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.
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 = _{A}^{B} � 100
Where B = weight of fraction passing
2.36 mm sieve A = weight of oven-dried sample