1 Characteristics of Good Building Stones
Good building stones must possess the following qualities :
1. Appearance. The stones should be of uniform colour and from architectural point of view, these should match with the surroundings. Lighter colours are preferred to darker ones as the latter are less durable. The property of the appearance of stones is extremely important for the face work of buildings. The good stones should also be able to receive good polish.
2. Strength. The stones used in the construction of buildings are generally subjected to compressive stress. These should therefore, be able to withstand the compression without getting crushed due to the load of the structure. Though all stones used for the construction of ordinary buildings, possess workable strength in compression but the strength of stones used for the construction of heavy structures should always be tested before use. Closed grained and uniform
textured stones are generally good in compressive strength.
3. Structure. A good stone when broken in a direction other than that of cleavage .(if any) should not give a dull appearance. These should be either closed grained or crystalline and should show uniformity of texture. These should also be free from cavities and cracks. If the stones are obtained from sedimentary rocks, their stratification should not be visible to naked eye.
4. Hardness. A good stone when used in floors, pavements and aprons of bridges, should be able to resist the abrasive forces caused due to wear and friction. Hardness of stones can be tested by Mohr's scale of hardness in the laboratory whereas in field, it can be tested by knife scratching. Hard
stones do not show any mark of scratching.
5. Toughness. Good stones should also be tough enough to withstand stresses developed due to vibrations of machineries and moving loads over them. Stones used in the construction of roads should be hard as well as tough.
6. Heaviness. The stones used for the construction of dams, weirs, barrages, docks and harbours, should be of heavier varieties. In case of dams and roof coverings, lighter varieties of stones are preferred to. The specific gravity of good building stones should lie between 2.4 to 2.8.
7. Durability. Good stones should resist the action of the atmosphere such as wind, rain and temperature. The effect of atmospheric conditions on stones is generally known as weathering. Durability of stones largely depends upon their chemical composition and physical structure. A durable stone should have a compact and crystalline structure, free from pores.
8. Porosity and absorption. A good stone should not be porous. It should not absorb water when immersed. Porous stones are unsuitable for the construction work as rain water falling on their exposed surface get driven in the pores of stones by the prevailing winds. The rain water generally becomes acidic due to atmospheric acidic gases and this acidic water reacts with the constituents of
stones causing them to crumble. In higher region water in the pores when freezes, disintegrates the stones because of increased volume after freezing. Porous stones should be used in the construction of structures with care and at places which are not exposed to frost, rain or moisture.
9. Resistance to fire. Stones when exposed to fire should be able to resist temperature. To ensure this, stones should be free from minerals which are likely to decompose on heating such as calcium carbonate and iron oxide. Stones should not be composed of minerals having different coefficients of thermal expansion. Quartz expands at low temperature and argillaceous variety of
stones resist high temperature.
10. Dressing. The art of converting a natural stone into a definite shape, is known as dressing. Stones should therefore possess good dressing properties for carving and structural work in building constructions. Marble is a good example. It may be noted that a stone possessing good dressing properties on the other hand, is weak in strength and durability, and also its hardness is low,
11. Seasoning. A good stone should be free from quarry sap. To ensure this, the stones after quarrying and dressing should be left for a period of 6 to 12 months for proper seasoning, before using it in the construction work.
Operations involved in the Manufacture of bricks
1. Preparation of clay. Good brick earth, a mixture of pure clay and sand along with a small quantity of finely divided lime is first dug out, broken up, watered and kneaded well under feet till it becomes a homogeneous mass. The tempered earth is then covered up with mat pieces and allowed to dry gradually till it is just soft-enough for moulding. For manufacturing superior bricks, the clay
is generally prepared by pug mills.
2. Moulding. The well prepared clay is moulded in rectangular steel or wood moulds without top and bottom, their longer sides project a few centimeters to act as handles. Bricks are usually moulded on a block of wood having a projection 6 mm deep and same length and breadth as the inside dimensions of the mould. Moulding of bricks done on the stock board, is called table moulding.
3. Drying. The moulded bricks are then allowed to dry so that these are sufficiently hard to be handled. When the bricks become sufficiently hard, these are stacked. Eight or ten layers of bricks on edge with intervals of about one metre between them, are generally stacked.
4. Burning. Well dried bricks are burnt in clamps or kilns to attain desired crushing strength and also to impart red or yellowish colour.
