include volcanic glasses, volcanic tuffs, calcined clays, and shales. Although many natural minerals are still being used in some part of the world (e.g. Zeolite minerals in finely ground tuff in China), many industrial by-products are the primary source mineral admixtures in use today. The common source of minerals from industrial by-products are fly ash, rice husk ash, silica fume or condensed silica fume, blast furnace slag and other slags. The use of these minerals results in ecological, economic and energy saving considerations. These minerals are described in more details below.


5.4.2 Effects of Mineral Admixtures on Concrete Properties Fresh concrete Generally the water content for mixing is reduced due to the inclusion of the fine particles of the mineral admixtures such as PFA and granulated blast furnace slag. But some coarse fly ash and fly ash with high loss-on-ignition, such as condense silica fume and rice husk ash, increase the water requirements. Bleeding may be reduced or eliminated by the addition of fine particles in the cement paste. Thus the reduction of the bleed water channels in concrete will decrease the tendency for segregation. As the mineral admixture replaces some of the Portland cement and fine aggregates, the content of the cement paste is increased hence the cohesiveness, workability and pumpability are also greatly increased. The inclusion of mineral admixtures retards concrete setting. The setting time varies with different mineral admixtures. The addition of pozzolans also requires longer curing periods before the benefits from the pore refinement process on strength and permeability become manifest. During curing of concrete heat produced due to cement hydration is lower for mineral concrete. Thermal stress can be reduced resulting in a reduction of thermal cracking. However, curing of concrete is important, as improper curing would affect the concrete strength development. Strength Development Concrete normally consists of voids and also microcracks at the transition zone between aggregates and cement paste. In the early stage of mixing the calcium silicate hydrate (C-S-H) phase in hydrated Portland cement is compact but contains capillary cavities or voids, which exist when the spaces originally occupied with water, do not get completely filled with the hydration products of cement. The mineral mixtures are very fine particles that reduce the voids and fill the microcracks in concrete. The pozzolanic reaction converts the compact C-S-H phase with large cavities to low-density C-S-H products with smaller voids. This pozzolanic cementitious reaction increases concrete properties in many ways. As the concrete is denser with the mineral admixture due to pore refinement, this also contributes to the improvement on the strength properties and its durability and permeability. Permeability As strength and permeability are inversely proportional to the volume of large pores in the hydrated cement paste, the considerable pore refinement leads to significant increase in impermeability of the concrete. The reduction in water diffusion depths improves the


resistance of concrete by chemical attack and the protection to steel reinforcement due to the higher concrete density. Durability Steel corrosion The low permeability of the concrete containing mineral admixtures also prevents the penetration of chloride ion, which will attack the steel reinforcement in the concrete. Generally the carbonation process from the carbon dioxide in the atmosphere will reduce the alkaline in the cement paste. Thus the corrosion process will start with a reduction in the alkalinity of the concrete. Pozzolanic reaction will consume calcium hydroxide and reduce the alkalinity of concrete. However the increase in impermeability of the concrete with mineral admixtures will improve the overall resistance to carbonation and thereby attack in steel by carbonation will be reduced. Acidic environment With the presence of large amounts of calcium hydroxide in the hydrated cement paste, Portland cement concrete is not durable in acidic environment. The mineral admixtures reduce the calcium hydroxide content of the cement paste and thus can improve the chemical resistance of concrete. Sulphate attack The presence of calcium hydroxide as well as certain aluminate and sulpho-aluminate hydrates in hydrated cement paste usually lead to deterioration of concrete when exposed to sulphate water. The rate of attack by sulphate ions can be reduced by limiting the proportion of tricalcium aluminate (C3A) in Portland cement. However the C3A content should not be less than 4 percent in order to avoid chloride attack on the reinforcement. An alternative approach is to replace at least 25% of Portland cement with PFA or to replace at least 70 % of the Portland cement with granulated blast furnace slag. Alkali-aggregate reaction The reaction between the soluble alkali from a high alkali Portland cement and certain forms of silica in the aggregates causes expansion and cracking. The partial replacement of the cement with mineral admixtures is found to be effective in reducing the risk of expansion due to alkali-aggregate reaction.


5.4.3 Silica Fume Condensed Silica Fume (CSF), also known as Silica Fume or Microsilica, is a by-product of the production of metallic silicon or ferrosilicon alloys in electric arc furnaces. CSF concrete has proved particularly beneficial where structures are subjected to attack in severe environments. These include hazardous ground conditions, marine exposure, chemical attack or physical degradation such as abrasion. Durability of the microsilica concrete is beyond the expectation of ordinary concrete. Usage in concrete CSF is usually added as an additional cementitious material at a percentage of the original cement content. The range of dosage for trial mixes is as follows: Pumping aid 2 to 3 % High quality 4 to 7 % High Strength* 7 to 15 % * Also for high impermeability and chemical resistance. Effects on Concrete Properties The superfine size of the condensed silica fume particles (mean particle size < 1 micron), 100 times finer than cement, combined with the very high content of reactive silica (> than 85% by weight), gives a powerful pozzolanic effect. The pozzolanic reaction of the condensed silica fume increases the calcium silicate hydrates (CSH) in the hardened concrete. There is a distinct change in the refinement of the pore structure in the condensed silica fume concrete giving less of the capillary pores and more of the finer gel pores due to the void filling action. Replacement of a small percentage (5 to 10 %) of the cement by CSF can significantly increase the impermeability and durability of the hardened concrete. Figure 5.4.1 shows the relation between permeability and microsilica content.

