Chapter 1
Stones
Stones form one of the most important building materials in civil engineering. Stones are derived from rocks, which form the earth's crust and have no definite shape or chemical combination but are mixtures of two or more minerals. The mineral is a substance which is formed by the natural inorganic process and possesses a definite chemical combination and molecular structure. They are strong, durable and descent in appearance.
The main uses of stone as a building material are:
- As a principal material for foundation of civil engineering works, and for the construction of walls, arches, abutments and dams.
- In stone masonry in places where it is naturally available.
- As coarse aggregate in cement concrete (crushed form of rock).
- As a roofing material in the form of slates.
- As a flag or thin slab for paving.
- As a soling material in the construction of highways and runways.
- As ballast for railway tracks.
- As a veneer for decorative front and interior of buildings.
- Limestone for construction of important buildings like temples, churches and mosques.
- Limestone for the manufacture of cement and as a flux in blast furnace.
Numerous examples of magnificent buildings made partly or wholly of stones can be given from different parts of the world. The Taj Mahal, Red Fort and temples of Jagannath, Puri and Mahabalipuram are best-known buildings of our country made up entirely of stones.
1.1 SOURCES OF STONES
Stones are obtained from rocks. A rock represents a definite portion of the earth's surface. They occur almost everywhere in mountainous and hilly areas. It has no definite chemical composition and shape. Stones are available in large quantities in different parts of our country. It is necessary to know the availability of good stones in and around the construction site to make maximum use of this naturally available and cheap building material.
It is known as monomineralic rock, if it contains only one mineral and it is known as polymineralic rock, if it contains several minerals. Quartz sand, chemically pure gypsum, magnesite, etc. are examples of monomineralic rocks and basalt, granite, etc. are examples of polymineralic rocks.
Minerals are the units of which the rocks are made up of. A mineral indicates a substance having definite chemical composition and molecular structure. It is formed by natural inorganic processes. The properties of a rock are governed by the properties of minerals present in the structure. The common rock forming minerals are quartz, feldspars, calcite and mica, hornblende, etc.
1.2 CLASSIFICATION OF ROCKS
Rocks are classified in four different ways:
- Geological classification
- Chemical classification
- Physical classification
- Practical classification
1.2.1 Geological classification
According to the mode of origin, rocks are divided into three principal classes or groups, namely:
- Igneous rocks
- Sedimentary rocks
- Metamorphic rocks
1.2.1.1 Igneous rocks
The molten material present in the inside portion of the earth's surface is known as magma and this magma occasionally tries to come out to the earth's surface through cracks or weak portions. The rocks which are formed by the cooling of magma are called igneous rocks. The portion of lava which comes outside the surface cools quickly and forms a rock of non-crystalline nature called as trap or basalt. The rest which remains inside the earth undergoes cooling at a slow rate and results in the formation of a rock of crystalline variety known as granite. The igneous rocks are classified into the following three types.
- Plutonic rocks: They are formed by the cooling of magma at a considerable depth from the earth's surface. The cooling is slow and the rocks possess coarsely grained crystalline structure. This rock is mostly used for construction purposes. Granite is the leading example of this type of rock.
- Hypabyssal rocks: They are formed by the cooling of magma at a relatively shallow depth from the earth's surface. The cooling is quick and, hence, the rocks possess a finely grained crystalline structure. Dolerite is an example of this type of rock.
- Volcanic rocks: In the case of these rocks, solidification of magma takes place on or near the surface of the earth. The cooling is very rapid as compared to the previous two cases. Hence, the rocks are extremely fine grained in structure. Basalt is an example of this type of rock.
1.2.1.2 Sedimentary rocks
These rocks are formed by the weathering action of natural elements on the original rock and subsequent transportation by air, river, glacier and sea and deposition at a different locality. The following four types of deposits occur:
- Residual deposits: Some portion of the products of weathering remains at the site of origin. Such deposit is known as a residual deposit.
- Sedimentary deposits: The insoluble products of weathering are carried away in suspension, and when such products are deposited, they give rise to sedimentary rocks.
- Chemical deposits: Some material that is carried away in solution may be deposited by some physio-chemical process such as evaporation and precipitation. It gives rise to chemical deposits.
- Organic deposits: Some portion of the product of weathering gets deposited through the agency of organisms. Such deposits are known as organic deposits.
Examples of sedimentary rocks are sandstone, limestone, gypsum, lignite, etc.
