BULKING OF SAND

The increase in volume of a given mass of fine aggregate caused by the presence of water is known as bulking.

Free moisture forms a film of water around each particle. The bulking of fine aggregate is caused by these films of water which pushes the particles apart. Therefore, no point of contact is possible between the particles. This causes increase in volume of the mass of fine aggregate. The extent of bulking depends upon the percentage of moisture present in the sand and the fineness of sand particles.

Figure 1 shows the effect of moisture content on bulking. It is seen that bulking increases with the increase in moisture content up to a certain limit and beyond that the further increase in the moisture content results in the decrease in the volume. This is due to the fact that after addition of certain amount of water in the fine aggregates, the further addition of water breaks the film around the particles and hence, volume gradually decreases.

Figure 1. Effect of moisture content on the bulking of sand

For ordinary sands the bulking usually varies between 15 to 30 percent. Fine sands bulk more and the maximum bulking is obtained at a higher water content that the coarse sand. In extremely fine sand, the bulking may be as much as about 40 percent at a moisture content of 10 percent but such sand is unsuitable for concrete. In case of coarse sand, the increase in volume is negligible due to the presence of free water as the thickness of the moisture film is very small compared with the size of the particle.

The percentage of bulking is to be determined as per IS: 2386-1963 (Part III). Put sufficient quantity of the sand loosely into a container until it is about two-thirds full. Level off the top of the sand and pushing a steel rule vertically down through the sand at the middle to the bottom, measure the height. Suppose this is h cm.

Empty the sand out of the container into another container where none of it will be lost. Half fill the first container with water. Put back about half the sand and rod it with a steel rod, about 6 mm in diameter, so that its volume is reduced to a minimum. Then add the remainder of the sand and rod it in the same way. Smooth and level the top surface of the inundated sand and measure its depth at the middle with the steel rule. Suppose this is h₁ cm.

The percentage of bulking of the sand due to moisture shall be calculated from the formula: