Exposure class (concrete)
Exposure class is the AS 3600 environmental grade (A1 to C2) driving minimum concrete strength, cover to reo, and mix design. Coastal sites carry the strictest.
Ask Chalkline about this →The exposure class is the environmental classification of concrete under AS 3600:2018 Section 4. It drives three design choices in lockstep: minimum concrete grade (strength), minimum cover to reinforcement, and the mix design (binder content, water-to-cement ratio, supplementary cementitious materials).
The exposure classes
| Class | Environment |
|---|---|
| A1 | Interior, fully protected from weather |
| A2 | Interior with some moisture exposure (e.g. residential bathroom slab edge) |
| B1 | Outdoor, exposed to rain and weather, more than 1 km from coast |
| B2 | Outdoor, within 1 km of coast (or saltwater spray) |
| C1 | Outdoor, within 50 m of saltwater spray |
| C2 | Permanently submerged in saltwater / aggressive marine |
| U | Other aggressive environments (industrial chemicals, sewerage), assessed case by case |
A residential slab on ground typically sits at A2 or B1 depending on geography; a coastal Class 1a build moves to B2; a beachfront foundation can hit C1 or C2 at the salt-spray face.
What it drives
AS 3600 Section 4 specifies for each class:
- Minimum concrete strength. N20 to N32 covers most residential; B2 typically needs N32+.
- Minimum cover to reinforcement. Tightens as exposure increases.
- Mix design. Water/cement caps, sometimes mandated SCMs (fly ash, slag) in chloride environments.
A spec written for A2 in a B2 location is non-compliant on all three axes at once.
Vs durability class
Durability class is a timber concept under AS 5604 measuring species decay resistance, not a concrete environmental grading. The two are sometimes confused but they apply to different materials.
For a builder
- Confirm the exposure class on the engineer’s notes before ordering concrete. “Standard residential mix” is not a spec.
- Check it again for coastal jobs. B2 vs B1 changes cost and cover.
- Match it on the docket. “N32, B2 mix” must appear; if not, ring the plant before the pour.
Category: Concrete / durability.
Related
See also
Last updated: 2026-05-29. Verified: 2026-05-29. Quarterly review for currency.