Testing Of Stones

Testing Of Stones

Building stones are available in large quantity in various parts of the country and to choose and utilize them for their satisfactory performance, it is necessary to test the stone for its strength properties, durability and quality.

Durability Test: Some of the tests to check the durability of stone are as follows. Of these tests, the crystallization test is prescribed by Bureau of Indian Standards. The durability (soundness) test is performed to find out the capacity of stone to resist disintegration and decomposition.

Smith Test: Break off the freshly quarried stone chippings to about the size of a rupee coin and put them in a glass of clean water, one-third full. If the water becomes slightly cloudy, the stone is good and durable. If water becomes dirty, it indicates that the stone contains too much of earthy and mineral

matter.

Brard's Test - for frost resistance - Few small pieces of freshly quarried stone are immersed in boiling solution of sulphate of soda (Glauber's salt) and are weighed. These are then removed and kept

suspended for few days and weighed again. The loss in weight indicates the probable effect of frost. Acid Test - to check weather resistance - confirms the power of stones to withstand the atmospheric conditions. 100 g of stone chips are kept in a 5 per cent solution of H₂SO₄ or HCI for 3 days. Then the chips are taken out and dried. The sharp and firm corners and edges are indication of sound stone. This test is used to test the cementing material of sand stone.

Crystallization Test (IS 1126): Three test pieces of 50 mm diameter and 50 mm height are dried for 24 hours and are weighed (W₁). The specimens are suspended in 14 per cent sodium sulphate solution (density 1.055 kg/m³) for 16 to 18 hours at room temperature (20 o to 30 o C). The specimens are then taken out of the solution and kept in air for 4 hours. They are then oven dried at a temperature of 105 o ± 5 o C for 24 hours and then cooled at room temperature. This process is repeated for 30 cycles. The specimens are weighed (W₂) and the difference in weight is found. This test is repeated thirty times and the loss in weight after every five cycles is obtained. The change in weight indicates the degree of decay of stone. Durability should be expressed in percentage as change in the weight. The average of three test results should be reported as durability value.

Change in weight = W₂/W₁

where W₁ is the original weight of the specimen and W₂ is the weight of the specimen after 30 cycles of the test.

Crushing Test

Compressive Strength Test (IS: 1121 (Part I)) Samples of stone weighing at least 25 kg each of the unweathered spcimen should be obtained from quarry. To test stone for compressive strength, specimen pieces in the form of cubes or cylinders are made from samples of rock. The lateral dimension or diameter of test piece should not be less than 50 mm and the ratio of height to diameter or lateral dimension should be 1:1. A minimum of three specimen pieces are tested in each saturated and dry conditions. Separate tests should be made for the specimen when the load to parallel to the rift and perpendicular to the rift. In all twelve test pieces should be used.

The specimen pieces of diameter or lateral dimension 50 mm are immersed in water at 20 to 30 o C for 72 hours and are tested in saturated condition. The specimen pieces are also tested in dry condition by drying them in an oven at 105 ± 5 o C for 24 hours and then cooled in a desiccator to 20 -30 o C. These are tested in universal testing machine. The load is applied gently at a rate of 14 N/mm² per minute until the resistance of the specimen piece to the increasing load breaks down and no greater load is sustained.

The compressive strength of the specimen piece is the maximum load in Newtons supported by it before failure occurs divided by the area of the bearing face of the specimen in mm². The average of the three results in each condition separately should be taken for the purpose of reporting the compressive strength of the sample.

When the ratio of height to diameter or lateral dimension differs from unity by 25 per cent or more, the compressive strength is calculated by the following expression.

Cc=Cp / EbO 0.778 0.222 É Ù Ê h Ú

where   C_c = compressive strength of standard specimen piece

C_p = compressive strength of the specimen having a height greater than the diameter or lateral dimension

b = diameter or lateral dimension h = height

The crushing strength of stones varies in the range of 15-100 N/mm².

Transverse Strength Test (IS: 1121 (Part II)): To test stone for transverse strength, specimen pieces are made in the form of blocks 200 × 50 × 50 mm. These are tested in saturated and dry conditions similar to as explained in the compressive strength test. Test apparatus used for testing is shown in Fig. 1. Each specimen piece is supported upon two self-aligning bearers

A and B, 40 mm in diameter, the distance between centres of bearers being 150 mm. Bearer A is supported horizontally on two bearer screws C, which carry hardened steel balls D. Bearer B is supported on one such bearer screw and ball. The load is then applied centrally on the specimen piece at a uniform rate of 2 kN/min through a third bearer E, also 40 mm in diameter, placed midway between the supports upon the upper surface of the specimen S and parallel to the supports.

The average of the three results (separately for saturated and dry condition) should be taken for the purpose of determining transverse strength of sample. Any specimen giving result as much as 15 per cent below the average value should be examined for defects.

