Bulking of Sand

Bulking of sand is the apparent increase in the bulk volume of fine aggregate when it is damp, caused by thin films of water around the particles that push them apart. The volume increase peaks at about 4-6% moisture (where damp sand can occupy 20-40% more volume than dry or fully saturated sand) and disappears when the sand is either oven-dry or fully inundated.

It matters critically for volume-batched concrete and mortar: if a fixed bucket/box of damp sand is used, far less actual sand goes in than intended, enriching the mix in cement and weakening the cement-sand ratio — a classic site error. IS 2386 Part 3 gives the field test (measuring settled volume of inundated vs damp sand). The practical fixes are weigh-batching, or inundation/field bulking correction added to the volumetric quantity for the day's moisture.