Figure 5.4.1 Relation between Permeability And Microsilica Content


Pumpability, cohesiveness and adhesion of the mix will be improved and bleeding will also be reduced to virtually nil. However, due to the large increase in surface area of the superfine particles, the addition of condensed silica fume will also require a higher dosage of superplasticier for a given workability. The increase in CSH leads to a reduction in capillary pores and contributes to the condensed silica fume concrete two major characteristics, increased strength and increased impermeability. This dual effect gives a concrete with increased resistance to physical attack; abrasion, erosion and impact damage; and to chemical attacks; water penetration, sulphates, chlorides, organic materials and acids. Sample Mix Proportions The table below shows the properties of hardened concrete in different mix proportions for different percentage of condensed silica fume that had been used in practice. Material (kg/M3) Mix 1 Mix 2 Mix 3 Mix 4 Cement 400 345 400 386 Microsilica 8 (2%) 35 (10%) 40(10%) 70 (18%) Aggregates 1028 1180 58% 971 Sand 860 960 42% 824 Water (litres) 165 152 167 166 Admixture (litres) 8 3 5.8 6 Water/Binder ratio 0.40 0.40 0.38 0.28 28-day Strength (MPa) 65 54.5 80 83 Slump (mm) 250 100 250 Water permeability - DIN 1048 Zero Chloride Permeability – ASTM 1202

481 coulombs

Chloride Diffusion The testing of chloride diffusion into concrete is a measurement indicating the quality of hardened concrete in terms of permeability/durability. Absorptivity or capillary suction is primary mechanism of water and salt penetration under conditions of wetting and drying or partial immersion. There is no such testing method in the British Standard.


However ASTM i.e. the AASHTO does have test (T277), to measure the variation of electric current passing through the concrete sample. Ordinary concrete will exhibit a range of 3000 to 4000 coulombs under the test. With the introduction of condensed silica fume, a five-percent replacement is able to lower the result to 1000 coulombs whereas a ten-percent replacement is able to lower the result further to 500 coulombs. Figure 5.4.2 shows the chloride ion permeability as a function of microsilica dosage.

Figure 5.4.2 Relation between Chloride Ion Permeability and Microsilica Dosage

5.4.4 Rice Husk Ash Rice husk ash is a by-product from combustion of rice shells produced during the de-husking operation of paddy rice in industrial furnaces and must be ground to fine particle sizes to develop pozzolanic property. The ash produced at low temperature is highly pozzolanic. 5.4.5 Pulverized Fuel Ash Pulverized Fuel Ash (PFA) is produced from the combustion of pulverized coal in coal fired power stations. Raw PFA is taken out of the furnace from the hot air stream by means of electrostatic precipitation. The particle sizes of the fly ash vary from under 1 µm to typically under 20µm. A simple and inexpensive way of reducing the cement content is to replace part of the cement by PFA. A maximum cement replacement of 25 % of the cement by weight causes little change in concrete strength (at 56-days) and workability. The reduction in OPC results a reduction in heat generation, thus shrinkage and thermal cracking during hardening can also be reduced.


The particles are generally finer than the cement. Thus they can fill up the inter-granular spaces between the cement particles. PFA contains about 65% of silica, which reacts with the lime during hydration of cement to produce more gel products to fill up the capillary pores. This filling effect reduces the pore sizes and thus the permeability of the hardened concrete. Secondly being lighter and finer, PFA helps reduce sedimentation of the solid particles and thus the bleeding and segregation of the fresh concrete mix. 5.4.6 Ground Granulated Blastfurnace Slag Blastfurnace slag is a waste product in the production of pig iron. Chemically, it is a mixture of lime, silica, aluminia and magnesia, i.e., the same as Portland cement but in different proportion. If slag is cooled slowly in air, it does not react with water at ordinary temperature. It has weak cementitious and pozzolanic properties if ground to very fine particles. However, when the liquid slag is rapidly quenched from high temperature by water the product is called granulated slag. The use of ground granulated blastfurnace slag (GGBS) as a cement replacement (usually varies from 25 to 75% by mass) is primarily as a result of environmental and economic considerations and also the enhanced concrete performance that can be achieved. The particle sizes of the slag should contain considerable particles below 10µm to contribute early strength development with others between 10µm and 40µm to continue the hydration thereafter. Influences of GGBS on properties of concrete GGBS concrete will normally exhibit a longer setting time than the corresponding OPC concrete at temperatures below 30°C because of the relatively slower hydration rate. Similar grade concrete containing GGBS generates less heat than OPC mix during the hydration process. This results in reducing thermal and shrinkage strain. GGBS concrete will require less water than the corresponding OPC concrete due to the low water absorption of the GGBS for concrete with the same workability. A reduction of up to 5% of mixing water can be expected. GGBS can also improve different properties of concrete especially in terms of various durability aspects i.e. reduction in temperature rise, sulphate resistance, chloride diffusion and alkali-silica reaction etc. A GGBS + OPC concrete contains more silica and less lime than OPC alone. As a result, the microstructure is much denser. Thus GGBS concrete has lower early strength than OPC concrete but the long term gain in strength is much higher. Concrete containing 50 to 65 percent GGBS by weight of total cementitious material exhibits somewhat greater drying shrinkage than plain Portland cement concrete.


