One of the first field investigations for a concrete construction project is to search for sources of aggregates which will give material of good quality at economical rates. Suitability of aggregate will depend upon the geological history of the region. The aggregate as per nature of formation may be divided into two types.
- Natural Aggregates
- Artificial Aggregates
These aggregates are obtained from natural deposit of sand and gravel or from quarries by cutting rocks. Cheapest among them will be the natural sand and gravel which have been reduced to their present size by natural agents such as water, wind and snow etc. River deposits are the most common and have good quality.
The second most commonly used source of aggregates is quarried bed rock material. Crushed aggregates are made by breaking down natural bed rocks into requisite graded particles through a series of blasting, crushing and screening, etc.
Amongst the artificial aggregates brick ballast and air cooled blast furnace slag are most common. Broken brick may be used for mass concrete but is not used for reinforced concrete work unless the crushing strength is high. Blast furnace slag is not commonly used on account of the possible corrosion of steel due to the sulpher content of slag. Concrete made with blast furnace slag aggregate has good fire resisting qualities. Other artificial aggregates such as foamed slag, expanded clay, shale and salte are also used for producing light weight concrete.