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Concrete In Australia : December 2013
Concrete in Australia Vol 39 No 4 37 Figure 1 shows a schematic representation for typical concrete assuming normal materials and consistence levels. In practice, there is considerably more overlap due to local conditions and materials. The influence of some of these factors can be summarised as follows: • increased water content reduces both yield shear stress and plastic viscosity • increased air content reduces plastic viscosity • increased stone content and angularity will increase yield shear stress • high fines content will increase plastic viscosity as will viscosity agents • increased slump using water reducing agents will reduce yield shear stress. Application of rheological principles to concrete mix design is generally only considered for major projects with resources and time to optimise materials and mixes and where there are high performance requirements.3 The technology can, however, be applied to most types of concrete production since the testing is fairly routine, although it does require some interpretation. 3.0 CONVENTIONAL CONCRETE Optimising the fresh properties of conventional concrete is well understood and there are plenty of guidelines for the design of concrete mixes.4 The rheology of these concretes does vary depending on local materials, especially with regard to aggregates used. Figure 2 shows the influence of aggregate shape on workability, which is as important as grading of aggregates.5 The concrete analysed was grade 30 and 35 MPa, had standard structural mixes and was tested at a similar consistence level of 120 mm slump. Voids ratio was the average values for both coarse and fine aggregate used in each mix and was determined from loose bulk density and specific gravity measurements.6 Pumped concrete must have a yield stress greater than about 150 Pa to maintain stability and prevent segregation. Concrete forms an undisturbed plug in the pipe with a shearing zone at the wall where slippage occurs. Pump pressures are largely dependent on the yield stress of conventional concrete such that long line pumps may struggle when the yield stress is above 1000 Pa. Slump of concrete has a direct influence on yield stress as shown schematically in Figure 3 for a range of concrete mixes. This shows how consistence and workability are linked but there is not a constant relationship between these properties since other factors such as grading, particle shape and mix proportions have an influence.7 Specifying unrealistic slumps is problematic for high performance concrete mixes that tend to be more viscous. For instance, a marine concrete was specified for a project in New Zealand using slag concrete with a maximum water/binder ratio of 0.34 and water content of 156 L/m3. The specified concrete Concrete type Rheology Property Slump ~ 200mm Slump ~ 150mm Slump ~ 100mm Comments Slag, w/b - 0.34 Water – 156 L/m3 τ0 (Pa) 491 1177 1855 Stiff at low slump Very sticky μ (Pa.s) 204 185 178 Slag, w/b – 0.34 Water – 165 L/m3 τ0 (Pa) 248 467 782 Workability OK Sticky μ (Pa.s) 162 163 158 Figure 2: Influence of aggregate shape on yield stress of fresh concrete. Figure 3: Influence of slump on yield stress of fresh concrete. Table 1: Rheology of 50 MPa marine concrete at different slump levels. 36-40 Mackechnie.indd 37 36-40 Mackechnie.indd 37 25/11/13 2:58 PM 25/11/13 2:58 PM