The conditions which govern the selection of stone for structural purposes arc cost, fashion, ornamental value and durability, although the latter property is frequently overlooked or disregarded. Cost is largely influenced by transportation charges, difficulties in quarrying and cutting, the ornamental features, and the durability of stone. The type of dressing of stone may make a difference to the cost, particularly with the stones derived from igneous rocks. When the cost of quarried stone to cost of finished stone is considered, it will be found that the labour cost is far greater than the price of the stone. Thus, a difference in the price between two alternative stones is unimportant and it would be unwise to reject a more durable stone on the grounds that it was costly.
Another factor which should he considered is the suitability of the stone for the type of design. For example, for a highly carved designs if, by mistake, a harder stone such as granite is selected the cost will be affected. Colour, arrangement and shape of mineral constituents greatly influence fashion and ornamental value. One of the first factors influencing the selection of stone for a particular work will be colour.
It is important that the designer is aware about how the colour is likely to change after long exposure and is particular how it may vary in polluted atmospheres. As an example limestone, being slightly soluble in water, will remain clean in portions facing rain but retain a film of root in sheltered areas This results in strong colour contrast Resistance to fire and weathering-factors which are largely influenced by the mineral constitution of the rock-are the most important determination of durability. It is very important to select a stone according to its exposure conditions. Limestones when used in areas not exposed to rain but acted upon by sulphur gases of polluted atmosphere form a hard and impermeable surface skin which subsequently blisters and flaskes off. It must be noted that flaking of this kind occurs mainly an external work only, although the air inside the building is almost equally polluted, probably due to the damper conditions inside.
Following are the characteristics of a stone
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.
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.
The coefficient of hardness, as worked out in hardness level, 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 rood work.
For face work it should have fine, compact texture; light coloured stone is preferred as dark colour are likely to fade out in due course of time.
For a good building stone, its fracture should be sharp, even bright and clean with grains well cemented together. A dull, chalky and earthly fracture of a stone indicates signs of early future decay.
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 retiming 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.
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 pressure 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.
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. Percentage wear in attrition test, if wear is more than 3%, the stone is not satisfactory. If it is equal to 3 /c. the stone is just tolerable. For a good building stone, the wear should be equal to or less than 3%.
For a good structural stone, the crushing strength should be greater than 300 N/mm-. The approximate values of crushing strength of some of the stones are shown in Table 2.5 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, reacts 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, rains or moisture.
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 type 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.