Tests for the Acceptance of Bricks for Building Construction
(i) Dimensions and tolerances test
1. Take twenty bricks out of the given samples.
2. Remove loose particles of clay and small projections from the bricks.
3. Arrange them on a level surface in contact with each other and in a straight line.
4. Measure the overall length of the bricks having size 19 x 9 x 9 cm laid by means of a steel tape.
5. The dimensions of 20 bricks should be within the following limits :
Class Length Width Height
Class A 368 to 392 cm 174 to 186 cm 174 to 186 cm
Class B 350 to 410 cm 165 to 195 cm 165 to 195 cm
. (ii) Compressive strength test:-This test is performed to determine the crushing strength of bricks.
1, Take five bricks out of the sample at random.
2. Immerse the bricks in water at room temperature for 24 hours.
3, Take out the bricks from the water and wipe off surplus water from their surfaces.
4. Fill the frogs and all voids in the bed and face with cement mortar 1:1( 1 cement: 1 clean sand).
5. Store the bricks under damp gunny bags for 24 hours and there-after immerse them in water for 72 hours.
6. Take out the bricks from water, wipe off dry. Place the bricks with flat surfaces horizontal and mortar filled frog face upward between two or three thin ply sheets and centre them between the plates of a compression testing machine.
Apply the load at a uniform rate of 140 kg/cm² per minute till the brick fails.
8. Take the average value of the compressive strengths of the five bricks.
9. The compressive strength of a common brick should be 50 Kg/cm²
Water absorption test. This test is performed to determine water absorption of the bricks. If the water absorption capacity of a brick is more, its strength will be comparatively low.
1. Select five bricks at random out of the given sample.
2* Dry them in a ventilated oven at 105° to 110° till they attain practically constant weight.
3. Remove the bricks from the oven and cool them to room temperature.
4. Weigh the bricks in a balance. Let it be W₁ kg.
5. Immerse the five bricks in water completely at 27° ± 2C for 24 hours.
6. Remove one brick from water and wipe off its surfaces with a damp cloth.
7. Weight the brick within three minutes after its removal from water. Let its weight be W₂ kg.
8. Water absorption capacity = (W₂ - W₁)/W₁ X 100
9. Take the average value of the water absorption capacities of the five bricks.
10. For 1st Class bricks, the water absorption capacity should not be more than 20% by weight.
Efflorescence test. This test is performed to know the presence of any alkaline matter in the bricks.
1. Take five bricks at random from the given sample.
2. Place each brick on end in a dish containing distilled water ensuring depth immersion at least 2.5 cm.
3. Keep the dish in a ventilated room (Temp. 20° to 30°C) till the whole of distilled water in the dish evaporates.
4. Again pour 2.5 cm depth of distilled water in the dish and keep it till the whole of water gets evaporated.
Terra-cotta which is a baked clay or baked earth is a superior variety of clay products and is usually moulded in the same manner as bricks. It is made from a mixture of fine clay (60%), crushed pottery (20%), white sand (14%) and powdered glass (6%), with a quantity of desired colouring substance.
For making a porous and sand proof terra-cotta, either sawdust or ground cork may be mixed with clay before moulding. Organic particles burn away during the burning of the moulded and dried terra-cotta and thus leaving behind small pores. Terra-cotta is used for architectural and ornamental parts ofsuperior buildings as a substitute for stones. It is used as sound proof material and its hollow blocks prevent dampness in the structure.
Glazing Of White Ware Products
Surfaces of white ware products are generally glazed to improve their appearance and also to protect them from the action of atmosphere, sewage and strong chemical agents. For providing transparent glazing, self glazing is the most important method. In this method a solution of sodium chloride is thrown in the kiln when the product is well burnt at a temperature 1200° to 1300°C. Due to high temperature, the sodium chloride evaporates and combines with silica of soil to make soda silicate. Soda silicate again combines with alumina, lime or iron of the clay to form a thin transparent layer. Vapours of volatilised salt get into every pore of the product and thus make it impermeable.
Characteristics Of Good Timber
1. Hardness. It should be hard.
2. Strength* It should be able to resist heavy loads in structural members.
3. Toughness. It should be tough enough to resist shocks due to vibrations. It should not break in bending and should resist splitting. Timbers with narrow annual rings, are generally the strongest.
4. Elasticity. It should be elastic so as to regain its original shape after removal of loads. This property is very important for the timber used in sports goods.
5. Durability. It should be able to resist the attacks of fungi and worms and also atmospheric effects for a longer duration.
