Coarse aggregate is the stone skeleton of concrete and makes up most of its volume. The right size and clean stone matter as much as the cement.
Sizes and Uses
| Size | Use in a house |
|---|---|
| 10 mm | Thin slabs, sunshades, congested steel, precast items |
| 20 mm | RCC slabs, beams, columns, footings — the default |
| 40 mm | PCC beds under footings, mass concrete, floor base |
IS 456 Size Rules
- Not more than one quarter of the minimum thickness of the member.
- For reinforced concrete, smaller than the clear distance between bars and the cover, generally by 5 mm, so stones can pass and surround the steel.
So a 100 mm sunshade suits 10–20 mm stone, while a 300 mm footing can take 20 or 40 mm.
Quantity per m³ of Concrete
| Grade (mix) | Aggregate per m³ |
|---|---|
| M10 (1:3:6) | 0.92 m³ (≈ 33 cft) |
| M15 (1:2:4) | 0.88 m³ (≈ 31 cft) |
| M20 (1:1.5:3) | 0.84 m³ (≈ 30 cft) |
Based on a 1.54 dry volume factor. Add 3–5% for handling.
Quality Checks
- Clean: no dust, clay or silt coating.
- Hard and angular: crushed stone, not soft or weathered.
- Well graded: a mix of sizes up to the maximum.
- Not flaky or elongated: these weaken concrete and pack poorly.
Wash dusty aggregate before use.
The Short Version
20 mm for RCC, 10 mm for thin or congested work, 40 mm for PCC; clean, hard, angular stone; about 0.84 m³ per m³ of M20.
