Bulking of Sand:

The moisture content of fine aggregates is also equally important. The fine aggregates in the damp condition may be subjected to bulking phenomenon. Increase in volume of a given weight of sand (fine aggregate) caused by the presence of moisture (within certain limit) is termed as bulking. The presence of moisture forms a film of water around the sand particles due to surface tension, which pushes the sand particles apart and volume of sand increases. The volume of sand goes on increasing with the increase in moisture content but only up to a certain limit, that is. 5-6 percent. Beyond this, the surface tension reduces to the extent that the water film is destroyed. The sand particles loose the repulsive action and volume starts reducing.

The bulking phenomenon of fine aggregates is explained in Figure. The figure clearly indicates that the finer sand bulks considerably more as compared to coarse and medium sand. A bulking phenomenon is almost absent in the case of coarse aggregates as they are larger in size and heavy in weight.

Figure: Bulking Phenomenon

Comments

The range of bulking for the commonly availabbule sand is 15-30 percent. The bulking plays an important role in volume batching of fine aggregates for manufacturing of concrete. If the necessary correction to compensate the bulking is not made, such concrete mix will be under-sanded, since the measured volume of bulked sand is greater than the actual volume of the sand.