1.2.1.3 Metamorphic rocks
These rocks are formed by the change in character of the pre-existing rocks. Igneous as well as sedimentary rocks change in character when they are subject to great heat and pressure. The process of change is known as metamorphism. Table 1.1 gives the names of the original and metamorphic rocks. Mineral composition and texture of a rock represent a system which is in equilibrium with its physio-chemical surroundings. Increase of temperature and pressure upsets this equilibrium and metamorphism results from an effort to re-establish a new equilibrium. In this process, original constituent minerals, which are unstable under the changed conditions, are converted into newer ones, which are more stable under the changed conditions. These minerals are arranged in a manner, that is more suitable to the new environment. It should, however, be noted that changes produced by weathering and sedimentation are not included in metamorphism.
There are three agents of metamorphism, namely heat, pressure and chemically acting fluids. Heat may be supplied by the general rise of temperature with depth or by igneous magma. Pressure may be developed due to the load of rocks or movement of the earth. Chemically acting fluids play a passive role only and they do not take active part in the process of metamorphism. Pressure may be uniform or directed. Uniform pressure may be applied to solids and liquids. Directed pressure or stress can exist only in solids and it is converted into uniform pressure if applied to liquids. Following are the four types of metamorphism that occur with various combinations of heat, uniform pressure and directed pressure.
- Thermal metamorphism: Heat is the predominant factor in this type of metamorphism.
- Cataclastic metamorphism: At the surface of the earth, temperature is low and metamorphism is brought about by directed pressure only. Such metamorphism is known as cataclastic metamorphism.
- Dynamo-thermal metamorphism: There will always be a rise in temperature with an increase in depth. Hence, heat in combination with stress brings about the changes in the rock. Such metamorphism is known as dynamo-thermal metamorphism.
- Plutonic metamorphism: Stress is effective only up to a certain depth. This is due to the fact that rocks become plastic in nature at certain depths. At great depths, a stage is reached when stress cannot exist as it is converted into uniform pressure because of the plasticity of rocks. Metamorphic changes at great depths are, therefore, brought about by uniform pressure and heat. Such metamorphism is known as plutonic metamorphism.
Various types of metamorphic rocks that originated from various types of rocks are given in Table 1.1.
Table 1.1 Various Types of Metamorphic Rock and Its Origin
| Name of the original rock | ||
|---|---|---|
| Igneous | Sedimentary | Name of the metamorphic rock |
| Gneiss | ||
| Granite | Limestone | Marble |
| Sandstone | Quartzite | |
| Clay | Slate | |
1.2.2 Chemical classification
On the basis of dominant chemical composition, the building stone may fall into any of the following three groups:
- Silicious rocks: In these rocks, silica predominates. These rocks are hard and durable. They are not easily affected by the weathering agencies. Silica, however, in combination with weaker minerals, may disintegrate easily. It is therefore necessary that these rocks should contain maximum amount of free silica for making them hard and durable. Granites, quartzite, etc. are examples of silicious rocks.
- Argillaceous rocks: In these rocks, clay predominates. Such rocks may be dense and compact or they may be soft. Slates, laterites, etc. are examples of silicious rocks.
- Calcarious rocks: In these rocks, calcium carbonate predominates. The durability of these rocks will depend upon the constituents present in the surrounding atmosphere. Limestone, marbles, etc. are examples of calcarious rocks.
Classification of Rocks According to Their Chemical Composition
| Chemical classification | Composition | Name of the rock |
|---|---|---|
| 1. Silicious rock | Predominance of silica | Granite, sandstone, basalt |
| 2. Argillaceous rock | Predominance of clay | Slate, laterite, schist |
| 3. Calcareous rock | Predominance of lime | Limestone, marbles, dolomite |
1.2.3 Physical classification
This classification is based on the general structure of rocks. According to this classification, the rocks are divided into three types.
- Stratified rocks: These rocks possess planes of stratification or cleavage and such rocks can easily be split up along these planes. Sedimentary rocks are distinctly stratified rocks.
- Unstratified rocks: These rocks are unstratified. The structure may be crystalline granular or compact granular. Igneous rocks of volcanic agency and sedimentary rocks affected by movements of the earth are of this type of rocks.
- Foliated rocks: These rocks have a tendency to be split up in a definite direction only. Foliated structure is very common in case of metamorphic rocks.
Classification of Rocks According to Their Structure
| Physical classification | Characteristics | Typical name |
|---|---|---|
| 1. Stratified rock | Has many strata | Slate |
| 2. Unstratified rock | Does not have strata | Granite |
| 3. Foliated rocks | Has foliated structure | Gneiss |
1.2.4 Practical classification
Practical classification is based on the usage. Practically stones have been classified as granite, basalt, laterite, marble, limestone, sandstone and slate.
