TIMBER AND INDUSTRIAL TIMBER PRODUCTS
Timber is a product of wood from felled trees suitable for construction purposes. Timber is one of the oldest and natural civil engineering construction materials. Three terms pertinent to timber one of interest. They are:
- Converted timber – This is the timber that is sawn and cut into required sizes for commercial use.
- Rough timber – This is the timber that is obtained after felling a tree.
- Standing timber – This is the timber that is in a living stage.
Wood, as a building material, has a number of valuable properties, viz., relatively high strength, low bulk density, low heat conductivity and amenability to mechanical working.
Despite its remarkable uses, wood has several shortcomings, viz., different strengths because of anisotropy, susceptibility to decay and inflammability.
Properties of timber and timber-based products as a building material are discussed in the subsequent sections.
10.2.1 Basic Structure of a Tree
A tree consists of a trunk, a crown and roots. The functions of the roots are to implant the tree in the soil, absorb moisture and mineral substances it contains and in turn supply them to the trunk. The trunk supports the crown and serves to provide water and nutrients from the roots to the leaves through branches and from the leaves back to the roots.
10.2.2 Classification of Trees
According to the manner of growth, trees may be broadly classified as Endogenous trees and Exogenous trees.
1. Endogenous Trees
These trees grow by the formation of layers. In such trees new wood crosses and penetrates the fibres of the wood previously formed, e.g., bamboo, palmyrah, coconut, etc. Such trees are not useful for engineering works.
2. Exogenous Trees
These trees grow outwards by the addition of rings of young wood. A cross-section of these trees shows distinct approximate concentric rings called annular rings. Timber from such trees is mainly used in engineering works.
They are further classified into two types, viz., softwood and hardwood.
Softwoods have needle-like leaves and are also known as conifers as they bear cone-shaped fruits. Trees under this group are pines, deodar, spruces, etc.
- The qualities of softwood are:
- Light in weight and colour
- Has straight fibres and is resinous
- Has distinct annular rings
- Is weak and can split easily
Hardwoods are mostly of broad-leafed trees. They are deciduous trees like teak, sap, etc.
- Qualities of hardwood are:
- Heavy in weight and dark in colour
- Close-grained and non-resinous
- No distinct annular rings
- Strong and durable
10.2.3 Structure of Exogenous Trees
A cross-section of an exogenous tree is shown in Fig. 10.1. It shows almost distinct concentric rings called annual rings. Pith is the innermost central portion consisting of cellular tissues. The inner annular area surrounding the pith is called heartwood. Heartwood is the one which is used for all engineering works. The portion between the cambium layer and the heartwood is called the sapwood, which can not be of any use for engineering purposes since it contains large quantities of water. A very small amount of cells that grow in the horizontal direction occurs as groups or bundles of cells known as medullary rays.
Figure 10.1 Typical cross-section of wood
Freshly cut trees contain large quantities of moisture. If the percentage of moisture is more than about 20%, then the cut trees are subjected to different types of fungus attacks and attacks by insects. Hence, the amount of moisture has to be reduced.
10.2.4 Growth of a Tree
A tree grows through the tops of branches. Just within the inner bark, the tree becomes thicker by the formation of cells. A young tree grows taller and thicker every year. The growth is rapid during spring and slower during late summer and early fall. In young trees, hollow cells are alive. During the growing season the cells are filled with sap, which circulates through each fibre by a series of openings and connecting channels. With time, the older channels become partly clogged and inactive and more mature tissues combine to form the heavier and stronger heartwood. Active growth takes place only in the cells of the inner bark. As long as the layer of cells just within the outer bark is healthy, growth continues. A few annual rings next to the inner bark are less mature, which is known as sapwood. Heartwood is more resistant to decay than sapwood.
10.2.5 Felling of Trees
The process of a cutting down or knocking down or causing trees to fall to the ground is called the felling of a tree. Trees have to be felled only when they are very close to maturity or have just matured. Generally, trees attain maturity between 50 and 100 years. Trees cut before maturity may not be durable. On the other hand, trees cut after maturity may cause the valuable heartwood to decay. The best time for felling trees is mid-summer or mid-winter when the sap quantity is minimum.
10.3 CLASSIFICATION OF TIMBER
As per Indian Standards (IS: 4021–1967), it is preferred that timber used for building construction purposes is of one of the following four types:
- Hardwood like teakwood for permanent structures.
