In this article we will discuss about:- 1. Sources of Stones 2. Common Rock Forming Minerals 3. Characteristics 4. Uses and Selection 5. Deterioration 6. Preservation 7. Tests 8. Natural Bed of Stone.
Contents:
- Sources of Stones
- Common Rock Forming Minerals
- Characteristics of Good Building Stones
- Uses of Stones and their Selection
- Deterioration of Stones
- Preservation of Stones
- Tests of Stones
- Natural Bed of Stone
1. Sources of Stones:
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The stones are derived from rocks which form the earth’s crust and have no definite shape or chemical composition but are mixtures of two or more minerals. The mineral is substance which is formed by the natural inorganic process and possesses a definite chemical composition and molecular structure.
The rocks from which stones are obtained are classified in the following four ways:
1. Geological
2. Physical
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3. Chemical, and
4. Practical.
1. Geological Classification:
Geologically the rocks are classified into three different types:
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(i) Igneous rocks.
(ii) Sedimentary rocks.
(iii) Metamorphic rocks.
The igneous rocks are formed by cooling of the molten lava on or inside the earth’s surface during the volcanic eruption. The portion of lava, which comes outside the surface, cools quickly and forms the rock of non-crystalline nature called as Trap or Basalt. The rest of the portion which remains inside the earth undergoes cooling at a slow rate and results in formation of a rock of crystalline variety known as Granite.
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Sedimentary rocks are formed by gradual deposition of disintegrated rocks (due to the atmospheric action such as rain, wind and temperature), vegetable matter and clay at the bottom of rivers, lakes or sea. These are also called stratified because these rocks are formed in layers. Limestone and sandstone belong to this category of rocks.
When sedimentary or even igneous rocks are subjected to great heat and pressure inside the earth, a new variety of rock is formed which is known as metamorphic rock. This change of structure is called metamorphism. For example limestone changes to marble, slate changes to Gneiss etc.
2. Physical Classification:
The basis of the classification are physical properties of rocks, the manner and arrangement of different particles and mass forming a stone.
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Physically the rocks are classified as:
(i) Stratified rocks.
(ii) Unstratified rocks.
Stratified rocks are those which exhibit distinct layers which can be separated. The plane along which the separation of the layers can be carried out is called ‘cleavage plane’. Limestone, slate and sandstone are the examples.
Unstratified rocks do not show any sign of strata and cannot be easily split into slabs. Examples are granite, trap, marble etc.
3. Chemical Classification:
The chemical constituents (in the rocks) are the basis of this classification.
Chemically the rocks are classified as:
(i) Silicious
(ii) Agrillaceous
(iii) Calcarious.
In case of Silicious rocks silica is the main constituent. The silica in Free State is called ‘sand’ and in combined state ‘silicate’. The rocks containing silica in free form are harder. While those having silica in combined from are likely to be disintegrated. Examples are sandstone, quartzite etc.
In argillaceous rocks clay or alumina is the main constituent. Slate, laterite, kaolin etc., belong to this category.
Calcarious rocks have calcium carbonate or lime as their leading constituent. These rocks are readily acted upon by even dilute hydrochloric acid. Limestone and the marble are the important examples.
4. Practical Classification:
Practical classification is based on the usage. Practically stones have been classed as- Granites, basalts, laterites, marbles, limestones, sandstones, slates.
2. Common Rock Forming Minerals:
The common rock forming minerals are discussed below:
1. Silica:
Quartz is pure or nearly pure silica and is a hard and glassy mineral. It occurs in granite in grey, white or colourless lumps which are sometimes crystalline. It is unaffected by weather. Quartz has a hardness of 7 and specific gravity of 2.66. Due to the presence of small quantities of metallic oxides has varying transparency and colours.
2. Felspar:
Felspar are the silicates of alumina, with alkaline substances like potassium, sodium and calcium. In a sound granite, it occurs in the form of grey or reddish brown bright – crystals, in a bad sample of granite the felspar is earthly in appearance. The hardness of felspars is 6 and specific gravity varies from 2.5 to 2.7. A stone readily meets the decay if it contains large proportions of felspars mixed with other softer minerals.
3. Mica:
It contains silicate of aluminium with potassium. It is of dark grey, black or brown colour. It is very soft and readily affected by atmosphere and chemicals. It has a perfect cleavage and could be split into very thin laminate or flakes. It has a hardness of 2.5 and specific gravity of about 3. The mica occurs in granite as semitransparent, glistening scales, which are sometimes dark grey, brown or black.
