Optimising the Fresh Properties of Concrete by Understanding Rheology

Transcription

1 Optimising the Fresh Properties of Concrete by Understanding Rheology James Mackechnie 1 1 South Island Plant Engineer, Allied Concrete, Christchurch, New Zealand Abstract: Project specifications have specific requirements to achieve the intended hardened properties but often do not make suitable allowance for fresh properties of concrete. Understanding the rheological properties of concrete allows better optimization of materials that influence workability. Characterizing 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. Keywords: Fresh properties, Mix design, Rheology, Specifications, Workability, 1. 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 100mm 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 100mm. 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 optimization to improve economics, 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. Rheology and rheocharts Rheology is the science of deformation and flow of material and is used in concrete technology to characterize 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. a) Yield shear stress (τ 0 ) is the boundary between liquid and solid behavior 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) b) 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 Pa.s while some high strength geopolymer concretes may be over 200 Pa.s)

2 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 characterize the optimum range for different concrete applications. A typical overview for concrete is shown in Figure 1 based on the authors experience and findings from researchers (2). Figure 1: Rheochart showing typical ranges of concrete after Wallevik (2) The above rheochart 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 summarized 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 optimize 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. Conventional concrete Optimizing the fresh properties of conventional concrete is well understood and there is 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 show 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, standard structural mixes and 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).

3 Figure 2: Influence of aggregate shape on yield stress of fresh concrete 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 proportional have an influence (7). Figure 3: Influence of slump on yield stress of fresh concrete

4 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/m 3. The specified concrete was found to have low workability when site slumps dropped below 120mm and was extremely sticky. An alternative concrete mix design was investigated, which had higher water content and produced a far more workable concrete as shown in Table 1. No adjustment was possible to the concrete during the project but research found similar durability potential when assessed using standard laboratory techniques (effective porosity of 10%, Nordic D nssm of 2x10-12 m 2 /s, oxygen permeability coefficient of 1x10-11 m/s). Table 1: Rheology of 50 MPa marine concrete at different slump levels Concrete type Rheology Slump Slump Slump Comments Property ~ 200 mm ~ 150 mm ~ 100 mm Slag, w/b τ 0 (Pa) Stiff at low slump Water 156 L/m 3 µ (Pa.s) Very sticky Slag, w/b 0.34 τ 0 (Pa) Workability ok Water 165 L/m 3 µ (Pa.s) Sticky 4. Self-compacting concrete Self-compacting concrete (SCC) is a highly flowable, non-segregating concrete that is designed to spread under its own weight to fill formwork and encapsulate reinforcing steel without mechanical vibration being required. SCC is characterised by the following properties: - flowability to completely fill formwork without vibration typically used in conventional concrete - passing ability to flow around congested reinforcement without blocking - segregation resistance to maintain suspension of coarse aggregate within the fluid paste SCC has low yield stress that is generally less than 60 Pa to ensure satisfactory flowability. Plastic viscosity varies depending on water and powder content with low viscosity SCC having high yield stress as shown in Figure 4 below. High viscosity SCC mixes tend to have much lower yield stress to provide the required flowability. Figure 4: General and optimum rheological range for SCC after Wallevik (8)

5 Simple assessment of the rheology of SCC can be obtained using standard control tests such that: - Slump flow or spread test provides a reasonable assessment of flowability and is related to the yield stress of SCC - T500 time provides a rough measure of plastic viscosity as shown in Figure 4 above - Segregation resistance and passing ability are best assessed directly rather than inferring performance from rheology properties. Koehler found a strong relationship between slump flow and yield stress and between T500 time and plastic viscosity (9). Regardless of which approach is used, it is important to characterize the rheological properties of SCC in order to understand the influence on other properties, such that: - Lower viscosity SCC has lower pumping pressures, produces good surface finishes but may be more vulnerable to segregation - Higher viscosity SCC increases pump pressures, may have improved segregation resistance and may reduce formwork pressure under some conditions Differences in self-compacting concrete mixes are illustrated in Table 2 where low shrinkage, moderate shrinkage and standard SCC are compared. Differences in fresh and hardened properties are engineered for specific applications such that hardened performance may require some compromises in fresh properties and visa versa. Table 2: Fresh and hardened properties of different SCC mixes Type Target flow (mm) T500 (secs) τ 0 (Pa) µ (Pa.s) Shrinkage (µstn) Application Low Stitch shrinkage joints Moderate Columns & shrinkage beams Standard SCC Walls 5. Fibre reinforced concrete Increasing the volume fraction and aspect ratio of fibres in concrete not only improves mechanical properties but also reduces workability of fresh concrete. The product of the fibre volume factor and its aspect ratio is defined as the fiber factor and has been shown to affect the consistence of concrete (e.g. slump or slump flow) when considering rigid fibres (10). Research on the rheological changes that occur when using steel fibres are used in concrete are well known and FRC mixes can be easily adjusted to maintain sufficient workability. This is shown in Table 3 where the change in yield stress and slump with increasing steel fibre dose are shown. A series of grade 32 MPa concrete mixes were used with varying fine aggregate contents. These were rheologically tested with different doses of steel fibres (cold drawn, hooked steel fibres with aspect ratio of 80 and length of 60 mm). An optimum yield stress of 500 Pa was assumed for this concrete using a maximum aggregate size of 13mm. Table 3: Fresh property changes of grade 32 FRC with increasing steel fibre dose Fine/Total Aggregate Property 0 kg 10 kg 20 kg 40 kg 60 kg 44 % τ Std. mix Slump % τ Std. mix Slump % τ Pump mix Slump % τ Soft pump Slump % τ Tremmie Slump kg

