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Normal plasticising admixtures reduce the amount of water needed to achieve a given workability in plastic concrete without significantly affecting the air content or setting characteristics. When water-reducing admixtures are used to increase the workability or consistence of the concrete, they are usually termed ‘plasticising admixtures’. Where no change is made to the water/cement ratio, plasticising/water-reducing admixtures increase workability. Typically the slump will be 60–100 mm higher than that of a plain concrete of equal water content.
When plasticising/water-reducing admixtures are used to reduce the water content while maintaining workability, the compressive strength of concrete is increased as a direct result of the lower water/cement ratio. When they are used to increase workability at equal water content, the compressive strength is generally similar to that of the equivalent plain concrete mix. The rate of slump loss is generally similar to or greater than that of the equivalent plain concrete mix of the same initial workability. If longer slump retention is required, the plasticising/ water-reducing admixture can be used to increase initial workability. The concrete will then maintain a high level of workability for longer after mixing and will still have the structural properties of the plain concrete.
When used to reduce the mix water content, plasticising/ water-reducing admixtures enhance the durability of the concrete by improving both the density and impermeability.Concrete Society Technical Report 61, Enhancing reinforced concrete durability, gives information on the effects of plasticising admixtures in concrete on the rates of carbonation and chloride ingress.
Acknowledgement: The Concrete Society
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