Bulking of Sand

The presence of moisture in sand increases the volume of sand. This is due to the fact that moisture causes film of water around sand particles which results in the increase of volume of sand. For a moisture content of about 5 to 8 per cent, this increase of volume may be as much as 20 to 40 per cent, depending upon the grading of sand. The finer the material, the more will be the increase in volume fora given moisture content. This phenomena is known as the bulking of sand and it can be expressed in a graphical way as shown in the following figure.

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 bulking of sand affects the volumetric proportioning of sand to a large extent.  It is more with fine sand and less with coarse sand. If proper allowance is not made for the bulking of sand, the cost of concrete and mortar increases and it results into under-sanded mixes which are harsh and difficult for working and placing.

Procedure to determine the bulking of sand

A very simple test, as shown in following figure, may be carried out to decide the percentage of bulking of sand. Following procedure is adopted: 

(i) A container is taken and it is filled two-third with the sample of sand to be tested.

(ii) The height is measured, say it is 200 mm.

(iii) The sand is taken out of container. Care should be taken to see that there is no loss of sand during this transaction.

(iv) The container is filled with water. 

(v) The sand is then slowly dropped in the container and it is thoroughly stirred by means of a rod.

(vi) The height of sand is measured, say it 160 mm.