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Concrete In Australia : December 2013
Concrete in Australia Vol 39 No 4 39 Macro-synthetic fibres are made from a variety of materials and can be relatively rigid when dispersed in cement paste. Even relatively flexible fibres such as polypropylene and cellulose fibres have an influence on the workability of concrete, especially fibres having higher aspect ratios. Most macro- synthetic fibres are relatively rigid in comparison to cement paste and even the most flexible fibres require extra mortar to coat and lubricate the extra surfaces. Typical changes in rheology that occur when macro-synthetic fibres are added to fresh concrete are shown in Figure 5. Higher doses of macro-synthetic fibres (e.g . volume fractions of over 0.5% or 4.5 kg/m3) have the potential to significantly reduce workability and may cause balling. This cannot always be overcome by increasing either water or super-plasticiser since this may increase slump but reduces viscosity and can lead to the paste segregating (typically found when slump exceeds 130mm). Concrete mixes need to be specially designed for higher fibre contents by adjusting aggregate contents and using chemical admixtures as is done with steel fibres.11 Rheology testing of FRC allows mix optimisation to be quickly done when dealing with new fibres or when dealing with challenging site requirements. A well designed FRC mix can be line pumped several hundred metres using the right equipment whereas a high dose of macro-synthetic fibres in a standard concrete mix can struggle to come down the truck chute. 6.0 SLIP-FORM CONCRETE Concrete fresh properties suitable for slip forming are a lower than normal plastic viscosity together with a reasonably high yield stress to maintain stability. A reduction in plastic viscosity allows a slip layer to be formed that reduces friction by generating liquid in the shearing zone.12 This combination of a relatively low viscosity together with a moderate yield stress is not easy to achieve for fresh concrete since most mix adjustments affect both properties proportionately. The most common approach to improving the slip-forming ability of concrete is to use air entrainment but this has a limited effect on reducing viscosity since strength is also compromised. Figure 6 shows the effect of several additives on the slip forming properties of fresh concrete. The advantage of using polymer-type materials in concrete can be clearly seen in the rheochart. Polymer latex modifiers change rheology by the combined effect of fine particle packing, air entrainment and polymerisation A number of proprietary emulsions are available from construction material suppliers and are used to make polymer modified mortars and concrete for special applications. These additives are expensive and are not widely used in ready mix concrete production. Waste latex paint (WLP) contains similar materials and the presence of polymers improves dispersion and provides air entrainment. Research has found that WLP significantly improves workability and reduces plastic viscosity of concrete.13 The consistency of the product is variable and attention needs to be given to homogenise the product before distribution. Research is currently beeing undertaken to investigate the potential advantages of using waste paint to improve slip-form performance of concrete in New Zealand. 7.0 CONCLUSIONS Optimising the fresh properties of concrete requires experience and an understanding of the influence of materials and mix Figure 5: Influence of macro-synthetic fibre dose on yield stress of fresh concrete. Fine/Total Aggregate Property 0kg SF80/60 10 kg SF80/60 20 kg SF80/60 40 kg SF80/60 60 kg SF80/60 80 kg SF80/60 44% Std. mix τ0 Slump 205 170 566 130 950 85 – 60 45% Std. mix τ0 Slump 176 180 236 160 578 100 985 60 47% Pump mix τ0 Slump 207 175 261 160 440 130 491 75 – 40 52% Soft pump τ0 Slump 147 170 170 160 228.5 130 287 95 541 75 60% Tremmie τ0 Slump 122 175 139 165 229 150 256 120 324 105 545 80 Table 3: Fresh property changes of grade 32 FRC with increasing steel fibre dose. 36-40 Mackechnie.indd 39 36-40 Mackechnie.indd 39 25/11/13 2:58 PM 25/11/13 2:58 PM