- Softwood like deodar wood for permanent structures.
- Hardwood like sal wood other than teakwood for permanent structures.
- Softwood other than deodar wood used for any temporary structures.
10.3.1 Classification of Teak
Teakwood is classified into three grades, viz., superior, first and second depending on the defects. The classification of teak based on the permissible defects, viz., cross grain, knots, pith, pockets or streaks, sapwood, pin holes and worm holes is presented in Table 10.1.
Table 10.1 Permissible defects for the classification of teak
Source: IS: 4021–1967.
10.3.2 Classification of Deodar
Only teak has been divided into three classes, whereas others are classified into two classes only, viz., first class and second class.
1. First-class Deodar
No individual ‘hard and sound knot’ shall be more than 25 mm in diameter. The total area of live knots shall not exceed 1% of the area of the piece. The wood should be free of sapwood.
2. Second-class Deodar
No individual ‘hard and sound knot’ shall be more than 40 mm in diameter. The total area of all the knots in the piece should not exceed 1.5% of the area of the piece. The wood should be free from sapwood. However, traces of sapwood are allowed.
10.3.3 Classification of Other Woods
Sal, another hardwood, grown in the sub-Himalayan regions and in Madhya Pradesh, is heavier than teak, which has to meet the requirements of first-class deodar. Timbers of Mango, Benteak, etc., are categorized as second-class.
10.4 CHARACTERISTICS OF GOOD TIMBER
Characteristics of good timber are given below in the order of decreasing significance:
The timber should be strong enough to effectively act as structural members, such as joists, beams, rafters, etc. Further, it should be in a position to sustain loads applied gradually or suddenly to it. Preferably, the strength should be adequate in direct and transverse directions.
The timber should be hard so that it is in a position to offer resistance when it is being penetrated by another body. As such hardness is imparted to the wood by chemicals present in the heartwood and the density of wood.
As timber is a bad conductor of heat, it should be dense enough so that it resists fire. A good timber should not contain resins and other inflammable oils since they accelerate the action of fire.
It should possess a uniform structure. This is ensured when there is firm adhesion of fibres and compact medullary rays. Further, the annual rings should be regular and closely located.
This is the property of a material which returns to its original shape when the load causing the deformation is removed. Timber should possess this property, as it is essential when timber is used for bows, carriage shafts, etc.
Another important quality required for timber is it should to be durable. It should have a long life. It should be capable of resisting the actions of fungi, insects, chemicals, physical agencies and mechanical agencies during its lifespan.
It should be free from defects such as shakes, flaws, dead knots or blemishes of any kind.
Quality timber should be of heavy weight and uniform along the length under consideration.
Good timber should possess straight and close fibres, which signify uniformity in the timber.
The timber should be easily workable, i.e., it should not clog the teeth of the saw. It should be capable of being easily planed. A good timber when planed shows a firm and bright appearance with a silky lustre.
The colour of quality timber should be uniform and of dark colour. Light-coloured wood usually indicates timber of low strength.
Good timber should produce a clear ringing sound when struck. Decayed timber produces a dull and heavy sound.
10.5 DEFECTS IN TIMBER
Different kinds of defects are found in timber. Generally, they are caused by the nature of the soil on which the tree is grown, the fertility it gained during growth and changes occurred in the environment during its growth. The defects to be generally looked into while selecting timber for woodwork are discussed below.
These are the sections of the branches of the tree present on the surface of the wood. This appears in the form of hard and dark pieces. Knots are the spots of weaknesses in timber, particularly when the timber has to bear a compressive force (Fig. 10.2). When the diameter of the knot is less than 6 mm, it is called a nail knot; when it is 6–20 mm, it is called a small knot; when it is 20–40 mm, it is called a medium knot; and when it is more than 40 mm, it is called a large knot. Tight knots are those which are joined to the wood securely. A knot is said to be a live knot or a sound knot when the formation of the knot is free from decay and other defects, and is also intact with the surrounding wood. A knot that is not held firmly in the wood is called a dead knot or a loose knot. Timber containing a large number of knots should be rejected as they are weak in strength and poor in appearance.
Figure 10.2 A typical knot
These are the cracks found in felled trees, which are formed due to some reason or the other. Different kinds of shakes are identified and are presented in Fig. 10.3.
