Qualities, Characterises and Properties of a Good Building Stone

Following are the qualities or characteristics or requirements properties of a good building stone:

1. Crushing Strength:

For a good structural stone, the crushing strength should be greater than 100 N/mm³. The approximate values of crushing strength of some of the stones are shown in table 2-1.

2. Appearance:

The stones which are to be used for face work should be decent in appearance and they should be capable of preserving their colour uniformly for a long time. The colour of the stones for face work should be chosen by keeping in mind the general get up of the surrounding area. It is desirable to prefer light coloured stones as compared to dark coloured stones because there are chances of the latter variety to

3. Durability:

A good building stone should be durable. The various factors contributing to durability of a stone are its chemical composition, texture, resistance to atmospheric and other influences, location in structure, etc.

Following are the important atmospheric agencies which affect the durability of a stone:

(i) Alternate conditions of heat and cold due to differences in temperature;

(ii) Alternate conditions of wetness and dryness due to rain and sunshine;

(iii) Chemical agencies such as dissolved gases in rain;

(iv) Growth of trees and creepers in the joints between stones;

(v) Wind with high velocity; etc.

For making stones durable, their natural bed should be carefully noted. The stones should be so arranged in a structure that the natural bed is perpendicular or nearly so to the direction of pressure.

4. Facility of Dressing:

The stones should be such that they can be easily- carved, moulded, cut and dressed. It is an important consideration from the economic point of view. However this property of stone is opposed to its strength, durability and hardness. Hence it is to be properly correlated with respect to the situation in which stone is to be used.

5. Fracture:

For a good building stone, its fracture should be sharp, even, bright and clear with grains well cemented together. A dull, chalky and earthly fracture of a stone indicates signs of early future decay.

6. Hardness:

The coefficient of hardness, as worked out in hardness test, should be greater than 17 for a stone to be used in road work. If it is between 14 and 17, the stone is said to be of medium hardness. If it is less than 14, the stone is said to be of poor hardness and such stone should not be used in road work.

7. Percentage Wear:

In attrition test, if wear is more than 3 per cent, the stone is not satisfactory. If it is equal to 3 per cent, the stone is just tolerable. For a good building stone, the wear should be equal to or less than 3 per cent.

8. Resistance to Fire:

The minerals composing stone should be such that shape of stone is preserved when a fire occurs. The failure of stones in case of a fire is due to various reasons such as rapid rise in temperature, sudden cooling, different coefficients of linear expansions of minerals, etc. The free quartz suddenly expands at a temperature lower than 600°C.

The limestone resists fire upto about 800°C and it then splits into CaO and CO₂. The sandstone with silicates as binding material can resist a fire in a better way. The argillaceous stones are poor in strength, but they can resist fire quite well.

9. Seasoning:

The stones should be well seasoned before putting into use. The stones obtained fresh from a quarry contain some moisture which is known as the quarry sap. The presence of this moisture makes the stone soft. Hence the stones quarried freshly are easy to work.

It is therefore desirable to do dressing, carving, etc. when stones contain quarry sap. The stones should be dried or seasoned before they are used in structural work. A period of about 6 to 12 months is considered to be sufficient for proper seasoning.

10. Specific Gravity:

For a good building stone, its specific gravity should be greater than 2.7 or so. The heavy stones are more compact and less porous and they can be used for various engineering applications such as dams, weirs, retaining walls, docks, harbours, etc. On the other hand, if stones are to be used for domes, roof coverings, etc., the lighter varieties of stones are preferred.

11. Texture:

A good building stone should have compact fine crystalline structure free from cavities, cracks or patches of soft or loose material. The stones with such texture are strong and durable.

12. Toughness Index:

In impact test, if the value of toughness index comes below 13, the stone is not tough. If it comes between 13 and 19, the stone is said to be moderately tough. If it exceeds 19, the toughness of stone is said to be high.

13. Water Absorption:

All the stones are more or less porous, but for a good stone, percentage absorption by weight after 24 hours should not exceed 0.60. The porous stones seriously affect the durability of stones. The rain water as it descends through the atmosphere absorbs some acidic gases forming light acids. Such rain water, if absorbed by porous stones, react with the constituents of stones causing them to crumble.

Similarly, in cold regions, if porous stones are used, the water remaining in pores will disintegrate stones because of its increase in volume on freezing. Hence the porous stones should not be recommended for places subjected to frost, rain or moisture.

14. Weathering:

A good building stone should possess better weathering qualities. It should be capable of withstanding adverse effects of various atmospheric and external agencies such as rain, frost, wind, etc. The best way to know the resisting power of a stone to the action of weather is to study the performance of buildings constructed with the similar stones in the locality or at a place having more or less similar atmospheric conditions. The stones having excellent weathering qualities should only be used in the construction of important buildings.

It should however be remembered that one kind of stone is not suitable in all types of construction. For instance, the soft stones are required for carving, the light stones are required for arches and the hard stones are necessary to stand high pressures.

It is therefore necessary to study carefully the situation in which stones are to be used before any recommendation is made. Other factors which affect the selection of stone are easy availability, nearness of quarry, facility of transport, reasonable price, climatic conditions of the construction site, etc.