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'.