Wednesday, May 10, 2017

Uses of Cement


Cement is a commonly used binding material in the construction. The cement is obtained by burning a mixture of calcarious (calcium) and argillaceous (clay) material at a very high temperature and then grinding the clinker so produced to a fine powder. It was first produced by a mason Joseph Aspdin in England in 1924. He patented it as portland cement.

read more about cement;


Types of Cement

In addition to ordinary portland cement there are many varieties of cement. Important varieties are briefly explained below;...

 White Cement; The cement when made free from colouring oxides of iron, maganese and chlorium results into white cement. In the manufacture of this cement, the oil fuel is used instead of coal for burning. White cement is used for the floor finishes, plastering, ornamental works etc. In swimming pools white cement is used to replace glazed tiles. It is used for fixing marbles and glazed tiles.

 Coloured Cement; The cements of desired colours are produced by intimately mixing pigments with ordinary cement. The chlorium oxide gives green colour. Cobalt produce blue colour. Iron oxide with different proportion produce brown, red or yellow colour. Addition of manganese dioxide gives black or brown coloured cement. These cements are used for giving finishing touches to floors, walls, window sills, roofs etc.

 Quick Setting Cement; Quick setting cement is produced by reducing the percentage of gypsum and adding a small amount of aluminium sulphate during the manufacture of cement. Finer grinding also adds to quick setting property. This cement starts setting within 5 minutes after adding water and becomes hard mass within 30 minutes. This cement is used to lay concrete under static or slowly running water.

 Rapid Hardening Cement; This cement can be produced by increasing lime content and burning at high temperature while manufacturing cement. Grinding to very fine is also necessary. Though the initial and final setting time of this cement is the same as that of portland cement, it gains strength in early days. This property helps in earlier removal of form works and speed in construction activity.

 Low Heat Cement; In mass concrete works like construction of dams, heat produced due to hydration of cement will not get dispersed easily. This may give rise to cracks. Hence in such constructions it is preferable to use low heat cement. This cement contains low percentage (5%) of tricalcium aluminate (C₃A) and higher percentage (46%) of dicalcium silicate (C₂S).

 Pozzulana Cement; Pozzulana is a volcanic power found in Italy. It can be processed from shales and certain types of clay also. In this cement pozzulana material is 10 to 30 per cent. It can resist action of sulphate. It releases less heat during setting. It imparts higher degree of water tightness. Its tensile strength is high but compressive strength is low. It is used for mass concrete works. It is also used in sewage line works.

 Expanding Cement; This cement expands as it sets. This property is achieved by adding expanding medium like sulpho aluminate and a stabilizing agent to ordinary cement. This is used for filling the cracks in concrete structures.

 High Alumina Cement; It is manufactured by calcining a mixture of lime and bauxite. It is more resistant to sulphate and acid attack. It develops almost full strength within 24 hours of adding water. It is used for under water works.

 Blast Furnace Cement; In the manufacture of pig iron, slag comes out as a waste product. By grinding clinkers of cement with about 60 to 65 per cent of slag, this cement is produced. The properties of this cement are more or less same as ordinary cement, but it is cheap, since it utilise waste product. This cement is durable but it gains the strength slowly and hence needs longer period of curing.

 Acid Resistant Cement; This cement is produced by adding acid resistant aggregated such as quartz, quartzite, sodium silicate or soluble glass. This cement has good resistance to action of acid and water. It is commonly used in the construction of chemical factories.

 Sulphate Resistant Cement; By keeping the percentage of tricalcium aluminate C3A below five per cent in ordinary cement this cement is produced. It is used in the construction of structures which are likely to be damaged by alkaline conditions. Examples of such structures are canals, culverts etc.

 Fly Ash Blended Cement; Fly ash is a byproduct in thermal stations. The particles of fly ash are very minute and they fly in the air, creating air pollution problems. Thermal power stations have to spend lot of money to arrest fly ash and dispose safely. It is found that one of the best way to dispose fly ash is to mix it with cement in controlled condition and derive some of the beneficiary effects on cement. Now-a-days cement factories produce the fly ash in their own thermal stations or borrow it from other thermal stations and further process it to make it suitable to blend with cement. 20 to 30% fly ash is used for blending.

