Concrete Grades — N20 to N40
Concrete Grades — N20 to N40
The N is normal-class, the number is the strength in MPa at 28 days. Which grade for which job, and the slump that comes as standard.
Standard grades (AS 3600)
| N20 | 20 MPa — paths and light-duty work (NCC minimum for a residential slab) |
| N25 | 25 MPa — driveways, house slabs, garage floors, footings |
| N32 | 32 MPa — exposed footings, structural slabs, retaining and pool walls |
| N40 | 40 MPa — commercial and multi-storey work |
Slump (how workable)
| Default for residential slabs & footings | 100 mm if not specified |
| Selectable range | 20–120 mm, in 10 mm steps |
| More slump is easier to place | but do not add water on site — it cuts strength |
Curing
- Keep it moist — AS 3600 expects at least 7 days for N20–N40
Reference only. The grade and slump for a structural element must come from the engineer's spec or the NCC, not a rule of thumb. Strength is the 28-day characteristic value.
Sources: AS 3600 (concrete structures) · CCAA — specifying & ordering concrete · NCC
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