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.
Admixtures in the UK supply chain
Admixtures used in UK concrete are declared against BS EN 934-2, which defines performance requirements for categories such as water reducers, high-range water reducers (superplasticisers), set retarders, accelerators and air-entraining agents. On most UK projects the admixture is chosen and dosed by the ready-mixed concrete producer as part of a certified mix, not added separately on site. That matters because the producer's quality system — typically under a third-party certification scheme such as QSRMC or BSI Kitemark — covers the combination of cement, SCM, aggregate and admixture that was tested, not the admixture on its own.
Common UK use cases
- Pumped and high-workability mixes: a superplasticiser lets the producer reach S4 or flow classes without adding water, keeping the water–cement ratio within the BS 8500 limit for the exposure class.
- Winter concreting: accelerators and higher early-strength cements help achieve striking strength in cold weather. Calcium chloride-based accelerators are not permitted in reinforced concrete because of corrosion risk.
- Summer pours and long travel times: retarders extend workable life where traffic or pump downtime is likely.
- External paving: air-entraining admixtures are used for freeze–thaw resistance, including in the PAV1 and PAV2 designated concretes defined in BS 8500.
- Basements: waterproofing or pore-blocking admixtures are sometimes specified as part of a Grade 2 or 3 basement strategy under BS 8102, alongside joint detailing — the admixture alone does not make a structure watertight.
Site-added admixture: when it is and isn't acceptable
Some UK producers allow a measured dose of superplasticiser to be added from the truck on arrival to restore workability. Where this is part of the producer's certified procedure, it is recorded on the delivery ticket and is very different from adding water. If a contractor adds admixture or water without the producer's agreement, the producer's conformity responsibility for that load may no longer apply, and the concrete supplied is no longer the concrete that was specified.
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.