Bulk density and bulking of fine and coarse aggregate

Very important properties of aggregates used in construction.

1. Government polytechnic, dahod Subject: Concrete technology (3350602) Topic: Bulk density and bulking of fine and coarse aggregate. Prepared by : Bhavesh Hathila

2. Index Introduction Bulk density Bulking of fine aggregate

3. The bulk density or unit weight of an aggregate is defined as mass of the aggregate per unit volume .it is expressed in kg/liter. The factors affecting bulk density of aggregate are 1.How densely the aggregates are packed . 2. particle size and shape 3.Greding of aggregate 4.Moisture content of aggregate. The shape of particles greatly affect the closeness of the packing. For a given specific gravity the angular aggregates show a lower bulk density. For a coarse aggregates of given specific gravity. Of the early methods of mix design make use of bulk density of aggregates in proportioning of concrete mix. BULK DENSITY:

4. The bulk density of aggregates determines the type of concrete for which it may be used it can be used for judging the quality of aggregate. The bulk density is also required for converting proportions by weight into the proportions volume.

5. . Size of largest particles Nominal capacity [liter] Inside diameter [cm] Inside height [cm] Metal thickness [mm] 4.75 mm and under 3 15 17 3.15 Over 4.75 to 40mm 15 25 30 4.0 Over 40mm 30 35 41 5.0 Determination of bulk density The bulk density of aggregate is determined in IS:2386{PART 3}-1963. The size of the container for measuring bulk density is show in table 1. SIZE OF CONTAINER FOR BULK DENSITY TEST

6. Fill the container in three equal layer. Each layer being temped with 25 stokes by a bullet ended tamping rod, 16 mm diameter and 60 cm long, the container is carefully struck off level using tamping rod as a straight edge. Bulk density of calculated as, Bulk density ϒ = weight of aggregate in kg capacity of the container in liter Percentage of voids={1- ϒ }x100 Where ϒ =bulk density of aggregate in kg/liter G=specific gravity of aggregate

7. Bulking is the phenomenon of increase in the volume of fine aggregates caused bay the presence of free moisture. free moisture from a films of water around each particle.This film of moisture exerts what is known as surface tension which keeps the neighbouring particles away from it. Therefore no point contact is possible between the particles. This causes increase in volume of the mass of fine aggregates . The extent of bulking depends upon the percentage of moisture present in the sand and the fineness of sand particles. BULKING OF FINE AGGREGATES

8. it is interesting to not that the bulking increases with the increase in moisture content up to a certain limit and beyond that the further increase in the moisture content results in the decrease in the volume and at a moisture content representing saturation point , the fine aggregate shows no bulking. This is due to fact that after addition of certain amount of water in fine aggregates, the further addition of water breaks the film around the particles And hence volume gradually decreases. For ordinary sands the bulking usually varies between 15 to 30 percent. Fine sand bulks more and the maximum bulking is obtained at a higher water content than the coarse sand. In case of coarse aggregates the increase in volume is negligible. Extremely fine sand and particularly the manufactured fine aggregate bulks as about 40 percent.

9. if the moist sand is measured buy volume and no allowance is made for bulking, the mix will be richer than that specified because for given mass moist sand occupies a considerably larger volume than the same mass of the dry sand . This results in a mix deficient in sand increasing the chances of the segregation and honey-combing of concrete . The yield of concrete will also be reduced . It is therefore , necessary to increase the measured volume of the sand bay the percentage bulking. If no allowance is made for the bulking of sand a nominal mix 1:2:4, for example, will correspond to 1:1.75:4 for a bulking of 15 percent, and strength of concrete will increase.

10. the percentage of bulking is to be determined as per IS 2386(part-3) 1963. a sample of moist sand is filled into a measuring cylinder in the normal manner. Record the level of sand in cylinder as h1. pour water in to the measuring cylinder and completely inundate the sand and shake it. since the volume of the saturated sand is the same as that of the dry sand, the inundated sand completely offsets the bulking effect. Record the level of sand as h2. percentage of bulking =h1-h2/h2x100

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