Fly ash blended cements have superior quality of resistance to weathering action. The ultimate strength gained is the same as that with ordinary portland cement. However strength gained in the initial stage is slow. Birla plus, Birla star, A.C.C. Suraksha are some of the brand mame of blended cement.

Properties of Ordinary Portland Cement

Chemical properties; Portland cement consists of the following chemical compounds:

There may be small quantities of impurifies present such as calcium oxide (CaO) and magnesium oxide (MgO).

When water is added to cement, C3A is the first to react and cause initial set. It generates great amount of heat. C3S hydrates early and develops strength in the first 28 days. It also generates heat. C 2S is the next to hydrate. It hydrates slowly and is responsible for increase in ultimate strength. C4AF is comparatively inactive compound.

Physical properties; The following physical properties should be checked before selecting a portland cement for the civil engineering works. IS 269–1967 specifies the method of testing and prescribes the limits:


 Setting time


 Crushing strength

Fineness; It is measured in terms of percentage of weight retained after sieving the cement through 90 micron sieve or by surface area of cement in square centimeters per gramme of cement. According to IS code specification weight retained on the sieve should not be more than 10 per cent. In terms of specific surface should not be less than 2250 cm²/gm.

Setting time; A period of 30 minutes as minimum setting time for initial setting and a maximum period of 600 minutes as maximum setting time is specified by IS code, provided the tests are conducted as per the procedure prescribed by IS 269-1967.

Soundness; Once the concrete has hardened it is necessary to ensure that no volumetric changes takes place. The cement is said to be unsound, if it exhibits volumetric instability after hardening. IS code recommends test with Le Chatelier mould for testing this property. At the end of the test, the indicator of Le Chatelier mould should not expand by more than 10 mm.

Crushing strength; For this mortar cubes are made with standard sand and tested in compression testing machine as per the specification of IS code. The minimum strength specified is 16 N/mm² after 3 days and 22 N/mm² after 7 days of curing.

Physical Tests on Cement

Soundness Test; It is conducted by sieve analysis. 100 gms of cement is taken and sieved through IS sieve No. 9 for fifteen minutes. Residue on the sieve is weighed. This should not exceed 10 per cent by weight of sample taken.

Setting Time; Initial setting time and final setting time are the two important physical properties of cement. Initial setting time is the time taken by the cement from adding of water to the starting of losing its plasticity. Final setting time is the time lapsed from adding of the water to complete loss of plasticity. Vicat apparatus is used for finding the setting times [Ref. Fig. 1.5]. Vicat apparatus consists of a movable rod to which any one of the three needles shown in figure can be attached. An indicator is attached to the movable rod. A vicat mould is associated with this apparatus which is in the form of split cylinder.

Before finding initial and final setting time it is necessary to determine water to be added to get standard consistency. For this 300 gms of cement is mixed with about 30% water and cement paste prepared is filled in the mould which rests on non porous plate. The plunger is attached to the movable rod of vicat apparatus and gently lowered to touch the paste in the mould. Then the plunger is allowed to move freely. If the penetration is 5 mm to 7 mm from the bottom of the mould, then cement is having standard consistency. If not, experiment is repeated with different proportion of water fill water required for standard consistency is found. Then the tests for initial and final setting times can be carried out as explained below:

Initial Setting Time: 300 gms of cement is thoroughly mixed with 0.85 times the water for standard consistency and vicat mould is completely filled and top surface is levelled. 1 mm square needle is fixed to the rod and gently placed over the paste. Then it is freely allowed to penetrate. In the beginning the needle penetrates the paste completely. As time lapses the paste start losing its plasticity and offers resistance to penetration. When needle can penetrate up to 5 to 7 mm above bottom of the paste experiment is stopped and time lapsed between the addition of water and end if the experiment is noted as initial setting time.

Final Setting Time. The square needle is replaced with annular collar. Experiment is continued by allowing this needle to freely move after gently touching the surface of the paste. Time lapsed between the addition of water and the mark of needle but not of annular ring is found on the paste. This time is noted as final setting time.

Soundness Test; This test is conducted to find free lime in cement, which is not desirable. Le Chatelier apparatus shown in Fig. 1.6 is used for conducting this test. It consists of a split brass mould of diameter 30 mm and height 30 mm. On either side of the split, there are two indicators, with pointed ends. The ends of indicators are 165 mm from the centre of the mould.
Le Chatelier’s
Le Chatelier’s apparatus

Properly oiled Le Chatelier mould is placed on a glass plate and is filled completely with a cement paste having 0.78 times the water required for standard consistency. It is then covered with another glass plate and a small weight is placed over it. Then the whole assembly is kept under water for 24 hours. The temperature of water should be between 24°C and 50°C. Note the distance between the indicator. Then place the mould again in the water and heat the assembly such that water reaches the boiling point in 30 minutes. Boil the water for one hour. The mould is removed from water and allowed to cool. The distance between the two pointers is measured. The difference between the two readings indicate the expansion of the cement due to the presence of unburnt lime. This value should not exceed 10 mm.

Crushing Strength Test; For this 200 gm of cement is mixed with 600 gm of standard sand confirming to IS 650–1966. After mixing thoroughly in dry condition for a minute distilled potable

water p/4+ 3 percentage is added where P is the water required for the standard consistency. They are
mixed with trowel for 3 to 4 minutes to get uniform mixture. The mix is placed in a cube mould of 70.6 mm size (Area 5000 mm²) kept on a steel plate and prodded with 25 mm standard steel rod 20 times within 8 seconds. Then the mould is placed on a standard vibrating table that vibrates at a speed of 12000 ± 400 vibration per minute. A hopper is secured at the top and the remaining mortar is filled. The mould is vibrated for two minutes and hopper removed. The top is finished with a knife or with a trowel and levelled. After 24 ± 1 hour mould is removed and cube is placed under clean water for curing.

After specified period cubes are tested in compression testing machine, keeping the specimen on its level edges. Average of three cubes is reported as crushing strength. The compressive strength at the end of 3 days should not be less than 11.5 N/mm² and that at the end of 7 days not less than 17.5 N/mm².

Uses of Cement

 Cement is used widely for the construction of various structures. Some of them are listed below:

 Cement slurry is used for filling cracks in concrete structures.

 Cement mortar is used for masonry work, plastering and pointing.

 Cement concrete is used for the construction of various structures like buildings, bridges. water tanks, tunnels, docks, harhours etc.

 Cement is used to manufacture lamp posts, telephone posts, railway sleepers, piles etc.

 For manufacturing cement pipes, garden seats, dust bins, flower pots etc. cement is commonly used.

 It is useful for the construction of roads, footpaths, courts for various sports etc.

Monday, April 17, 2017

Types of Lime Used in Construction

Types of Lime Used in Construction


It is an important binding material used in building construction. Lime has been used as the material of construction from ancient time. When it is mixed with sand it provides lime mortar and when mixed with sand and coarse aggregate, it forms lime concrete.
Types of Lime Used in Construction

Types of Limes and their Properties

The limes are classified as fat lime, hydraulic lime and poor lime:

Fat lime:

It is composed of 95 percentage of calcium oxide. When water is added, it slakes
vigorously and its volume increases to 2 to 2 ¹/₂ times. It is white in colour. Its properties are:

 hardens slowly
 has high degree of plasticity
 sets slowly in the presence of air
 white in colour
 slakes vigorously

Hydraulic lime:

It contains clay and ferrous oxide. Depending upon the percentage of clay present, the hydraulic lime is divided into the following three types:

Feebly hydraulic lime (5 to 10% clay content)
Moderately hydraulic lime (11 to 20% clay content)
Eminently hydraulic lime (21 to 30% clay content)

The properties of hydraulic limes are:

Sets under water 
Colour is not perfectly white
Forms a thin paste with water and do not dissolve in water.
Its binding property improves if its fine powder is mixed with sand and kept in the form of heap for a week, before using.

Poor lime:

It contains more than 30% clay. Its colour is muddy. It has poor binding property. The mortar made with such lime is used for inferior works.