4. Hornblende:
It is a very complex silica and its colour varies from dark green to black. It has hardness of 5.5 and specific gravity of 3.2.
5. Calcite:
It is the leading constituent of the limestones and marbles. Its colour varies from white to grey and lustre varies from vitreous dull to earthy. Hardness = 3 and specific gravity = 2.7.
6. Dolomite:
It is CaMg (CO3)2 or magnesium carbonate. It is commonly found in dolomitic limestone and dolomitic marbles. Its stability and weather resisting property is better than pure calcium carbonate.
3. Characteristics of Good Building Stones:
Stones find their applications in the construction of heavy structures such as dams, docks and harbours, weirs, bridgepiers, building etc.
To find the suitability of stones under different conditions, the following characteristics should be considered:
1. Appearance and Colour:
The stones which are to be employed for decoration work should have uniform and appealing colour and should be free from flaws and clay holes. The use of stones which contain much iron should be discouraged as the formation of iron oxide disfigures it and brings about disintegration. The stones should also have the ability to receive good polish.
2. Weight:
The specific gravity hence the weight of the good building stone should be high as heavier stones can resist the force of bigger magnitude. A heavy stone possesses more compactness and less porosity.
3. Porosity and Absorption:
Porosity (which is decided by the number of pores) exists in all the stones but if it is present in greater extent it makes the stone unsuitable for building construction because during rain, water seeping into pores contains acids and fumes (absorbed from atmosphere) which destroy the stone. When the climate is cold, water entering the pores may even freeze and split the stone.
4. Fineness of Grain:
The stones which are fine grained are suitable for moulding works. If the stones are non-crystalline they are likely to disintegrate under the action of natural agencies.
5. Compactness:
Stone’s durability is decided by their compactness or density of composition. A compact stone can withstand the effects of external agencies effectively.
6. Resistance to Fire:
For resistance against fire, the stone-
(i) Should have homogeneous composition, and
(ii) Should be free from calcium carbonate or oxide of iron.
7. Electrical Resistance:
The electrical resistance of a stone decreases when it gets wet. Thus to have steady and high electrical resistance, the stone must be non-absorbent like slate.
8. Hardness and Toughness:
The stone must be adequately hard and tough so that they may resist wear and tear (as in the case of railway ballast, road metal etc.). Hardness may be tested by scratching with a pen knife which should not be able to produce an impression on hard stone like granite. Toughness of the stone can be tested by subjecting it to a hammer action.
9. Strength:
Stones used in the structures are usually subjected to compression; so they should have sufficient strength to cope with the requirements. Generally all the stones possess a reasonable degree of strength as far as their use in building work is concerned but for heavy structures only those stones are suitable which have high strength comparatively.
10. Durability:
A stone is more durable in case it is-
(i) Compact,
(ii) Homogeneous, and
(iii) Free from any material affected by dilute hydrochloric and sulphuric acids. It should also have negligible water absorption.
11. Dressing:
The art of shaping a stone is known as dressing. Stones should possess uniform texture and softness so that they may be easily dressed, if it is too hard, finish will be poor and dressing uneconomical.
4. Uses of Stones and their Selection:
In the table below are given the important uses and the selection of stones for variety of works.
5. Deterioration of Stones:
The various agencies which bring about the deterioration of stones are as follows:
1. Rain:
A stone gets wet due to rain and becomes dry due to sun; this alternate wetting and drying go a long way in deteriorating the stone. The harmful gases and acids contained in rain water also exercise a damaging effect on the stone.
2. Temperature:
The stone also gets deteriorated due to frequent changes in temperature of atmosphere (due to these changes expansion and contraction take place frequently, which results in the production of cracks).
3. Wind:
A strong wind carrying with it grits and dust strikes the stones and brings about the wear of the stones. Also the water particles in the wind penetrate into the pores of the stones and cause dampness which results in deterioration.
4. Frost:
When the stone is used in the cold climatic conditions, the water in the pores freezes; consequently it expands and thereby splits the stone. Therefore, porous stones should not be, used in very cold places.
5. Atmospheric Impurities:
In an Industrial town the presence of acids and fumes adversely affect the stones containing carbonate of lime.
6. Vegetable Growth:
Certain acids secreted by the roots of certain plants and trees in the joints of the stone, attack it. The vegetable growth also keeps the stone in wet condition; this constant dampness decays the stone.
7. Living Organism:
In the stones, sometimes holes are bored by certain insects due to which they become weak. These insects are found in sea and thus stones near shores are affected by them.
