Bulking of sand means an increase in volume. Fine aggregate (i.e., sand) increases in volume when they possess some moisture.
This volume increase is not due to the increase in the volume of the individual sand particle in the presence of moisture. It is due to the formation of a thin film of water around the
aggregate or sand particles. The thickness of this film goes on increasing with more moisture. That is, the volume of sand mass continues to increase with moisture content. This increase in volume varies 20-30% (vol. basis) depending upon the fineness of sand. The finer particles will undergo a greater increase in volume. After a certain percentage of water (5-8%, also depends on the fineness of
sand), the volume of sand starts decreasing with the further increase in water and eventually, the increased volume completely vanishes, and at this moment, the volume occupied by sand becomes equal to the volume of dry sand.
Bulking of sand effects water-cement ratio and also the proportioning of aggregate.
