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Cement concrete is an artificial stone obtained by mixing cement, aggregate and water in predetermined proportion. When these ingrediants are mixed, they form a plastic mass which can be poured in suitable moulds, called forms, and set on standing into hard solid mass. The chemical reaction of cement and water in the mix is relatively slow and requires time and favourable temperature for its setting. This time is known as setting time. It may be divided into three distinct phases. The first phase, known as the time of initial set, requires 30 minutes to 60 minutes. During this period, the mixed concrete decreases its plasticity and develops pronounced resistance to flow. The second stage, known as final set, may vary between 5 to 6 hours after mixing operation. During this phase, concrete appears to be relatively soft solid without surface hardness. The third phase consists of progressive hardening and increse in strength. The process is rapid in the initial stage, until about one month after mixing, at which time the concrete almost attains the major portion of its potential hardness and strength.
Cement concrete is considered as a major building material in all types of construction because
- It can be readily moulded into durable structural items of various sizes and shapes.
- It is possible to control the properties of cement concrete within a wide range by using appropriate ingredients and by applying special mechanical, chemical and physical processing techniques.
- It is possible to mechanise completely its preparation and placing processes.
- It possesses adequate plasticity for mechanical working.
Cement concrete hardness with age and the process of hardening continues for a long time after the concrete has attained sufficient strength. Because of this property, cement concrete has a distinct place among the building materials. It is proved to be more economical because sand and pebbles or crushed rock, forming the bulk of cement concrete to the extent of about 80 to 90 percent are available at moderate cost.
Cement concrete has a tendency to shrink. The initial shrinkage is mainly due to loss of water through forms, absorption by surfaces of forms and so on. The shrinkage of cement concrete occurs as it hardness. This tendency of cement concrete can be minimized by proper curing of concrete.
The cement concrete has a tendency to be porous. This is due to the presence of voids which are formed during and after its placing. To avoid this tendency.
- There should be proper grading and consolidating of the aggrements.
- Minimum water – cement ratio should be adopted.
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