6. Defects. It should be from the heart of a sound tree and be free from sap, dead knots, shakes and other similar defects.
7. Fibres and Structure. It should have straight and closed fibres and compact medullary rays. It should give a clear ringing sound when struck. Dull heavy sound is an indication of internal decay. Its annual rings should be uniform in shape and colour.
8. Appearance and colour. Freshly cut surface should give sweet smell and present shining surface. It should have preferably dark colour, as light coloured timbers are generally weak in strength.
9. Shape and weight. It should retain its shape during the process ofseasoning. Heavy timbers are always stronger than light weight timbers.
10. Workability. It should be well seasoned and easily workable. Teeth of saw should not get clogged during the process of sawing. It should provide smoothened surface easily.
Structure of a timber
The cross-section of the trunk of a timber tree may be distinctly divided into four parts.
1. Pith, heart or medulla. Inner most part or core of the stem, which consists entirely of cellular tissues, is called pith.
2. Medullary sheath. The portion consisting of vascular tissues and which encloses the pith, is called medullary sheath. Medullary rays are vertical layers of cellular tissues and spider-like radial lines originating from the pith to the bark. Medullary rays bind the annual rings to one another. Large and distinct radial lines are called silver grains or flowers.
3. Annual rings. These consist of cellular tissues and woody fibres arranged in distinct concentric circles round the pith. Annual rings are generally formed in every year, due to the deposition ofsap below dark. Number of annual rings indicates the age of a tree in a tropical climate. Sap wood consists of outer annual rings. Heart wood consists of inner annual rings round the pith.
4. Bark or Cortex. It consists of cells of wood fibre and is the outermost cover or skin of the stem.
3. Annual rings
4. Pith or heart
5. Medullary sheath
6. Heart wood or duramen
7. Sap wood
8. Medullary rays.
Characteristic Difference s of Sap Wood and Heart Wood
1. Sap wood, (i) Sap wood is younger in age and
lighter in colour
(ii) It is easily attacked by insects.
(iii) Its annual rings are far apart.
(iv) It possesses less strength.
2. Heart wood, (i) Heart wood is older in age and darker in colour.
(ii) It is hard core of the stem and is not attacked by insects.
(iii) Its annual rings are nearer to each other.
(iv) It possesses more strength.
Preservation of Timber
A properly seasoned timber is most durable. If it is not seasoned properly, it is likely to be attacked by insects i.e. white ants, dry and wet rots. Timber should be used either fully dried in well ventilated positions or well immersed in water. In water the timber does not decay though it becomes soft and weak. In case timber is not seasoned before it is used, it should be preserved by the application of preservatives. In masonry construction, the timber should not be used in direct contact with limemortar.
Preservation of timber may be done by one of the following methods :
1. Charring. Lower ends of the timber posts before embedding in ground are generally charred to a depth of 15 cm and quenched in water, to prevent attack from dry rots and worms.
2.Tarring. Embedded portion of timber fence posts, ends of door and window frames, bettons and beams built in walls are usually tarred.
3. Painting. Painting the surface of timber members, protects it from moisture and thus prolongs its life. Paints possess excellent preservative properties and protect the timber against the attack of white ants. Paints are available in varieties of shade with different trade marks.
4. Creosoting. Creosote oil is a dark brown thick liquid. By applying creosotes to timber, chances of attacks of white ants and rots are reduced considerably. In this method well seasoned timber driedfor 24 hours, is kept in air tight chamber and air is exhausted. Creosote is then pumped in at a pressure of 9 kg/cm²
at a temperature of 50°C till it is fully saturated with oil. Creosoting is done for
railway sleepers, piles and transmission poles.
5. Wolmen Salt. A timber treated with wolmen salt which consists of creosote and sodium fluoride dissolved in water, is extremely fire resistant and free from fungi attacks. Zinc chloride, sodium fluoride, magnesium silco-fluoride and copper sulphate when applied to timber also help it from the attacks of fungi. On drying, such timbers are suitable for painting.
6. Ascu. A timber treated with Ascu powder developed by F.R.I. (Forest Research Institute) Dehra Dun is immune to the attacks of white ants and may be painted, varnished and polished.
7. Fire proofing of timber. A timber to some extent, may be made fire proof by soaking it in ammonium sulphate, ammonium chloride, ammonia phosphate, sodium arsenate or zinc chloride.
Defects in Timber
The following are the most common defects in timber.