The transverse strength of the specimen is given by

          R =    3WL/2bd³  

where          R = transverse strength in N/mm²

W = central breaking load in N

L = length of span in mm

b = average width in mm of the test piece at the mid section

d = average depth in mm of the test piece at the mid section

Tensile Strength Test (IS: 1121 (Part III))

Three cylindrical test pieces of diameter not less than 50 mm and the ratio of diameter to height 1:2 are used to determine the tensile strength of the stone in each saturated (kept in water for 3 days at 20 to 30 o C) and dry condition (dried in an oven at 105 ± 5 o C for 24 hours and cooled at room temperature). The general arrangement for testing tensile strength of stone is shown in Fig. 3.11. Each test piece to be tested is sandwiched in between two steel plates of width 25 mm, thickness 10 mm and length equal to the length of test piece. The load is applied without shock and increased continuously at a uniform rate until the specimen splits and no greater load is sustained. The maximum load applied to the specimen is recorded.

Split tensile strength,

S= 2W/SdL

where

S = split tensile strength (N/mm²) W = applied load (N)

d = diameter of specimen (mm), and L = length of specimen (mm)

The average of three results separately for each condition should be reported as split tensile strength of the sample. In case any test piece gives a value of as much as 15 per cent below the average, it should be examined for defects and if found defective the test piece should be rejected.

Shear Strength Test (IS: 1121 (Part IV))

The test is carried out either in Jhonson shear tool (Fig. 3) or Dutton punching shear device (Fig. 4). Three test pieces are used for conducting the test in each of the saturated and dry condition.

Test piece for use in Jhonson shear tool should be bars 50 × 50 mm in section and not less than 100 mm in length and that for use with the Dutton punching shear device should be slabs 30 mm in thickness, 100 mm in width and not less than 100 mm in length.

Using Jhonson Shear Tool

The test piece is carefully centred in the shear tool and the bolts drawn up tightly. The tool is then centred in the testing machine with the centre of the spherical block in contact with the centre of the top portion of the plunger of the shear tool. The speed of the moving head of the testing machine during load application should not be more than 1 mm/min. During the test, the beam of the testing machine should be kept constantly in floating position. The shear strength of test piece is calculated by

S=W/2A

where

S = Shear strength (N/mm²) W = total maximum load (N)

A = area of the centre cross-section of test piece (mm²)

The average of all the three results separately for each condition is calculated and taken as the shear strength of the test piece.

Using Dutton Punching Shear Device

Centre lines are laid over one surface of the slab. Thickness of the slab is measured at three points approximately equidistant around the circumference of a 50 mm circle centred on the intersection of the two center lines. The test piece is centred in the punching device keeping it under the plunger. The punching device is then centred in the testing machine with the centre of the spherical bearing block in contact with the centre of the top portion of the plunger of the shear device. The speed of the moving head of the testing machine during load application should not be more than 1 mm/min. During the test, the beam of the testing machine should be kept constantly in floating position. The shear strength of the test piece is calculated by

S= W_tW_i SDT

where

S = Shear strength (N/mm²) W_t = total maximum load (N)

W_i = initial load (N) required to bring the plunger in contact with the surface of specimen D = diameter (mm) of the plunger

T= thickness (mm) of the specimen

The average of all the three results separately for each condition is calculated and taken as shear strength of the test piece.

Absorption Test (IS: 1124)

The selected test pieces of stone are crushed or broken and the material passing 20 mm IS Sieve and retained on 10 mm IS Sieve is used for the test. The test piece weighing about 1 kg is washed to remove particles of dust and immersed in distilled water in a glass vessel at room temperature 20 to 30 o C for 24 hours. Soon after immersion and again at the end of  soaking period, entrapped air is removed by gentle agitation achieved by rapid clock-wise and anti-clock-wise rotation of the vessel. The vessel is then emptied and the test piece allowed to drain.The test piece is then placed on a dry cloth and gently surface dried with the cloth. It is transferred to a second dry cloth when the first one removes no further moisture. The test piece is spread out not more than one stone deep on the second cloth and left exposed to atmosphere away from direct sunlight or any other source of heat for not less than 10 minutes untill it appears to be completely surface dry. The sample is then weighed (B).

The sample is then carefully introduced in a 1000 ml capacity measuring cylinder and distilled water is poured by means of 100 ml capacity measuring cylinder while taking care to remove entrapped air, untill the level of water in the larger cylinder reaches 1000 ml mark. The quantity of water thus added is recorded in ml or expressed in gram weight (C).

The water in the larger cylinder is drained and the sample is carefully taken out and dried in an oven at 100 to 110 o C for not less than 24 hours. It is then cooled in a desiccators to room temperature and weighed (A). The room temperature during the test is recorded.

Apparent specific gravity = A/1000C

Water absorption = BA - 100 / A

Apparent Porosity = BA-100 / C  . 1000

The true porosity shall be calculated from the following formula:

True Porosity = [ True specific gravity - Apparent specific gravity  ] / True Specific gravity

Where

A = Weight of oven-dry test piece (g)

B = Weight of saturated surface-dry test piece (g)

C = Quantity of water added in 1000 ml jar containing the test piece (g)

Hardness: This test is performed by scratching a stone with knife on Mohs scale.

Toughness: This test is performed by breaking the stone with a hammer. Toughness is indicated by resistance to hammering.