Plain concrete is known as concrete without steel reinforcement. It is an intimate mixture of cement, fine aggregates, coarse aggregates and water. It can be easily molded to desired shape and size before it hardens. Plain concrete is strong in compression but weak in tension.
Major ingredients of plain concrete are;
- Binding materials such as cement, lime, polymer.
- Fine aggregate material such as sand.
- Coarse aggregate materials like crushed stone, gravel, and granite.
- Clean water.
Common cement, sand and coarse aggregate ratios are;
1:1:2, 1:2:4, 1:3:6, 1:4:8
Constituents of plain concrete.
Cement is the binding material. It binds and hardens when water is added. Generally a concrete mix with more cement gives more strength.
Coarse aggregates consists of crushed stones of igneous origin. They should be clean, hard, angular and sharp. They give mass to the concrete and prevent shrinkage of cement.
River sand is the common fine aggregate for plain concrete. Its gives mass to concrete and fills voids created by coarse aggregates.it adds density to concrete. A denser concrete is higher in strength.
Water used for making concrete should be clean. It hydrates the cement to form a plastic mass which hardens gradually. Water makes it possible to mix the concrete with ease and place it in final position. The more water the better, however, excess water reduces the strength of concrete. A water – cement ratio of 0.5-0.6 is used to achieve desired concrete strength when hand mixed.
Preparing and placing of concrete.
Batching by volume or weight are the 2 common methods of measurement.
Thorough mixing to produce good quality concrete. Cement, sand and coarse aggregates is mixed in dry condition, then in wet condition after adding water. Hand and machine mixing are 2 common methods of mixing of concrete mixing
Transport and placing:
Fresh concrete is transported on site using head pans, wheel barrow on small scale work while concreting chutes, bell conveyors or pumps are used in large scale construction.
Compaction of concrete is necessary to remove entrapped air during placement of concrete. Compaction can be carried out by hand or with help of concrete vibrators