Fine aggregate (sand) has a tendency to increase in volume (bulk) depending upon the moisture present in it.
Bulking Test for Fine Aggregate (IS: 2386 (Part III))
Fine aggregate (sand) has a tendency to increase in volume (bulk) depending upon the moisture present in it. In making concrete mix, if the batching in done by volume, the actual quantity of sand in each batch will be less than the recommended volume and actual quantity of sand in each batch will be less than the recommended volume and consequently the mix will be rich in cement, i.e. the proportions of cement and sand will be different to be provided. This will result in uneconomy and may affect adversely teh concrete also. Therefore, the amount of sand will have to be increased by percentage bulking (B).
The field method of determination of bulking is explained in Sec. 6.3. In the laboratory test sufficient quantity of oven dried sand is filled in graduated cylinder up to a certain fixed mark. The sand is emptied in a container and the weigh of dry sand is determined 1% water is added in the sand mixed thoroughly and filled in the graduated cylinder. The volume of sand will be found to increase. The process is repeated by increasing the percentage of water in steps of 1% each time till a decrease in the volume of sand is observed. Still this addition of water is continued till the volume of sand comes back to the original volume (fully saturated).
A graph is plotted between moisture content and percentage increase in volume.The percentage bulking is given by,
B = bulked volume - dry volume ¥ 100 dry volume