The maximum bulking of sand occurs at a water content between
Correct Answer: A. 4% and 5%
📚 Detailed Explanation: Bulking of Sand
Bulking is the phenomenon of apparent volume increase in fine aggregate (sand) caused by thin films of moisture forming around each particle. These capillary water films generate surface tension that pushes grains apart, so the same mass of sand occupies more volume than it would when bone-dry or fully submerged.
Why A (4%–5%) is correct: Capillary tension is greatest when the moisture film is thin but unbroken and continuous around each particle. At 4%–5% moisture (by weight of dry sand), the film thickness hits this sweet spot, and bulking can reach 20–30% above the dry volume. Below 4%, films are too thin to push grains far apart. Above 5%, excess water begins to fill the air voids, destroys the menisci, and the sand settles back toward its saturated-and-submerged volume.
Moisture Content vs. Bulking Behaviour
| Moisture Content | Bulking Behaviour |
|---|---|
| 0% (bone dry) | No bulking — baseline volume |
| 2%–3% | Moderate bulking begins |
| 4%–5% | Maximum bulking — up to 20–30% volume increase |
| >5% | Bulking decreases as voids fill with water |
| Fully submerged | Volume approaches the dry baseline |
Key Concepts for Students
- Always batch sand by mass — volume batching without a bulking correction under-doses sand and weakens the mix.
- Finer sands bulk more than coarser sands at the same moisture level because their larger surface area holds more capillary water.
- IS 2386 (Part III) gives the standard test method for measuring bulking using a graduated measuring cylinder.
- On-site correction: if volume batching is unavoidable, increase the measured sand volume by the bulking percentage determined by test.