6 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. Figure 5: Influence of macro-synthetic fibre dose on yield stress of fresh concrete Higher doses of macro-synthetic fibres (e.g. volume fractions of over 0.5% or 4.5 kg per cubic metre) 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 130 mm). 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 optimization 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. 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 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 polymerization

7 Figure 6: Rheological properties required for slip forming concrete 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 given to homogenize the product before distribution. Research is currently been undertaken to investigate the potential advantages of using waste paint to improve slip-form performance of concrete in New Zealand. 7. Conclusions Optimizing the fresh properties of concrete requires experience and an understanding of the influence of materials and mix designs. Characterizing the fresh properties of concrete using rheology can provide valuable information and relies on a direct scientific assessment rather than using an empirical approach. Data presented in this paper was used to draw four main conclusions: - Workability of concrete cannot be accurately measured by using consistence tests such as the slump test and allowance must be made for harsh and/or viscous concrete mixes. High performance concrete in particular may require much higher consistence levels to ensure not just sufficient workability but that other properties such as durability are not compromised - Rheology of self-compacting concrete can be roughly inferred from empirical tests such as slump flow and T500 time but these do not provide definitive guidance for all SCC mixes. Characterizing the rheology of SCC also allows other properties of the material, such as pumping pressures and surface finish to be predicted before production. - Inclusion of fibres into concrete, whether rigid or flexible, will affect the rheology of FRC and allowance needs to be made in concrete mix designs. Allowance for higher fibre doses involves increasing the fine aggregate content of FRC mixes together with the use of chemical admixtures. - Slip-form performance of concrete is not easy to achieve since the material must be relatively stiff without becoming too viscous. Achieving both of these properties simultaneously can be achieved using polymer additions that significantly reduce plastic viscosity at relatively low slump levels required for slip-forming.

8 Having access to rheological testing equipment is not essential to develop a better understanding of fresh properties of concrete. Understanding the framework within which fresh testing is based can however be improved by knowing the relevance of rheological properties on fresh properties. This will in turn provide a more scientific basis for comparison and improve optimization of concrete mixes. 8. Acknowledgements The use of the concrete viscometer based at the University of Canterbury, Christchurch, New Zealand is gratefully acknowledged and was used to generate rheological properties discussed in this paper. The sponsorship of Holcim Cement, New Zealand who funded attendance of this conference is gratefully acknowledged by the author. 9. References 1. Tattersall, GH and Banfill, PFG, The rheology of fresh concrete, Pitman, London, Wallevik, OH and Wallevik JE, Rheology as a tool in concrete science: The use of rheographs and workability boxes, Cement and Concrete Research, 41, 2011, pp Yammine, J, Chaouche, M et al, From ordinary rheology concrete to self compacting concrete: A transition between frictional and hydrodynamic interactions, Cement and Concrete Research, 38, 2008, pp Ryan, WS and Samarin, A, Australian Concrete Technology, Sydney, Longman Cheshire, Cortes, DD, Kim, H-K et al, Rheological and mechanical properties of mortars prepared with natural and manufactured sands, Cement and Concrete Research, 38, 2008 pp New Zealand Standards, NZS3111 Method of test for water and aggregates for concrete, Wellington Feys D, De Schutter,G et al, Parameters influencing pressure during pumping of self compacting concrete, Material and Structures, 46, 2013, pp Wallevik OH, Rheology A scientific approach to develop self-compacting concrete, Proc. Third Int. RILEM Symp.on Self-Compacting Concrete, Reykjavik, 2003, pp Koehler E, Use of rheology to design, specify and manage self-consolidating concrete Proc of 10 th Int. Conf. on Recent Advances in Concrete Technology and Sustainability Issues, Martinie, L, Rossi P et al, Rheology of fiber reinforced cementitious materials: classification and prediction, Cement and Concrete Research, 40, 2010, pp Bartos PJM and Hoy CW, Interactions of particles in fibre reinforced concrete, Production methods and workability of concrete, E and FN Spon, London, 1996, pp Choi, M, Roussell, N, et al, Lubrication layer properties during concrete pumping, Cement and Concrete Research, 45, 2013, pp Haigh, CJ, Latex and acrylic based waste paint as admixture in concrete masonry blockfill, Masters thesis, University of Auckland, 2008.