In this article we will discuss about:- 1. Uses of Stones 2. Natural Bed of Stones 3. Dressing 4. Deterioration 5. Retardation 6. Preservation 7. Artificial Stones.
Uses of Stones:
The stones are used in the construction of buildings from the ancient times and most of the ancient temples and forts of our country were built with stones. The Taj Mahal at Agra and Red Fort, Jama Masjid, Parliament House, Central Secretariat and Rashtrapati Bhawan at Delhi and various other prominent structures spread throughout the length and breadth of our country furnish us the splendid examples of contribution of stones as a building material. Even at present, they form a basic material for cement concrete and bricks.
Following are the various uses to which stones are employed:
(1) Structure:
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The stones are used for foundation, walls, columns, lintels, arches, roofs, floors, damp-proof courses, etc.
(2) Face- Work:
The stones are adopted to give massive appearance to the structure. The walls are of bricks and facing is done in stones of desired shades. This is known as the composite masonary.
(3) Paving:
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The stones are used to cover floor of buildings of various types such as residential, commercial, industrial, etc. They are also adopted to form paving of roads, footpaths, etc.
(4) Basic Material:
The stones are disintegrated and converted to form a basic material for cement concrete, marum of roads, calcareous cements, artificial stones, hollow blocks, etc.
(5) Miscellaneous:
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In addition to above uses, the stones are also used as:
(i) Ballast for railways,
(ii) Flux in blast furnaces,
(iii) Blocks in the construction of bridges, piers, abutments, retaining walls, light houses, dams, etc.
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It should however be remembered that the stones are gradually losing their popularity as the building material because of the following facts:
(i) The dressing of stones proves to be tedious, laborious and time-consuming.
(ii) The stones of desired strength and quality are not easily available at moderate rates, especially in plain areas.
(iii) The alternatives to stories, namely R.C.C. and steel, have proved to be stronger, less bulky, more durable and more suitable for present day construction of multi-storied and important buildings.
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(iv) The structures constructed of stones cannot be rationally designed as in case of R.C.C. or steel structures.
Natural Bed of Stones:
(1) Definition:
The building stones are obtained from rocks. These rocks have a distinct plane of division along which stones can easily be split. This plane is known as the natural bed of stone and it thus indicates the plane or bed on which the sedimentary stone was originally deposited. The natural bed of stone need not necessarily be horizontal.
For sedimentary rocks, it is easy to observe and locate the natural bed as it lies along the planes of stratification. For igneous rocks, the natural bed is of little significance or importance and it is also difficult to determine.
(2) Importance:
In stone masonry, the general rule to be observed is that the direction of natural bed of all sedimentary stones should be perpendicular or nearly so to the direction of pressure. Such an arrangement gives maximum strength to the stonework.
The natural beds of stones can be detected by pouring water and examining the directions of layers. The magnifying glass may also be used for this purpose. An experienced worker can easily locate the direction of natural bed of stones from the resistance offered to the chisel. The stones break easily along these natural beds.
With respect to natural bed, the stones are placed in different situations as follows:
(i) Arches:
In stone arches, the stones are placed with their natural beds radial as shown in fig. 2-1.
With such an arrangement, the thrust of arch acts normal to the direction of natural beds.
(ii) Cornices, String Courses, etc.:
The stones are partially unsupported in case of cornices, string courses, etc. Hence they should be placed with direction of natural beds as vertical. This principle will not hold good for corner stones. It would be desirable, in such cases, to adopt stones without natural beds.
(iii) Walls:
The stones should be placed in walls with the direction of their natural beds horizontal as shown in fig. 2-1.
Dressing of Stones:
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:
(i) To get the desired appearance from stone work,
(ii) To make the transport from quarry easy and economical,
(iii) To suit to the requirements of stone masonry,
(iv) To take advantage of local men near quarry who are trained for such type of work, etc.
With respect to the place of work, the dressing can be divided into two types, namely, quarry dressing and site dressing.
At the quarry place, the stones are roughly dressed to secure the following advantages:
(i) At quarry site, it is possible to get cheap labour for the process of dressing of stones.
(ii) It is possible to sort out stones for different works, if quarry dressing is practised.
(iii) The irregular and rough portions of the stones are removed which decrease the weight of stones and it also facilitates easy transportation of the stones.