Heart shakes are those which split or crack with more width at the centre and diminishes towards the circumference. This type of crack occurs in over-matured trees. Star shakes are radical splits or cracks wider at the circumference, which reduce towards the centre of the tree. These are caused mostly due to severe frost or more heat. These cracks are confined to sapwood only. Cup shakes are presumably formed by the rupture of the tissues in a circular direction across the cross-section of log, usually along the annular rings. Radial shakes are similar to star shakes, but they are thin, irregular and numerous. They are caused when there is a delay in sawing after felling. They usually radiate to a small distance from the bark towards the centre.
Figure 10.3 Types of shakes in timber
10.5.3 Twisted Fibres
These are also known as twisted grains or wandering hearts. They are caused by the twisting of young trees by fast-blowing winds. The fibres of the wood are twisted in one direction only which causes problems while sawing. However, they may be used as poles or posts without any planing (Fig. 10.4).
Figure 10.4 Typical twisted fibres and upset timbers
It is also called as rupture. This is caused due to some injury during the growth of the tree. It may be caused due to heavy winds and due to faulty felling of the tree.
10.5.5 Rind Galls
Rind means bark, and gall represents abnormal growth. Thus, rind galls are peculiar curved swellings formed on the body of a tree. These are caused due to the growth of layers over the injuries left over after improper felling of trees. The timber taken from this part is very weak and is not durable (Fig. 10.5).
Figure 10.5 Rind galls
10.5.6 Wind Cracks
The exterior surface of a tree shrinks when it is exposed to atmospheric agencies. Such a shrinkage results in cracks known as wind cracks (Fig. 10.6).
Figure 10.6 Typical wind cracks of timber
10.5.7 Sloping Grains
The cells in living trees do not always grow perfectly vertical or straight and parallel to the length of the truck but in a sloping manner. Such an action causes the grains to slope when the timber is sawn parallel to the pitch. Such sloping grains are considered to be a defect.
10.5.8 Presence of Sapwood
As discussed earlier sapwood is less durable than heartwood and hence its presence should be avoided. The presence of sapwood can be identified by the colour, which is much lighter than that of heartwood. Further, it does not take good polish as compared to heartwood.
Apart from the natural causes discussed above there may be some defects that occur in the process of seasoning, which is also to be considered while selecting timber for building construction works.
After felling trees, the branches are removed and the trunk is cut into logs. Conversion is the process of cutting and sawing logs into suitable sections of timber. In earlier days, it was done manually using saws. Now-a-days, cutting is done by band and circular saws run by machines.
Sawing is done by four methods, viz., ordinary sawing, quarter sawing, tangential sawing, or radial sawing.
10.6.1 Ordinary Sawing
In this method of sawing the log is simply sawed along the diameter. This is the quickest and commonly adopted method. This approach is more economical as wastage of useful timber is minimum. Each cut plank has an outer portion of sapwood and an inner portion of heartwood. But there will be differential shrinkage and warping.
10.6.2 Quarter Sawing
The disadvantage mentioned in ordinary sawing is to some extent avoided in quarter sawing. This method of sawing produces fine timber when the wood has no distinct medullary rays. However, timber cut by this method has a tendency to get bent in the transverse direction (Fig. 10.7(a)).
Figure 10.7 Ordinary and quarter sawing
10.6.3 Tangential Sawing
It is also called plain sawing or flat-grained sawing. This is done tangential to the annular rings. This method produces planks that are susceptible to warping as the medullary rays that give strength to the longitudinal fibres are cut. It is difficult to polish planks evenly (Fig. 10.8).
Figure 10.8 Tangential and radial sawing
10.6.4 Radial or Rift Sawing
This is the method adopted for sawing hardwood. This method of sawing produces planks, which do not shrink much. Because of the interplay of grains, it gives a decorative finish. However, maximum wastage occurs by this method of sawing.
Trees felled recently will contain large amounts of sap and moisture. Seasoning is the process of removing all the sap and moisture content under controlled conditions to avoid any splitting and distortion in the wood.
10.7.1 Objectives of Seasoning
Although the primary aim is to remove sap and moisture, the other objectives of seasoning are as follows:
- As most causes of decay and other problems are more or less related to moisture, the main objective is to remove moisture thereby increasing the resisting power of timber.
