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. The finer the material the more will be the increase in volume for the given moisture content. Bulking of sand can be expressed in a graphical way as shown in Figure 2.1.
For a moisture content of 5–8 per cent, the increase in volume may be about 20–40 per cent depending upon the gradation of sand. When the moisture content is further increased, the sand particles pack near each other and the amount of bulking is decreased. Hence, dry sand and the sand completely flooded with water have practically the same volume.
The volumetric proportioning of sand depends upon the extent of bulking. It is more with fine sands than with coarse sands. If proper allowances are not made for bulking of sand, the cost of concrete and mortar increases and it results in mixes with inadequate sand. This makes the mix harsh and difficult for working and placing.