IS 712-1973 classifies lime as class A, B, C, D and E.

 Class A Lime: It is predominently hydraulic lime. It is normally supplied as hydrated lime and is commonly used for structural works.

 Class B Lime: It contains both hydraulic lime and fat lime. It is supplied as hydrated lime or as quick lime. It is used for making mortar for masonry works.

 Class C Lime: It is predominently fat lime, supplied both as quick lime and fat lime. It is used for finishing coat in plastering and for white washing.

 Class D Lime: This lime contains large quantity of magnesium oxide and is similar to fat lime. This is also commonly used for white washing and for finishing coat in plastering.

✖ Class E Lime: It is an impure lime stone, known as kankar. It is available in modular and block form. It is supplied as hydrated lime. It is commonly used for masonry mortar.

Tests on Limestones

The following practical tests are made on limestones to determine their suitability:

 Physical tests
 Heat test
✒ Chemical test
✒ Ball test

 Physical tests ; Pure limestone is white in colour. Hydraulic limestones are bluish grey, brown or are having dark colours. The hydraulic lime gives out earthy smell. They are having clayey taste. The presence of lumps give indication of quick lime and unburnt lime stones.

 Heat test ; A piece of dry stone weighing W1 is heated in an open fire for few hours. If weight of sample after cooling is W2, the loss of weight is W2 – W1. The loss of weight indicates the amount of carbon dioxide. From this the amount of calcium carbonate in limestone can be worked out.

 Chemical test ; A teaspoon full of lime is placed in a test tube and dilute hydrochloric acid is poured in it. The content is stirred and the test tube is kept in the stand for 24 hours. Vigourous effervescence and less residue indicates pure limestone. If effervescence is less and residue is more it indicates impure limestone.

If thick gel is formed and after test tube is held upside down it is possible to identify class of lime as indicated below:

Class A lime, if gel do not flow.
Class B lime, if gel tends to flow down.
Class C lime, if there is no gel formation.

 Ball test ; This test is conducted to identify whether the lime belongs to class C or to class B. By adding sufficient water about 40 mm size lime balls are made and they are left undisturbed for six hours. Then the balls are placed in a basin of water. If within minutes slow expansion and slow disintegration starts it indicates class C lime. If there is little or no expansion, but only cracks appear it belongs to class B lime.

Uses of Lime

The following are the uses of lime in civil works:


 For white washing.
 For making mortar for masonry works and plastering.
 To produce lime sand bricks.
 For soil stabilization.
 As a refractory material for lining open hearth furnaces.
 For making cement. 
Types of Lime Used in Construction

Waiting For Next Lesson, Next Lesson about CEMENT...

Saturday, April 15, 2017

Bricks and Types of Bricks

 Bricks and Types of Bricks

Brick is obtained by moulding good clay into a block, which is dried and then burnt. This is the oldest building block to replace stone. Manufacture of brick started with hand moulding, sun drying and burning in clamps. A considerable amount of technological development has taken place with better knowledge about to properties of raw materials, better machinaries and improved techniques of moulding drying and burning.

The size of the bricks are of 90 mm x 90 mm x 90 mm and 190 mm x 90 mm x 40 mm. With mortar joints, the size of these bricks are taken as 200 mm x 100 mm x 100 mm and 200 mm x 100 mm x 50 mm. However the old  size of 8 ³/₄" × 4 ¹/₂" × 2 ⁵/₈"  giving a masonary Size of 9" x 4 ¹/₂ 9" x 4 ¹/₂" x 3" is still Commonly used in india. 

 Types of Bricks

Bricks may be broadly classified as:

1. Fire bricks
2. Paving bricks
3. Building bricks
4. Special bricks

1. Fire bricks ; These bricks are specially made to withstand furnace temperature. Silica bricks belong to this category

2. Paving bricks ; These are vitrified bricks and are used as pavers.

3. Building bricks ; These bricks are used for the construction of walls.

4. Special bricks ; These bricks are different from the commonly used building bricks with respect to their shape and the purpose for which they are made. Some of such bricks are listed below:

⒜ Specially shaped bricks
⒝ Facing bricks
⒞ Perforated building bricks
⒟ Burnt clay hollow bricks
⒠ Sewer bricks
⒡ Acid resistant bricks

⒜ Specially shaped bricks ; Bricks of special shapes are manufactured to meet the requirements of different situations. Some of them are shown in Below image.