1. Heart Shakes. [Fig(a)]. These are splits occurring in the centre of the tree and running from the pith towards the sapwood in the direction of medullary rays. In some timbers, these splits are hardly visible and in some timbers these are quite permanent. Heart shakes are caused due to shrinkage of the interior parts due to age. A heart shake straight across the trunk, is not a seriousdefect.
2. Star Shakes [Fig(b)]. These are splits which radiate either from the centre of timber or from the bark, running in the planes of medullary rays. Star shakes are mostly confined to sapwood and are caused due to severe frost and scorching heat of the sun.
3. Cup Shakes. [Fig(c)]. These are curved splits which separate the whole or part of one annual ring from another. These are caused due to unequal growth of timber.
4. Radial Shakes [Fig(d)]. These are similar to star shakes and occur in felled timbers when exposed to sun during seasoning. Radial shakes are generally irregular, fine and numerous. Many splits appear to start a few centimeters within the bark, run a short distance towards the centre, then following the course of an annual ring, approach the centre radially.
5. Rind-galls [Fig(e)]. These are typical enlarged swellings caused generally by the growth of layers over the wounds left after the branches have been cut off.
6. Rupture. These are caused due to fibres having been injured by crushing.
7. Twisted Fibres. The twisting of fibres is caused due to the action of prevalent wind twisting the young tree constantly in one direction.
8. Wind cracks. These are shakes or splits on the sides of a bark of timber due to the shrinkage of the exterior surface exposed to atmospheric influences.
9. Knots. These are the roots of small branches of the tree. These break the continuity of fibres. These are not much harmful if small, hard and rounds. Timber with large dead (loose) knots of many smaller ones, should be rejected as these do not provide specified strength.
10. Dead wood. It is deficient in strength and weight and is the result of trees being felled after maturity.
Seasoning of Timber And Its Necessity
Definition. The process of drying timber or removing moisture or sap from a freshly felled tree, is called seasoning of timber, A well seasoned timber may contain about 10 to 12 per cent moisture which is necessary for
proper retention of the shape and size of the articles manufactured from the timber. On the other hand if a timber is not properly seasoned before use, it is liable to shrink warp, crack, rot and decay. This is why properly seasoned timber need only be used for high class timber work. Necessity of seasoning a timber.
Seasoning of timber is done for the following purposes:
1. To reduce the weight of the timber for achieving economy in its transportation from the place of felling to the place of manufacturing the articles.
2. To minimise the tendency to shrink, split and warp in the manufactured wood work.
3. To increase the strength and durability of the timber and also to make the timber electrically resistant.
4. To improve the wood working qualities in timber for gluing, painting and polishing the surfaces of finished articles.
5. To enable to provide proper preservation treatment of the timber,
6. To make the timer free from the danger of being attacked by insects, fungus, etc.
7. To achieve good characteristics in timber,
1. Natural or air seasoning. A felled log if left on the ground for long is likely to be attacked by insects and fungi. It should therefore be converted by sawing into desired dimensions and stacked on well drained place in the shade. The main principle of seasoning is to remove the moisture from the timber. While stacking, it should be ensured that there is free circulation of fresh air around each piece.
For natural air seasoning a suitable concrete foundation, a few centimeters above the ground is provided to stack the logs under shade. Care should be taken not to expose the freshly converted timber stacked for seasoning to sun or to severe winds. To avoid the tendency of splitting of hard wood during seasoning, cleats are fixed and nailed to their ends. This method of seasoning is the best as it gives very strong and durable timber but it takes longer time. It generally takes more than six months for timber to season to moderate climates. Timber seasoned
by natural seasoning method, generally contains 18% of moisture.
2. Artificial or kiln seasoning. Seasoning of timber by this method is done in masonry chamber equipped with an arrangement for heating, controlling humidity and circulating the air in the kiln. Steam is generally used for heating and humidifying the air in the chamber. The timber in the chamber is stacked as for natural seasoning. In the beginning, the seasoning is started at a comparatively lower temperature and high humidity. Initially, moisture content in the timber is more and hence at higher temperature, the wood shrinks and cracks develop. As the moisture from the timber decreases, the temperature of the chamber is increased. At times, the sample pieces of wood are taken out of the seasoning chamber and their percentage of moisture content is checked.
As the timber dries, at the end of seasoning, the temperature of the air inside the chamber is raised fairly high and humidity is reduced. The seasoned timber is allowed to cool in the chamber within 20°C of the outside temperature before removal. Seasoning by this method generally takes four to five days under normal conditions.
