Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is bulking of sand? When dry sand gets slightly damp, it swells in volume. A box of damp sand therefore holds less actual sand than a box of dry sand.
Why it matters: if sand is measured by volume (boxes, head pans) without allowing for bulking, the mix gets less sand than designed. The concrete or mortar becomes harsh, honeycombed and uses more cement paste than planned.
This guide follows IS 2386 (Part 3).
Why Damp Sand Swells
Bulking vs Moisture
Apparatus Required
Two containers
Straight-sided, so that height shows volume (a glass jar or measuring cylinder works well).
Steel scale
Measures the height of sand (mm).
Steel rod (6 mm)
Prods the sand under water to settle it fully.
Water
Floods the sand so its bulking disappears.
Procedure
- Pour the damp sand loosely (without pressing or shaking) into a container until it is about two-thirds full.
- Level the top gently and measure the height of sand at the centre with the steel scale: h.
- Tip all the sand into the second container, without losing any.
- Fill the first container about half full of water.
- Pour in about half the sand and prod it with the 6 mm rod to settle it to its smallest volume. Add the rest of the sand and prod again.
- Level the top of the sand (now under water) and measure its height at the centre: h′.
Calculation
h = height of damp, loose sand; h′ = height of the same sand flooded with water. Do two tests and report the mean to two decimal places.
Try It: Bulking Calculator
Enter your heights in mm. The values filled in are a worked example.
Bonus: correct the sand boxes for volume batching
Precautions
🤲 Fill loosely
Pour the damp sand gently. Pressing or shaking it hides the bulking.
🧹 Lose nothing
Move every grain between containers; lost sand makes h′ too small and bulking too high.
🥢 Settle fully
Prod the flooded sand well so no air remains and it reaches its smallest volume.
⏱️ Test the sand you’re using
Take the sample from the stockpile being used today; moisture, and so bulking, changes through the day.
Practical Tips for Site Engineers
1. Typical Bulking (Fine Sand)
| Moisture content | Typical bulking |
|---|---|
| 2% | about 15% |
| 3% | about 20% |
| 4% | about 25% |
| 5% | about 30% |
Rough guide only; the real value depends on the sand. Always test the sand on site.
2. Quick Site Method with a Measuring Cylinder
- Pour damp sand loosely into a 250 ml measuring cylinder up to the 200 ml mark (h = 200).
- Empty it, add about 100 ml of water to the cylinder, pour the sand back in a little at a time and stir to remove air.
- Read the level of the settled sand (h′). Example: 160 ml → bulking = (200 ÷ 160 − 1) × 100 = 25%.
3. Good Site Habits
- Weigh batch if you can: batching sand by weight (with a moisture correction) avoids the bulking problem completely.
- If batching by volume: increase the sand boxes by the bulking %. Example: 2 boxes of dry sand with 25% bulking → 2.5 boxes of damp sand. The calculator above does this.
- Test at least daily, and again after rain or when a new sand load arrives.
- Don’t correct coarse aggregate: bulking is a problem for sand only; stone hardly bulks.
- Watch the mix: a harsh, stony mix that is hard to finish or shows honeycombing often means too little sand. Check bulking.
- Reduce the mixing water too: damp sand brings its own water, so the water added should be reduced.
Key Takeaways
- Damp sand swells in volume (bulks), often by 20–30% at 4–6% moisture; flooded sand doesn’t bulk.
- Measure loose damp sand height (h), then the height after flooding (h′).
- Bulking (%) = (h ÷ h′ − 1) × 100.
- On site: in volume batching, increase sand boxes by the bulking %; prefer weigh batching; test daily and after rain.
References: IS 2386 (Part 3): Methods of Test for Aggregates – Specific Gravity, Density, Voids, Absorption and Bulking; IS 456: Plain and Reinforced Concrete; IS 383:2016: Coarse and Fine Aggregate for Concrete.