There are various kinds of lab and field tests available for building stones or rocks. These stone or rock tests are generally performed to determine the physical quality of stone materials used for construction work. Few tests such as Acid test may be performed to determine the chemical quality of building material or stones. Many of the known building stone & rock tests are explained briefly below:
1. Hardness test
- Hardness is the resistance of a stone to indentation, rebound or scratch.
- It is tested by a pen knife with the aid of Moh’s scale of hardness.
| Minerals | Moh’s Scale | Hardness test |
| Talc | 1 | easily scratched with the thumb-nail |
| Gypsum | 2 | scratched by the thumb-nail |
| Calcite/ Marbles | 3 | not scratched by thumb-nail but easily cut by knife |
| Fluorite | 4 | can be cut by knife with greater difficulty than calcite |
| Apatite | 5 | can be cut only with difficulty by knife |
| Orthoclase/ Feldspar | 6 | can be cut with knife with great difficulty on thin edges |
| Quartz | 7 | not scratched by steel, scratches glass |
| Topaz | 8 | |
| Sapphire/ Corundum | 9 | |
| Diamond | 10 |
- Hardness test is also carried out in Dorry’s testing machine.
2. Crushing test
- Three cube samples of size are cut and placed in water for 72 hours.
- They are tested in crushing test machine with loading 13.7 N/mm2 per minute.
3. Transverse strength test
- A specimen of the stone whose length is ten times its depth is placed on wedge-shaped supports near its ends. A vertical load applied at the centre is gradually increased until failure occurs. The transverse strength, called the modulus of rupture, in MPa is computed as
Where,
W = breaking load applied at the centre in N,
L = distance between the supports in mm,
b = width of the specimen block in mm and
d = depth of the specimen block in mm
- Transverse strength tests are usually made on specimens of 25mm square in section. The approximate values for ordinary stone are: Granite 10 – 17.5 MPa; limestone 3.5 – 20 MPa and sandstones in the range 4 – 15 MPa.
4. Impact test
- For toughness
- A steel hammer of 20 N is allowed to fall axially in the specimen.
- The blow at which the specimen breaks represents the toughness of the stone.
5. Fire resistance test
- The stone which is free from calcium carbonate can resist fire.
- Presence of calcium carbonate can be detected by few drops of dilute sulphuric acid which will produce bubbles.
6. Attrition test
- To determine the rate of wear of stones employed.
- Also known as abrasion test and is carried out in Deval’s Attrition Testing Machine.
7. Acid test
- Kept in solution of 1% H2SO4 or HCl and checked for deposits on surface.
- The stones having high percentage of lime content exhibit efflorescence when subjected to action of acids.
8. Porosity and water absorption test
- The porosity of the commonly used stones varies from 0 to 20%.
- The percentage of water absorbed by an air dried stone when immersed in water for 24 hours is termed as absorption of stone (air dry basis).
- A good building stone must absorb less than 5% water and those stones that absorb more than 10% of water should be rejected.
- Sandstones should not absorb water more than 10%, 17% in case of limestones and 1% in granites of their volume when dipped for 3-4 hours.
9. Smith’s test
- This test indicates the presence of earthy matter.
10. Crystallization test:
- To determine the durability or weathering quality of the stone.
- A sample of stone is immersed in solution of sodium sulphate at room temperature and dried in hot air.
- The process of wetting and drying is carried out for two hours; the difference in weight if any is recorded. Little difference in weight indicates durability and good weathering quality of the stones.