Fine Aggregate and Coarse Aggregate

 Fine aggregate

Fine aggregate is the inert or chemically inactive material, most of which passes through a 4.75 mm IS sieve and contains not more than 5 per cent coarser material. They may be classified as follows:

1. Natural sand: Fine aggregate resulting from the natural disintegration of rocks and which has been deposited by streams or glacial agencies.
2. Crushed stone sand: Fine aggregate produced by crushing of hard stone.
3. Crushed gravel sand: Fine aggregate produced by crushing of natural gravel.

The fine aggregates serve the purpose of filling all the open spaces in between the coarse particles. Thus, it reduces the porosity of the final mass and considerably increases its strength. Usually, natural river sand is used as a fine aggregate. However, at places, where natural sand is not available economically, finely crushed stone may be used as a fine aggregate.

Coarse aggregate

The inert material, most of which is retained on a 4.75 mm sieve and contains not more than 0–10 per cent of finer materials, is known as coarse aggregate. They may be put under the following categories:

1. Uncrushed gravel or stone which results from the natural disintegration of rocks.
2. Crushed gravel or stone which results from crushing of gravel or hard stone.
3. Partially crushed gravel or stone which is a product of the mixture of the above two types.

The function of the coarse aggregate is to act as the main load-bearing component of the concrete. The nature of work decides the maximum size of the coarse aggregate. For thin slabs and walls, the maximum size of the coarse aggregate should be limited to one-third the thickness of the concrete section. The aggregates to be used for cement concrete work should be hard, durable and clean. The aggregates should be completely free from lumps of clay, organic and vegetable matter, fine dust, etc. The presence of all such debris prevents adhesion of aggregates and, hence, reduces the strength of concrete.