Durability The incorporation of GGBS, normally in excess of 50% by mass of cement, in concrete improves its resistance to sulphate attack and chloride penetration when exposed to sulphate and marine environments. The improvement is attributed to the lower permeability from a denser microstructure and also the alumina content of the concrete mix. Slag concrete can be used in class 4 sulphate conditions whereas PFA concrete cannot be used in that situation. The use of GGBS has generally been accepted as a possible means to reduce the deleterious expansion caused by alkali-aggregate reaction due to the low permeability and lower release of alkali from GGBS concrete.

Figure 5.4.3 Chloride Penetration by Diffusion; OPC and GGBS Concrete

Demand of GGBS is limited in Hong Kong; thus only one ready mixed concrete depot currently stocks the materials. However GGBS is usually cheaper than OPC and PFA. The chloride penetration by diffusion for OPC and GGBS concrete is shown in Figure 5.4.3 5.4.7 Other Slags These include the steel slags, the by-product of the conversion of pig iron to steel, and those in the production of metallic copper, nickel and lead in various smelting furnaces. All these slags contain unusually high content of iron oxide and slow pozzolanic behaviour. 5.4.8 Combined Use of Mineral Admixtures More than one mineral admixture are usually used together and combined with chemical admixtures to produce concrete with various strength and durability levels. The following combinations are very often used: I. PFA + CSF II. CSF + GGBS


In evaluating the results of current research work and mix design from concrete suppliers, it appears the combination of fly ash with reduced quantities of condensed silica fume plus the chemical admixtures offers very cost effective and superb results in strength and low chloride permeability. 5.5 Air-Entraining Admixtures Air-entraining agent produces air bubbles in the cement paste of concrete. The existence of bubbles improves the workability of concrete. In the hardened state, the bubbles provide resistance to frost action and the detrimental effects of de-icing salts. Its advantages are utilized in watertight construction. 5.5.1 Materials Air-entraining agents are a type of chemical namely surfactant which is a surface-active substance. 5.5.2 Effects of Air Entraining Agents on Concrete Properties In hardened concrete paste, water is present on the surface of solid particles, in the gel pores and the capillaries. In the situation when the areas are saturated, frost action can cause failure of the saturated concrete. The bubbles produced by air-entraining agent are 0.05 mm to 1.25 mm in diameter. They are in much larger size than the capillaries and provide discontinuity of the capillaries. Due to the surface tension force of water, water from the capillaries can not fill the voids of the bubbles. Under freezing condition, the frost action can do no damage to the cement paste. When thawing, surface tension of water causes the water inside the bubble to withdraw back to the capillaries. The same happens when de-icing salts are used, the freezing zone in the concrete is shifted further into concrete away from the solution of de-icing salt on the concrete surface. The air bubbles lubricate the fresh concrete. It improves the workability of concrete and some times is used as a plasticizer. Although the air bubbles produce voids in concrete, the increase of workability leads to a lower w/c ratio; hence an increase of strength is obtained. As the bubbles are not interconnected, the absorption and permeability characteristics are not impaired and are found improved.


5.6 Miscellaneous Admixtures 5.6.1 Pumping Aids In the case where concrete has to be transported by pumps, concrete needs to be highly workable to avoid blocking of pumps. Three types of pumping aids are listed below: I. Water-thickening Type – Materials include polyethylene oxides, cellulose ethers and

alginates. Their functions are to increase the viscosity of water and/or to flocculate the cement particles

II. Air-entraining plasticizing type – Synthetic materials such as neutralized wood resin

and alkyl sulphonate that have surface-active structures to produce air bubbles to lubricate the concrete mixtures.

III. Non-air-entraining plasticizing type – Materials include lignosulphonates, salts of

hydroxy carboxylic acids, melamine formaldehyde condensates and sulponated napthalenes. They are surface-reaction agents which when absorbed onto the cement particles, leave partly charged groups on the surface to repel the adjacent cement particles, providing better dispersion of cement particles in suspension.

Type I or II admixtures are used and the improvement of pumpability is noticeable. For richer mixes with high friction of grout in the concrete would require an admixture of type III to modify the flow characteristics of cement to obtain optimum flow conditions. This type of admixture does not have direct effects on watertightness in waterproofing concrete but it does facilitate the construction on pumping work. 5.6.2 Corrosion Inhibitors Corrosion inhibitors are chemical compounds that interfere with the corrosion reaction preferentially at the anodic or cathodic sites or both. The ability to accept electrons is provided by anodic inhibitors. Materials include calcium and sodium nitrite, sodium benzoate and sodium chromate. Dosage of this type is critical. The effect is only achieved in high concentrations. When used with insufficient quantity the steel corrodes. Too much quantity causes pitting locally. It is therefore not recommended to use this type. Cathodic inhibitors act either by slowing the cathodic reaction or by selectively precipitating cathodic sites. Materials are bases that increase the pH of the medium. However, a combination of the two types would have a more effective result.


