Aggregate is the granular material — sand, gravel, crushed stone or recycled concrete — that makes up the bulk of concrete, typically 60–75% of its volume. It is divided into fine aggregate (sand, passing a 4.75 mm sieve) and coarse aggregate (stone or gravel, retained on it).
Aggregates are not just inert filler: their strength, shape, grading and cleanliness strongly affect the concrete. Well-graded aggregate (a good spread of particle sizes) packs densely and needs less cement paste to fill the gaps, giving stronger, more economical concrete. Angular crushed stone interlocks better than rounded gravel, while flaky or elongated particles weaken the mix.
For house construction, 20 mm is the most common maximum size of coarse aggregate for slabs and beams, with 10 mm used in thin or heavily reinforced sections. Coarse aggregate has a bulk density of roughly 1500 kg/m³. In estimating, the aggregate quantity follows directly from the concrete's dry volume and mix ratio.