Aggregate Impact Value Test
AGGREGATE
IMPACT VALUE TEST.
( IS : 2386
– PART – 4 )
INTRODUCTION:
Toughness
is the property of a material to resist impact. Due to traffic loads, the road
stones are subjected to the pounding action or impact and there is possibility
of stones breaking into smaller pieces. The road stones should therefore be
tough enough to resist fracture under impact. A test designed to evaluate the
toughness of stones i.e., the resistance of the fracture under repeated impacts
may be called an impact test for road stones.
Object:
To determine the toughness of road stone materials by Impact test.
Apparatus:
a) Impact testing machine:
b) Measure: A cylindrical
metal measure having internal diameter 75 mm and depth 50 mm for measuring
aggregates.
c) Tamping rod: A straight
metal tamping rod of circular cross section, 10 mm in diameter and 230 mm long,
rounded at one end.
d) Sieve: IS sieve of sizes
12.5 mm, 10 mm, and 2.36 mm for sieving the aggregates.
e) Balance: A balance of
capacity not less than 500 gm to weigh accurate up to 0.1 gm.
f) Oven: A thermostatically
controlled drying oven capable of maintaining constant temperature between 100 0 C
to 110 0 C.
Procedure:
The test sample consists of aggregates passing 12.5 mm sieve and
retained on 10 mm sieve and dried in an oven for four hours at a temperature 100 0 C
to 110 0 C and cooled. Test aggregates are filled up to about
one-third full in the cylindrical measure and tamped 25 times with rounded end
of the tamping rod. Further quantity of aggregates is then added up to
two-third full in the cylinder and 25 stocks of the tamping rod are given. The
measure is now filled with the aggregates to over flow, tamped 25 times. The
surplus aggregates are struck off using the tamping rod as straight edge. The
net weight of the aggregates in the measure is determined to the nearest gram
and this weight of the aggregates is used for carrying out duplicate test on
the same material. The impact machine is placed with its bottom plate flat on
the floor so that the hammer guide columns are vertical. The cup is fixed
firmly in position on the base of the machine and the whole of the test sample
from the cylindrical measure is transferred to the cup and compacted by tamping
with 25 strokes.
The hammer is raised until its lower face is 380 mm above the upper
surface of the aggregates in the cup, and allowed to fall freely on the
aggregates. The test sample is subjected to a total 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 sieved on the 2.36 mm sieve until
no further significant amount passes. The fraction passing the sieve is weighed
accurate to 0.1 gm. The fraction retained on the sieve is also weighed and if
the total weight of the fractions passing and retained on the sieve is added it
should not be less the original weight of the specimen by more than one gram,
if the total weight is less than the original by over one gram the results
should be discarded and a fresh test made.
Calculations:
The aggregate impact value is expressed as the percentage of the fines
formed in terms of the total weight of the sample.
100 W2 Where, W1 = Original weight of the
sample.
Aggregate Impact Value = W1 W2 = Weight of fraction
passing 2.36mm IS sieve.
Results: The mean of the three results is reported as the
AIV(Aggregate Impact Value) of the specimen to the nearest whole number.
Limits: < 10% Exceptionally strong, 10–20% Strong, 20–30%
Satisfactory for road surfacing & > 35%Weak for road surfacing.
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