Test on Stones
In order to find out the essential properties of stones, the following tests can be done:
- Crushing Strength Test
- Water absorption test
- Abrasion test
- impact test
- Acid test
(ii) Water Absorption Test: For this test a cubic sample weighing about 50 grams is prepared and the test is done in the steps given below:
(A) Note the weight of the dry sample as W1.
(b) Place the sample in water for 24 h.
(C) Remove the sample, wipe the surface with a piece of cloth and weigh the sample. Let its weight be W2.
(d) Freely suspend the sample in water and weigh it. Let its weight be W3.
(e) Place the sample in boiling water for 5 h. Then take it out, wipe the surface with a cloth and weigh it. Let this weight be W4. Then,
Percentage of absorption by weight = W2-W1/W1× 100 ...(1)
Percent absorption by volume = W2-W1/W2-W3× 100 ...(2)
Percentage of porosity by volume = W4-W1/W3-W2× 100 ...(3)
Density = W1/W2-W1 ...(4)
Specific gravity = W1/W2-W3 ...(5)
Saturation coefficient = water absorption / total porosity
= W2-W1/W4-W1.
(iii) Abrasion Test: This test is performed on stones which are used as aggregates for road construction. The test results indicate the suitability of the stones against the grinding action under traffic. One of the following tests may be performed to determine the suitability of the aggregate:
(i) Los Angeles abrasion test
(ii) Dewal friction test
(iii) Abrasion test of string.
However, the Los Angeles abrasion test is preferred because these test results correlate well with the performance of the pavement.
The Los Angeles apparatus [fig] consists of a hollow cylinder 0.7 m in diameter and 0.5 m long, with both ends closed. It is mounted on a frame so that it can be rotated about a horizontal axis. IS Code has standardized the testing procedure for various gradations of samples. A specified number of cast iron balls of diameter 48 mm are placed in a cylinder with the specified weight of the sample.
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| Fig. Los Angeles test machine |
The cylinder is then rotated at a speed of 30 to 33 rpm for the specified time (500 to 1000). The aggregate is then removed and sieved at 1.7 mm. Have to sieve The total passing weight is found. Then the value of Los Angeles is found as
= Weight of the aggregate passing through the sieve/original weight × 100.
The following values are recommended for road works:
For bituminous mix - 30%
For Base Course - 50%
(iv) Impact test: The resistance to impact of stones is found by conducting tests in impact testing machine (Fig. 1.2). It consists of a frame with guides in which a metal hammer weighing from 13.5 to 15 kg can freely fall from a height of 380 mm.
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| Fig. Aggregate impact testing machine |
Aggregates ranging in size from 10 mm to 12.5 mm are filled in the cylinder in 3 equal layers; Each layer is being tamped 25 times. The same is then transferred to the cup and tamped again 25 times. The hammer is then allowed to fall freely on the sample 15 times. The sample is then filtered through a 2.36 mm sieve.
Then, Impact Value = W2/W1
Where, W2 = Weight of penalty
W1 = Original weight.
The recommended impact values for various tasks are:
(i) for wearing Course >/ 30%
(ii) for bituminous mechadum >/35%
(iii) for water bound mechadam >/40%
(v) Acid test: This test is usually performed on sandstones to check for the presence of calcium carbonate, which weakens the weather resistant quality. In this test a stone sample weighing about 50 to 100 grams is taken and kept in a solution of one percent hydrochloric acid for seven days. The solution is agitated at intervals. A good building stone retains its sharp edges and keeps its surface intact. If the edges are cracked and powder forms on the surface, this indicates the presence of calcium carbonate. Such stones will have poor weather resistance.