In our history, stone had been used as a building material during the early years of civilization. Being a naturally occurring material, stone is available in the form of blocks and can be cut to required sizes and shapes when used in building.Be it residential buildings or large palaces even temples around the world used stone as a building material.
Types of natural stones
Stone is classified as geophysical, physical and chemical in civil engineering works.
Stone is classified into three main groups based on their origin of formation as igneous rocks, sedimentary rocks and metamorphic rocks.
Igneous Rocks – are formed from cooling and solidifying of molten materials. Igneous rocks are strong and durable like granite and basalt. Granite stone have crystalline surface because of the slow cooling of lava under thick cover at the top of the surface of the earth. When the cooling of lava on top of the surface of the earth results to non – crystalline and glassy texture, basalt are formed.
Sedimentary Rocks – are formed from deposits of eroded and pre –existing rock that settle in layers on sea beds. These deposits merge together by pressure and heat. Chemical agents contributes to the cementing of these deposits. Thus, rocks formed are more uniformed, fine grained and compact in nature. Sandstone and limestone are examples of these type of stone.
Metamorphic Rocks – are formed when igneous and sedimentary rocks go through metamorphic changes because of pressure and internal heat. Granite becomes gneiss and basalt and trap change to schist and laterite, limestone to marble, sandstone to quartzite and mudstone becomes slate because of metamorphic action.
Properties of Stones
When selecting stone for engineering works, stonemasons and builders should take a deeper look on the properties of stones.
- Structure: Stones may be stratified or unstratified. Structured stones should be easily dressed and should be suitable when creating a super structure. Unlike unstratified stone which are hard and difficult to dress and are preferred for foundation works.
- Texture: Fined grained stones look attractive and mostly used in carving. They have a homogenous distribution and usually strong and durable.
- Density: Denser and compact stones are stronger while light weight ones are weaker. Stones with a specific gravity of 2.4 or less are considered unsuitable for buildings.
- Appearance: Stones with uniform and attractive colour are durable if their grains are compact. Marble and granite are great in appearance when polished and are mostly used for facades, floors and benches.
- Strength: It is important to look into the strength of stone before selecting it as a building block specifically their crushing strength to make sure that they can be safely used.
- Hardness: Stone’s hardness is a very important especially when considering to use it as a flooring and pavement. Testing the stone’s hardness using the Dory’s testing machine is one way of finding the coefficient of hardness. Coefficient of hardness for road works should be at least 17 and it should not be less than 14 when it is to be used on building works.
- Percentage Wear: This is an essential property of stone that can be measured by attrition test. This is to be considered when selecting aggregate for road works and railway ballast. A good stone must not show wear of more than 2%.
- Porosity and Absorption: All stones absorb water because pores and capillaries are found in all stones. To test the percentage of water absorption by a stone, an absorption test should be performed by immersing the stone under water for 24 hours. The percentage of absorption alone will be a great indicator on how porous the stone is. This can be measured and compared using weight by weight method.
- Weathering: The good appearance of stones can be lost with rain, wind and all other external factors. Stones that have good weather resistance should be used for facade works.
- Toughness: A stone’s resistance to impact is called toughness. Toughness is determined by performing an impact test. For road works, toughness index that is more than 19 is preferred. 13 to 19 toughness indexes are considered medium tough and less than 13 toughness index is a poor stone.
- Resistance: Argillaceous materials are poor in strength but they are very good in resisting fire.Sandstone resist fire better.
- Ease of Finishing: The cost of finishing plays an important role when it comes to the cost of stone masonry to a very great extent. An engineer should consider sufficient strength rather than high strength when selecting stone for building works because it’s finishing is easy with lesser strength.
- Seasoning: The process of removing moisture from pores is called seasoning. Stones obtained from quarry contain moisture in the pores. If this moisture is removed, the stone’s strength is improved. Allowing the stone to the action of nature for 6 to 12 months is the best way of seasoning. Laterite stones require seasoning the most.