Mixing

• Concrete  shall be mixed in a mechanical mixer 
• The mixing  shall he continued until there is a uniform  distribution of the materials and the mass is uniform in colour and consistency 
• The mixing time may be taken as about 2 minutes 
• Carry out  slump test  and where possible  compacting factor test to determine workability of concrete 
• sequence of charging the  mixer 

The most common sequence of charging the mixer is - 

1. To begin with, half the quantity of coarse aggregate is placed in the skip over which  half of tine Aggregate is poured 
2. Now the complete quantity of cement is spread over, followed by remaining coarse and tine aggregate. The sequence prevents  spilling of cement  while discharging into the drum and also prevents  blowing off away of cement in strong wind. Mineral  admixtures are generally added with the cement, but water soluble admixtures should be dissolved in mixing wafer. 
3. Now the drum is wetted with 25 per cent of the mixing water. This prevents any sticking of cement on the blades  or to the drum. 
4. Next the material lying in the skip is discharged into the drum followed by remaining 75 per cent of'mixing water. The time of mixing is counted from the moment the complete quantity of water is fed into the drum.