Aggregate Impact Value Test (IS:2386 (Part IV))
The aggregate impact value gives a relative measure of the resistance of an aggregate to sudden shock or impact, which in some aggregate differs from its resistance to a slow compressive load.
The test sample consists of aggregate the whole of which passes a 12.5 mm sieve and is retained on a 10 mm sieve. The aggregate comprising the test sample is dried in an oven for a period of four hours at a temperature of 100-110 o C and cooled.
The measure is filled about one-third full with the aggregate and tamped with 25 strokes of the rounded end of the tamping rod. A further similar quantity of aggregate is added and a further tamping of 25 strokes is given. The measure is finally filled to overflowing, tamped 25 times and the surplus aggregate is struck off, using the tamping rod as a straight-edge. The net weight of aggregate in the measure is determined to the nearest gram (weight A).
A cup, 102 mm internal diameter and 50 mm deep, is fixed firmly in position on the base of the machine and the whole of the sample is placed in it and compacted by a single tamping of 25 strokes of the tamping rod.
The hammer is raised until its lower face is 380 mm above the upper surface of the aggregate in the cup, and allowed to fall freely on to the aggregate. The test sample is subjected to a total of 15 such blows each being delivered at an interval of not less than one second.
The crushed aggregate is then removed from the cup and the whole of it is sieved on 2.36 mm IS sieve until no further significant amount passes in one minute. The fraction passing the sieve is weighed to an accuracy of 0.1 g (weight B). The fraction retained on the sieve is also weighed (weight C) and, if the total weight (B + C) is less than the initial weight (A) by more than one gram, the result is discarded and a fresh test made. Two tests are made.
The ratio of the weight of fines formed to the total sample weight in each test are expressed as a percentage, recorded to the first decimal place:
Aggregate impact value = AB ¥ 100
Where B = weight of fraction passing 2.36 mm sieve A = weight of oven-dried sample
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