Water retaining structures are subjected to either saturation or wet and dry conditions. Reinforcement corrosion is a major concern. Should this type of admixture be included in the concrete, a longer lasting life of the water retaining structure can be achieved. 5.6.3 Shrinkage Reducing Admixtures This type of admixtures is used to offset the shrinkage of concrete in plastic and hardening states by an expansion producing effect and is in three categories: I. Gas forming or gas generating materials to control settlement and provide expansion

in the plastic stage of concrete. II. To control settlement by providing expansion in the plastic and hardening states.

These are composed of calcium sulphoaluminate and lime based materials. III. Not controlling settlement, but provide expansion only in the hardened state. These

consist of granulated iron and chemicals that promote oxidation of iron in the presence of moisture and air.

The gas forming type generates bubbles in concrete during the plastic state to produce an expansion effect. Once the concrete is set, little effect would be gained. The applications are mainly for grouting of base plates and void filling in repair works. The type with granulated iron filings causes expansion after hardening of concrete. The iron particles and rust promoting chemicals produce oxidation of the particles during the first few hours of hardening of concrete to provide volume increases to compensate the settlement before initial set. Its applications are also for grouting base plates and hard wearing concrete floors. The expansion effect produced by the second type is the crystal growth in the formation of ettringite in the liquid phase of cement. Most of the expansion effect takes place at the wet curing stage and decreases with time in air curing. Its applications are suitable for large floor construction in minimizing construction joints and for water retaining structures. Admixtures of the above which function as expanding materials to reduce shrinkage are expansion producing admixtures. However, there are admixtures that do not cause expansion of concrete but control the shrinkage. During concrete curing the capillary tension of pore water is a main cause of dry shrinkage. These types of admixtures can reduce the capillary tension so that shrinkage can be reduced. The cracks caused by shrinkage force are also reduced. It is suitable for large floor construction and water retaining structures.


6.0 Waterproofing Admixtures There are two ways for water to penetrate through the concrete. When concrete is under hydrostatic pressure on one surface, water passes through the channels formed by the interconnecting cracks and voids to the other surface. The other way for the passage of moisture through the concrete from the wet side to the dry side is by capillary action. It should be noted that currently there are no nationally recognized standards covering the performance and use of these admixtures. 6.1 Integral Waterproofing Admixtures An integral waterproofing admixture is a combination of admixtures that have the ability of producing concrete with reduced permeability. Generally, waterproofing admixtures are classified in the following groups: I. Finely divided solids such as fullers earth, talc, bentonite and other siliceous powder

which are inert pore filling materials. II. Chemically reactive finely divided solids such as the mineral admixtures mentioned in

section 5.4. III. Conventional chemical admixtures such as water reducers, accelerators, air entraining

agents, and superplasticizers. The combination of the above admixtures results in lower water content, highly workable and dense concrete with reduced permeability. 6.2 Hydrophobic and Pore Blocking Ingredients Water Repellents or Hydrophobers These are damp proofing admixtures that can produce hydrophobic effects to repel water. Normally, water surface tension keeps water inside the pores. However, in the presence of certain type of permeability reducing admixtures, the surface of the concrete and the pores are coated with a layer of molecules in the case of stearic acid and other fatty acids, or a layer of coalesced or separate particles of materials in the cases of waxes and bitumen.


Materials in this group reduce the passage of water through the dry concrete which would normally occur as a result of capillary action and not as a result of an external water pressure. The water repelling admixtures alter the surface tension force of water to produce hydrophobic effects. The end result is that the hydrophobic coating produces a reversed angle of contact so that the surface tension forces push the water out of the capillary pores in the concrete. Thus, the concrete can be kept dry. The admixture is only good for resisting water splash such as in the tidal zone on concrete walls. They are not designed to resist hydrostatic pressure. Water repellent admixtures should be used in conjunction with a water reducer and cement content of at least 330 kg/m3 for optimum effect. Pore Blocking Admixtures These admixtures rely on the physical blocking the pores and capillaries in the concrete. The addition of bituminous or wax emulsions to concrete results in colloidal particles collecting in the capillaries formed by bleed water. When subjected to water under pressure the emulsion particles are pushed into the capillaries to a point where they form a physical plug preventing further penetration. The effectiveness of such a system depends on the volume of the capillaries to be blocked and the amount of blocking materials added to the mix. Hydrophobic and Pore Blocking Ingredients Some admixture systems combine both water repellent and pore blocking effects to provide both water repellent and pore blocking effects to provide protection against both capillary suction and water under pressure. This effect of high contact angle can resist hydrostatic head up to 14 metres as claimed by a product manufacturer before water can penetrate through the surface capillaries. It sounds a good water-repellent. However, in actual concrete structures the presence of voids in concrete is inevitable, and the incomplete hydrophobic coating can greatly reduce this resistance. It is therefore essential that the quality control in the mix design and on site in terms of concrete placing is more stringent in order to minimize the defect in the concrete. These admixtures are very expensive but are considered suitable in special situations such as casting basement walls against contiguous bored piles or fast track construction with a tight time frame that prohibit the installation of waterproof membranes and where difficult site conditions are encountered. There is however still a number of other concerns in adopting these types of damp proofing admixtures. These areas that should be thoroughly investigated on a specific nature and on a project basis to see if they are suitable for a particular project, are listed below:


I. If the admixture is so effective in stopping water movement within concrete and from outside, internal self-dessication may occur or imbibing water cannot enter. In both cases a portion of cement cannot be hydrated completely. Thus the initial curing must be properly done to avoid loss of strength.

