Calculating the Bulk, apparent Specific gravity and water absorption in coarse aggregates

 Specific Gravity of Coarse Aggregates

It is defined as the ratio of the mass of the solid in given volume of sample to the mass of the equal volume of water at same temperature.It generally varies from 2.6 to 2.9 range. Specific gravity of aggregates generally defines the strength of aggregates . Stones of low specific gravity are generally considered to be weak. The specific gravity helps for choosing right stone for pavement and concrete mixes.It is also useful for the weight volume conversion and calculating the void content in aggregates.In case of Apparent Specific Gravity the volume measurement only includes the volume of the aggregate particle; it does not include the volume of any water permeable voids. The mass measurement only includes the aggregate particle. Apparent specific gravity is intended to only measure the specific gravity of the solid volume, therefore it will be the highest of the aggregate specific gravities. It is formally defined as the ratio of the mass of a unit volume of the impermeable portion of aggregate (does not include the permeable pores in aggregate) to the mass of an equal volume of gas-free distilled water at the stated temperature.In case of Bulk Specific Gravity The volume measurement includes the overall volume of the aggregate particle as well as the volume of the water permeable voids. The mass measurement only includes the aggregate particle. Since it includes the water permeable void volume, bulk specific gravity will be less than apparent specific gravity. It is formally defined as the ratio of the mass of a unit volume of aggregate, including the water permeable voids, at a stated temperature to the mass of an equal volume of gas-free distilled water at the stated temperature.

Apparatus

Weighing machine up to 3 kg capacity, wire basket having mesh size not more than 6.3 mm,oven, container up to 5 liter capacity, soft cloth , tray etc.

Specific gravity apparatus frame setup with wire mesh bucket

Procedure

1. Take about 2 kg of sample and sieve it from 10 mm sieve to remove fines. Now immerse the sample with wire bucket in a water filled container.
2. Immediately after immersion the entrapped air is removed from the sample by lifting the basket containing it 25 mm above the base of the tank and allowing it to drop at the rate of about one drop per second. The basket and aggregate should remain completely immersed in water for a period of 24 hour afterwards at temperature of 22° – 32°C. 
3.  Now the water is drained out and The basket and  the sample are weighed while suspended in water at a temperature of 22° – 32°C. The weight while suspended in water is noted W1g. 
4. Basket and aggregates are removed from water and allowed to drain for a few minutes after which the aggregates are transferred to the dry absorbent clothes. The empty basket is then returned to the tank of water dipped 25 times and weighed in water W2 g.
5. Aggregates placed on the absorbent clothes are surface dried till no further moisture could be removed by this cloth. Then aggregates are dried for at least 10 minutes until the aggregates are completely surface dry. The surface dried aggregate is then weighed  W3 g.
6. The aggregate is placed in a shallow tray and kept in an oven maintained at a temperature of 110° C for 24 hrs. It is then removed from the oven, cooled in an air tight container and weighted=W4
   
   

Observations and Calculations

Weight of saturated aggregate suspended in water with basket = W1 g.

Weight of basket suspended in water = W2 g.
Weight of saturated surface dry aggregate in air = W3 g.
Weight of oven dry aggregate = W4 g.
Weight of saturated aggregate in water = W₁ – W₂ g
Weight of water equal to the volume of the aggregate = W₃–( W₁–W₂)g

Bulk Specific gravity = W₃ / (W₃– (W₁– W₂))
Apparent specific gravity = W₄/ (W₄– (W₁– W₂))
 Water Absorption = ((W₃ – W₄) / W₄) x 100