Properties of Stones
The following properties of the stones should be looked into before selecting them for engineering works:
(vii) Percentage Wear.
(viii) Porosity and Absorption.
(xi) Resistance to Fire.
(xii) Ease in Dressing.
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.
Fine grained stones with homogeneous distribution look attractive and hence they are used for carving. Such stones are usually strong and durable.
Denser stones are stronger. Light-weight stones are weak. Hence stones with a specific gravity less than 2.4 are considered unsuitable for buildings.
A stone with uniform and attractive colour is durable if grains are compact. Marble and granite get a very good appearance, when polished. Hence they are used for face works in buildings.
Also, read: What Is Flooring | Types of Flooring.
Strength is an important property to be looked into before selecting stone as a building block. Indian standard code recommends, a minimum crushing strength of 3.5 N/mm2 for any building block.
Table 1 shows the crushing strength of various stones. Due to the non-uniformity of the material, 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 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.
|Name of Stone||Crushing Strength in N/mm2|
|Trap||300 to 350|
|Basalt||153 to 189|
|Granite||104 to 140|
|Slate||70 to 210|
|Laterite||1.8 to 3.2|
Table 1. Crushing Strength of Common Building Stones
It is an important property to be considered when a stone is used for flooring and pavement. The coefficient of hardness is to be found by conducting a test on a standard specimen in Dory’s testing machine.
For road works coefficient of hardness should be at least 17. For building works stones with a coefficient of hardness less than 14 should not be used.
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%.
Porosity and Absorption
All stones have pores and hence absorb water. The reaction of water with a material of stone cause disintegration.
The absorption test is specified as the 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.
Also, read: What Is Stone | Type of Stones | Uses of Stones
Rain and wind cause loss of the good appearance of stones. Hence stones with good weather resistance should be used for face works.
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 medium tough and stones with toughness index less than 13 are poor stones.
Resistance to Fire
Sand-stones resist fire better. Argillaceous materials, though poor in strength, are good in resisting fire.
Ease in Dressing
The cost of dressing contributes to cost of stone masonry to a great extent. The 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.
The stones obtained from the 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. The best way of seasoning is to allow it to the action of nature for 6 to 12 months. This is very much required in the case of laterite stones.
Requirements of Good Building Stones
The following are the requirements of good building stones:
(v) Specific Gravity
(vi) Porosity and Absorption
The stone should be able to resist the load coming on it. Ordinarily this is not of primary concern since all stones are having good strength.
However in case of a large structure, it may be necessary to check the strength.
Stones selected should be capable of resisting adverse effects of natural forces like wind, rain, and heat.
The stone used in floors and pavements should be able to resist abrasive forces caused by the movement of men and materials over them.
Building stones should be tough enough to sustain stresses developed due to vibrations. The vibrations may be due to the machinery mounted over them or due to the loads moving over them.
The stone aggregates used in the road construction should be tough.
Heavier variety of stones should be used for the construction of dams, retaining walls, docks, and harbors. The specific gravity of good building stone is between 2.4 and 2.8.
Porosity and Absorption
Building stone should not be porous. If it is porous rainwater enters the pour and reacts with stone and crumbles it.
In higher altitudes, the freezing of water in pores takes place and it results in the disintegration of the stone.
Giving the required shape to the stone is called dressing. It should be easy to dress so that the cost of dressing is reduced.
However the care should be taken so that, this is not at the cost of the required strength and the durability.
In case of the stones to be used for face works, where appearance is a primary requirement, its colour and ability to receive polish is an important factor.
Good stones should be free from the quarry sap. Laterite stones should not be used for 6 to 12 months after quarrying.
They are allowed to get rid of quarry sap by the action of nature. This process of removing quarry sap is called seasoning.
Cost is an important consideration in selecting a building material. The proximity of the quarry to the building site brings down the cost of transportation and hence the cost of stones comes down.