II. The autogenous healing which may be able to seal creaks up to 0.2mm and also

leakage at construction joints in normal concrete may however be less effective for concrete containing water-repellent admixtures.

III. The construction joints will require special coating to break the water-repellent effect

and the quality of the treatment depends on site quality control. IV. The dosage of the admixture must be carefully administered on site. V. The reinforcement based on the design of water retaining structures will be adequate

for the use of this admixture. However for roof structures designing to BS8110 additional steel is needed if water repellent admixtures are used to replace the traditional waterproof membrane.

Some commercial products claimed to have a complete system for waterproofing are briefly discussed below: Everdure Caltite This Cementaid's product was developed in 1950's in Australia and the company has a long list of job reference for projects around the world. The waterproofing system obtained a British Agr�ment Certificate in 1994. Its hydrophobic poreblocking ingredient is claimed to be a highly effective water repellent/permeability reducer preventing the flow of water and moisture under pressure. The system is much more expensive than 'ordinary concrete'. However the supplier claims that the avoidance of tanking and drained cavity space, can lead to a more competitive overall construction. The supplier also offers guarantees and supervision on site. Proof Marine This hydrophobic waterproof system is marketed by Master Builders Ltd. Proof Marine is very much a Caltite clone with similar chemistry. Conplast WP500 The manufacturer, Fosroc claims the system is different in chemistry to Caltite and states the problem at the construction joint does not exist.


7.0 Commercial Admixture Products Commercial products that are available or can be supplied to the ready mix concrete depot in Hong Kong are listed below for reference. It should be noted that the performance requirements and uniformity tests that required by the relevant British Standard are carried out by the manufacturers and compliance certificates should be obtained from the manufacturers. Products not complying with the relevant parts of BS5075 but complying with other standards, such as ASTM, are marked with an asterisk. 7.1 Retarding to BS5075: Part 1 Manufacturer Brand Name 1 2 3 4 5 6 7 8

Grace Daratard88# Liq Lignosulphonate

2-4 S 1 yr. 1.18 .002

Grace Daratard17# Liq Lignosulphonate

0.4-1.2

S 1 yr. 1.25 .01

Grace Daratard17D# Liq Lignosulphonate

2-4 S 1 yr. 1.10 .002

Master Builder Pozzolith 300R Liq Lignin-gluco 1-3 N 1 yr. 1.24 N Nil Sika Sika Retarder Liq Phosphate 2-4 yes 1.13 Nil 7.2 Normal Water Reducing to BS5075: Part 1 Manufacturer Brand Name 1 2 3 4 5 6 7 8

Grace WRDA88# Liq 2-3 Y 1 yr. 1.1 .002 Grace WRDA+Hycol# Liq 0.6-1 Y 1 yr. 1.19 .004 Master Builder Pozzolith 80 Liq Lignosulpho

nate 2-4 N 1 yr. 1.14 N Nil

Master Builder Pozzolith 322N Liq Lignosulphonate

2-4 N 1 yr. 1.21 N Nil

Master Builder WP74# Liq

Tricosal BV# Liq Lignosulphonate

1 N 1 yr. 1.2 N 0.032


7.3 Accelerating Water Reducing to BS5075: Part 1 Manufacturer Brand Name 1 2 3 4 5 6 7 8

Master Builder Glenium SP8N Liq 2-12 N 6 m 1.05 N Nil Master Builder Rheobuild 1000 Liq Polymer 3-5 N 1 yr. N Nil 7.4 Retarding Water Reducing to BS5075: Part 1 Manufacturer Brand Name 1 2 3 4 5 6 7 8

Grace MIRA Liq Org.Polymer 3-7 N 1 yr. 1.16 Nil Master Builder Pozzolith 311RD Liq Lignosulphonate 2-5 N 1 yr. 1.06 N Nil Master Builder Rheobuild 600 Liq Lignosulphonate 2-5 N 1 yr. 1.15 N Nil Master Builder Pozzolith748ME# Liq Sika Plastiment-VZ# Liq Lignosulphonate 1.5-3 Inf. 1.13 Nil Tricosal VZ020# Liq Lignosulphonate 24 N 1 yr. 1.22 N 0.023

7.5 Air Entraining to BS5075: Part 2 Manufacturer Brand Name 1 2 3 4 5 6 7 8

Master Builder Microair VR* Liq Vinsol Resin .05-.5 Y 1 yr. N Nil Master Builder Rheomix 700A* Liq 0.5- 1 Y 1 yr. 1.00 N Nil Sika Sika Aer 0.1-.6 Y 2 yr. 1.01


