Particle size is the diameter of the particles that make up a soil or an aggregate. It is the most important property by which soil is classified and named, and in concrete technology it helps determine the strength, workability and appearance of the mix.
The classification
| Particle size | Name |
|---|---|
| smaller than 0.002 mm | clay |
| 0.002 – 0.063 mm | silt, loess |
| 0.063 – 2 mm | sand |
| 2 – 63 mm | gravel |
| 63 – 200 mm | cobbles |
| 200 – 630 mm | boulders |
| larger than 630 mm | large boulders |
Within these categories there is further subdivision: fine sand runs to roughly 0.2 mm, coarse sand from about 0.5 to 2 mm.
Why particle size governs how ground behaves
Particle size is not a label but the cause of the soil’s behaviour:
- Permeability — sand and gravel let water through; clay effectively does not. That difference decides whether an infiltration system will work, whether a crawl space stays damp and whether an excavation needs dewatering.
- Settlement — sand settles almost immediately and hardly at all afterwards. Clay and peat go on settling for years. An extension founded on a clay layer settles differently from the existing house on piles: the classic cause of cracking at the junction.
- Bearing capacity — coarse, angular material carries far more load than fine, rounded material.
- Frost susceptibility — silt and fine sand draw water up by capillary action and are therefore the most frost-susceptible. Gravel barely is.
In concrete and sub-base layers
- The maximum particle size is limited by bar spacing, concrete cover and the thickness of the section being cast.
- A blend of sizes fills the void space better, so less cement paste is needed and the concrete comes out denser and stronger.
- In sub-base and capping layers, particle size determines whether the layer can be compacted and whether it drains or holds water.
Practical tip
If in doubt about the ground, have a particle size analysis carried out rather than judging by appearance. “Sandy ground” that turns out to be a third silt behaves very differently from clean sand.
Related terms
- Aggregate size fraction
- Particle size distribution
- Soil type
- Soil mechanics
- Clay
- Gravel
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