Properties of Stones
The following properties of the stones should be looked into before selecting them for engineering works:
(i) Structure: The structure of the stone may be stratified (layered) or unstratified. Structured
stones should be easily dressed and suitable for super structure. Unstratified stones are hard and difficult to dress. They are preferred for the foundation works.
(ii) Texture: Fine grained stones with homogeneous distribution look attractive and hence they are used for carving. Such stones are usually strong and durable.
(iii) Density: Denser stones are stronger. Light weight stones are weak. Hence stones with a specific gravity less than 2.4 are considered unsuitable for buildings.
(iv) Appearance: A stone with uniform and attractive colour is durable, if grains are compact. Marble and granite get very good appearance, when polished. Hence they are used for face works in buildings.
(v) Strength: Strength is an important property to be looked into before selecting stone as building block. Indian standard code recommends, a minimum crushing strength of 3.5 N/mm2 for any building block, usually a factor of safety of 10 is used to find the permissible stress in a stone. Hence even laterite can be used safely for a single storey building, because in such structures expected load can hardly give a stress of 0.15 N/mm2. However in stone masonry buildings care should be taken to check the stresses when the beams (Concentrated Loads) are placed on laterite wall.
(vi) Hardness: It is an important property to be considered when a stone is used for flooring and
pavement. Coefficient of hardness is to be found by conducting a test on the standard specimen in Dory’s testing machine. For road works coefficient of hardness should be at least 17. For building works stones with coefficient of hardness less than 14 should not be used.
(vii) Percentage wear: It is measured by the attrition test. It is an important property to be considered in selecting aggregate for road works and railway ballast. A good stone should not show the wear of more than 2%.
(viii) Porosity and Absorption: All stones have pores and hence absorb water. The reaction of
water with material of stone cause disintegration. Absorption test is specified as a percentage of water absorbed by the stone when it is immersed underwater for 24 hours. For a good stone, it should be as small as possible and in no case more than 5.
(ix) Weathering: Rain and wind cause loss of good appearance of stones. Hence stones with good weather resistance should be used for face works.
(x) Toughness: The resistance to impact is called toughness. It is determined by the impact test.
Stones with toughness index more than 19 are preferred for road works. Toughness index 13 to 19 are considered as medium tough and stones with toughness index less than 13 are poor stones.
(xi) Resistance to Fire: Sandstones resist fire better. Argillaceous materials, though poor in
strength, are good in resisting fire.
(xii) Ease in Dressing: Cost of dressing contributes to the 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.
(xiii) Seasoning: The stones obtained from quarry contain moisture in the pores. The strength of
the stone improves if this moisture is removed before using the stone. The process of removing moisture from pores is called seasoning.
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*Source- Internet, Books and Self-Analysis
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