3.Kiln seasoning is a quick method of seasoning timber to the desired moisture contents.
Advantages of kiln seasoning
1. Perfect control of drying
2. Economy of time
3. Moisture content may be reduced to desired level
4. Unlikely to be attacked by fungi and insects.
5. Wood receives paints well.
Disadvantages of kiln seasoning
1. It requires skilled supervision
2. Expensive in cost
3. Due to carelessness, the wood develops surface cracks,
warping and splitting
Constituent Parts of Paint and their Functions
The constituent parts of paints are the following :
1. Base. It is very finely grounded metallic oxide. It acts as a body of paint. Because of film of base, the paint becomes hard and resistive to weathering friction. The most commonly used bases in paints are : White lead, Lead sulphate, Sublimed lead, Red lead, Zinc oxide, and Titanium oxide.
2. Vehicle, The material used in paints to help it to spread the base over the surface is called vehicle. It acts as a binder between base and pigment and causes it to adhere to the surface to be painted. Vehicle is mixed with the bases to form a paste. Most commonly used vehicle, is Linseed Oil of the following four types i.e.
(i) Row Linseed Oil, (ii) Refined Linseed Oil. (iii) Pale boiled Linseed Oil. (iu) Wouble boiled Linseed Oil.
3. Colouring Pigments. The materials added to the paints to obtain desired final colour, is called colouring pigments. These are used to obtain the final colour of the paint different from that of the base. Depending upon the final colour of paints, the colouring pigments may be used. Such as lamp black, bone black, graphite, Indian red, chrome yellow, etc.
4. Thinner. The material used in paints to reduce its consistency, is called thinner. It enables the paint to be spread over the surface to be painted with the brush and to penetrate into the surface. Most commonly used thinner is turpentine oil which dries rapidly and helps to dry paint soon, Naptha and spirit are also sometimes used as thinner.
5. Drier. The material used in paints, to accelerate the action of drying is called drier. Paints need be dried soon to avoid the risk to catch dust and dirt. Most commonly used drier is Litharge whose use in finishing coat should be avoided, otherwise colour of paint may change due to change in atmospheric conditions.
6. Adulterants. The material which is used to reduce the cost of paint and also to reduce the weight and to increase its durability, is called Adulterant. Barium sulphate is widely used as adulterant because of its cheapness and its property not to react with paint. Calcium Carbonate, Magnesium Silicate and Silica are also used as adulterants.
1. Manufacture of Cement by Wet Process
For the manufacture of cement, following ingredients are required.
1. Lime stone 2. Clay 3. Coal 4. Gypsum.
Process. The cement is prepared by mixing 75% of limestone and 25% clay. Hard lime stone is powdered in a crusher. Clay is thoroughly mixed with water in a wash mill. Powdered lime stone and clay solution are then mixed and ground in a wet grinding mill to form a slurry having a moisture 32 to 40%, The slurry is stirred in a collecting basin, and is tested for its chemical composition as
(i) Take the slurry in tube and mix HCl to it.
(ii) * The mixture is heated till precipitation occurs.
(iii) The mixture is cooled to obtain a jelly like material.
if the jelly so formed is hard, it indicates that the proportion of the mix is not correct. Either clay or limestone is then added as required in collecting basin itself. This test is very important and must be carried out by an expert, because if proportion is not correct, the properties of resulting cement will adversely change. From the correcting basin, slurry is dumped to a storage basin, where it is
constantly stirred by mechanical process. From storage basin, the slurry is pumped to upper chamber of the rotary kiln, regularly.
Rotary kiln consists of an inclined cylinder supported on masonry chamber 15 metres apart. Its length varies from 90 to 120 metres and diameter varies from 3 to 3.5 metres. The diameter of the cylinder in burning zone, is comparatively larger than that of other zones. Slurry is admitted from the upper chamber of the kiln, to the higher portion of the cylinder which makes one revolution per minute and pulverised coal is entered from other end. When the slurry reaches the burning zone (temp. 1500° to 1600°C), CO2 gas is evolved after heating and the moisture
evaporates. Hot chamber is then cooled by blowing in cool air in the outlet pipe. To delay the setting time of the resulting cement, gypsum (3 to 4%) is added at this stage.
Klinker is then ground in ball mill and tube mill in which balls grind the klinkers to a very fine powder, called cement.