Key points
- ASTM C494 defines 7 types: A water-reducing, B retarding, C accelerating, D water-reducing and retarding, E water-reducing and accelerating, F high-range water-reducing, G high-range water-reducing and retarding.
- Types A, D and E must reduce mixing water by at least 5%; types F and G (superplasticisers) by at least 12%.
- Retarders offset the effect of high temperature on setting, but they are no substitute for good hot-weather concreting practice.
- Modern superplasticisers use polycarboxylate chemistry and are used to produce self-consolidating concrete (SCC).
An admixture is a liquid or powder added to concrete during mixing, in a small dose relative to the cement, to improve a particular property. Because the market is full of brand names, it is better to ask for the admixture type under the standard than for its name.
1) The seven types in ASTM C494
The American standard ASTM C494 classifies chemical admixtures by function1:
| Type | Function | Water-reduction requirement |
|---|---|---|
| A | Water-reducing (normal plasticiser) | At least 5% |
| B | Retarding | — |
| C | Accelerating | — |
| D | Water-reducing and retarding | At least 5% |
| E | Water-reducing and accelerating | At least 5% |
| F | High-range water-reducing (superplasticiser) | At least 12%, possibly over 30% |
| G | High-range water-reducing and retarding | At least 12% |
When buying an admixture, ask for the data sheet and check it states the type under ASTM C494 (or the equivalent European standard EN 934-2) and the recommended dosage.
2) Why reduce water?
Extra water makes concrete easier to place but weaker and more porous, and high porosity lets chlorides and carbon dioxide reach the steel so it rusts. The US National Ready Mixed Concrete Association therefore recommends a water-to-cementitious ratio in the range of 0.40 to 0.50 for low-permeability concrete, and about 0.40 where chlorides are present2. That is where plasticisers come in: a workable consistency with less water.
3) Retarders and hot weather
High temperatures speed up setting and shorten working time. Retarders are used to offset the effect of high temperature and improve workability, but they are not a substitute for good hot-weather concreting procedures1: cooling mix ingredients, placing in cooler hours, and curing immediately. See concrete basics and curing.
4) Superplasticisers and self-consolidating concrete
Modern superplasticisers are based on polycarboxylate chemistry rather than the older melamine and naphthalene formulas, offer a wide range of slump retention, and are effective for producing self-consolidating concrete (SCC)1, which flows and fills forms between dense reinforcement with little or no vibration.
5) Other additions you'll hear about
- Corrosion inhibitors: delay the onset of steel corrosion2, used in structures exposed to salts.
- Supplementary cementitious materials (SCM): silica fume, fly ash and slag cement reduce concrete permeability; typical dosages cited are 5% silica fume, 25% fly ash and 50% slag2.
- Curing compounds: a liquid sprayed on the concrete surface after finishing that forms a moisture-retaining film; the association recommends starting curing as soon as possible, either by spraying one or by covering with wet burlap kept moist for at least 3 days3.
- Synthetic fibres: help reduce plastic shrinkage cracking (ASTM C1116)4.
6) Pros and cons in brief
Pros
- Stronger, less permeable concrete at the same consistency.
- Control over setting time for weather and haul distance.
- Easier placing in heavily reinforced elements.
Cons and cautions
- Overdosing can cause segregation or excessive retardation.
- Some admixtures are incompatible with each other or with the cement; trial mixes are needed.
- They don't fix poor workmanship: compaction and curing remain essential.
7) Checklist for contractors and owners
- Ask the plant for the mix design, including admixture type and dosage.
- Refuse water additions on site without the engineer's approval.
- In summer: ask about the retarder and the curing plan before placing.
- Keep the delivery tickets for every truck.
Sources
The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.
- 1Water-Reducing and Set-Controlling Admixtures (ASTM C494 Types A–G) — National Precast Concrete Association (NPCA)
- 2CIP 25 — Corrosion of Steel in Concrete — National Ready Mixed Concrete Association (NRMCA)
- 3CIP 4 — Cracking Concrete Surfaces — National Ready Mixed Concrete Association (NRMCA)
- 4CIP 5 — Plastic Shrinkage Cracking — National Ready Mixed Concrete Association (NRMCA)