7.6 Superplasticizers to BS5075: Part 3 Manufacturer Brand Name 1 2 3 4 5 6 7 8

Grace ADVA Liq Org. polymer 1-3 N 1 yr. 1.06 Nil Grace Super 20 Liq NSFC 2-8 N 1 yr. 1.21 Nil Master Builder Rheobuild 1100* Liq BNS 3-5 N 1 yr. 1.21 N Nil Master Builder Glenium SP8S Liq PEP 3-10 N 6 m 1.02 N Nil Sika Sikament-NN Liq NFS 2-12 Inf. 1.2 Nil Sika Sikament-LA Liq Synthetic 2-14 Inf. 1.2 <0.1Tricosal Tricosal Fluid Liq Lignin/naphthal 1.8 N 1 yr. 1.12 N 0.013

Tricosal TCAcosal Fluid 307 Liq Melamine 2.8 N 1 yr. 1.12 N 0.006

7.7 Retarding Superplasticizers to BS5075: Part 3 Manufacturer Brand Name 1 2 3 4 5 6 7 8

Grace Daracem 100 Liq NSFC 2-7 N 1 yr. 1.20 Nil Master Builder Rheobuild 561 Liq BNS 3-5 N 1 yr. 1.18 N Nil Sika Sikament-R4 Liq Synthetic 2-8 Inf. 1.18 Nil Sika Sikament-163EX Polymer 2-8 1 yr. 1.18 Nil 7.8 Pumpability Aids Manufacturer Brand Name 1 2 3 4 5 6 7 8

Sika Sika Pump Liq polymer 2-4 S 1 yr. 0.99 T Tricosal Tricosal Fluid Liq Lignin/naphthal 3.6 N 1 yr. 1.12 N 0.013

Tricosal TCAcosal Fluid 307 Liq Melamine 4.0 N 1 yr. 1.12 N 0.006


7.9 Integral Waterproofers Manufacturer Brand Name 1 2 3 4 5 6 7 8

Grace Hydratite WR Liq Liq Mod.Polymer 3-4hr S 1 yr. 1.15 Nil Master Builder Rheomac 788 Liq 1-3 N 1 yr. 1.18 N Nil Master Builder Rheomac 725 Liq 1-4 N 1 yr. N Nil Sika Sika-1 Liq 2-4 2 yr. 1.05 N Nil Sika Plastocrete N Liq Lignosulphonate 1-4 Inf. 1.19 T Nil Tricosal Tricosal Normal Liq Proteine 2.8 Y 1 yr. 1.03 N 0.121

7.10 Hydrophobic and Pore Blocking Ingredients Manufacturer Brand Name 1 2 3 4 5 6 7 8

Cementaid Caltite# Grace Darapel Liq Stearate S 1.02 Nil Grace Hydratite WR Pdr Stearate S Nil Fosroc Conplast WP500 Liq 4-6 1 yr. 1.10 Nil Master Builder Proof Marine Liq 15-30 S 1 yr. N Nil Tricosal BC Dichtungsmitted

Mowilitl Liq N 1 yr. 1.04 N 0.013

7.11 Silica Fume Manufacturer Brand Name 1 2 3 4 5 6 7 8

Grace Force 10000D Pdr Silica Fume 5-10% N 2.2 Nil Grace Force 10000 Liq Silica Fume 5-10% N 1.4 Nil Master Builder MB-SF Pdr Silica Fume N 1 yr. N Nil Sika Sikafume# Silica Fume


7.12 Curing Compounds Manufacturer Brand Name 1 2 9

Grace Daracure CM90A Liq (S) Resin >90% Master Builder Masterkure

181FD# Liq (S) Resin 85%

Master Builder Masterkure 191 Liq Resin 95% Remarks 1 Liq=Liquid, Pdr=Powder, W=water based, S=solvent based 2 Active ingredients 3 Dosage in litres per M3 of concrete 4 Contains air entraining properties Y=yes, N=no, S=slight 5 Shelf life 6 Specific gravity 7 Safety precautions T=toxic, N= non-toxic 8 Chloride ion content by mass of admixture 9 Curing efficiency # Product on Arch S. D. lists of accepted/probation materials or materials on trial 8.0 Review of Current Specification 8.1 Materials In clause 6.36 of the General Specification (GS), it is stated that accelerating, retarding, normal water reducing admixtures and the combinations of these shall comply with BS 5075. There is no mention of the superplasticizers in admixtures in the GS.


