Aggregate (concrete)
Aggregate is the coarse stone and fine sand in concrete, 60-75% of mix volume. 20 mm nominal is residential default; 10-14 mm for slim sections; 40 mm mass-concrete only.
Ask Chalkline about this →Aggregate in concrete is the coarse stone (gravel, crushed rock) and fine sand component of the mix, occupying 60 to 75% of the concrete volume. The remainder is cement paste (cement + water) plus admixtures. Aggregate provides the structural skeleton of the cured concrete; cement paste binds it together. AS 2758.1:2014 specifies the requirements for concrete aggregates supplied in Australia.
Nominal size and what it means:
The “20 mm aggregate” or “14 mm aggregate” label on a concrete mix is the nominal maximum aggregate size: the largest stone consistently present in the mix. The actual size distribution is graded from the nominal max down through smaller sizes.
| Nominal size | Typical residential application | Section thickness constraint |
|---|---|---|
| 40 mm | Mass concrete only (foundations under heavy structures, dam-style work) | Sections over 200 mm thick |
| 20 mm | Residential default (slabs, footings, driveways) | Standard 100 to 150 mm slabs |
| 14 mm | Slim residential sections (raised beams, narrow beams) | 90 to 120 mm sections |
| 10 mm | Architectural / decorative concrete, blinding layers | Very thin sections (under 75 mm) |
Why aggregate size constrains the mix:
- The aggregate must fit between reinforcement bars without bridging. AS 3600 requires a clear bar-to-bar gap of at least 1.5x the nominal aggregate size, or the maximum aggregate diameter plus 5 mm, whichever is greater. A 20 mm aggregate mix cannot be poured into a heavily reinforced beam with tight bar spacing.
- The aggregate must pass through the pump pipe. Concrete pumps typically need aggregate to be 20 mm or smaller to avoid blockages and excessive pressure. 40 mm aggregate is essentially wheelbarrow-and-shovel only.
- The slab’s surface finish requirement: 20 mm aggregate can be floated smooth without exposing stones; 40 mm aggregate cannot.
What’s in “aggregate” for AS 2758 purposes:
- Coarse aggregate: stones above 4.75 mm (typically gravel or crushed rock).
- Fine aggregate: sand below 4.75 mm.
- A residential concrete is typically 60% coarse + 40% fine by volume.
Source matters:
- Crushed limestone, basalt, granite: standard quarry products, varied geographically.
- River gravel: rounded particles, lower interlocking strength than crushed rock.
- Recycled crushed concrete: AS 2758.1 permits at defined replacement rates; commercial use rising.
- Marine sand: contains salt; must be washed and tested before residential use. Untreated marine sand corrodes reinforcement.
Common defects:
- Mix specified at 20 mm but the supplier delivers a 14 mm mix without authorisation. Pump pipe doesn’t block but the mix may have higher cement content (different shrinkage).
- 40 mm aggregate poured into a slab with reo at 100 mm bar centres. Stones bridge between bars; voids around reo.
- Aggregate-and-sand specification mixed up: builder ordered “20 mm aggregate” thinking it was a single 20 mm stone, supplier delivered a 20 mm-nominal mix with proper grading. The latter is correct.
- Marine sand used untreated in a coastal residential slab. Reinforcement corrosion in 5-10 years.
Also known as: stone (coarse only); sand (fine only); concrete aggregate; cobble (40 mm and up).
Category: Materials.
Related
See also
Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.