1.3 DRESSING OF STONES
A place where exposed surfaces of good quality natural rocks are abundantly available is known as ‘quarry’ and the process of taking out stones from the natural bed is known as ‘quarrying’. This is done with the help of hand tools like pickaxe, chisels, etc., or with the help of machines. Blasting using explosives is another method used in quarrying.
The stones after being quarried are to be cut into suitable sizes and with suitable surfaces. This process is known as the dressing of stones and it is carried out for the following purposes:
- To make the transport from the quarry easy and economical.
- To suit the requirements of stone masonry.
- To get the desired appearance for the stonework.
The different stages of dressing are:
- Sizing: This is reducing the irregular blocks to the desired dimensions by removing extra portions. It is done with help of hand hammers and chisels.
- Shaping: This follows sizing and involves removing of the sharp projections. Many stones are used in common construction after shaping.
- Plaining: This is rather an advanced type of dressing in which the stone is cleared off all the irregularities from the surface.
- Finishing: This is done in case of specially dressed stones only and consists of rubbing of the surface of stones with suitable abrasive materials such as silicon carbide.
- Polishing: This is the last name in dressing and is done only on marbles, limestone and granite.
1.3.1 Types of dressing
- Hammer dressing: A hammer-dressed stone has no sharp and irregular corners and has a comparatively even surface to fit well in the masonry (Figure 1.1).
Figure 1.1 Hammer dressing - Chisel drafting: In this method drafts or grooves are made with help of chisels at all the four edges and any excessive stone from the centre is then removed. These stones are specially used in plinths and corners of buildings (Figure 1.2).
Figure 1.2 Chisel drafting - Fine tooling: This involves removing most of the projections and a fairly smooth surface is obtained (Figure 1.3).
Figure 1.3 Fine tooling - Rough tooling: A rough-tooled surface has a series of bands, 4-5 cm wide, more or less parallel to tool marks all over the surface (Figure 1.4).
Figure 1.4 Rough tooling - Punched dressing: A rough-tooled surface is further dressed to show the series of parallel ridges. Chisel marks are left all over the face (Figure 1.5).
Figure 1.5 Punched dressing - Closed picked dressing: A punched stone is further dressed so as to obtain a finer surface (Figure 1.6).
Figure 1.6 Closed picked stone surface
1.4 PROPERTIES OF GOOD STONE
- Appearance: The appearance of a stone in relation to the design is of great importance from an architectural point of view. Appearance depends upon the colour and ease with which the stone can be dressed, rubbed or polished. Deep colours, however, in sedimentary rocks are due to oxides of iron in the cementing material, which, upon exposure to the atmospheric influences, either fade away or disfigure and strain the surface on account of the rusting of iron. The stones which are to be used for face work should be attractive in appearance and should be of uniform colour and free from clay holes, spots of other colour, bands, etc. Stones with lighter shades are preferable, because even if they fade a little they will not show a striking difference and spoil the appearance.
- Durability: It denotes the period in years for which a stone may stand practically unaltered after being used in construction. A variety of factors affect the durability of a stone. Of these, the mineral composition, texture and structure of rocks and their capacity to absorb moisture are very important.
- Texture: Texture relates to the grains or particles composing the stone in the strata. A good building stone should have a compact fine crystalline structure, free from cavities, cracks or patches of soft or loose material.
- Crushing strength: It is also called compressive strength of a stone and is defined as the load per unit area at which a given stone starts cracking or failing. For a good structural stone the crushing strength should be greater than 100 N/mm2.
- Resistance to fire: The fire resistance of a stone may be defined as its capacity to withstand very high temperature without disintegrating. For this requirement, the different materials constituting the composition of a stone should have different coefficients of expansion. In igneous rocks, like granite, free quartz is the most dangerous material as it undergoes a sudden expansion at less than 600°C and flies into splinters; even timber is able to withstand a much higher temperature (about 800°C). It then crumbles to powder and also increases in bulk. Sandstone with silicates as binding material are fire resistant. Clay stones have good fire resistance but are poor in strength and durability.
- Specific gravity: For a good building stone, the specific gravity should be greater than 2.7 or so. Heavy stones are suitable for construction of abutments, dams, docks, harbours, etc. while lighter varieties are used in building construction.
- Hardness: For use in structures subjected to very heavy loadings, such as for constructing bridges, piers and abutments and marine structures, and particularly where they are subjected to abrasion, hardness of the stone is a necessary requirement.