Also, cement replacement materials such as ground granulated blastfurnace slag and silica fume have not been included in the GS. 8.2 Mix Proportions For reinforced concrete liquid retaining structures with nominal maximum aggregate size in 20mm or less, a comparison of the performance specification of the concrete mix design of Arch S. D. and other specifications is listed below: Mix Proportions Arch S. D. BS8007 BS6349 CED MTRC

Concrete grade (MPa) 30 35 45 40(Cat. A)

Min cementitious content (kg/m3)

325 325 (OPC) 400 (OPC) 380 265+PFA

Max cementitious content (kg/m3)

400 400 (OPC) 450 330+PFA

Max cementitious content with PFA (kg /m3)

400 450 450 440

Max water/cement ratio 0.45 0.55* 0.45 0.38 0.38

Min slump (mm) 75 Ensure full compaction

Ensure adequate compaction

75 >140

Remarks Liquid retaining structures

*0.5 when PFA is used

Maritime structures exposed to sea spray

Marine Environment

1500 coulombs at 35 days & water absorption <1.5%

There is a very low demand for the application of admixtures to produce a mix that complies with the Arch S. D. specification. Conventional retarding and mid-range water reducing admixtures are used with a small proportion of polymer and superplasticizer in the mix design and the mix works well in many situations. However there have been a number of failures that require making good mainly due to poor workmanship. Another problem associated with the mix design is that quite often although the slump arrives on site within the acceptable limit it drops to a much lower value after say one hour. To help rectify the situation, it will be preferable if the initial slump can be increased to 150mm minimum. This can be achieved with the application of high-range water reducing admixtures or high quality superplasticizers.


Discussions have been held with a number of admixtures/concrete suppliers. Most of them share the same view that the specified minimum slump value dictates the range of concrete mix design. This is because the slump value controls the concrete price. Most contractors would mainly consider the lowest possible cost of the concrete that complies with the minimum requirements of the specification, rather than the quality of the mix which improves the working conditions. With a specified slump value of 75 mm, the choice of admixtures and the range of mix are limited and so is the concrete performance. An increase in slump value would give more freedom in concrete mix design to produce more workable concrete with the use of admixtures. Furthermore, the maximum cementitious content of 400kg/m3 is less than the 450kg/m3 that allowed in BS 8007 for water retaining structures if PFA is used in the mix design. It would therefore be preferable if the cementitious content was increased from the current 400kg/m3 to 440kg/m3 and the use of PFA is made compulsory. 8.3 Curing To ensure low permeability, curing is very essential for the concrete to complete the hydration process. This is particularly important if the cement content in the mix is high and/or the water/cement content is low, as is the case for concrete in watertight construction. It should be noted that BS6349 requires that the curing period recommended in BS8110 be increased by 50% to ensure low permeability. Arch S. D. specification requiring to maintain a moist surface for four days complies with the above requirements in view of the average curing temperature in Hong Kong. However, the increase in curing period by 25% when PFA is used in the GS Clause 6.52 is less than the increase in the range of 33% to 75% that are specified in BS8110. According to the study conducted by PWCL in 1995, if concrete with 25-35% PFA replacement is water cured for 7 days then air cured, there is little effect on the 28-day strength. Thus it is beneficial to increase the curing period in Arch S. D. specification to 7 days for practical purposes when PFA is used. Moreover, a more stringent curing regime including adequate enforcement on site must be established for watertight construction. The traditional method of wetting and with polythene covers is seldom carried out properly nor maintained adequately. Sprayed curing compounds are rarely used, mainly due to the cost considerations. Proper curing is particularly important when slags or silica fume are included in the concrete mix design.


8.4 Durability Tests For concrete in watertight construction, the concrete strength is not a major concern. Instead, the durability and impermeability are more important. There are tests in various standards to verify the concrete properties in this respect. For water absorption test, BS 1881: Part 122 Method for Determination of Water Absorption is normally used. For chloride permeability test, there are no British Standards. However a number of test standards exist in United States namely; AASHTO T277 Rapid Determination of the Chloride Permeability of Concrete; ASTM 1202 Electrical Indication of Concrete Ability to Resist Chloride Ion Penetration; AASHTO T259 Resistance of Concrete to Chloride Ion Penetration with long term ponding test; and Chloride Diffusion Tests as recommended by Taywood. It should be noted that the above tests might not represent the actual site conditions. For example, absorption tests measure the permeability of concrete under flow pressure in a saturated condition. It does not cater for capillary suction, which occurs in unsaturated conditions. In saturated concrete, the supply of oxygen for steel reinforcement corrosion is limited. But in unsaturated concrete the supply of oxygen and chloride to the steel reinforcement through the capillaries is great. Under wet and dry conditions the situation would be worse. Steel corrosion would occur in early age concrete. The AASHTO T277 method of measuring the extent of chloride ion in terms of coulombs passed as a durability value is adopted by US standards. In the chloride diffusion tests, the specimen is first immersed in salt solution, and then the surface is ground off in increments of 1mm and the salt content is determined for each layer. From these results the chloride content profile and the depth of concrete free from chloride attack can be established. However, it has been reported that further studies are required by PWCL for the repeatability and correlation of the test results. Thus these test are intended as reference tests rather than acceptance tests. In view of the above, it is believed that appropriate tests to verify the concrete quality for durability are yet to be developed. Thus the adaptation of one of the above standards will only give a relative indication of the concrete durability.


