DETERMINATION OF SPECIFIC GRAVITY AND WATER ABSORPTION OF FINE AGGREGATE

The specific gravity of aggregates can be found out by Pycnometer Method in the field. Pycnometer is large density bottle and its shape is similar to typical Indian pickle jar with a brass conical top to fit the jar by means of a rubber gasket. 

The method involves placing the aggregate sample in the bottle and filling it with water and the results depend for their accuracy upon the thoroughness with which air is removed from the aggregate by stirring.

The Pycnometer should first be dried carefully and weighed (W1) after which it is filled with clean tap water and weighed again (W2). 

The water is then emptied and the bottle is again dried. About 500 grams. Of dried aggregate are introduced and the Pycnometer and contents are again weighed (W3). 

Water is then added to the aggregate, which is stirred with a glass rod during the process to allow entrapped air to be released. Finally, the jar is filed completely with water, dried on the outside and weighed again (W4). 

The specific gravity of the aggregate can then be calculated from the formula.

\[{sp.~Gravity} = {{W3 – W1 } \over {W2 –W1 + W3 – W4 } }\]

Specific Gravity Values:

Generally the specific gravity of fine and coarse aggregate is about 2.6 to 2.7 and 2.5 to 2.6 respectively.

Specific gravity of Portland cement is generally about 3.15. Portland blast furnace slag and Portland Pozzolona cement may have specific gravity of values of about 2.90. The specific gravity of cement is not an indication of the cement’s quality.

Water absorption:

Generally the water absorption of fine aggregate need not be taken as we get the river sand in a moist condition. 

However if we get crushed sand it will be dry and water absorption is to be calculated. Also if river sand is heaped for days together in hot summer it may absorb water. 

The method to check absorption is to take a sample of about 2 kg and to heat it up in oven. It will become bone dry. Its weight (W1) should be taken. 

Later this should be immersed in water for 24 hours. After that it should be taken and should be spread on a plate not exposed to sun and let it dry for 2 to 3 hours. Thus it gets air-dried. It weight (W2) should be taken. 

The difference between the weights gives the water absorption.