Crushing Value of aggregate

Aim
To determine crushing value of course aggregate 

Apparatus
A 15-cm diameter open-ended steel cylinder, with plunger and base-plate,A straight metal tamping rod, A balance of capacity 3 kg, readable and accurate to one gram, IS Sieves of sizes 12.5, 10 and 2.36 mm, For measuring the sample, cylindrical metal measure of sufficient rigidity to retain its form under rough usage and of the following internal dimensions: Diameter 11.5 cm and Height 18.0 cm

Theory: The aggregate crushing value‘ gives a relative measure of the resistance of an aggregate to crushing under a gradually applied compressive load. With aggregate of aggregate crushing value‘ 30 or higher, the result may be anomalous, and in such cases the‗ten percent fines value‘ should be determined instead.



Procedure :


1. The material for the standard test shall consist of aggregate passing a 12.5 mm IS Sieve and retained on a 10 mm IS Sieve, and shall be thoroughly separated on these sieves before testing.

2. The aggregate shall be tested in a surface-dry condition. If dried by heating, the period of drying shall not exceed four hours, the temperature shall be 100 to 110°C and th aggregate shall be cooled to room temperature before testing.

3. The appropriate quantity may be found conveniently by filling the cylindrical measure in three layers of approximately equal depth, each layer being tamped 25 times with the rounded end of the tamping rod and finally leveled off, using the tamping rod as a straight-edge.

4. The weight of material comprising the test sample shall be determined (Weight A) and the same weight of sample shall be taken for the repeat test.

5. The apparatus, with the test sample and plunger in position, shall then be placed between the platens of the testing machine and loaded at as uniform a rate as possible so that the total load is reached in 10minutes. The total load shall be 400 kN.

6. The load shall be released and the whole of the material removed from the cylinder and sieved on a 2.36 mm IS Sieve for the standard test. The fraction passing the sieve shall be weighed (Weight B).

Calculation
The ratio of the weight of fines formed to the total sample weight in each test shall be expressed as a percentage Aggregate Crushing Value =
A = weight (gm. ) of saturated surf ace - dry sample,
B = weight (gm) of fraction passing through appropriate sievs

Result

The aggregate crushing value of given sample of coarse aggregate is ……….. %
The aggregate crushing value should not be more than 45 per cent for aggregate used for concrete other the wearing surfaces, and 30 per cent for concrete used for wearing surfaces such a runways, roads and pavements.


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