 Facing bricks ; These bricks are used in the outer face of masonry. Once these bricks are provided, plastering is not required. The standard size of these bricks are 190 × 90 × 90 mm or 190 × 90 × 40 mm.

⒞ Perforated building bricks ; These bricks are manufactured with area of perforation of 30 to 45 per cent. The area of each perforation should not exceed 500 mm². The perforation should be uniformly distributed over the surface. They are manufactured in the size 190 × 190 × 90 mm and 290 × 90 × 90 mm.

⒟ Burnt clay hollow bricks ; Below image shows a burnt clay hollow brick. They are light in weight. They are used for the construction of partition walls. They provide good thermal Insulation to buildings. They are manufactured in the sizes 190 × 190 × 90 mm, 290 × 90 × 90 mm and 290 × 140 × 90 mm. The thickness of any shell should not be less than 11 mm and that of any web not less than 8 mm.

⒠ Sewer bricks ; These bricks are used for the construction of sewage lines. They are manufactured from surface clay, fire clay shale or with the combination of these. They are manufactured in the sizes 190 × 90 × 90 mm and 190 × 90 × 40 mm. The average strength of these bricks should be a minimum of 17.5 N/mm² . The water absorption should not be more than 10 per cent.

⒡ Acid resistant bricks ; These bricks are used for floorings likely to be subjected to acid attacks, lining of chambers in chemical plants, lining of sewers carrying industrial wastes etc. These bricks are made of clay or shale of suitable composition with low lime and iron content, flint or sand and vitrified at high temperature in a ceramic kiln.

 Properties of Bricks

The following are the required properties of good bricks:

Color ; Color should be uniform and bright.

Shape ; Bricks should have plane faces. They should have sharp and true right angled corners.

Size ; Bricks should be of standard sizes as prescribed by codes.

Texure ; They should possess fine, dense and uniform texture. They should not possess fissures, cavities, loose grit and unburnt lime.

Soundness ; When struck with hammer or with another brick, it should produce metallic sound.

Hardness ; Finger scratching should not produce any impression on the brick.

Strength ; Crushing strength of brick should not be less than 3.5 N/mm². A field test for strength is that when dropped from a height of 0.9 m to 1.0 mm on a hard ground, the brick should not break into pieces.

Water Absorption ; After immercing the brick in water for 24 hours, water absorption should not be more than 20 per cent by weight. For class-I works this limit is 15 per cent.

Efflorescence ; Bricks should not show white patches when soaked in water for 24 hours and then allowed to dry in shade. White patches are due to the presence of sulphate of calcium, magnesium and potassium. They keep the masonry permanently in damp and wet conditions.

Thermal Conductivity ; Bricks should have low thermal conductivity, so that buildings built with them are cool in summer and warm in winter.

Sound Insulation ; Heavier bricks are poor insulators of sound while light weight and hollow bricks provide good sound insulation.

Fire Resistance ; Fire resistance of bricks is usually good. In fact bricks are used to encase steel columns to protect them from fire.

 Tests on Bricks

The following laboratory tests may be conducted on the bricks to find their suitability:

Crushing strength
Shape and size

Crushing strength ; The brick specimen are immersed in water for 24 hours. The frog of the brick is filled flush with 1:3 cement mortar and the specimen is stored in damp jute bag for 24 hours and then immersed in clean water for 24 hours. The specimen is placed in compression testing machine with 6 mm plywood on top and bottom of it to get uniform load on the specimen. Then load is applied axially at a uniform rate of 14 N/mm². The crushing load is noted. Then the crushing strength is the ratio of crushing load to the area of brick loaded. Average of five specimen is taken as the crushing strength.

