Bulking of sand means increase in it's volume due to presence of surface moisture . The volume increases with increase in moisture content . The volume may increase up to 20 to 40% when moisture content is 5 to 10 %
When mixes are specified by volume, the sand is assumed to be dry. The volume of a given weight of sand, however, varies according to its moisture content. Equal weights of dry and inundated sand have practically the same volume but the same weight of sand in a damp condition can occupy a volume as much as 40% greater. This phenomenon is known as 'bulking'.
The increase in the volume of sand due to the presence of moisture is known as bulking of sand. This is due to the fact that moisture forms a film of water around the sand particles and this results in an increase in the volume of sand. The extent of bulking depends on the grading of sand.