Stone is a ‘naturally available building material’ which has been used from the early age of civilization.It is available in the form of rocks, which is cut to required size and shape and used as building block.It has been used to construct small residential buildings to large palaces and temples all over the world.
Type of Stones
Stones used for civil engineering works may be classified in the following three ways:
•- Geological
•- Physical
•- Chemical
Geological Classification
Based on their origin of formation stones are classified into three main groups—Igneous, Sedimentary and Metamorphic Rocks.
(i) Igneous Rocks:
These rocks are formed by cooling and solidifying of the rock masses from their molten magmatic condition of the material of the earth. Generally igneous rocks are strong and durable. Granite, trap and basalt are the rocks belonging to this category, Granites are formed by slow cooling of the lava under thick cover on the top. Hence they have crystalline surface. The cooling of lava at the top surface of earth results into non-crystalline and glassy texture. Trap and basalt belong to this category.
(ii) Sedimentary Rocks:
Due to weathering action of water, wind and frost existing rocks disintegrates. The disintegrated material is carried by wind and water; the water being most powerful medium. Flowing water deposits its suspended materials at some points of obstacles to its flow.
These deposited layers of materials get consolidated under pressure and by heat. Chemical agents also contribute to the cementing of the deposits. The rocks thus formed are more uniform, fine grained and compact in their nature. They represent a bedded or stratified structure in general. Sand stones, lime stones, mud stones etc. belong to this class of rock.
(iii) Metamorphic Rocks:
Previously formed igneous and sedimentary rocks undergo changes due to metamorphic action of pressure and internal heat. For example due to metamorphic action granite becomes greisses, trap and basalt change to schist and laterite, limestone changes to marble, sandstone becomes quartzite and mudstone becomes slate.
Physical Classification
Based on the structure, the rocks may be classified as:
-• Stratified rocks
– •Unstratified rocks
(i) Stratified Rocks:
These rocks are having layered structure. They possess planes of stratification or cleavage. They can be easily split along these planes. Sand stones, lime stones, slate etc. are the examples of this class of stones.
(ii) Unstratified Rocks:
These rocks are not stratified. They possess crystalline and compact grains. They cannot be split in to thin slab. Granite, trap, marble etc. are the examples of this type of Rocks.
(iii) Foliated Rocks:
These rocks have a tendency to split along a definite direction only. The direction need not be parallel to each other as in case of stratified rocks. This type of structure is very common in case of metamorphic rocks.
Chemical Classification
On the basis of their chemical composition engineers prefer to classify rocks as:
• -Silicious rocks
• -Argillaceous rocks and
• -Calcareous rocks
(i) Silicious rocks: The main content of these rocks is silica. They are hard and durable. Examples of such rocks are granite, trap, sand stones etc.
(ii) Argillaceous rocks: The main constituent of these rocks is argil i.e., clay. These stones are hard and durable but they are brittle. They cannot withstand shock. Slates and laterites are examples of this type of rocks.
(iii) Calcareous rocks: The main constituent of these rocks is calcium carbonate. Limestone is a calcareous rock of sedimentary origin while marble is a calcareous rock of metamorphic origin.
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 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. Table 1.1 shows the crushing strength of various stones.
Due to 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 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 stone is used for flooring and pavement. Coefficient of hardness is to be found by conducting test on 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 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 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 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.
(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 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:
Sand stones resist fire better. Argillaceous materials, though poor in strength, are good in resisting fire.
(xii) Ease in Dressing:
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.
(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. 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.
