Admixture Dose Calculator
Percentage dose to kg per m³ and per batch, with the new w/b ratio.
Inputs
Result
Formula
| Binder = cement + fly ash + slag + silica fume | not cement alone |
| Admixture = binder × dose% | per m³ |
| Water = base water × (1 − reduction) | measured reduction |
| w/b = water / binder |
Cement 300 + fly ash 60 gives 360 kg/m³ of binder at 2.0%, so 7.2 kg/m³ of admixture. With base water 175 and 15% reduction, water falls to 148.8 kg and w/b goes from 0.486 to 0.413. For a two-bag batch the factor is 0.333 and the dose is 2.40 kg.
FAQ
Why dose on total binder rather than cement?
Admixtures act on the surface of all hydrating powder. Fly ash and slag hydrate slowly but still adsorb the plasticiser. Dosing on cement alone under-doses by the SCM percentage, which typically shows as an acceptable initial slump that collapses within half an hour - and then someone adds water on site, which ruins the strength.
Is a higher water reduction always better?
No. Higher reduction usually comes with more entrained air and retarder: above 25% you see bleeding, segregation, very long setting times (delaying striking and the next lift), and extreme sensitivity to clay in the aggregate - raising fines from 1% to 3% can halve the effective reduction at the same dose. Practical range is 15-25% with moderate air.
Two kilograms per batch is hard to weigh accurately - what to do?
Weigh it on a 0.1 kg scale, or pre-dilute and meter by volume (converting through the concentration, mixing fresh to avoid settling). Never eyeball it: a 20% dose error is enough to completely change slump loss and setting time.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.