Absorption Test ; Brick specimen are weighed dry. Then they are immersed in water for a period of 24 hours. The specimen are taken out and wiped with cloth. The weight of each specimen in wet condition is determined. The difference in weight indicate the water absorbed. Then the percentage absorption is the ratio of water absorbed to dry weight multiplied by 100. The average of five specimen is taken. This value should not exceed 20 per cent.

Shape and size ; Bricks should be of standard size and edges should be truely rectangular with sharp edges. To check it, 20 bricks are selected at random and they are stacked along the length, along the width and then along the height. For the standard bricks of size 190 mm × 90 mm × 90 mm. IS code permits the following limits:

Lengthwise: 3680 to 3920 mm
Widthwise: 1740 to 1860 mm
Heightwise: 1740 to 1860 mm

The following field tests help in acertaining the good quality bricks:

uniformity in size
uniformity in colour
hardness test
sound test
strength test

uniformity in size ; A good brick should have rectangular plane surface and uniform in size. This check is made in the field by observation.

uniformity in colour ; A good brick will be having uniform colour throughout. This observation may be made before purchasing the brick.

structure ; A few bricks may be broken in the field and their cross-section observed. The section should be homogeneous, compact and free from defects such as holes and lumps.

hardness test ; For this a simple field test is scratch the brick with nail. If no impression is marked on the surface, the brick is sufficiently hard

sound test ; If two bricks are struck with each other they should produce clear ringing sound. The sound should not be dull.

Efflorescence ; The presence of alkalies in brick is not desirable because they form patches of gray powder by absorbing moisture. Hence to determine the presence of alkalies this test is performed as explained below:

Place the brick specimen in a glass dish containing water to a depth of 25 mm in a well ventilated room. After all the water is absorbed or evaporated again add water for a depth of 25 mm. After second evaporation observe the bricks for white/grey patches. The observation is reported as ‘nil’, ‘slight’, ‘moderate’, ‘heavy’ or serious to mean

Nil: No patches

Slight: 10% of area covered with deposits

Moderate: 10 to 50% area covered with deposit but unaccompanied by flaking of the surface.

Heavy: More than 50 per cent area covered with deposits but unaccompanied by flaking of the surface.

Serious: Heavy deposits of salt accompanied by flaking of the surface.

 Classification of Bricks Based on their Quality

The bricks used in construction are classified as:

⏎ First class bricks
 Second class bricks
 Third class bricks
 Fourth class bricks

First class bricks ; These bricks are of standard shape and size. They are burnt in kilns. They fulfill all desirable properties of bricks.

Second class bricks ; These bricks are ground moulded and burnt in kilns. The edges may not be sharp and uniform. The surface may be some what rough. Such bricks are commonly used for the construction of walls which are going to be plastered.

Third class bricks ; These bricks are ground moulded and burnt in clamps. Their edges are somewhat distorted. They produce dull sound when struck together. They are used for temporary and unimportant structures.

Fourth class bricks ; These are the over burnt bricks. They are dark in colour. The shape is irregular. They are used as aggregates for concrete in foundations, floors and roads.

 Uses of Bricks

 Bricks are used in the following civil works: 
 As building blocks.
 For lining of ovens, furnaces and chimneys.
 For protecting steel columns from fire.
 As aggregates in providing water proofing to R.C.C. roofs.
 For pavers for footpaths and cycle tracks.
 For lining sewer lines.

Bricks and Types of Bricks

Monday, April 3, 2017



Timber refers to wood used for construction works. In fact the word timber is derived from an old English word ‘Timbrian’ which means ‘to build’. A tree that yields good wood for construction is called 'Standing Timber'. After felling a tree, its branches are cut and its stem is roughly converted into pieces of suitable length, so that it can be transported to timber yard. This form of timber is known as rough timber. By sawing, rough timber is converted into various commercial sizes like planks, battens, posts, beams etc. Such form of timber is known as converted timber.

Timber was used as building material even by primitive man. Many ancient temples, palaces and bridges built with timber can be seen even today.

Classification of Timber

Various defects which are likely to occur in timber may be grouped into the following three:

1.) Due to natural forces.
2.) Due to defective seasoning and conversions.
3.) Due to attack by fungi and insects.

