In ordinary structural concrete the aggregates occupy about 70 to 75 % of the volume of harden mass. The reminder consist of hardened cement paste uncombined water (i.e. water not involved in the hydration of cement) air voids. The later evidently do not contribute to the strength of concrete. In general more densely they can be packed, the better the durability and economy of concrete.
For this reason gradation of particle size to produce close packing is of considerable importance.
It is important that they has good strength durability and water resistance that its surface is free from impurities such as loam, clay, silt and organic matter that may weaken the bond with cement paste.
Types of Aggregate
Normally they are classified as
1) Fine Aggregate
It may include sand, clay, silt and surkhi.
2) Coarse Aggregate
Coarse include crushed stone, brick ballast and gravels