Admixture (concrete)
Concrete admixtures: chemicals added at the batch plant (water reducer, superplasticiser, accelerator, retarder, fibre). Adding water on site breaks spec.
Ask Chalkline about this →An admixture is a chemical added to a concrete batch at the batch plant (rarely at the truck, never on site) that modifies the set time, workability, durability, strength, or other property of the resulting concrete. Admixtures are specified separately from the concrete grade (the strength class) and are listed on the delivery docket that arrives with each truck of concrete. AS 1478.1 defines the test methods and classifications; AS 1379 governs the supply (verified 2026-05-16).
The main admixture families used in residential:
| Family | Function | Common use |
|---|---|---|
| Water reducer (plasticiser) | Reduces water needed for given workability, increases strength without increasing water-cement ratio | Standard inclusion in most residential mixes |
| Superplasticiser | Strong water reducer; allows very high slump at low w/c | Self-compacting concrete, pump mixes |
| Accelerator | Speeds set and early strength gain | Cold-weather pours, fast strip times |
| Retarder | Delays set | Hot-weather pours, long transport distances |
| Air entrainer | Stabilises microscopic air bubbles | Freeze-thaw resistance, durability |
| Fibre (steel or synthetic) | Crack control, impact resistance | Slabs, industrial floors (residential less common) |
| Plasticiser/Superplasticiser combinations | Workability + strength | Architectural concrete, exposed finish |
| Calcium chloride (avoid) | Accelerator; corrosive to reinforcement | Historic use; superseded by non-chloride accelerators |
Reading the delivery docket:
A typical 32 MPa residential mix delivery docket might show:
N32-20-100 + WR-100 + Retarder
Translation: N32 grade (32 MPa characteristic strength), 20 mm maximum aggregate, 100 mm target slump, with water reducer at 100 mL/100 kg cement, plus retarder (no quantity = supplier default).
Critical builder takeaway: adding water on site is not an admixture, it’s a defect.
Concrete arriving at the slab with the design slump is engineered to that slump. The water-cement ratio is the dominant determinant of final strength. Adding 10 to 20 L of water to a 6 m³ load to “make it pumpable” or “easier to work” cuts the 28-day strength by 10 to 30%, doesn’t add workability for long (the cement quickly absorbs the extra water), and is invisible at the post-pour inspection. The certifier checks the docket against the design; the docket says N32 but the actual concrete may be N22.
If concrete is stiff on arrival, the correct action is:
- Re-call the supplier; the truck may need to return with a fresh batch.
- Request a workability admixture (water reducer or superplasticiser) added at the site under the supplier’s authority. Driver inputs it at the chute.
- Never the site water hose into the drum.
Common defects on the docket:
- Wrong slump for the application (high slump for a vertical pour will run; low slump for a deep pour will not consolidate).
- Calcium chloride accelerator on a reinforced mix (causes long-term corrosion).
- Air entrainer omitted on alpine cold-cycle exposure.
- Generic “high-flow” labelling with no quantified admixture call-out.
Also known as: concrete additive (loose); chemical admixture; AS 1478 admixture.
Category: Materials.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.