Mixing the Ingredients:

Mixing time is an important parameter as far as mixing operation is concerned. Mixing time depends upon the type of concrete, capacity of mixer, number of revolutions of the drum per minute, etc. Basically, these parameters, i.e. number of revolution per minute, capacity of mixer and time of mixing are inter-dependent parameters, which are generally specified by the manufacturer of the mixer. The mixing time reduces as the number of revolution per minute (rpm) of mixing drum increases. On the other hand, it increases with the capacity of drum. Also, harsh mix requires more mixing time as compared to wet mix. In normal circumstances, the mixing time of 1-2 min is sufficient. The time is measured from the moment when all the solid ingredients are fed into the mixers and further, it should be ensured that all water is added within one quarter of mixing time. Generally, the speed of a mixer drum varies between 16 rpm to 20 rpm.

IS 456: 2000 recommends that concrete shall be continued to mix in a mechanical vibrator until there is a uniform distribution of the materials and the mass is uniform in color and consistency. If segregation is observed after unloading from the mixer, the concrete should be re-mixed.