aggregate

Aggregate is granular material, typically ranging from 2 mm to 100 mm in diameter, that makes up the bulk of concrete, asphalt, and mortar. It is mixed with a binder (cement, bitumen, or lime) to create a composite material that is stronger and more economical than the binder alone. Aggregate does not hydrate or chemically react with most binders; it simply provides volume, wear resistance, and structural support.

Aggregates are classified by origin and size. Coarse aggregate consists of stone fragments larger than 4.75 mm, typically produced by crushing quarried rock (granite, limestone, basalt) or recycled concrete. Fine aggregate, or sand, is material passing a 4.75 mm sieve. Some aggregates are water-worn gravel from riverbeds or glacial deposits, which require no crushing. Slag, a byproduct of iron smelting, is also used. Density varies: standard crushed stone is around 2,600 to 2,700 kg/m3; lightweight aggregates such as expanded clay may be as low as 1,200 kg/m3.

In concrete, aggregate typically comprises 70 to 80 percent of the mix by volume. The grading (size distribution) of aggregate particles affects workability, strength, and durability. Well-graded aggregate packs more efficiently and requires less cement paste. In asphalt pavements, aggregate size and shape influence skid resistance and fatigue life. Angular particles lock together better than rounded ones, but rounded aggregate is easier to place.

Common problems and specifications

Aggregate quality defects include excess fines (particles under 75 micrometers), which increase water demand and reduce strength; flat or elongated particles, which segregate during placement; and reactive silica minerals that can expand when exposed to alkaline conditions in concrete, causing damage over years. Standards such as ASTM C33 (in North America) or EN 12620 (in Europe) specify maximum chloride content, organic impurities, and other contaminants that compromise durability or appearance.

The term "aggregate" derives from the Latin aggregare, meaning to gather together. In roofing applications, aggregate is applied to the surface of built-up tar or bitumen membranes to provide UV protection, fire resistance, and slip resistance. The same word spans multiple trades because the underlying principle is identical: binding small particles into a larger structure.