Bulking of sand

Bulking is the increase in the volume of sand (fine aggregate) when it contains a certain amount of surface moisture, compared with the same mass of dry sand.

Cause

Moisture forms a thin film around each sand grain. Surface tension of this film pushes the particles apart and keeps them from packing closely, so voids increase and the volume rises. Bulking is about 20-40 per cent at a moisture content of 4-6 per cent for fine sand. When the sand is fully flooded (saturated), the film is broken and the volume falls back to the dry volume. Finer sand bulks more than coarse sand.

Bulking %
  |        _____
  |      /       \
  |    /          \
  |  /              \___ (flooded:
  |/                      bulking = 0)
  +----------------------------> moisture %
     4-6% peak

Effect

• If the mix is batched by volume, the actual quantity of sand is less than intended, so the mix becomes lean in fine aggregate: the concrete is harsh, honeycombed, less workable, lower in strength, and the yield per bag is lower.
• Wrong sand-cement proportion affects economy and quality.

Remedial measures

• Batch sand by weight, not volume.
• Make a bulking allowance: increase the sand volume by the bulking per cent found by test.
• Dry the sand or flood it fully before batching.

Determination (field test)

Fill a measuring cylinder to a height $h_1$ with moist sand, then add water and stir so that the sand is flooded; the height becomes $h_2$.

$$\text{Bulking}\ \% = \frac{h_1 - h_2}{h_2} \times 100$$

Role in concrete manufacture

Knowing bulking, the sand quantity measured in volume batching is corrected as: required volume of damp sand = volume of dry sand $\times\ (1 + \text{bulking}/100)$. The free moisture carried by the sand is also deducted from mixing water, so the water-cement ratio stays correct.