(iv) The natural bed surface of stones can be made prominent during the quarry dressing.
(v) The stones when quarried freshly contain quarry sap and hence they are comparatively soft and can be easily dressed.
Following are the varieties of finishes obtained by the dressing of stones:
(1) Axed Finish:
The surfaces of hard stones such as granite are dressed by means of an axe. Such a finish is termed as an axed finish.
(2) Boasted or Droved Finish:
In this type of finish, the boaster is used to make non-continuous parallel marks on the stone surface as shown in fig. 2-11. These marks may be horizontal, inclined or vertical. A boaster is a chisel having an edge of width Boasted or droved finish about 60 mm.
(3) Chisel-Draughted Margins:
In order to obtain uniform joints in stone work, the margins are placed which may be either squared or pitched or chamfered.
(4) Circular Finish:
In this type of finish, the surface of stone is made round or circular as in case of a column.
(5) Dragged or Combed Finish- In this type of finish, a drag or a comb, which is a piece of steel with a number of teeth, is rubbed on the surface in all directions and surface, as shown in fig. 2-12, is obtained. This finish is suitable for soft stones only.
(6) Furrowed Finish:
In this type of finish, a margin of about 20 mm width, is sunk on all the edges of stone and the central portion is made to project about 15 mm.
A number of vertical or horizontal grooves about 10 mm wide are formed in this projected portion as shown in fig. 2-13. This finish is generally adopted to make the quoins prominent.
(7) Moulded Finish:
The surface of stone can be moulded in any desired shape so as to improve the appearance of the work. The mouldings can be made either by hand or machine.
(8) Hammer-Dressed Finish:
In this type of finish, the stones are made roughly square or rectangular by means of a Waller’s hammer as shown in fig. 2-14. The hammer-dressed stones have no sharp or irregular corners and have comparatively even surface so as to fit well in masonry.
(9) Plain Finish:
In this type of finish, the surface of the stone is made approximately smooth with a saw or with a chisel.
(10) Polished Finished:
The surface of the stones such as marbles, granites, etc. can be polished either with hand or with machine.
(11) Punched Machine:
On the stone surface, the depressions are made by using a punch. The surface of the stone takes the form of a series of hollows and ridges.
(12) Reticulated Finish:
This type of finish presents a net-like appearance as shown in fig. 2-15. A margin, about 20 mm wide, is marked on the edges of stone and irregular sinking are made on the enclosed space. A margin, about 10 mm wide, is provided around the irregularly shaped sinking, having a depth of about 5 mm. A pointed tool is used to put the marks on the sunk surface so as to present a pock-marked appearance.
(13) Rubbed Finish:
This type of finish is obtained by rubbing a piece of stone with the surface or by rubbing the surface with the help of a suitable machine.
The water and sand are freely used to accelerate the process of rubbing.
(14) Scabbling Finish:
In this type of finish, the irregular projections are removed with a scabbling hammer and in this way, the stones are roughly dressed.
(15) Tooled Finish:
The stone surface is finished by means of a chisel and parallel continuous marks, either horizontal or inclined or vertical, are left on the surface.
(16) Self-Faced or Rock-Faced or Quarry-Faced Finish:
Some stones, as obtained from the quarry, possess smooth surface and they can be directly placed on the work. Such a stone surface is termed as the self-faced or rock-faced or quarry-faced finish.
(17) Sunk Finish:
This finish is obtained by sinking the surface below the original level in the form of wide grooves, chamfers, inclined surfaces, etc.
(18) Vermiculated Finish:
This finish is just similar to reticulated type except; that the sinking are more curved. This finish presents a worm-eaten appearance.
Deterioration of Stones:
The stones with exposed faces are acted upon by various atmospheric and deterioration.
Following are the causes of decay of stones:
(1) Alternate wetness and drying
(2) Frost
(3) Impurities in atmosphere
(4) Living organisms
(5) Movements of chemicals
(6) Nature of mortar
(7) Rain water
(8) Temperature variations
(9) Vegetable growth
(10) Wind.
(1) Alternate Wetness and Drying:
The stones are made wet by various agencies such as rain, frost, dew, etc. Such wet surface is dried by sunshine. It is found that stones subjected to such alternate wetness and drying wear out quickly.
