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.