6. Preservation of Stones
:
The durability of stone increases to a considerable extent if it is properly preserved.
Preservation means making the stone strong enough to face the atmospheric agencies which are the root cause of its deterioration and can be effected in the following ways:
1. Filling up the stone pores.
2. Providing the stone with a coat of preservative to prevent the ingress of moisture into the pores.
3. The use of stones containing carbonate of lime in industrial town should be discouraged.
4. In a structure, as far as possible limestone and sandstones, magnesium limestone and granular limestone should not be laid close to each other.
5. Only that binding material, which does not have any, adverse effect on a particular stone, should be employed.
6. Growth of plants and trees on the stone should be checked.
Some of the preservatives are:
(i) Coal tar,
(ii) Linseed oil,
(iii) Bartya solution,
(iv) Sczerelmy’s solution and Alum soap solution.
7. Tests of Stones
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To determine the suitability of a stone for its use in engineering works, the following tests are performed:
1. Hardness Test:
Hardness of a stone is tested by a pen knife which will not be able to produce a scratch on a hard stone (like granite). Hardness is determined with aid of the Moh’s scale of hardness. If a pocket knife makes a mark on a fresh surface of a stone say on limestone, the hardness of that limestone may be taken as H = 3, a scratch with the aid of a finger nail indicates a hardness of H = 2. Hard silicious rocks which cannot be scratched by a knife, represent a hardness of H = 7.
2. Crushing Test:
This test in particular, is performed for the stone which is to be used under high compression (such as the stone to be used at the bottom of a heavy structure). It is carried out on blocks, 10 cm cube, in a testing machine.
3. Toughness Test:
Toughness of a stone can be determined by subjecting it to a hammer blows.
4. Fire Resistance Test:
The stone which is free from calcium carbonate can resist the tire. The presence of calcium carbonate in the stone can be detected by dropping a few drops of dilute sulphuric acid which will produce bubbles.
5. Attrition Test:
This test also known as abrasion test is carried out in Deval’s testing machine. Broken pieces of stone nearly 5 kg in weight are placed in a cylinder along with cast iron balls. The cylinder is kept at inclination of 30° with the horizontal and is rotated at 30 r.p.m. for 5 hours.
At the end of this interval the pieces are removed, cleaned and weighed. Loss of weight determines the abrasion resistance property of the stone. The stones which undergo a heavy loss of weight are not suitable for road metal.
6. Acid Test:
In this test, a stone is kept for one week in the solution of sulphuric acid and hydrochloric acid having 1% strength. The corners of stones which have high alkaline content turn roundish and loose particles will get deposited on its surface. Such type of stones are unsuitable for smoky atmosphere. The stones having high percentage of lime content exhibit efflorescence when subjected to action of acids.
7. Electrical Resistance Test:
As the electric resistance of a wet stone is less, therefore, the stone should be non-absorbent. Sandstones should not absorb more than 10 percent of water, 17 percent in case of limestones and 1 percent in granites of their volume of water when dipped for 3-4 hours.
8. Smith’s Test:
This test indicates the presence of earthly matter. The stone’s sample is broken into small pieces and put into a test tube containing clear water. It is then shaken vigorously, the dirty colour will show presence of argillaceous matter.
9. Crystallization Test:
This test determines the durability or weathering quality of a stone. A sample of stone is immersed in the solution of sodium sulphate at room temperature and dried in hot air. The process of wetting and drying is carried out for two hours; the difference in weight if any is recorded. Little difference in weight indicates durability and good weathering quality of the stone.
10. Porosity or Absorption Test:
In this test the stone is first weighed when dry and weighed again after immersing it in water for a few hours.
The percentage of absorption, then is given by:
8. Natural Bed of Stone:
Natural bed of stone is the original position occupied by the stratified rock from which the stone is obtained. It is also known as the plane of cleavage. The strength and durability of the stratified stone depends on its position in a structure, i.e., how it is placed. It should always be placed in such a fashion that the load or thrust acts perpendicular to ‘natural bed of stone’ or plane of cleavage.
If the stone is placed with its natural bed parallel to the direction of load, it will get destroyed by the effects of frost and rain (which scale off the face layer by layer). In case of a column or a wall where the load line is vertical, the stones should be placed with the planes of bedding radial so that thrust acts normal to the bedding plane while in cornices with undercut mouldings, the natural bed should be placed vertically and perpendicular to the face otherwise the layers of overhanging portions being horizontal will drop off.