Mixing of Concrete

Mixing of concrete may be done by hand or by a machine. Mixing should be done thoroughly so that the ingredients are uniformly distributed, and this can be judged by uniform colour and the consistency of concrete.

On a clean, hard and water-tight platform cement and sand are mixed dry using shovels until the mixture shows a uniform colour. Then aggregates are added and the ingredients are thoroughly mixed. Based on the water–cement ratio, the quantity of water required is calculated and added to the dry mix. The mass is then turned to obtain a workable mass and placed in the required area within 30 minutes. Hand mixing can be used for small quantities of concrete, or due to the non-availability of a machine or where the noise of the machine should be avoided. In general, extra cement of 10% is used to compensate the possible inadequacy.

Mixing by machine is always preferred. Concrete mixers are used for mixing concrete and are of two types, viz., (i) continuous mixers and (ii) batch mixers.

Continuous mixers are used for purposes where large quantities of concrete are needed such as dams, bridges, etc. Batch mixers are also called drum mixers, which consist of drums with blades or baffles inside them, and they are rotated. In the batch mixer, all required materials are fed into the hopper of the revolving drum in correct quantity. When the mix has attained the desirable consistency, the mix is discharged from the drum and conveyed to the concreting yard.