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Concrete In Australia : December 2013
38 Concrete in Australia Vol 39 No 4 FEATURE: RHEOLOGY was found to have low workability when site slumps dropped below 120 mm 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 Dnssm of 2 x10-12 m2/s, oxygen permeability coefficient of 1x10-11 m/s). 4.0 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. High viscosity SCC mixes tend to have much lower yield stress to provide the required flowability. 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 • 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 characterise 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 SCC 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. 5.0 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 fibre 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 13 mm. Type Target flow (mm) T500 (secs) τ0 (Pa) μ (Pa.s) Shrinkage (μstn) Application Low shrinkage 720 9.0 40 115 600 Stitch joints Moderate shrinkage 700 6.0 20 70 750 Columns & beams Standard SCC 680 3.5 15 40 850 Walls Figure 4: General and optimum rheological range for SCC after Wallevik .8 Table 2: Fresh and hardened properties of different SCC mixes. 36-40 Mackechnie.indd 38 36-40 Mackechnie.indd 38 25/11/13 2:58 PM 25/11/13 2:58 PM