The following characteristics are considered while making selection of a good building stone for different works:-
1. Crushing Strength
A good building stone should have sufficient crushing strength to withstand the superstructure load. The crushing strength of a good building stone should not be less than 100N/mm². The approximate values of the crushing strength of some stones are given below:
|Rock||Stone||Crushing Strength in N/mm²|
|Igneous||Basalt||150 to 185|
|Diorite||90 to 150|
|Granite||75 to 127|
|Syenite||90 to 150|
|Trap||330 to 380|
|Laterite||1.8 to 3.1|
|Shale||0.2 to 0.6|
|Metamorphic||Gneiss||206 to 370|
|Slate||75 to 207|
A good building stone should be of uniform colour and free from clay holes. The stones which are to be used for face work sould be decent in appearance and they should be capable of preserving their colour uniformly for a long time.
A good building stone must be durable enough to resist the effect of weathering agencies e.g. wind, rain, and alternate conditions of heat and cold. It should have a fine-grained, compact and crystalline structure.
The following are important atmospheric agencies which affect the durability of a stone:-
- Alternate conditions of heat and cold due to differences in temperature.
- Alternate conditions of wetness and dryness.
- Chemical agencies such as dissolved gases in rain.
- Growth of trees and creepers in the joints.
- Wind with high velocity.
For making stone durable,the stones should be so arranged in a structure that the natural bed is perpendicular to the direction of pressure.
4. Hardness, Toughness and Resistance to Abrasion.
The stone to be used in stairs, pavements, and railway ballast must be sufficiently hard, tough and able to resist wear and tear. The coefficient of hardness should not be less than 17 for a stone to be used in road works.
A good building stone should have a closed-grained and compact texture.
The stone to be used for ornamental carvings and architectural appearance should be easily and economically dressed.
The stone should be well seasoned before its use. The presence of moisture makes the stone soft and can decay when used.
A good building stone should not absorb more than 5% of water. If it absorbs more than 10% it will be rejected.
9. Specific Gravity
For a building stone, its specific gravity would be greater than 2.7 or so. Heavy and less porous stone can be used in dams retaining walls, harbours etc. Lighter varieties can be used for domes roof coverings etc.
10. Resistance to fire
A good building stone should be able to resist high temperatures. Its mineral constituents should have an equal coefficient of expression.