- To impart hardness, stiffness and strength, which improve the adoptability of timber for many building purposes.
- To improve the resisting power of timber against electrical power and attack by fungi and insects.
- To make the timber easily workable.
- To totally remove the possibility of shrinking, warping and splitting.
- To maintain the size and shape of the components of the timber structure of articles that are expected to be unchanged during the life span.
- To make timber suitable for treating with paints, polishes, preservatives, etc.
- To make the timber suitable for effectively joining with the use of glues.
- To reduce the weight of the timber for easy handling and transportation to other places.
- To give high order of durability for more years after construction.
10.7.2 Methods of Seasoning
There are two broad approaches to seasoning, viz.,
- Natural seasoning
- Artificial seasoning
1. Natural Seasoning
There are two methods of natural seasoning, viz., air seasoning and water seasoning.
(i) Air Seasoning
It is also known as air drying. In this method timbers intended for seasoning are stacked in a dry place under cover about 30 cm above the floor level. Here the longitudinal and cross pieces are arranged one upon the other leaving space for free circulation of air (Fig. 10.9). It is better to provide a foundation to keep the timber clean off the ground. Wood seasoned by such a process can be used for carpentry work after two years.
Figure 10.9 Typical air seasoning of timber
(ii) Water Seasoning
This method of seasoning timber is to completely immerse the logs of wood in water soon after cutting. This is better done in a running stream of water with the longer ends of the logs being kept pointing upstream. By this arrangement the sap, sugar, gum, etc. are leached out of the logs and in turn replaced by water. After adequate soaking the logs are dried in an open place. Although it is a quick process, it reduces the durability of the timber.
2. Artificial Seasoning
Artificial seasoning may be done by kiln seasoning, chemical seasoning and electrical seasoning. By artificial seasoning the moisture content can be brought under control from 4% to 12%. This is a quick process.
(i) Kiln Seasoning
In this system, complete control of temperature and humidity is maintained with proper air circulation and ventilation system. The timber is kept inside an airtight chamber. As a first step fully saturated air at 35–38°C is circulated. Then the humidity is reduced slowly followed by an increase of temperature till the moisture in the timber is reduced to the degree of moisture required. Then the kiln is slowly cooled. This process takes about a fortnight during which the timber is kept on a trolley. The quality of wood obtained by this method is inferior to those seasoned by natural methods.
(ii) Chemical Seasoning
This method of seasoning is also known as salt seasoning. In this method the timber is immersed in salt solution. After a specific period it is taken out and seasoned in the ordinary way. In this method of seasoning the interior surface of the timber dries before the exterior dries out. Here the chances of formation of external cracks are reduced.
(iii) Electrical Seasoning
This method is based on the principle that heat is produced when poor conductors are placed in the field of high frequency. Here, an induction coil producing field of high frequency is set up. The timber is made to pass through the coil. Due to electric induction the moisture is removed instantly and the wood is dried quickly. By this process the moisture gets evaporated uniformly and results in superior quality of timber. Because of high cost this method is not generally recommended.
10.8 DECAY OF WOOD
Once the woods get deteriorated they lose their engineering properties and the wood is said to be decayed. Several reasons contribute to the decay of wood, which are explained below:
- Decay may be caused due to alternate dry and wet conditions.
- Formation of fungi, which is responsible for the development of various diseases in wood, viz., dry rot, wet rot, blue stain, sap stain, etc.
- Improper removal of sapwood from the wood.
- Keeping timber in contact with a damp wall, damp earth, etc.
- Improper and incomplete seasoning.
- Non-application of preservatives on the surface of seasoned timber.
- Using unseasoned wood after application of oily paint.
- While younger, timber would have been subjected to shocks and impacts.
- Improper storage while stacking the timber.
- Permitting insects such as beetles, marine borers, termites, etc., to affect the timber.
10.9 PRESERVATION OF TIMBER
To increase the life span of wood, they should be treated with chemicals–this process is known as the preservation of timber.
10.9.1 Requirements for a Good Preservative
A good preservative should fulfil the following requirements:
- It should be safe to the user.
- It should not have unpleasant smell.
- It should be non-inflammable.
- It should be capable of penetrating into the fibres.
- It should not corrode the metal pieces attached to the timber.
- It should allow other materials such as paints or varnishes to be applied on it.
- It should not be washed away by water.
- It should not get deteriorated by heat, light, etc.