(2) Frost:
In hill stations or very cold places, the moisture present in the atmosphere is deposited in pores of stones. At freezing point, this moisture freezes and in doing so, it expands in volume and causes the splitting of stone.
(3) Impurities in Atmosphere:
The atmosphere contains various impurities which have adverse effects on stones. For instance, the acids and fumes are predominant in an industrial town. These impurities act on carbonate of lime and cause the deterioration of stone.
(4) Living Organisms:
Some living organisms like worms and bacteria act upon stones and deteriorate them. These organisms make holes in stones and thus weaken them. They also secrete organic acids which have a corrosive action on stone minerals.
(5) Movements of Chemicals:
If stones of different variety, such as limestone and sandstone, are used side by side in the same structure, the chemicals formed by the action of atmospheric agencies on one variety may move on the other and cause the deterioration of that other.
For instance, if sandstone is placed below limestone, the chemicals brought down from the limestone by rain water or any other reason will cause decay of the sandstone. In a similar way, if granular limestone and magnesium limestone are used together, the granular limestone may deteriorate due to the absorption of magnesium sulphate from magnesium limestone.
(6) Nature of Mortar:
The nature of mortar used as a binding material in stone masonry may be such that it may react chemically with any one of the constituents of stones and thus it may lead to the disintegration of stones.
(7) Rain Water:
The action of rain water on stones is too fold – physical and chemical. The rain wets the surface of stone and it is dried by sunshine. Such alternate wetness and drying result in the disintegration of stone. This is the physical action of rain water.
The rain water, as it descends through the atmosphere to the surface of earth, absorbs carbon dioxide (CO2), hydrogen sulphide (HgS) and other gases present in the atmosphere. These gases act adversely on stones and they cause decay of stones. This is the chemical action of rain water.
(8) Temperature Variations:
The rise of temperature results in expansion of stones. The fall of temperature causes contraction of stones. If rise and fall of temperatures are frequent, the stones are easily deteriorated because of the setting up of internal stresses.
(9) Vegetable Growth:
The creepers and certain trees develop on stone surface with their roots in joints between stones. Such roots attract moisture and keep the stone surface damp. At the same time, they try to expand. Such actions then accelerate the decay of stones.
(10) Wind:
The wind contains fine particles of dust. If it is blowing with high velocity, such particles will strike against the stone surface and thus the stones will be decayed. The wind also allows rain water to enter pores of stones with force. Such water, on freezing, expands and splits the stones.
Retardation of Decay of Stones:
Following precautions should be taken to retard the decaying action of the weathering agencies on the stones:
(1) Compact Silicious Stones:
It is desirable to use only compact silicious stones for the external surfaces of important buildings. These stones must have a dense crystalline texture. The use of sandstones cemented with silicious binding material should be made and use of lime stones or calcareous sandstones with open texture should be avoided for the external surfaces in industrial towns.
(2) External Renderings:
For ordinary buildings, the external renderings such as pointing or plastering should be given to the stone surface at the time of construction.
(3) Joints:
All the joints in the stone masonry should be completely filled in so as to have a sound and solid structure without hollows or cavities.
(4) Natural Beds:
The stones should be placed in position on their natural beds.
(5) Qualities of Stones:
The use of finished, polished and well-dressed stones should be preferred to the rough stones.
(6) Seasoned Stones:
The freshly quarried stones contain quarry sap which accelerates the decaying action and hence such stones should be seasoned for a sufficient time by exposing them before they are placed in position.
(7) Size of Stones:
It is advisable to employ large size stones as far as possible to minimize the number of joints which are signs of weakness and through which water or moisture gets easy entry.
(8) Washing with Water:
The exposed stone surface should be kept as clean as possible and for this purpose, it should be washed with water at regular intervals.
Preservation of Stones:
The decay of building stones of inferior quality is to some extent prevented, if they are properly preserved. For this purpose, the preservatives are applied on the stone surfaces.
An ideal preservative has the following properties:
(i) It does not allow moisture to penetrate the stone surface.
(ii) It does not develop objectionable colour.
(iii) It hardens sufficiently so as to resist effects due to various atmospheric agents.
(iv) It is easily penetrated in stone surface.
(v) It is economical.
(vi) It is non-corrosive and harmless.
(vii) It remains effective for a long time after drying.
(viii) Its application on stone surface is easy.
