Ingredients of Concrete and Their Functions

Cement concrete is a composite mixture that consists mainly of relatively inert mineral matter in the form of particles or fragments held together by a binding medium, which gives concrete its solidity and strength. Sand and stone chips or boulders are inert materials and a combination of Portland cement and water is the binder. The inert material, called ‘aggregate’, is normally graded in size from fine sand to boulders or fragments of stone.

There are four ingredients, which make up the composite material of cement concrete:

1. Cement
2. Sand (fine aggregate)
3. Stone chips or boulders (coarse aggregate)
4. Water

1. Cement: The function of cement in the concrete is to bind the coarse and fine aggregate particles together by setting and hardening around such particles. There are different types of cement and each type is used under certain conditions due to its special properties. However, for ordinary construction, generally Ordinary Portland Cement is used. When water is mixed with cement, a chemical reaction takes place because of which the cement paste first loses its plasticity and becomes stiff, at the same time it acquires hardness and strength.
2. Fine aggregate: This 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. The fine aggregates serve the purpose of filling all the open spaces in between the coarse particles, and thus by decreasing the porosity of the final mass, its strength is considerably increased. Sand is universally used as a fine aggregate although many other materials have been developed for special-purpose concretes.
3. 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. The function of the coarse aggregate is to act as the main load-bearing component of the concrete. When a good number of coarse aggregate fragments are held together by a binding material, their behaviour towards the imposed loads is just like a rock mass. Gravels and crushed stones are commonly used for this purpose.
4. Water: This is the least expensive but most important ingredient of concrete. It governs the important properties related to cement concrete such as durability, strength and watertightness. The purposes of mixing water are (a) to damp the aggregates and prevent them from absorbing the water vitally necessary for the chemical combination between cement and water which is called ‘hydration’ (b) to flux the cementing material over the surface of the particles of aggregates and (c) to make the concrete workable so that it can be placed easily and uniformly between the reinforcing bars and in the corners.