- It should not reduce the strength of timber and warp the structure.
- It should be cheap and easily available in the market.
10.9.2 Types of Preservatives
There are three types of preservatives with different bases as prescribed in the Code (IS: 401–2001).
1. Oil Types
In this type coal tar creosote with or without admixtures of soluble oils is used.
2. Organic Solvent Types
These are toxic chemicals in non-aqueous solvents such as copper napthenate, zinc napthenate, benzene hexachloride, etc.
3. Water-base Types
These are toxic chemicals in water, such as zinc chloride, boric acid, copper chrome arsenic composition, etc.
10.9.3 Preservative Treatment of Timber
The method of preservative treatment to be used depends on the use of wood in construction. The general methods adopted are as follows:
In this method, the ends of the timber are burnt to about 1.5 cm and then quenched in water. This charred end act as a protective coat. This is particularly done to prevent dry rot and attacks by insects.
This is applying a coat of tar or tar mixed with pitch. It is generally applied to rough types of works such as ends of door and window frames that are to be fixed to the walls, timber fences, etc.
Creosote oil is one which has wood preservative oil in it. The timber to be treated is placed in cylinders and closed tightly. Creosote oil under a pressure of 9 bars and at a temperature of 50°C is pumped into the cylinder until the desired absorption is reached. By this treatment the timber is preserved against rot and attack by white ants.
4. Treatment Using Preservatives
Some of the methods where preservatives are directly used are as follows:
(i) Surface Application Method
Surface Application Method is also known as the brush and spray method. Solvents or emulsions are used for this purpose. The timber to be treated is brushed or sprayed. At least two coats should be applied. This method is adopted only when naturally resistant timbers are used.
(ii) Soaking Treatment
Soaking Treatment is also known as steeping. In this treatment the timber is submerged in the preservative solution till the required absorption is obtained. Here, a water-soluble type preservative is used to avoid excessive evaporation loss. This treatment is recommended for timbers containing a lot of sapwood and for light and medium density timbers.
(iii) Hot and Cold Treatment
Hot and Cold Treatment is one in which the timber is placed in a steel tank immersed in a preservative (creosote) and heated to about 85–90°C. Then the tank is cooled slowly after some time with the timber immersed in the solution. This treatment is adopted for timbers containing sapwood and easily treatable heartwood.
(iv) Pressure and Vacuum Treatment
Pressure and Vacuum Treatment is of two types, viz., full cell process and empty cell process.
In the full cell process, the timber to be treated is placed in a closed vessel and is subjected to a low vacuum for about an hour. During this process, a preheated preservative is introduced till the vessel is full. A pressure up to 14 kg/sq.cm is gradually applied and kept constant for several hours. The pressure is then reduced and the solution is drained out. Timbers for marine works, railway sleepers, etc. are treated by this process.
In the empty cell process the quantity of preservative required is less and hence cheaper than the full cell process. The timber to be treated is stacked in a cylinder. After closing the door, the timber in the cylinder is then subjected to an air pressure of 1.75–5 bars from half an hour to one hour. The pressure is maintained while the cylinder is filled with preservatives. When the cylinder is completely filled, a pressure of 5–12 bars is applied and this pressure is maintained till the required absorption has taken place. The cylinder is drained off and vacuum is applied to remove the preservative dripping from the timber. This method is used for preserving timber of mixed species and timber containing sapwood and heartwood.
5. Treatment by Diffusion
This treatment is carried out for timber which has moisture content of over 50%, i.e., for green timber, which is felled recently. Extremely soluble boron compounds are applied to the surface of the timber. The timber pieces are stacked and covered with an impermeable cover to prevent evaporation. After a period of 1–2 months, the boron diffuses into the wet timber and acts as a preservative.
10.10 TESTING OF TIMBER
Generally, wood for building construction is not tested in the laboratory. It is a practice to classify them by their species and by visually examining them for defects. However, the following tests are conducted for important works:
- Tensile Strength Test
- Compressive Strength test
- Shear Strength Test
- Bending Strength Test
- Moisture Content Test
10.10.1 Tensile Strength Test
The specimen used for tensile strength test is of size 5 cm × 5 cm × 20 cm long. The testing is done in a wood testing machine. The load may be applied either parallel to the grains or perpendicular to the grains. The tested result is compared with the standard value.