- Water absorption: Moisture reduces the strength of the rocks and as such rocks that contain or absorb great amounts of moisture show lower strength values. All the stones are more or less porous, but for a good stone percentage absorption by weight after 24 hours should not exceed 0.60.
- Weathering: A good building stone should possess better weathering qualities. It should be capable of withstanding adverse effects of various atmospheric and external agencies such as rain, frost, wind, etc.
- Facility of dressing: For facility of dressing, a stone must be comparatively soft, yet durable, compact-grained and homogeneous. It must be free from veins and planes of cleavage. Such a stone is called freestone. The stones should be such that they can be easily carved, moulded, cut and dressed. It is an important consideration from the economic point of view.
1.5 COMMON BUILDING STONES IN INDIA
The commonly used building stones in India include granite, basalt and trap, limestone, marbles, gneiss, laterite, slate, etc.
- Granite: Granite is essentially an igneous rock and is hard and durable. Most of these rocks possess excellent building properties, like high strength, very low abrasion value, good resistance to frost and other weathering agencies, and are available in different appealing colours. It is used for facing work, walls, bridge piers, columns, steps, etc. These rocks are mainly found in Kashmir, Rajasthan, Punjab, Uttar Pradesh, Bihar, Orissa, Kerala, Tamil Nadu and Gujarat.
- Limestone: They are sedimentary rocks composed mainly of calcium carbonate. They show great variation in their properties and, hence, all types are not useful as building stones. They can be used as road metal for construction of floors, steps, walls, etc. Limestone is also used for the manufacture of cement and lime. The use of limestone as facing stones should be avoided in areas where the air is polluted with industrial gases or in coastal regions where air from the sea can attack them. India has extensive deposits of limestone in Maharashtra, Rajasthan, Delhi, Andhra Pradesh, Madhya Pradesh, Bihar and Bengal.
- Marble: These are metamorphic rocks and have been formed from limestone under high temperatures. Marbles vary greatly in colour, structure and texture and most of them are suitable both as an ornamental stone and as a construction material. These stones can take brilliant polish. India has got fairly widespread deposits of marble and it is mainly exploited from Rajasthan, Maharashtra and Gujarat.
- Basalt and Trap: These are also igneous rocks and are generally heavier and darker than granites and also stronger, but may contain cavities and pores within them. They are extensively used for rubble masonry, foundation work and road construction. They are mainly found in Maharashtra, Bihar, Bengal and Madhya Pradesh.
- Sandstone: These are sedimentary rocks and consist mainly of quartz. Sandstones occur in many colours. The most suitable and durable type is that which is light coloured, having silica, cement and a homogeneous, compact texture. It can be used for steps, facing work, flooring, columns, etc. India has immense reserves of really good quality sandstones in Madhya Pradesh, Uttar Pradesh, Orissa and Bihar.
- Laterite: It is a sedimentary rock composed mainly of oxides of iron and aluminium. Laterites are of dull red or brown colour and their important property is that they are quite soft and plastic when cut from the natural bed rock but become hard on exposure. It is used for rough stone masonry work and is sometimes used in place of bricks. They are mainly found in Maharashtra, Tamil Nadu, Kerala, Karnataka and Andhra Pradesh.
- Gneiss: Gneiss is a metamorphic rock and closely resembles granite in its building properties. But sometimes it may be rich in mica and useless as a building stone. They can be used for street paving, rough stone masonry work, etc. They are mainly found in southern states like Tamil Nadu, Kerala, Karnataka, Andhra Pradesh and also in states like Bihar, Bengal and Orissa.
- Slate: Slate is a metamorphic rock and splits into thin sheets having smooth surfaces along the natural bedding planes. In building construction their use is limited to roofing purpose for ordinary buildings or as paving or insulating materials. Slate occurs in Gujarat, Rajasthan, Andhra Pradesh and Bihar.
- Kankar: Kankar is a sedimentary rock and is a form of impure limestone. It is used as road metal for the manufacture of hydraulic lime, etc. It occurs among the different parts of north and central India.
- Murum: Murum is a metamorphic rock. It is a form of decomposed laterite and is deep brown or red in colour. It is a soft rock and can be used for fancy paths and garden walls. It occurs mainly in Maharashtra, Tamil Nadu, Orissa, Bihar and Madhya Pradesh.
REVIEW QUESTIONS
- What are the main uses of stone in building construction?
- How are rocks classified? Briefly discuss the different classification of rocks.
- Give short notes on
- Chemical classification of rocks
- Different stages of dressing of stones
- Common building stones in India
- What are the purposes of dressing of stones?
- What are the different types of dressing of stones?
- Briefly discuss the properties of a good stone.
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