Following are the engineering and physical properties of stones which should be observed before selecting them for engineering works:
The stone structure can be stratified (layered) or unstable. Structured stones should be easily prepared and suitable for super structure. Lined stones are hard and hard to wear. They are given priority for foundation works.
Fine-grained stones with homogeneous distribution look attractive and are therefore used for carving. Such stones are usually strong and durable.
Dense stones are stronger. Light weight stones are weak. Therefore stones with a specific gravity less than 2.4 are considered unsuitable for buildings.
A stone with a uniform and attractive color is durable if the grain is compact. Marble and granite look great when polished. Hence they are used for face work in buildings.
Strength is an important asset before selecting stone as a building block. Indian standard code recommended, minimum crushing strength 3.5 N/mm2 For any building block. The table below shows the crushing power of different stones. Due to the non-uniformity of the material, a safety factor of 10 is usually used to find the permissible stress in a stone. Hence laterite can also be safely used for a single storey building, as the load required in such structures can hardly deliver a stress of 0.15 N/mm.2, However in stone masonry buildings care should be taken to check the tension when beams (concentrated loads) are placed on a laterite wall.
|Crushing Strength of Common Building Stones|
|name of stone||n / mm. crushing power in2|
|Net||300 to 350|
|bajalat||153 to 189|
|granite||104 to 140|
|slate||70 to 210|
|laterite||1.8 to 3.2|
This is an important asset when stone is used for floors and sidewalks. The coefficient of stiffness is to be found by testing it on a standard sample in a string testing machine. The coefficient of hardness for road works should be at least 17. Stones with a coefficient of hardness not less than 14 should be used for building works.
7. wear percentage
It is measured by attrition test. This is an important property when selecting aggregates for road works and railway ballasts. A good stone should not have more than 2% wear.
8. porosity and absorption
All stones have pores and therefore absorb water. The reaction of water with the stone leads to dissolution. The absorption test is specified as the percentage of water absorbed by the stone when it is immersed in water for 24 hours. For a good stone it should be as small as possible and in no case be more than 5.
Good appearance of stones is lost due to rain and wind. Therefore stones with good weather resistance should be used for facial works.
Resistance to impact is called toughness. This effect is determined by testing. Stones with toughness index above 19 are preferred for road works. Toughness index 13 to 19 is considered medium tough and stones with toughness index less than 13 are poor stones.
1 1 fire resistance
Sandstones resist fire better. Argilaceous material, though poor in strength, is good at resisting fire.
12. ease of dressing
The cost of dressing contributes substantially to the cost of stone masonry. Dressing is easier in low strength stones. Hence an engineer should focus on adequate strength rather than high strength while selecting stones for construction works.
The quarry stones contain moisture in the pores. The strength of the stone is improved if this moisture is removed before the stone is used. The process of removing moisture from the pores is called seasoning. The best way to season is to allow it to be in nature’s action for 6 to 12 months. This is much needed in the case of laterite stones.