10.10.2 Compressive Strength Test
For this test also the specimen size is as that of tensile test. The load is applied parallel to the grains. The specimen shall not have a slope of grain more than 1 in 20 parallel to its longitudinal edges. The test result is compared with the standard value.
10.10.3 Shear Strength Test
The size of the specimen is 5 cm × 5 cm × 6.25 cm and notched in a corner so as to produce a failure on 5 cm × 5 cm surface in the radial or tangential surface as desired. Shear strength is determined only parallel to the grain. Shear strength is important when the timber is used as beams or slabs.
10.10.4 Bending Strength Test
The Bending Test is performed on specimens of size 5 cm × 5 cm × 7.5 cm. The slope of the grain should not be more than 1 in 20 parallel to the longitudinal edges. This test is performed to determine the Modulus of Rupture and Modulus of Elasticity of timber.
10.10.5 Moisture Content Test
The Moisture Content Test is an important test that has to be performed for all types of works. The presence of high moisture produces shrinkage of wood work, attack by organisms, causes warping, etc. Even blistering of paints occurs if the moisture content is high.
In this test, the weight of moist wood and the weight of oven dry wood are determined. Then
10.11 INDUSTRIAL TIMBER PRODUCTS
To meet the minor needs of wood works, natural wood can be made into many other industrial products by industrial processes for building construction. These industrial-made products are also called composite boards. The common industrial timber products are:
- Particle board or chip board
- Hard board
- Fibre board
- Block board
- Decorative laminates
Plywoods are boards which are prepared from thin layers of wood or veneers. Veneers are thin sheets of slices of wood of superior quality. The thickness of veneers varies from 0.4 to 6 mm.
To make plywood, round logs of 1.5–2.5 m lengths are first cut. They are then steamed and veneers are cut from these logs with a wide sharp knife. About three or more veneers in multiples of odd numbers are glued one over the other. Care is taken to place the direction of the grains in one sheet at right angles to the other. This arrangement is needed to provide adequate longitudinal and transverse strengths. The arranged veneers with adhesives in between are pressed under a pressure of 0.7–1.40 N/mm2 under hot or cold conditions (IS: 303–1989).
Three-ply plywood is made using only three plies. The thickness varies from 6 to 25 mm (Fig. 10.10). Multiply plywood is made using more than three plies but keeping the number of veneers used as odd in number.
Figure 10.10 Typical three-ply plywood
10.11.2 Particle Board or Chip Board
These boards are made of particles of wood embedded in synthetic resins, which are then pressed under heat. They are made by extrusion pressing or by pressing between parallel plates. In the extrusion pressing process, the particles are oriented in the direction perpendicular to the plane of the board, whereas in the other case the particles are oriented parallel to the plane of the board.
Particle board is heavier than solid wood. Further, it provides broad and stable panels of reasonable strength. They can be sawn just like regular wood. Particle boards are preferred for furniture works than for construction works.
10.11.3 Hard Board
Hard Board is made as per the Indian Standard (IS: 1658–1966). It is made out of wood pulp that is compressed to make sheets usually of 3-mm thickness. Its top surface is smooth and hard and the rear side is rough with patterns or cross lines.
As per the CPWD specification, they are classified as given in Table 10.2.
Table 10.2 CPWD specification of hard board
For door shutters only tempered hard board should be used.
10.11.4 Fibre Board
The technique adopted for the manufacture of fibre board is the combination of the process used is making particle board and hard board. In this case, wood chips are steamed to separate the fibres from each other. These fibres are blended with resin and wax. These are turned into sheets by pressing under controlled heat and pressure. The boards are available in thicknesses of 25–32 mm and as sheets of 2.44 m × 1.22 m size. They are suitable for flush doors, cabinets, etc.
10.11.5 Block Board
Block board is also known as batten-board or solid-core board. Batten board consists of a core solid block of thin wood sawn from natural wood. The central block is then glued between two or more outer veneers on either side (Fig. 10.11). Thicknesses are from about 19 mm upwards. They are suitable for door panels, partitions, table tops, etc.
Figure 10.11 Typical batten board and lamin board
Lamin board is similar to that of batten board with the difference being the core is made up of multiply veneers. The thickness of veneers is about 6 mm, and the total thickness of the board is about 50 mm.
