When the fine aggregate is moist, volume batching is not considered as a good method for proportioning because of the ______
Correct Answer: C. Bulking of sand
📚 Detailed Explanation: Bulking of Sand Makes Volume Batching Unreliable
Why C (Bulking of sand) is correct: Moist sand has thin water films around each grain. Surface tension in these films pushes grains apart, increasing the apparent volume. For example, sand at 5% moisture content can bulk by as much as 30%. If you measure 2 units of moist sand by volume, the actual dry sand content could be only 2/1.30 = 1.54 units — a 23% deficiency. This causes the actual concrete mix to have less sand than specified, producing an unworkable or segregated mix.
Bulking of Sand — Effect on Volume Batching
| Moisture Content (%) | Approximate Bulking | Actual Sand in “2 volumes” |
|---|---|---|
| 0 (bone dry) | 0% | 2.00 volumes |
| 5% | ~25–30% | 2.0/1.30 = 1.54 volumes |
| 10% | ~10% (decreasing) | 2.0/1.10 = 1.82 volumes |
| Saturated | 0% (bulking disappears) | 2.00 volumes |
- Bulking of sand makes volume batching inaccurate — the measured volume overstates the actual sand content.
- Weight batching avoids this problem since the weight of sand is unaffected by moisture (or can be easily corrected).
- If volume batching is used, correction must be applied: measured vol. = specified vol. × (1 + bulking fraction).