When hand mixing of concrete is adopted, the excess cement to be added (over the designed quantity) is:
Correct Answer: B. 10%
📚 Detailed Explanation: 10% Extra Cement for Hand Mixing
Why B (10%) is correct: Machine mixing produces a more uniform and efficient mix with less material wastage. Hand mixing is less thorough and some cement may adhere to the mixing platform or be unevenly distributed. To achieve the same strength as a machine-mixed batch, 10% extra cement is added as a compensation factor.
Why Extra Cement Is Needed for Hand Mixing
| Mixing Method | Efficiency | Uniformity | Cement Compensation |
|---|---|---|---|
| Machine mixing (drum) | High (~100%) | Very good | None needed |
| Pan mixer | Excellent | Best | None needed |
| Hand mixing | Lower (~85–90%) | Fair (uneven spots possible) | +10% cement |
Hand Mixing Procedure (IS Standard)
| Step | Action |
|---|---|
| 1 | Measure aggregates and cement on a watertight, smooth platform |
| 2 | Mix sand and cement dry until uniform colour |
| 3 | Add coarse aggregate and mix again |
| 4 | Add water gradually; mix until workable and uniform |
| 5 | Use the mixed concrete within 30 minutes |
- Hand mixing: +10% cement added to compensate for mixing inefficiency.
- Hand mixing is only acceptable for minor, non-structural works (M10/M15).
- IS 456 specifies machine mixing for M20 and above — hand mixing is not permitted for structural grades.