Theory and Scope:
This is one of the major Mechanical properties required
in a road stone. The test evaluates the ability of the Aggregates used in road
construction to withstand the stresses induced by moving vehicles in the form
of crushing. With this the aggregates should also provide sufficient resistance
to crushing under the roller during construction and under rigid tyre rims of
heavily loaded animal drawn vehicles. The crushing strength or aggregate
crushing value of a given road aggregate is found out as per IS-2386 Part- 4.
The aggregate crushing value provides a relative measure of resistance to
crushing under a gradually applied compressive load. To achieve a high quality
of pavement aggregate possessing low aggregate crushing value should be
preferred. The aggregate crushing value of the coarse aggregates used for
cement concrete pavement at surface should not exceed 30% and aggregates used
for concrete other than for wearing surfaces, shall not exceed 45% as specified
by Indian Standard (IS) and Indian Road Congress (IRC).
Aim:
To determine crushing strength of a given aggregate as
per IS: 2386 part – IV
Apparatus:
Ø A
steel cylinder of internal diameter 15.2 cm (Steel cylinder with open ends)
Ø A
square base plate, plunger having a piston diameter of 15 cm.
Ø A
cylindrical measure of internal diameter of 11.5 and height 18 cms.
Ø Steel
tamping rod having diameter of 1.6 cms length 45 to 60 cms.
Ø Balance
of capacity 3 kg with accuracy up to 1 gm.
Ø Compression
testing machine capable of applying load of 40 tonnes at a loading rate of 4
tonnes per minute.
Procedure:
Ø The
aggregate in surface-dry condition before testing and passing 12.5 mm sieve and
retained on 10 mm sieve is selected.
Ø The
cylindrical measure is filled by the test sample of the aggregate in three
layers of approximately equal depth, each layer being tamped 25 times by the
rounded end of the tamping rod.
Ø After
the third layer is tamped, the aggregates at the top of the cylindrical measure
are leveled off by using the tamping rod as a straight edge. Then the test
sample is weighed. Let that be w1 gm.
Ø Then
the cylinder of test apparatus is kept on the base plate and one third of the
sample from cylindrical measure is transferred into cylinder and tamped 25
times by rounded end of the tamping rod.
Ø Similarly
aggregate in three layers of approximately equal depth, each layer being tamped
25 times by rounded end of the tamping rod.
Ø Then
the cylinder with test sample and plunger in position is placed on compression
testing machine.
Ø Load
is then applied through the plunger at a uniform rate of 4 tonnes per minute
until the total load is 40 tonnes and the load is released.
Ø Aggregates
including the crushed position are removed from the cylinder and sieved on a
2.36mm IS. sieve and material which passes this sieve is collected and weighed.
Let this be w2 gm.
Ø The
above step is repeated with second sample of the same aggregate. The two tests
are made for the same specimen for taking an average value.
Ø Total
weight of dry sample taken is w1 gm weight of the portion of crushed material
passing 2.36mm IS sieve be w2 gm.
Ø Then
the aggregate crushing value is defined as the ratio of weight of fines passing
the specified IS sieve to the total weight of the sample (w1).
Result:
The mean (average) crushing value of the given
aggregate sample is ______________%.
thanks for this upload.
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