Specific gravity test of aggregates is done to measure the strength or quality of the material while water absorption test determines the water holding capacity of the coarse and fine aggregates.
The main objective of these test is to,
- To measure the strength or quality of the material.
- To determine the water absorption of aggregates.
Specific Gravity is the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. It is the measure of strength or quality of the specific material. Aggregates having low specific gravity are generally weaker than those with higher specific gravity values.
Procedure of Water Absorption and Specific Gravity Test on Aggregates
There are three methods of testing for the determination of the specific gravity of aggregates, according to the size of the aggregates larger than 10 mm, 40 mm and smaller than 10 mm. For Samples larger than 10 mm, 40 mm, the below given test method is used and for samples smaller than 10 mm Pycnometer test is done.
- A balance of capacity about 3kg, to weigh accurate 0.5g, and of such a type and shape as to permit weighing of the sample container when suspended in water.
- A thermostatically controlled oven to maintain temperature at 100-110° C.
- A wire basket of not more than 6.3 mm mesh or a perforated container of convenient size with thin wire hangers for suspending it from the balance.
- A container for filling water and suspending the basket
- An air tight container of capacity similar to that of the basket
- A shallow tray and two absorbent clothes, each not less than 75x45cm.
- About 2 kg of aggregate sample is washed thoroughly to remove fines, drained and placed in wire basket and immersed in distilled water at a temperature between 22- 32º C and a cover of at least 5cm of water above the top of basket.
- 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.
- 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.
- The 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 jolted 25 times and weighed in water= W2 g.
- The aggregates placed on the absorbent clothes are surface dried till no further moisture could be removed by this cloth. Then the aggregates are transferred to the second dry cloth spread in single layer and allowed to dry for at least 10 minutes until the aggregates are completely surface dry. The surface dried aggregate is then weighed = W3 g
- 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 g.
Observations of Test
Weight of saturated aggregate suspended in water with basket = W1g
Weight of basket suspended in water = W2 g
Weight of saturated surface dry aggregate in air = W3g
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
(1) Specific gravity = W3 / (W3– (W1– W2))
(2) Apparent specific gravity = W4/ (W4– (W11– W2))
(3) Water Absorption = ((W3 – W4) / W4) X 100
** The size of the aggregate and whether it has been artificially heated should be indicated.
** Though high specific gravity is considered as an indication of high strength, it is not possible to judge the suitability of a sample aggregate without finding the mechanical properties such as aggregate crushing, impact and abrasion values.
Recommended Values of Specific Gravity and Water Absorption for Aggregates
The specific gravity of aggregates normally used in road construction ranges from about 2.5 to 3.0 with an average of about 2.68.
Water absorption shall not be more than 0.6 per unit by weight.
Read more : Specific Gravity of Fine Aggregates