Bulking of sand is the increase in volume of a given weight of sand cause by the films of water pushing the sand particles apart.
Free moisture forms a film around each particle. This film of moisture exerts surface tension which keeps the neighboring particles away from it.
The force exerted by surface tension keeps every particle away from each other. This causes bulking of the volume. The extent of bulking depends on the percentage of moisture present in the sand and on its fineness.
With increase in the moisture content up to 5 to 8 %, bulking of 15 to 40 % takes place. Beyond this, the films of water merge and water moves into the voids between the particles resulting in decrease in the volume of sand.
Bulking does not effect the proportioning of materials by weight. In case of volume batching bulking results in a smaller weight of sand occupying the fixed volume of the measuring box.
For this reason the mix becomes deficient in sand and appears ‘stony’ and the concrete may be prone to segregation and honeycombing. Also the yield of the concrete is reduced. The remedy, of course, lies in increasing the apparent volume of sand to allow for bulking.