Concrete Admixture Dosage
Admixtures affect a mix design in two different ways depending on their type — chemical admixtures change the water demand, mineral admixtures change the cementitious content itself. This page explains both and how they are dosed. Open the calculator with an admixture switched on.
Chemical and mineral admixtures
Chemical admixtures (water reducers and superplasticisers) are dosed as a percentage of the cementitious content, commonly in the range of about 0.5% to 2% by mass — always follow the manufacturer’s recommended dosage and confirm it by trial. Their main effect in mix design is a reduction in free water for the same workability; use the water reduction actually measured in your trial rather than a generic figure. Mineral admixtures (fly ash, GGBS) work differently: they replace part of the cement and are counted within the cementitious content itself. Compatibility between an admixture and the cement being used should always be checked by trial.
Worked example
With a cementitious content of 350 kg/m³ and an admixture dosed at 2%, the admixture mass is 7 kg/m³. If the same admixture allows a 20% water reduction on trial, the water content for the mix falls correspondingly, which in turn lowers the cement needed for the same adopted w/c ratio.
Questions this page answers
Is more admixture always better?
No — overdosing can cause segregation, retardation or bleeding. Stay within the manufacturer’s recommended dosage range and confirm performance by trial.
Does admixture replace cement?
Chemical admixtures do not replace cement. Mineral admixtures such as fly ash or GGBS do, partly, since they form part of the cementitious content.