When water is added to sand, by the absorption effect, a thin film of water is formed around the sand particles. The film exerts surface tension on particles making them move apart. This leads to increase in volume because there is no direct contact between particles.

For a moisture content percentage of 5 to 8 there will be an increase in volume up to 20 to 40 percent depending upon the sand quality. If the sand is finer there will be greater increase in volume. This is known as bulking of sand.

Test for determining sand bulking

The percentage of bulking of sand can be easily calculated by following this field method.

• Pour the sample of damp sand in a graduated glass cylinder of 250 ml capacity.
• Consolidate the sand by shaking. When the sample has reached to 200 ml mark, fill the cylinder with water and stir well.
• After stirring, the level of sand will drop below the 200 ml mark. The height of submerged sand is measured and marked as “X” ml. The percentage of bulking of sand is calculated by formula: % Bulking = (200-X/X) * 100

With the increase of moisture initially the volume of sand increases, but after certain time further increase in moisture results in decrease of volume.

When moisture content is 5 -10% the volume increases 20 – 40%.

Moisture content less than 5% should be preferred for construction purpose. At about 6 – 10%, the film starts breaking and volume reduces.

It has been found that 4% moisture content (By weight) of dry sand may increase its volume up to 25 -30% depending upon the grading of sand. Finer sand results in extreme bulking.

This phenomenon is explained in the following Figure

Important facts in connection with the bulking of sand are as follows:

• When moisture content is increased by adding more water, the sand particles pack near each other and the amount of bulking of sand is decreased. Thus, the dry sand and the sand completely flooded with water have practically the same volume.
• The coarse aggregate is hardly affected by the moisture content.
• One of the reasons of adopting proportioning by weight is the bulking of sand as proportioning by weight avoids the difficulty due to the bulking of sand.
• The bulking of sand should be considered when volumetric proportioning of the aggregates is adopted. Otherwise, less quantity of concrete per bag of cement will be produced, which naturally will increase the cost of concrete. Also, there will be less quantity of fine aggregate in the concrete mix which may make the concrete difficult to place/pump.

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