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Superplasticising admixtures

Superplasticising/high-range water-reducing admixtures significantly reduce the amount of water needed to achieve a given workability in plastic concrete. This effect can be used to reduce water content (for increased strength and reduced permeability/improved durability), i.e. as a high-range water-reducer, or to increase consistence and workability, i.e. as a superplasticiser.

High-range water-reducing admixtures used to increase the workability or consistence of the concrete are usually termed ‘superplasticising admixtures’ but these names are frequently interchanged. They function in a similar way to ‘normal plasticising admixtures’ but can be used at higher dose without unwanted side-effects such as air entrainment or set retardation.

When superplasticisers are used as high-range water-reducers, the compressive strength of concrete is increased as a direct result of the lower w/c 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.

Where the w/c ratio is not altered, water-reducing/plasticising admixtures increase workability. Typically the slump will be more than 120 mm higher than for a plain concrete of equal water content. At higher doses, the concrete can be made flowing or even self-compacting but care is needed over mix design or the mix may be subject to significant bleed and segregation.

Concrete Society Technical Report 61, Enhancing reinforced concrete durability, gives information on the effects of superplasticising admixtures in concrete on the rates of carbonation and chloride ingress.


Acknowledgement: The Concrete Society

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TR18 A guide to the selection of admixtures for concrete

TR61 Enhancing reinforced concrete durability