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Concrete In Australia : December 2013
36 Concrete in Australia Vol 39 No 4 FEATURE: RHEOLOGY Optimising the fresh properties of concrete by understanding rheology James Mackechnie, South Island Plant Engineer, Allied Concrete, New Zealand Project specifications have specific requirements to achieve intended hardened properties but often do not make suitable allowance for fresh properties of concrete. Understanding the rheological properties of concrete allows better optimisation of materials that influence workability. Characterising the yield shear stress and plastic viscosity of concrete provides a more scientific assessment of fresh properties and complements the intuitive feel developed by experienced concrete technologists. Design of special concrete mixes is currently done using a heuristic approach and product development is therefore fairly conservative. This paper presents findings from rheological studies of concrete where the effects of materials, mix designs and specifications are compared, and the advantage of this approach is shown. The influence of supplementary cementitious materials, aggregates, chemical admixtures and fibres on concrete properties is discussed and implications for these factors on specifications are explained. 1.0 INTRODUCTION Fresh properties of concrete are often still specified prescriptively in terms of consistence as measured by the slump of concrete. This approach may be suitable for standard concrete mixes of moderate grade strength but is not recommended for special concretes where higher performance is required, especially if unrealistically low slump levels are specified (e.g . slumps of 100 mm or less for higher strength concrete). Some concrete specifications allow the consistence level to be nominated by the contractor, which is a more flexible and practical approach. Prescriptive specifications for fresh concrete properties become problematic with special concrete mixes that are often significantly less workable at normal consistence levels such as a slump of 100 mm. This is because many of these concretes contain high powder contents and chemical admixtures that significantly increase viscosity of the material. The result can be that site limitations on slump ultimately reduce the hardened properties of concrete due to inadequate compaction, poor productivity and other early-age problems. Concrete mixes require some optimisation to improve economics and ensure appropriate performance while retaining sufficient robustness for practical use on site. This process is currently done by experienced concrete technologists who have a good feel for their materials and concrete mix designs. A complementary technique is presented using rheological testing of fresh concrete, which provides scientific backing and allows better understanding of the issues to other engineers. This paper seeks to present rheology in a practical manner that reinforces some of the issues commonly encountered by readymix (premix) concrete suppliers. 2.0 RHEOLOGY AND RHEOCHARTS Rheology is the science of deformation and flow of material and is used in concrete technology to characterise the fresh properties of the material. Two properties were measured using a Contec BML4 viscometer that effectively shears concrete by applying a stepped rotational cycle: • Yield shear stress (T0) is the boundary between liquid and solid behaviour and effectively provides an estimate of the plastic stiffness of concrete (e.g . self-compacting concrete would be close to zero while kerb concrete would be over 2000 Pa). • Plastic viscosity (μ) represents the change in resistance of the concrete at increasing shear and represents the stickiness of fresh concrete (e.g. normal concrete viscosity is between 40- 80 Pa.s while some high strength geopolymer concretes may be over 200 Pa.s). The application of rheology to concrete technology is not new and the science is well developed internationally.1 Probably the most useful application of rheology is in rheocharts, which help characterise the optimum range for different concrete applications. A typical overview for concrete is shown in Figure 1 based on the author’s experience and findings from researchers.2 Figure 1: Rheochart showing typical ranges of concrete after Wallevik.2 Plastic viscosity (Pa.s) Yieldshearstress(Pa) 36-40 Mackechnie.indd 36 36-40 Mackechnie.indd 36 25/11/13 2:58 PM 25/11/13 2:58 PM