Tests on Stones

TESTS ON STONES

Following are different tests on conducted on building stones:

❖ Water absorption test

❖ Crushing  strength test.

❖ Impact test

❖ Abrasion test

❖ Acid test

(i) Water absorption test:

Water absorption test is conducted on a test cube specimen and the following steps are carried out:

❖From the sample of stone, a cube weighing about 50gm is prepared. Take actual weight as W₁ gm.

❖ Cube specimen is then immersed in distilled water for a period of 24 hrs.

❖ Specimen is taken out of water and surface water is wiped off and weighed again. Let the weight be W₂ gm.

❖ Suspend the specimen freely in water and weight it. Let its weight be W₃ gm.

❖ Water is boiled and the specimen is kept in boiling water for 5 hours.

❖ Cube is removed and surface water is wiped off and is weighted again. Let it be W₄ gm.

❖ From the above observations, values of the following properties of stones are obtained.

Percentage absorption by weight after 24 hours 

= [ (W₂- W₁ / W₁)] x 100

Percentage absorption by volume after 24 hours

 =  [(W₂-W₁ /W₂-W₃) ] x 100

Volume of displaced water

 = (W₂-W₃)

Percentage porosity by volume

= [(W₄-W₁ /W₂-W₃) ] x 100

Density 

= W₁ / (W₂- W₁)  kg/m³

Specific Gravity 

= W₁ / (W₂- W₃)  

Saturation Coefficient  = (Water Absorption /Total Porosity) =  = (W₂- W₁) / (W₄ - W₁)

(ii) Crushing Strength Test:

The crushing strength of a stone can be determined using crushing test machine as follows:

❖ Stone specimen is cut into 40 x 40 x 40 mm sizes of cubes and are finely dressed and finished.

❖ Such specimen should be placed in water for about 72 hours (3 Days) prior to test and therefore tested in saturated condition.

❖ Load bearing surface is then covered with plaster of paris of about 5mm thick on its top and bottom surfaces to get even surface so that load applied is distributed uniformly.

❖ In certain cases 5 mm thick plywood is also used instead of plaster of paris for getting uniform load distribution.

❖ Load is applied axially on the cube in a crushing test machine at the rate of 14 N/mm2 per minute.

❖Crushing strength of the stone per unit area is the maximum load at which the sample crushes or fails divided by the area of the bearing face of the specimen.

❖ Generally 3 specimens should be tested and average values are considered as crushing strength.

(iii) Impact strength test:

The toughness of a stone can be determined by conducting impact strength test as follows:

❖ Impact testing machine 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.

❖ Aggregates of size 10 mm to 12.5 mm are filled in cylinder in 3 equal layers, after being tamped 25 times.

❖ The sample is then transferred to a cup of diameter 102 mm and height 50mm and again tamped 25 times. The original weight is taken as W₁ gm.

❖The hammer is then allowed to fall freely on the specimen 15 times.

❖ Finally the sample is then sieved through 2.36 mm sieve. The weight of the fines is taken as W2 gm.

❖ Impact strength value is obtained by W₂/W₁

❖ The impact value should not be greater than 30% for wearing course, 30% for bituminous macadam and 40% for water bound macadam.

(iv) Abrasion Test:

The suitability of stones which are used as aggregates for road works can be determined by conducting abrasion test as follows:

❖ Abrasion test can be conducted using Los Angeles abrasion test apparatus which consists of a hollow cylinder of 0.7 m inside diameter and 0.5 m long with both ends being closed.

❖ The cylinder is mounted on a frame so that it can be rotated about horizontal axis.

❖ A specified number of cast iron balls of 48 mm diameter are placed in the cylinder, along with specified weight of oven dried specimen (W₁ gm).

❖ Then the cylinder is rotated at a speed of 30 to 33 rpm for specified no 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 (W₂ gm).

❖ Aggregate abrasion value =  [(W₁,- W₂) / W₁ x 100 ]The nominal values are found to be 30% for bituminous mixes, 50% for base course and 60% for sub-base course.

(v) Acid Test:

Acid test is carried out on sandstones to understand the presence of calcium carbonate in building stone.

❖ A sample of stone weighing about 50 to 100 gm is taken.

❖ The sample is placed in a solution of 1% hydrochloric acid for seven days at periodical agitation.

❖ A good building stone maintains its sharp edges and keeps its surface free from any powder at the end of 7 days.

❖ If the 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.