The following tests are conducted for determining the qualities of a good building stone!
(1) Acid test
(2) Attrition test
(3) Crushing test
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(4) Crystallisation test
(5) Freezing and thawing test
(6) Hardness test
(7) Impact test
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(8) Microscopic test
(9) Smith’s test
(10) Water absorption test.
Each of these test will now be briefly described.
1. Acid Test:
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In this test, a sample of stone weighing about 0.50 to 1 N is taken. It is placed in a solution of hydrochloric acid having strength of one per cent and it is kept there for seven days. The solution is agitated at intervals. A good building stone maintains its sharp edges and keeps its surface free from powder at the end of this period.
If edges are broken and powder is formed on the surface, it indicates the presence of calcium carbonate and such a stone will have poor weathering quality. It is natural that this test cannot be applied to the lime stones. This test is usually carried out on the sandstones.
2. Attrition Test:
This test is done to find out the rate of wear of stones which are used in road construction. The results of test indicate the resisting power of stones against the grinding action under traffic.
Following procedure is adopted:
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(i) The sample of stone is broken into pieces of about 60 mm size.
(ii) Such pieces, weighing 50 N, are put in both the cylinders of Deval’s attrition test machine. The diameter and length of cylinder are respectively 200 mm and 340 mm.
(iii) The cylinders are closed. Their axes make an angle of 30° with the horizontal.
(iv) The cylinders are rotated about horizontal axis for 5 hours at the rate of 30 R.P.M.
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(v) After this period, the contents are taken out from the cylinders and they are passed through a sieve of 1.50 mm mesh.
(vi) The quantity of material which is retained on the sieve is weighed.
(vii) The percentage wear is worked out as follows –
3. Crushing Test:
The compressive strength of stone is found out with the help of this test.
The sample of stone is cut into cubes of size 40 mm x 40 mm x 40 mm. The sides of cubes are finely dressed and finished. The minimum number of specimens to be tested is three. Such specimens should be placed in water for about 72 hours prior to test and thereafter tested in saturated condition.
The load-bearing surface is then covered with plaster of Paris or 5 mm thick plywood. The load is applied axially on the cube in a crushing test machine. The rite of loading is 13.72 N/mm2 per minute. The crushing strength of the stone per unit area is the maximum load at which its sample crushes or fails divided by the area of the bearing face of the specimen.
4. Crystallisation Test:
In this test, at least four cubes of stone with side as 40 mm are taken. They are dried for 72 hours and weighed. They are then immersed in 14 per cent solution of Na2SO4 for 2 hours. They are dried at 100°C and weighed. The difference in weight is noted. This procedure of drying, weighing, immersing and reweighing is repeated at least five times. Each time, the change in weight is noted and it is expressed as a percentage of original weight.
It is to be noted that the crystallisation of CaSO4 in pores of stone causes the decay of stone due to weathering. But, as CaSO4 has low solubility in water, it is not adopted in this test.
5. Freezing and Thawing Test:
The specimen of stone is kept immersed in water for 24 hours. It is then placed in a freezing mixture at -12°C for 24 hours. It is then thawed or warmed at atmospheric temperature. This should be done in shade to prevent any effect due to wind, sun rays, rain, etc. Such a procedure is repeated several times and behaviour of stone is carefully observed.
6. Hardness Test:
To determine the hardness of a stone, the test is carried out as follows:
(i) A cylinder of diameter 25 mm and height 25 mm is taken out from the sample of stone.
(ii) It is weighed.
(iii) It is placed in Dorry’s testing machine and pressed with a pressure of 12.50 N.
(iv) The annular steel disc of machine is then rotated at a speed of 28 R.P.M.
(v) During the rotation of disc, the coarse sand of standard specification is sprinkled on the top of disc.
(vi) After 1000 revolutions, the specimen is taken out and weighed.
(vii) The coefficient of hardness is found out from the following equation –
7. Impact Test:
To determine toughness of a stone, the impact test is carried out in page impact machine as follows:
(i) A cylinder of diameter 25 mm and height 25 mm is taken out from the sample of stone.
(ii) It is placed on cast-iron anvil of machine.
(iii) A steel hammer of weight 20 N is allowed to fall axially in a vertical direction over the specimen.
(iv) The height of first blow is 1 cm; that of second blow is 2 cm; that of third blow is 3 cm; and so on.
(v) The blow at which specimen breaks is noted. If it is nth blow, n represents the toughness index of stone.
8. Microscopic Test:
In this test, the sample of stone is subjected to the microscopic examination.
The thin sections of stone are taken and placed under the microscope to study various properties such as:
(i) Average grain size;
(ii) Existence of pores, fissures, veins and shakes;
(iii) Mineral constituents;
(iv) Nature of cementing material;
(v) Presence of any harmful substance;
(vi) Texture of stone; etc.
9. Smith’s Test:
This test is performed to find out the presence of soluble matter in a sample of stone. The few chips or pieces of stones are taken and they are placed in a glass tube. This tube is then filled with clear water. After about an hour, the tube is vigorously stirred or shaken. The presence of earthy matter will convert the clear water into dirty water.
If water remains clear, the stone will be durable and free from any soluble matter. On the other hand, if the water becomes dirty, it will indicate that the stone contains too much of soluble earthy and mineral matters.
10. Water Absorption Test:
Following procedure is adopted for this test:
(i) From the given sample of stone, a cube weighing about 0.50 N is prepared. Its actual weight is recorded. Let it be W1N.
(ii) The cube is then immersed in distilled water for a period of 24 hours.
(iii) The cube is taken out of water and surface water is wiped off with a damp cloth.
(iv) It is weighed again. Let its weight be W2N.
(v) The cube is suspended freely in water and its weight is recorded. Let it be W3N.
(vi) The water is boiled and cube is kept in boiling water for five hours.
(vii) It is then removed and surface water is wiped off with a damp cloth. Its weight is recorded. Let it be W4N.
From these data, the values of the following properties of stone are obtained:
