Properties of Stones

PROPERTIES OF STONES

The criteria for selection of stone as a building material is based upon the following properties:-

(i) Structure:

❖ Structure of a stone layer may be either stratified or unstratified.

❖ Structured stones are easily dressed and are suitable for the superstructures.

❖ Unstratified stones are hard and difficult to dress and are preferred for the foundation works.

(ii) Texture:

❖ Fine grained stones are usually strong and durable.

❖ For carving, attractive fine-grained stones with homogeneous distribution are used.

(iii) Density:

❖ Light weight stones are weak and denser stones are stronger.

❖ Stones with specific gravity less than 2.4 are not suitable for buildings.

(iv) Appearance:

❖ Appearance is a chief requirement for the selection of the stone.

❖ A stone with uniform and attractive colour is durable, if grains are compact.

❖ The colour and ability to receive polish greatly influence the appearance.

❖ Marble and granite get very good appearance, when polished. Hence they are used for face works in buildings.

(v) Colour

❖The colour of stone along with its shape and arrangement of mineral constituents greatly influence its fashion and ornamental value.

❖ A stone with uniform and the attractive colour is durable if its grains are compact.

❖The engineer selecting the stone should be aware of the variation of colour of the stone after long exposure and in polluted atmospheres.

(vi) Strength:

❖  For the selection of stone as a building block, strength is an important property to be looked for.

❖ For any building block, minimum crushing strength should be 3.5 N/mm2 as recommended by Indian Standard Code.

❖ Due to non-uniformity of the material, usually a factor of safety of 10 is used to find the permissible stress in a stone.

(vii) Hardness:

❖ When stone is used for flooring and pavement, this is an important property to be considered

❖ Coefficient of hardness should be less than 14, for building work stones.

❖ Coefficient of hardness should be at least 17, for road works.

❖The stones used in floors and pavements should be able to resist abrasive movement caused due to the movement of men and materials over them.

(viii) Durability:

❖When stone is used as aggregate for road works and railway ballast, this is an important property to be considered.

❖The durability of the stone is significantly determined by its resistance to fire and weathering.

❖ Stones selected should be capable of resisting adverse effects of natural forces like wind, rain and heat.

❖ A good stone should not show wear of more than 2%.

(ix) Toughness:

❖ Building stones should have more toughness to sustain stresses developed due to vibrations.

❖In general, the resistance to impact is known as toughness, and can be found by impact test.

❖ Stones having toughness index 13 to 19 are medium tough and less than 13 are poor.

❖For road works, the toughness index of the stone should be more than 19.

(x) Porosity and Absorption:

❖ Building stones should not be porous, since rain water might enter the pores and deteriorate it.

❖All stones have pores and hence absorb water. The percentage of water absorbed by the stone when it is immersed underwater for 24 hours should not be more than 5% for a good stone.

(xi) Resistance to Fire and Weathering:

❖ Rain and wind cause loss of good appearance of stones.

❖ Stones with good weather resistance should be used for face works.

❖ Sand stones resist fire better.

❖ Argillaceous materials, though poor in strength, are good in resisting fire.

(xii) Specific Gravity:

❖For the construction of dams, retaining walls, harbours, etc. a heavier variety of stones should be used.

❖ For a good building stone, specific gravity is between 2.4 and 2.8.

(xiii) Ease in Dressing:

❖ Dressing is the process of giving required shape to the stone.

❖ Cost of dressing contributes to cost of stone masonry to a great extent.

❖ Dressing is easy in stones with lesser strength. Hence an engineer should look into sufficient strength rather than high strength while selecting stones for building works.

(xiv) Seasoning:

❖ The process of removing moisture from pores is called seasoning.

❖ Quarry stones contain moisture in the pores. If this moisture is removed before using the stone, the strength of the stone improves.

❖The optimum way of seasoning is to let it exposed in the nature for 6 to 12 months. Seasoning is very much required in the case of laterite stones.

(xv) Cost:

1. Cost is an essential consideration while selecting a building material.

2. Cost of dressing has a great contribution in the total cost of stone masonry.

3. Transportation charges, quarrying and cutting costs, adding ornamental features and the durability of a stone mostly persuades the total cost.

Nevertheless, it is prominent that a single stone cannot satisfy all the criteria's of a good building stone. One property may contradict with another, like strength and durability criteria oppose the ease of dressing requirement. Therefore it is essential that, the engineer has to consider the criteria based on its properties before selecting the stone for particular work.