Properties of Good Building Stones
The following properties of the stones should be looked into before selecting them for engineering works.
1) Structure
The structure of the stone may be stratified (layered) or unstratified. Stratified 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.
2) Texture
Fine grained stones with homogeneous distribution look attractive and hence they are used for carving. Such stones are usually strong and durable.
3) Density
Denser stones are stronger. Light weight stones are weak. Hence stones with specific gravity less than 2.4 are considered unsuitable for buildings.
4) Appearance
A stone with uniform and attractive colour is durable, if grains are compact. Marble and granite get very good appearance, when polished.
5) 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. 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 are placed on laterite wall. A good building stone should have better crushing strength i.e. greater than 100 N/mm2.
6) 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.
7) 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%.
8) Porosity and Absorption
All stones have pores and hence absorb water. The reaction of water with material of stone causes 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.
9) Weathering
Rain and wind cause loss of good appearance of stones. Hence stones with good weather resistance should be used for face works.
10) 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 indexes 13 to 19 are considered as medium tough and stones with toughness index less than 13 are poor stones.
11) Resistance to Fire
Sand stones resist fire better. Argillaceous materials, though poor in strength, are good in resisting fire.
12) 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.
13) 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.
14) Durability
A good building stones must be durable long lasting nature.
15) Water Resistance
They should have less water absorption properties.
16) Economy
They should be economical and easily available.
Requirements of Good Building Stone
The following are the quality requirements of good building stones.
1) Strength
Strength is an important property to be looked into before selecting stone as a building block. Generally, most of the building stones have high strength to resist the load coming on it. Therefore, it is not of prime concern when it comes to check the quality of stones. But when the stones are to be used in large structures, it becomes necessary to check the compressive strength of stones. Compressive strength of building stones generally falls within the range of 60 to 200 N/mm2. Indian standard code recommends, a minimum crushing strength of 3.5 N/mm2 for any building block.
2) Durability
Building stones should be capable to resist the adverse effects of natural forces like wind, rain and heat. It must be durable and should not deteriorate due to the adverse effects of the above natural forces.
3) Hardness
It is an important property to be considered when a stone is used for flooring, pavement or aprons of bridges, they become subjected to wearing and abrasive forces caused by movement of men or machine over them. So it is required to test hardness of stone. 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.
4) Toughness
Toughness of stones means it ability to resist impact forces. It is determined by the impact test. 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 constructions should be tough. Stones with toughness index more than 19 are preferred for road works.
5) Specific Gravity
The more the specific gravity of stone, the heavier and stronger the stone is. Therefore, stones having higher specific gravity values should be used for the construction of dams, retaining walls, docks and harbours. The specific gravity of good building stone is between 2.4 and 2.8. Stones with a specific gravity less than 2.4 are considered unsuitable for buildings.
6) Porosity and Water Absorption
All stones have pores and hence absorb water. The reaction of water with a material of stone cause disintegration. Porosity of building stones depend upon the mineral constituent and structural formation of the parent rock. If stones used in building construction are porous then rain water can easily enter into the pore spaces and cause damage to the stones. Therefore, building stone should not be porous. Water absorption of stone is directly proportional to the porosity of rock. If a stone is more porous then it will absorb more water and cause more damage to stone. In higher altitudes, the freezing of water in pores takes place and it results into the disintegration of the stone. 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.
Table 1 - 24 Hours Water Absorption of Stones by Volume
|
Sl. No |
Types of Stone |
Water absorption (% not greater than) |
|
1 |
Sandstone |
10 |
|
2 |
Limestone |
10 |
|
3 |
Granite |
1 |
|
4 |
Trap |
6 |
|
5 |
Shale |
10 |
|
6 |
Gneiss |
1 |
|
7 |
Slate |
1 |
|
8 |
Quartzite |
3 |
7) Dressing
Giving 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 being at the cost of the required strength and the durability.
8) Appearance
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. 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. Light coloured stones are more preferred than dark coloured stones as the colour are likely to fade out with time.
9) Seasoning
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. Good stones should be free from the quarry sap. Lateritic 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.
10) Workability
Stone should be workable. Stone is said to be workable when the work involved in stone working (such as cutting, dressing & shaping) is economical and easy to conduct.
11) Cost
Cost is an important consideration in selecting a building material. Proximity of the quarry to building site brings down the cost of transportation and hence the cost of stones comes down.
12) Fire Resistance
Stones should be free from calcium carbonate, oxides of iron and minerals having different coefficients of thermal expansion. Igneous rock show marked disintegration principally because of quartz which disintegrates into small particles at a temperature of about 575°C. Limestone can withstand a little higher temperature; i.e. up to 800°C after which they disintegrate. Sandstones resist fire better. Argillaceous materials, though poor in strength, are good in resisting fire.
13) Percentage Wear
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%.
14) Weathering
Rain and wind cause loss of the good appearance of stones. Hence stones with good weather resistance should be used for face works.
15) Structure
The structure of the stone may be stratified (layered) or unstratified. Stratified 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.
0 comments:
Post a Comment