To get better efficiency, the sequence of charging the loading skip is as under
1. First place half the quantity of coarse aggregate in the skip.
2. Place half the quantity of fine aggregate over the previously placed coarse aggregate.
3. Then pour full quantity of cement (i.e. one bag of cement) over it.
4. Then deposit the remaining portion of coarse aggregate and fine aggregate over it. This prevents spilling of cement, while discharging into the drum and also this prevents the blowing away of cement in windy weather.
5. Before the loaded skip is discharged to drum, about 25 percent of the total quantity of water is introduced into the mixer drum to wet the drum and to prevent any cement sticking to the blades or at the bottom of the drum.
6. Then discharge the dry material into the drum.
7. Immediately, on discharging the dry material into the drum, the remaining 75 percent of water is added to the drum.
Notes To Remember
The mixing time is counted from the moment all the materials; particularly the complete quantity of water is fed into the drum.
When plasticizers or super plasticizers are to be used in the concrete, then calculated quantity of plasticizers or super plasticizers is mixed with one litre of water (held back from the remaining 75 percent of mixing water) and the same is added to the mixer drum after about one minute of mixing.
It is desirable that concrete is mixed little longer so that the plastcizing effect is fully achieved by proper dispersion.