Concrete admixtures: types, limits and when to use them

Concrete admixture: what it is

Concrete admixtures are chemicals added to the mixer in small quantities, liquid or powder, to change the fresh or hardened concrete. They are dosed by cement content, written as a percentage by weight of cement (% bwc). The Sika guide to concrete admixtures describes them as acting chemically, physically or both on cement hydration.

An admixture is not the same as a mineral addition such as silica fume, which the Sika handbook doses at 5–10% of the cement weight, well above the admixture limit below. Nor is it a coating applied to the finished concrete: a waterproofing admixture works inside the mix, while products such as a cementitious waterproof coating go on the surface afterwards.

Types of admixtures under EN 934-2

The European concrete standard EN 206 refers to the admixture types defined in EN 934-2. As listed in the Sika guide:

Types of admixtures under EN 934-2

Type	What it does
Water reducer / plasticiser	allows less water at the same consistence, or more flow at the same water
High-range water reducer / superplasticiser	the same effect, much stronger
Retarder	extends the time to initial set and the workable period
Set accelerator	shortens the time to initial set and raises early strength
Hardening accelerator	speeds early strength gain with little effect on setting
Air-entraining agent	creates evenly spread small air bubbles that stay in the hardened concrete
Viscosity modifier (stabiliser)	reduces bleeding, the loss of mix water to the surface
Water-resisting admixture	reduces capillary water absorption of the hardened concrete
Combined types	water reducer + retarder, superplasticiser + retarder, water reducer + set accelerator

Sika adds products that fall outside EN 934-2: shrinkage-reducing, pumping aid, corrosion-inhibiting, blowhole-reducing and alkali-silica reaction control admixtures.

Plasticiser and superplasticiser: the numbers

The difference between the two is a test result, not a brand name. A Master Builders Solutions declaration quotes the EN 934-2 requirement: a plasticiser must reduce the water of a test mix by at least 5% against a control mix, a superplasticiser by at least 12%.

The EN 934-2 definition allows the effect to be used either way: the same consistence with less water, or more flow with the same water, or both at once. Which one a mix needs is set in the mix design. Sika’s guide notes that modern superplasticisers based on polycarboxylate ether (PCE) are designed polymer by polymer for water reduction, slump retention and early strength.

ASTM C494 types

Specifications written to American standards use letter types. ASTM C494, as summarised in the Sika guide, defines:

• Type A — water-reducing
• Type B — retarding
• Type C — accelerating
• Type D — water-reducing and retarding
• Type E — water-reducing and accelerating
• Type F — high-range water-reducing
• Type G — high-range water-reducing and retarding
• Type S — specific performance, such as slump retention

Other ASTM standards cover air-entraining admixtures (C260), pigments for coloured concrete (C979), corrosion-inhibiting admixtures (C1582), cold-weather admixture systems (C1622) and shotcrete admixtures (C1141). When comparing products, ask which type and which standard each one meets.

Dosage limits

EN 206 sets the frame for all admixtures in a mix, as summarised in the Sika Concrete Handbook, 2020 edition, table 3.3.1:

1. Total dose no more than 5% by weight of cement, unless the effect of a higher dose on performance and durability is verified.
2. Doses below 0.2% of cement only if dissolved in part of the mixing water.
3. If liquid admixtures exceed 3 L per m³ of concrete, their water counts in the water-cement ratio.
4. Where more than one admixture is used, their compatibility must be checked by testing.

For a mix with 350 kg of cement per m³, 5% is 17.5 kg and 0.2% is 0.7 kg (our calculation). The product data sheet gives the working dose inside that frame.

Admixtures in hot weather

The Sika handbook gives two temperature limits that matter in warm conditions: fresh concrete should not be hotter than 30°C when placed without protective measures, and above 25°C ambient, concrete without a retarder can start to cause problems because hydration speeds up and the mix stiffens before it can be placed. Its usual countermeasure is a retarding superplasticiser, or a superplasticiser combined with a set retarder. The handbook adds that the retarding effect depends on the cement and mix design, and only preliminary testing gives certainty.

A retarded mix also stays soft for longer, which affects how soon formwork can be struck; see formwork and shuttering. Fibres lower workability, and the American Concrete Institute notes that water-reducing admixtures are often used with them; see concrete fibre.

What to ask before buying an admixture

1. The type under EN 934-2 or ASTM C494, from the data sheet.
2. The recommended dose in % bwc and the maximum.
3. Whether it is compatible with the cement you use and with any other admixture in the mix.
4. Its effect on setting time at your expected concrete temperature.
5. Shelf life and storage conditions.

Trial mixes with your own cement and aggregates answer points 3 and 4 better than any catalogue.