Concrete Admixtures Explained
Admixtures are added in small quantities to modify fresh or hardened concrete properties without changing the basic cement-aggregate-water proportions. BS EN 934-2 defines the common categories used with EN 206 concrete, and sets performance requirements each type must meet.
Common admixture types
- Water-reducing / plasticising: reduces water demand for a given workability (or increases workability at the same water content), letting the w/c be lowered without losing consistence.
- High-range water-reducing / superplasticising: achieves a much larger water reduction than a standard plasticiser, used for high-strength, self-compacting or very fluid mixes.
- Retarding: slows the setting time, useful in hot weather, long hauls, or where large pours need extended working time.
- Air-entraining: deliberately introduces microscopic, stable air bubbles, primarily to give freeze-thaw resistance (see air entrainment).
Dosage and the calculator
Admixture dosage is normally expressed as a percentage of cement mass, within a range the manufacturer states for that product (overdosing can cause excessive retardation, segregation or other side effects, so the stated range should not be exceeded without trial data). This calculator’s useAdmix and waterReduction fields let a water-reducing admixture’s effect be modelled directly — the stated water reduction lowers the free water content (and so the w/c, for the same cement content) compared with an unadmixed mix.
Frequently asked questions
Can I just add a superplasticiser to increase workability without a mix redesign?
In principle a superplasticiser can add workability at the same w/c, but the dosage and its interaction with the specific cement and other admixtures should be confirmed by trial mix, not assumed — see trial mixes.
Do all admixtures work with all cement types?
Not necessarily — compatibility between a given admixture and a given cement or combination can vary, which is one of the things initial testing is meant to catch before full-scale use.
Is air-entraining admixture the same as a water-reducing admixture?
No — they serve different purposes. Air-entraining admixtures are used for freeze-thaw durability; water-reducing admixtures are used to lower water demand or w/c. Combination products exist, but the two functions are distinct.