What are aggregates?
Aggregates are granular materials such as sand, gravel and crushed stone that, together with a binder (usually cement) and water, make up concrete or mortar. In concrete, aggregates form the largest part of the volume. They therefore largely determine its strength, weight, shrinkage and workability. In Europe, the requirements for concrete aggregates are set out in the standard EN 12620.
Types
- Fine aggregate: sand, with small grains.
- Coarse aggregate: gravel (rounded, from rivers or the sea) or crushed stone and chippings (broken rock, angular).
- Recycled aggregate: crushed concrete and mixed granulate from demolition waste.
- Lightweight aggregate: expanded clay or pumice, for lightweight concrete.
- Heavyweight aggregate: for example iron ore, for radiation-shielding concrete.
What to look out for
- Grain size and grading: a good mix of fine and coarse grains fills the voids, so less cement paste is needed. The largest grain must fit between the reinforcement and the formwork.
- Cleanliness: clay, loam, humus and salts weaken the concrete or attack the reinforcement.
- Shape: rounded gravel makes concrete easier to work; angular crushed stone gives better bond.
- Moisture content: wet sand already contains water. Allow for this when measuring the mixing water.
Rules of thumb for DIY
- For a small batch of concrete, a volume ratio of roughly 1 part cement, 2 parts sand and 3 parts gravel is often used, with only as much water as needed for good workability. For structural work, choose ready-mixed concrete with a specified strength class.
- Do not use coarse sand or gravel for bricklaying mortar; use building sand.
- Beach sand and unwashed sand are unsuitable: salt and contamination attack concrete and reinforcement.
Related terms
- Aggregate size fraction
- Gravel
- Gravel and chippings
- Granulate
- Cement
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