Another type of block board called the metal-faced plywood is the one where the core is sandwiched between thin sheets of aluminium, steel, copper, etc. This is a rigid type of plywood (Fig. 10.12).
Figure 10.12 Metal-faced block board
10.11.6 Decorative Laminates
Laminates are the products made by bonding together two or more layers of materials. High-pressure decorative laminates are pasted on plywood sheets, which are used for large areas like cabinets, wide door shutters, tables, etc.
10.11.7 Advantages and Disadvantages of Processed Timber
- Unusable trees such as branches of good wood and timber of fast-growing tress cannot be used for regular purposes. Such timber can be used as processed timber.
- By conversion of expensive wood into thin veneers, large surface areas can be covered with less cost.
- Processed timber is better dimensionally stable and does not absorb moisture from the air with the exception of particle board.
- Properly made plywood and block board with better gluing properties are highly durable even in wet environment. Further, the durability depends on the grade of the adhesive used.
- Workability is better than that of regular wood.
- Fasteners can be fixed easily as that of wood.
Only processed timber can be used provided they are made out of reliable materials. However, doors and other wooden fittings should be restricted to wet areas like the bathroom.
- Timber is a product of wood from felled trees, which is suitable for construction purposes.
- Converted timber is that which is sawn and cut into required sizes for commercial use.
- Rough timber is that which is obtained after felling a tree.
- Standing timber is the timber which is in a living stage.
- A tree consists of a trunk, a crown and roots.
- Endogenous trees grow by the formation of layers.
- Exogenous trees grow outwards by the addition of rings of young wood.
- Softwoods have needle-like leaves and are also known as conifers as they bear cone-shaped fruits.
- Hardwoods are mostly of board-leaved trees.
- Pith is the innermost central portion consisting of cellular tissues.
- The inner annular area surrounding the pith is called the heartwood.
- The portion between the cambium layer and the heartwood is called the sapwood.
- A very small amount of cells that grow in the horizontal direction occurs as groups or bundles of cells known as medullary rays.
- Growth of a tree occurs through the growth at the tips of the branches.
- The process of cutting down or knocking down or causing the trees to fall to the ground is called felling of a tree.
- Defects in timber are knots, shakes, twisted fibres, upset, rind galls, wind cracks, sloping grains and the presence of sapwood.
- Timber is sawed by four methods, viz., ordinary sawing, quarter sawing, tangential sawing or radial sawing.
- Seasoning is the process of removing all the sap and moisture content under controlled conditions to avoid any splitting and distortion to the wood.
- Methods of seasoning are natural seasoning and artificial seasoning.
- Natural seasoning methods include air seasoning and water seasoning. Artificial seasoning methods comprise kiln seasoning, chemical seasoning and electrical seasoning.
- Types of preservatives are: Oil types, organic solvent type and water-based type.
- Preservative treatment of timber is done by charring, tarring, creosoting and by using preservatives. Treatment using preservatives are: surface application method, soaking treatment, hot and cold treatment, pressure and vacuum treatment and treatment by diffusion.
- The following tests are conducted on timber. Tensile Strength Test, Compressive Strength Test, Shear Strength Test, Bending Strength Test and Moisture Content Test.
- Industrial timber products are: plywood, particle board or chip board, hard board, fibre board, block board and decorative laminates.
- What is timber? Name three types of timbers.
- Explain the basic structure of a tree.
- How are trees classified?
- With a sketch explain the different parts of an exogenous tree.
- What are the factors that govern the growth of a tree?
- What do you understand by the felling of a tree?
- Name the types of timber used for building constructions as per the Indian Standards.
- How is teakwood classified? Explain.
- What are the characteristics of soft and hard timber?
- What is the difference between first- and second-class deodar?
- Enumerate the characteristics of good timber.
- Explain the defects in timber.
- Distinguish between dry rot and well rot in timber. What steps are taken in executing wood work to see that these defects do not occur.
- What are the methods of conversion of timber? Discuss their merits.
- Define seasoning. What are the objectives of seasoning?
- Explain briefly the methods of seasoning.
- Explain different causes for the decay of wood.
- What are the requirements of a good preservative?
- Name the types of preservatives.
- How is the preservative treatment of timber done?
- What are the different types of tests conducted to find the suitability of timber for building construction purposes?
- Explain different types of industrial timber products.
- Explain with neat sketches, the different forms of plywood.