Explain abrasion test and impact test with equation and diagram.
1 Answer


(1) Abrasion Test:

This test is carried out on stones which are used as aggregates for road construction. The test result indicates the suitability of stones against the grinding action under traffic.

Any one of the following tests may be conducted to find out the suitability of aggregates:

(i) Los Angeles abrasion test

(ii) Deval abrasion test

(iii) Dorry’s abrasion test.

The Los Angeles apparatus figer consists of a hollow cylinder 0.7 m inside diameter and 0.5 m long with both ends closed.

It is mounted on a frame so that it can be rotated about horizontal axis. IS code has standardised the test procedure for different gradation of specimen.

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Along with specified weight of specimen a specified number of cast iron balls of 48 mm diameter are placed in the cylinder.

Then the cylinder is rotated at a speed of 30 to 33 rpm for specified number of times (500 to 1000). Then the aggregate is removed and sieved on 1.7 mm. IS sieve.

The weight of aggregate passing is found. Then Los Angeles value is found as.

$ =\frac{\text { Weight of aggregate passing through sieve }}{\text { Original weight }} \times 100 .\\ $

The following values are recommended for road works:

For bituminous mixes $\quad-30 \%$

For base course $-50 \%$

(2). Impact Test:

The resistance of stones to impact is found by conducting tests in impacting testing machine Figer.

It consists of a frame with guides in which a metal hammer weighing 13.5 to 15 kg can freely fall from a height of 380 mm.

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Aggregates of size 10 mm to 12.5 mm are filled in cylinder in 3 equal layers; each layer being tamped 25 times.

The same is then transferred to the cup and again tamped 25 times. The hammer is then allowed to fall freely on the specimen 15 times. The specimen is then sieved through 2.36 mm sieve. Then,

Impact value $=\frac{\mathrm{W}_2}{\mathrm{~W}_1}$

where $\mathrm{W}_2=$ weight of fines

$\mathrm{W}_1=$ original weight.

The recommended impact values for various works are:

for wearing course $\ngtr 30 \%$

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