The admixture dose is established by trial within the manufacturer's recommended range and then controlled accurately on site. Overdosing — especially of superplasticisers and retarders — causes severe over-retardation (set failure), segregation/bleeding, or excess entrained air (strength/yield loss). Accurate dispensing (calibrated dosing, not site eyeballing) and dose discipline are as important as the choice of admixture.
Key Requirements
•Working dose set by trial (compatibility clause) within the manufacturer's range
•Accurate calibrated dispensing — not manual eyeballing/buckets at the mixer
•Guard against overdosing: severe retardation/non-set, segregation/bleeding, excess air
•Account for the admixture water in the W/C if it is a liquid (do not double-count/ignore)
•Control the dose batch-to-batch; re-trial on material/temperature change
Practical Notes
✓Overdosed superplasticiser/retarder is a classic site disaster — concrete that won't set for a day or segregates badly; the cause is dispensing/dose control, not the product.
✓Liquid-admixture water counts toward W/C — ignoring it quietly raises the effective water content.
Common Mistakes
⚠Eyeballed/uncalibrated dosing at the mixer.
⚠Overdosing for extra workability → over-retardation/segregation.
⚠Ignoring the liquid-admixture water in the W/C ratio.