Test of Stones

Test of Stones

To acertain the required properties of stones, the following tests can be conducted:

(i) crushing strength test (ii) water absorption test (iii) abrasion test (iv) impact test (v) acid test.

Crushing Strength Test

For conducting this test, specimen of size 40 × 40 × 40 mm are prepared from parent stone. Then the sides are finely dressed and placed in water for 3 days. The saturated specimen is provided with a layer of plaster of paris on its top and bottom surfaces to get even surface so that load applied is distributed uniformly.

Uniform load distribution can be obtained satisfactorily by providing a pair of 5 mm thick playwood instead of using plaster of paris layer also. The specimen so placed in the compression testing machine is loaded at the rate of 14 N/mm2 per minute. The crushing load is noted.

Then crushing strength is equal to the crushing load divided by the area over which the load is applied. At least three specimen should be tested and the average should be taken as crushing strength.

Water Absorption Test

For this test cube specimen weighing about 50 grams are prepared and the test is carried out in the steps given below:

  1. Note the weight of dry specimen as W1.
  2. Place the specimen in water for 24 hours.
  3. Take out the specimen, wipe out the surface with a piece of cloth and weigh the specimen. Let its weight be W2.
  4. Suspend the specimen freely in water and weight it. Let its weight be W3.
  5. Place the specimen in boiling water for 5 hours. Then take it out, wipe the surface with cloth and weigh it. Let this weight be W4. Then,

Abrasion Test of Stones

This test is carried out on stones which are used as aggregates for road construction. The test result indicate the suitability of stones against the grinding action under traffic. Any one of the following test may be conducted to find out the suitability of aggregates:

  • Los Angeles abrasion test
  • Deval abrasion test
  • Dorry’s abrasion test.

However Los Angeles abrasion test is preferred since these test results are having good correlation with the performance of the pavements. The Los Angeles apparatus [Fig. 1] 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.

Fig. 1. Los Angeles testing machine.

IS code has standardised the test procedure for different gradation of specimen. 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 = (Weight of aggregate passing through sieve ÷ Original weight) × 100.

The following values are recommended for road works:

  • For bituminous mixes – 30%
  • For base course – 50%

Impact Test of Stones

The resistance of stones to impact is found by conducting tests in impacting testing machine (Fig. 2).

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.

test of stones
Fig. 2. Aggregate impact testing machine.

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 = W2 ÷ W1

Where W2 = weight of fines, W1 = original weight.

The recommended impact values for various works are:

  • for wearing course >/ 30%
  • for bituminous mechadam >/ 35%
  • for water bound mechadam >/ 40%

Acid Test of Stones

This test is normally carried out on sand stones to check the presence of calcium carbonate, which weakens the weather resisting quality. In this test, a sample of stone weighing about 50 to 100 gm 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 maintains its sharp edges and keeps its surface intact. If edges are broken and powder is formed on the surface, it indicates the presence of calcium carbonate. Such stones will have poor weather resistance.

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