Determination of Bulk Density and Voids (IS: 2386 Part III)
The bulk density, measured in kilograms per litre is affected by several factors, including theamount of moisture present and the amount of effort introduced in filling the measure. Thisis laboratory test intended for comparing properties of different aggregates. It is not generallysuitable for use as a basis for quoting mix design conversion factors.The test is carried out on dry material when determining the voids, but when bulking testsare required material with a given percentage of moisture may be used.The measure is filled with thoroughly mixed aggregate to about one-third and tamped with25 strokes of the rounded end of the tamping rod.
A further similar quantity of aggregate isadded with a further tamping of 25 times and the surplus aggregate is struck off, using thetamping rod as a straight edge. The net weight of the aggregate in the measure is determinedand the bulk density is calculated.The measure is then filled to overflowing by means of a shovel or scoop, the aggregate beingdischarged from a height not exceeding 50 mm above the top of the measure.
The surface ofthe aggregate is then levelled with a straight edge.
The net weight of the aggregate in themeasure is determined and the bulk density is calculated.The percentage of voids are calculated as follows:
Percentage of voids =(Gs-¥ /Gs)*100
where Gs=specific gravity of the aggregate
¥=bulk density in kg/litres
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). 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 /dry volume)
0 comments:
Post a Comment