1.) Defects due to Natural Forces

The following defects are caused by natural forces:

✤ Knots
✤ Wind Cracks
✤ Shakes
✤ Upsets

When a tree grows, many of its branches fall and the stump of these branches in the trunk is covered. In the sawn pieces of timber the stump of fallen branches appear as knots. Knots are dark and hard pieces. Grains are distorted in this portion. Figure 1.9 shows some varieties of knots. If the knot is intact with surrounding wood, it is called live knot. If it is not held firmly it is dead knot.

 Wind Cracks
These are the cracks on the outside of a log due to the shrinkage of the exterior surface. They appear as shown in below picture.

✤ Shakes
The shakes are cracks in the timber which appear due to excessive heat, frost or twisting due to wind during the growth of a tree. Depending upon the shape and the positions shakes can be classified as star shake, cup shake, ring shakes and heart shakes.

✤ Upsets
Below picture shows a typical upset in a timber. This type of defect is due to excessive compression in the tree when it was young. Upset is an injury by crushing. This is also known as rupture.

2.) Defects due to Defective Seasoning and Conversion

If seasoning is not uniform, the converted timber may warp and twist in various directions. Sometimes honey combining and even cracks appear. This type of defects are more susceptible in case of kiln seasoning.

In the process of converting timber to commercial sizes and shapes the following types of defects are likely to airse: chip marks, torn grain etc.

3.) Defects due to Fungi and Insects Attack

Fungi are minute microscopic plant organism. They grow in wood if moisture content is more than 20°C and exposed to air. Due to fungi attack rotting of wood, takes place. Wood becomes weak and stains appear on it.

Beetles, marine borers and termites (white ants) are the insects which eat wood and weaken the timber. Some woods like teak have chemicals in their compositions and resist such attacks. Other woods are to be protected by chemical treatment.

Preservation of Timber

Preservation of timber means protecting timber from fungi and insects attack so that its life is increased. Timber is to be seasoned well before application of preservatives. The following are the widely used preservatives:

 Chemical salt

Hot coal tar is applied to timber with brush. The coating of tar protects the timber from the attack of fungi and insects. It is a cheapest way of protecting timber. Main disadvantage of this method of preservation is that appearance is not good after tar is applied it is not possible to apply other attractive paints. Hence tarring is made only for the unimportant structures like fence poles.

Two to three coats of oil paints are applied on clean surface of wood. The paint protects the timber from moisture. The paint is to be applied from time to time. Paint improves the appearance of the timber. Solignum paint is a special paint which protects the timber from the attack of termites.

 Chemical salt
These are the preservatives made by dissolving salts in water. The salts used are copper sulphate, masonry chloride, zinc chloride and sodium fluoride. After treating the timber with these chemical salt paints and varnishes can be applied to get good appearance.

✢ Creosote
Creosote oil is obtained by distillation of coal tar. The seasoned timber is kept in an air tight chamber and air is exhausted. Then creosote oil is pumped into the chamber at a pressure of
0.8 to 1.0 N/mm² at a temperature of 50°C. After 1 to 2 hours timber is taken
out of the chamber.

This preservative is developed by the Forest Research Institute, Dehradun. It consists of 1 part by weight of hydrated arsenic pentoxide (As₂O₅, 2 H₂O), 3 parts by weight of copper sulphate (CuSO₄⋅5 H₂O) and 4 parts by weight of potassium dichromate (K₂Cr₂O₇) or sodium dichromate (Na₂Cr₂O₇⋅2 H₂O). This preservative is available in powder form. By mixing six parts of this powder with 100 parts of water, the solution is prepared. The solution is then sprayed over the surface of timber. This treatment prevents attack from termites. The surface may be painted to get desired appearance.

Uses of Timber

 For heavy construction works like columns, trusses, piles

 For light construction works like doors, windows, flooring and roofing.

 For other permanent works like for railway sleepers, fencing poles, electric poles and gates.

 For temporary works in construction like scaffolding, centering, shoring and strutting, packing of materials.

 For decorative works like showcases and furnitures.

 For body works of buses, lorries, trains and boats

 For industrial uses like pulps (used in making papers), card boards, wall papers

For making sports goods and musical instruments.