9.0 Recommendations This report examines the effects of chemical admixtures and mineral admixtures on fresh and hardened concrete and also reviews the criterion for adopting appropriate admixtures in concrete in watertight construction. A review of the Arch S. D. specification and other specifications relating to the admixtures materials, concrete mix proportions in watertight construction, curing and durability tests are also included. It is recommended that the following suggestions be further discussed so that the advance in concrete technology to produce high performance and durable concrete can be applied to enhance the structural performance of Arch S. D. projects. 9.1 Chemical Admixtures I. It is recommended that concrete admixtures such as superplasticizers to BS 5075: Part

3: 1985 be included in the General Specification so that concrete suppliers are allowed a wider choice of admixtures in the concrete mix design. The specification would preferably be reviewed to allow application of superplasticizers, including second dosages if necessary on site.

II. It should be noted that currently there are no British Standards on integral

waterproofing admixtures or hydrophobic type admixtures. 'Waterproofing' type admixtures, that are mainly water reducing admixtures conforming to BS 5075: Part 1, are likely to produce concrete with reduced permeability and therefore can be considered acceptable.

9.2 Mineral Admixtures I. It is recommended that the compulsory use of PFA as a constituent in the concrete mix

design be specified for partial cement replacement in watertight construction to help reducing thermal cracking.

II. If high performance concrete is intended for specific durable structures with improved

impermeability, reduced water/chloride absorption, and improved abrasion resistance, etc. then silica fume should be specified.

III. The use of ground granulated blastfurnace slag, as a cement replacement should be

considered in areas of high sulphate conditions.


9.3 Hydrophobic and Pore Blocking Admixtures These damp proofing and pore blocking types of water repellant admixtures may suit some projects of a special nature. However due to its deficiency associated with the hydrophobic character, it should not be used until its properties are better understood. 9.4 Performance Requirements in the Mix Design To encourage a wider application of admixtures to improve the workability and impermeability of concrete, it is suggested that the concrete mix in the current specification for watertight construction be revised to type C35S shown below. The performance requirements can be achieved with a higher quality superplasticizer or high-range water reducing admixture, rather than the current mid-range water reducing admixture, with a minimal increase in cost. For salt water tanks, manhole and waste traps that are suspended to form part of the building structures, beach structures subject to sea water spray, special structures that are exposed to chemical cleansing such as slaughter houses and structures requiring long term durability, it is suggested that concrete mix in type C45S below shall be considered. The performance requirements can be achieved with a higher quality superplasticizer, without the addition of silica fume. If it is necessary to ensure that the mix must perform well for the maximum durability, permeability tests such as the chloride diffusion test should then be specified. In such cases, 5 to 10 % of silica fume should be added to the mix design. This will have only a moderate increase in cost of the ready mix concrete. MIX PROPORTIONS C35S C45S

Concrete grade (MPa) 35 45

Minimum OPC content (kg/m3) 265 265

Maximum OPC content (kg/m3) 330 330

Maximum cementitious content (kg/m3) 440 440

Water cement ratio 0.45 0.38

Minimum slump (mm) 100 100

Remarks Water tanks, swimming pool and basement construction

Salt water tanks, suspended manholes and maritime structures


9.5 Compliance Tests I. The compliance tests certificates of the admixture as specified in BS5075 shall be

obtained from the concrete suppliers or the admixture manufacturers. The manufacturers shall also provide the results of any uniformity tests. Where two or more admixtures are used in a concrete mix, the compatibility shall be verified, if necessary by trial mixes, by the supplier.

II. It should be noted that a more stringent quality control should be established on site

and at the ready mix concrete depot to ensure that the procedures in the application of admixtures are strictly in accordance with the manufacturer's recommendations.

III. There are no British Standards available for the chloride diffusion test. If testing is

required for quality control, the sampling/testing procedures such as the AASHTO T277 to ASTM standard shall be examined and agreed with the testing laboratories.

9.6 Curing I. When PFA is used in the concrete for water retaining structures it is suggested that the

curing period shall be increased to 7 days. II. A more stringent curing regime such as adequate enforcement on site shall be

established for watertight construction. If the traditional wetting method is consistently abused or if silica fume is specified for durability reasons in the mix design, sprayed curing membrane should then be specified.


References: 1. Admixture for Concrete: Improvement of Properties - E. Vazquez, Chapman & Hall 2. Concrete Admixtures - Peter Russell, Cement and Concrete Association 3. Concrete Admixtures Handbook: Properties, Science, and Technology - V. S.

Ramachandran, Noyes Publication 4. Concrete Admixtures: Use and Applications - M. R. Rixom. The Cement Admixtures

Association, The Construction Press 5. Chemical Admixtures - American Concrete Institute Compilation 23 6. Chemical Admixtures for Concrete - M.R. Rixom and N.P. Mailraganam, E. & F.N. Spon 7. Properties of Concrete - A.M. Neville 8. MTRC - Materials and Workmanship Specification for Civil Engineering Works 9. CED - Recommended Specification for Reinforced Concrete in Marine Environment 10. BS 5075 : Part 1, Specification for Accelerating Admixtures, Retarding Admixtures and

Water Reducing Admixtures 11. BS 5075 : Part 3, Specification for Superplasticizing Admixtures 12. BS 6349 : Maritime Structures 13. BS8007: Code of Practice for design of Concrete Structures for Retaining Aqueous Liquid 14. BS8110 : Structural Use of Concrete

Documents

Report on Concrete Admixtures for Waterproofing Construction