An aggregate size fraction — also called a grain group — states the smallest and the largest particle size of an aggregate. It is the commercial designation by which sand, gravel and recycled granulate are ordered and delivered. The notation is d/D, where lower-case d is the smallest particle size and upper-case D the largest.
The classification
| Category | Particle size | Examples | Common fractions |
|---|---|---|---|
| Fine aggregate | smaller than 4 mm | building sand, concreting sand, pit sand | 0/1, 0/2, 0/4 |
| Coarse aggregate | 4 mm and larger | gravel, chippings, granulate | 4/8, 4/16, 4/22, 4/32, 8/11, 8/16, 16/22, 16/32 |
The boundary values are not arbitrary: they belong to the standard sieve series, in which sizes are rounded to figures such as 5, 11, 22 and 32 mm. In concrete work, aggregate is rarely coarser than 32 mm.
Why the choice matters
- Bar spacing — the largest particle has to pass between the reinforcement bars and between reinforcement and formwork. As a rule of thumb the maximum aggregate size stays below the clear bar spacing and below the concrete cover.
- Cement content — the larger the maximum size, the less cement paste is needed to coat all the particles. Coarser aggregate is therefore more economical, but less workable in thin or heavily reinforced sections.
- Surface finish — in fair-faced and exposed-aggregate concrete, the fraction directly determines the appearance.
- Price and availability — standard fractions are common and cheap; unusual fractions cost more and can be hard to source.
Ordering in practice
- Always state the fraction. “A cubic metre of gravel” is not an order; “4/16” is.
- Building sand is not concreting sand. Building sand is finer and more rounded, concreting sand coarser and more angular. They are not interchangeable.
- Combine fractions deliberately. A single fraction gives a gap-graded mix with a lot of void space; a blend of fractions fills that space and produces stronger, denser concrete.
Related terms
- Particle size
- Particle size distribution
- Fraction
- Gravel and chippings
- Ready-mixed concrete
- Concrete
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