Sunday, August 12, 2012

Requirement of good building stones.

Hello there,

This post discusses the various requirements of a good building stone. The following are the qualities of the stones used in the building construction:

  • Compressive strength:  The compressive strength or the crushing strength of the stone is tested with the help of a crushing strength testing machine and for a good building stone the value should not be less than 100N/mm2. 

The various igneous rocks and the metamorphic rocks have the required amount of the strength that is why some of them makes it up to as a building stone.

  •  Appearance:  The stones should have a homogeneous color and should be resistant to the weathering agencies. The stone used in the face work should be of decent appearance. The color of the stone chosen should be according to the surrounding environment.

The light colored stones are preferred because they are weathering resistant. A good building stone is of uniform color and do not possess any clay spots or other color spots.

  •  Durability:  In general durability is defined as the ability to resist the weathering action of the surroundings. A good building stone should be durable. The various factors contributing to the durability of a stone  are its chemical composition, texture, resistance to atmospheric and other influences, location in the structure etc.

The various important environmental agents which affect the durability of a stone are alternate heating and cooling, alternate drying and wetting, chemical agencies such as dissolved gases in the environment, growth of the trees and creepers in the joints between the stone and wind and the high velocity; etc.
For making stone durable some people suggest that the natural bed of the stone should be noted and the stones should be so arranged in a structure such that the natural bed is perpendicular to the direction of the pressure.

  • Work-ability/ Facility of dressing: The stones should be such that they can be easily carved, molded, cut and dressed. It is 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.

  • Fracture: For a good building stone its fracture should be sharp, even, bright and the grains well cemented together, or as suggested by the several books. A dull, chalky and earthly fracture will indicate an early future decay of the stone.
  •  Hardness: The stone to be used in the building should possess the good hardness. The co-efficient of hardness, as worked out in the hardness test, should be greater than 17 for a stone to be used in road work. If it is between 14 and 17 then the stone is said to be of medium hardness. If the hardness is less than the 14 then the stone is said to be of poor hardness.

  • Porosity/ Percentage water: 

 The porosity of stone is defined as the ratio of the volume of the air plus water voids present in the stone to the total volume of the stone. If we put the stone in the water for 24 hours it should not absorb the water more than 3% by weight.

The porosity may affect the durability of the stone. The rain water as it descends through atmosphere absorbs some acidic gases forming light acids. Such rain water if absorbed by stone reacts with the constituents of the stone causing them to crumble.

 Similarly, in cold regions the low temperature turns the water into ice which has more volume than the water. The absorbed water will further increase the cracks  as it needs more volume in the ice form.  So the porous stones should not be used in the places which are subjected to rain, frost or moisture.

  • Resistant to fire: 

The minerals composing the stone should be such that the shape of the stone is preserved when a fire occurs. The failure of stone in case of a fire is due to various reasons such as rapid rise in the temperature, sudden cooling, different co-efficient of linear expansion of minerals, etc.

 The limestone resists fire up to a temperature of 800 degrees C and then it is split in to CaO and Co2. The sandstone with silicates as binding material can resist a fire in a better way. The argillaceous stones are weak in strength but they can resist fire quite well.

  • Seasoning: 

 The stones should be well seasoned before putting into use. The stone obtained fresh from the 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 advised to dress them when the stone contain the quarry sap. The stones should be dried or seasoned before they are used in the structural use. A period of about 6 to 12 month is advised for the proper seasoning.

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