STONES | PROPERTIES OF GOOD BUILDING STONES

Properties of good building stones

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

1.   Crushing strength: For a good structural stone, the crushing strength should be greater than 100 N/mm². The approximate values of crushing strength of some of the stones are shown in table below.

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. It is desirable to prefer light coloured stones as compared  to  dark  coloured stones as these are more likely to be attacked by weathering agents. A good building stone should be of uniform colour and free from clay holes,  spots  of  other  colour, bands, etc.

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.

4.      Facility of dressing: The stone should be such that it can be easily carved,  moulded, cut, and dressed  and at  the same time it  should have sufficient strength .  It is an important consideration from the economic- point of view.

5.      Fracture: For a good building stone, its fracture should  be  sharp, even,  bright and clear with grains well cemented together.

6.      Hardness: The coefficient of hardness, as worked out in hardness test by Dorry’ testing machine, 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 argillaceous stones are poor in strength,  but they can resist fire quite well. The sandstone resists a fire in a better way.

9.      Seasoning: the stones should be dried or seasoned before they are used in structural work. 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. 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.

12.      Toughness index: The resistance to impact is called toughness. It is determined by impact test. Stones with toughness index more than 19 are preferred for road works. Toughness index 13 to 19 is considered as medium tough and stones with toughness index less than 13 are poor stones.

13.      Porosity and Absorption:  All stones have pores and hence absorb water. The reaction of water with material of stone causes disintegration . Absorption test is specified as percentage of water absorbed by the stone when it is immersed under  water  for  24 hours. For a good stone it should be as small as possible and in no case more than 5 percent.

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.