It should however be remembered that there is not a single preservative which is suitable for all types of stones. The choice of a preservative therefore requires careful consideration. Depending upon the chemical composition of stones and their location in structure, a particular preservative should be recommended. Each case should be properly studied before a final choice is made.
Following are the preservatives which are commonly adopted to preserve the stones:
(1) Coal Tar:
If coal tar is applied on stone surface, it preserves stone. But the colour of coal tar produces objectionable appearance and surface coated with coal tar absorbs heat of the sun. Hence this preservative is not generally adopted because it spoils the beauty of stones.
(2) Linseed Oil:
This preservative may be used either as raw linseed oil or boiled linseed oil. The raw linseed oil does not disturb the original shade of stone. But it requires frequent renewal, usually once in a year. The boiled linseed oil lasts for a long period, but it makes the stone surface dark.
(3) Paint:
An application of paint on stone surface serves as a preservative. The paint changes the original colour of stone. It is applied under pressure, if deep penetration is required.
(4) Paraffin:
This preservative may be used alone or it may be dissolved in neptha and then applied on stone surface. It changes the original colour of stone.
(5) Solution of Alum and Soap:
The alum and soft soap are taken in proportion of about 0.75 N and 0.50 N respectively and they are dissolved in a litre of water. This solution, when applied on stone surface, acts as preservative.
(6) Solution of Baryta:
A solution of barium hydroxide Ba(OH)2, when applied on stone surface, acts as a preservative. This preservative is used when the decay of stone is mainly due to calcium sulphate, CaSO4.
Following chemical reaction takes place –
Ba(OH)2 + CaSO4 = BaSO4 + Ca(OH)2
The barium sulphate is insoluble and it is least affected by atmospheric agencies. The calcium hydroxide absorbs carbon dioxide from atmosphere and forms calcium carbonate CaCO3 which adds to the strength of stone.
Artificial Stones:
These are also known as the cast stones or reconstructed stones.
(1) Procedure for Making an Artificial Stone:
Following procedure is generally adopted in making an artificial stone:
(i) The natural stone is crushed into sizes less than 6 mm.
(ii) The stone dust is removed.
(iii) A mixture of 1½ A parts of stones of size 3 mm to 6 mm, 1½ parts of stones of size less than 3 mm and 1 part of cement by volume is prepared.
(iv) The necessary pigment to produce the desired colour effect is added to the above mixture. Its proportion should not exceed 15% of cement by weight.
(v) The water in required quantity is added and thorough mixing of materials is done.
(vi) The mixture thus prepared is transferred to specially constructed moulds.
(vii) It is allowed to harden and its surface is kept wet. The artificial stone is then ready in block form.
(viii) The polishing is done, if required.
(ix) The white cement may be used in place of ordinary cement to produce colour of light shade.
(2) Forms of Artificial Stones:
The artificial stones may take up various forms as follows:
(i) Cement Concrete:
This is a mixture of cement, fine aggregate, coarse aggregate and water. It may be cast-in-situ or pre-cast. It is widely used at present. If steel is used with cement concrete, it is known as the reinforced cement concrete or R.C.C. construction.
(ii) Mosaic Tiles:
The pre-cast concrete tiles with marble chips at top surface are known as the mosaic tiles. They are available in different shades and are widely adopted at present.
(iii) Terrazzo:
This is a mixture of marble chips and cement. It is used for bath rooms, residential buildings, temples, etc.
(3) Advantages of Artificial Stones:
Following are the advantages of artificial stones:
(i) The cavities may be kept in the artificial stones. These cavities are used to convey pipes, electric wires, etc.
(ii) The grooves can be kept in an artificial stone, while it is being cast. These grooves are useful for fixing various fittings.
(iii) It can be cast in desired shape to suit the architectural requirements.
(iv) It can be made in a single piece and hence the trouble of getting large blocks of stones for lintels, beams, etc. is avoided.
(v) It can be made stronger than the natural stone.
(vi) It is cheap and economical as stones of smaller sizes are profitably consumed.
(vii) It is equally good in resisting deterioration and disintegration caused by various atmospheric agencies like rain, frost, etc.
(viii) It is more durable than the natural stone.
(ix) The natural bed is absent in artificial stones and hence the question of taking precautions with regard to the natural bed of stones does not arise.