Concrete is a mixture of Cement, Sand and Coarse Aggregate (Stone chips) mixed in water. These ingredients are mixed in a suitable proportion (advised by the Architect) either by volume or by weight. The quantity of water is taken as per the fixed water-cement ratio. All the four ingredients when mixed produce a plastic mass which can be poured into suitable moulds or formworks to give desired shape to the resulting stone like solid mass. The whole process from mixing of all ingredients to hardening of concrete is called Concreting.
In concrete, cement and water undergo chemical reaction which is responsible for setting and hardening of the plastic concrete, whereas sand and coarse aggregate do not take part in the chemical reaction. They simply provide mass volume to the concrete and reduce shrinkage effects.
The mixing of large quantity of concrete mix, mixing should preferably be done in mixer machine, as it is economical and produces more homogeneous mix as compared to hand mixing.
Setting and Hardening of Concrete:
During concreting, the process of loosing plasticity is called Setting and process of gaining strength is called Hardening.
The setting of concrete can be divided in two parts – Initial Setting and Final Setting. The Initial setting of concrete may take from 30 minutes to about 10 hours after mixing of water, whereas Final setting may take about 5 to 20 hours after mixing of water, depending upon the type of cement used and weather conditions. Generally, the cement which we use for construction of our homes and buildings the initial setting of concrete takes about 30 minutes and final setting takes 10 hours after mixing water. Therefore, as soon as the water is mixed, the concrete should be poured in the required mould or formwork and compacted within 30 minutes before it starts setting.
Though the process of setting of concrete gets completed within 24 hours of mixing water but process of hardening continues for years. Within first 7 days the concrete gains strength rapidly but with the passage of time it goes on slowing down. It is seen that the concrete achieves almost 90% strength within first 28 days after mixing water.