Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is bulk density? It is the weight of aggregate that fills a container of known volume, including the air gaps between the pieces. It is expressed in kg/litre (or kg/m³).
What are voids? The empty spaces between the aggregate pieces, as a percentage of the total volume. These gaps must be filled by sand, cement paste or bitumen.
Why it matters: used to convert between weight and volume (batching, stock and truck measurement), in mix design, and to judge grading and shape. This guide follows IS 2386 (Part 3).
The Idea in One Picture
Apparatus Required
Cylindrical measure
Watertight metal measure with handles; size depends on the aggregate (table below).
Tamping rod
Steel rod with a rounded end, used to compact each layer.
Balance
Weighs the measure with and without aggregate.
Scoop / shovel
Fills the measure gently for the loose test.
Glass plate
Covers the measure full of water when finding its volume.
Oven
Dries the aggregate before testing.
Size of Measure
| Largest particle size | Capacity | Inside diameter × height |
|---|---|---|
| Up to 4.75 mm (sand) | 3 litres | 15 cm × 17 cm |
| 4.75 to 40 mm | 15 litres | 25 cm × 30 cm |
| Over 40 mm | 30 litres | 35 cm × 31 cm |
As per IS 2386 (Part 3). Metal thickness 3.15, 4.00 and 5.00 mm respectively.
Procedure
Step 1: Find the Measure’s Weight and Volume
- Weigh the clean, dry, empty measure: M1.
- Fill it with water at about 27 °C, slide a glass plate over the top so no air is trapped, and weigh. The weight of water in kg = volume in litres: V.
Step 2: Rodded (Compacted) Bulk Density
- Fill the measure in three equal layers.
- Give each layer 25 strokes of the rounded end of the tamping rod, spread evenly over the surface.
- Strike off the surplus using the rod as a straight edge, so the top is level with the rim.
- Weigh the measure with aggregate: M2.
Step 3: Loose Bulk Density
- Empty the measure. Fill it again with a scoop, letting the aggregate fall from not more than 5 cm above the rim, until it overflows.
- Strike off the surplus with the rod without pressing down.
- Weigh: M3.
Calculation
Gs = specific gravity of the aggregate (from the specific gravity test); γ = bulk density in kg/litre (same as g/cm³). Report to two decimal places.
Try It: Bulk Density & Voids Calculator
Enter weights in kg and volume in litres. The values filled in are a worked example for 20 mm aggregate in a 15-litre measure.
Bonus: volume ↔ weight converter (uses the loose density)
Precautions
☀️ Use dry aggregate
Test oven-dry or surface-dry aggregate. Moisture, especially in sand, changes the result a lot (bulking).
🎯 Even rodding
Spread the 25 strokes over the whole layer. On the first layer, don’t strike the bottom of the measure hard.
📏 Level strike-off
Level the top with the rod; for coarse aggregate, balance any pieces sticking out with hollows below the rim.
🚫 No jolting
For the loose test, don’t tap or shake the measure; it settles the aggregate and raises the value.
Practical Tips for Site Engineers
1. Typical Loose Bulk Densities (Dry)
| Material | Loose bulk density | Typical voids |
|---|---|---|
| Coarse aggregate 20 mm | 1.45–1.60 t/m³ | 40–45% |
| Coarse aggregate 10 mm | 1.40–1.55 t/m³ | 42–47% |
| Sand (dry) | 1.50–1.70 t/m³ | 35–42% |
Typical ranges only. Use values tested on your own material for any calculation or payment.
2. Good Site Habits
- Check truck deliveries: if material is bought by volume but weighed at a weighbridge (or the other way round), use the tested loose bulk density to convert. The converter above does this.
- Volume batching: convert the mix design weights into boxes using loose bulk density. For sand, allow for bulking when it is damp.
- Measure stockpiles: stockpile volume × loose bulk density gives a quick estimate of tonnage for stock-taking.
- Low voids = good grading: well-graded, cubical aggregate packs tighter (higher rodded density, fewer voids) and needs less cement paste. High voids point to poor grading or flaky stones.
- Compare loose and rodded: the difference shows how much the material will settle in trucks and stockpiles; allow for it when measuring.
- Retest when the source or grading changes, and keep the result with the delivery records.
Key Takeaways
- Bulk density = weight of aggregate filling a known volume, including gaps.
- Measure size depends on the largest particle: 3 L (sand), 15 L (up to 40 mm), 30 L (over 40 mm).
- Rodded: 3 layers × 25 strokes → M2. Loose: pour from ≤ 5 cm → M3.
- γ = (M − M1) ÷ V; voids (%) = (Gs − γ) ÷ Gs × 100.
- On site: use loose bulk density to convert m³ ↔ tonnes for trucks, stock and volume batching.
References: IS 2386 (Part 3): Methods of Test for Aggregates – Specific Gravity, Density, Voids, Absorption and Bulking; IS 383:2016: Coarse and Fine Aggregate for Concrete.