Friday, 28 July 2017

Different test conduct on Stones to select the appropriate stone for construction purpose.

Stones form one of the most important building materials in civil engineering. Stones are derived from rocks, which form the earth's crust and have no definite shape or chemical combination but are mixtures of two or more minerals. The mineral is a substance which is formed by the natural inorganic process and possesses a definite chemical combination and molecular structure. They are strong, durable and descent in appearance.

The main uses of stone as a building material are:

As a principal material for foundation of civil engineering works, and for the construction of walls, arches, abutments and dams.In stone masonry in places where it is naturally available.As coarse aggregate in cement concrete (crushed form of rock).

Tests which are to be conducted on stones for selecting it as a building material.

Acid Test: Acid test is used to investigate how much atmospheric action can be resisted by stone.  In this test 100 grams of stones in chipped form are kept in a 5% solution of hydrochloric acid or sulphuric acid. After 3 days stones in chipped form are taken out and dried. If the edges of stones are sharp as earlier, it indicates that stone can resist weathering actions.

Smith's Test: This test is used for finding out the presence of soluble matter in stones. In this test few sample of stones are place in a glass or test-tube filled with clean water. Stones are kept in water for 1 hour. After this the glass or test-tube is vigorously shaken. Due to presence of earthy material and clay impurities water is converted to dirty water. Slightly cloudiness of water will prove that the stones are good and durable. If water becomes too dirty, it indicates that stone contains too much soluble impurities and it is not suitable for construction.

 
Crushing Strength: Crushing test is used to investigate the compressive strength of stone. In this test stone is cut into cubes of dimension 40mm. Sides of cube are finely dressed and finished. Cubes of stones are then kept in water for 72 hours. Then 5mm thick layer of plywood or plaster of paris is applied on the load bearing surface. Load is applied axially on load bearing surface using universal testing machine or crushing testing machine until cracks appear on the stone or stone starts crushing.
     Crushing strength of the stone is the maximum load at which it crushes divided by the area of the load bearing surface. 

Water Absorption Test: In this test, 50 grams of stones in chipped form are places in an oven at 105 degree celsius for 3 hours then cooled at room temperature. Weight of stones is then taken (W1). Then stones are places in distilled water for 3 days. After 3 days weight of stones is taken (W2).

     Percentage '%' of water absorption should not exceed 15%, otherwise stone is not suitable for construction.

Crystallization Test: 4 cubes of stone with dimension 40mm are taken. Stones are dried for 3 days and weighed. Then stones are immersed in 14% solution of Sodium Sulfate (Na2SO4) for 2 hours. After this stones are dried at 100 degree Centigrade  and weighed. Difference in weight is noted .Process of drying, weighing, immersion and reweighing is repeated at least 5 times. Each time, change in weight is noted and it is expressed as a percentage of original weight.

PERCENTAGE OF WEAR SHOULD NOT EXCEED 2% FOR GOOD STONE.

Hardness Test: This test is carried out to determine the hardness of stone. First weight of specimen is taken (W1). The specimen is filled in a test cylinder of diameter 25 mm and height 25 mm. Then cylinder is placed in Dorry's testing machine and force of 12.50 N is applied. The disc of testing machine is rotated at 28 revolutions per minute.During the rotation of the disc, coarse sand of standard specification is sprinkled on the top of disc. After 1000 revolutions specimen is weighed (W2).

Impact Test: This test is carried out to determine the toughness of stone. This test requires an 'Impact Testing Machine'. In this test stones are filled in test cylinder of diameter 25 mm and height 25 mm. The cylinder is placed on machine and steel hammer of weight 20 N is allowed to fall on the specimen in cylinder. The height of first fall is 1 cm, height of second fall is 2 cm and so on. The height at which specimen breaks is recorded. If specimen breaks at 'n cm' then 'n' is the toughness index of stone.  
Microscopic Test: In this test specimen of stones is placed under microscope and various properties are studied such as grain size, texture of stone, pores, veins, shakes etc.

Attrition Test: This test is carried out to test the resistance to abrasion (ability to withstand grinding action)  of stone. This test is carried out in 'Attrition Test Machine'. In this test specimen of stone is weighed (W1). Then stones are transferred to drum and drum is inclined to 30 degree to the horizontal. Then stones are revolved ate 2000 revolution per hour for 5 hours. After this stones are sieved on a 2 mm sieve. Stones retained on sieve are weighed (W2) and loss in weight percentage gives the percentage of wear. 

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