Timber Defects 2w63g
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UNIT 4 TIMBER
Timber
Structure 4.1
Introduction Objectives
4.2
Definition of Timber
4.3
Uses of Timber
4.4
Classification of Trees 4.4.1 4.4.2
Endogenous Trees Exogenous Trees
4.5
Growth of a Tree
4.6
Structure of a Tree 4.6.1 4.6.2
Macrostructure Microstructure
4.7
Characteristics of Good Timber
4.8
Defects in Timber
4.9
Decay of Timber
4.10 Felling of Trees 4.11 Seasoning of Timber 4.11.1 4.11.2
Objects of Seasoning Methods of Seasoning
4.12 Stacking of Timber 4.12.1 4.12.2
Methods of Stacking Precautions to be taken in Stacking Timber
4.13 Preservation of Timber 4.14 Measurements of Timber 4.15 Tests of Timber 4.15.1 4.15.2
Moisture Content Test Specific Gravity Test
4.16 Summary 4.17 Answers to SAQs
4.1 INTRODUCTION Timber is one of the important construction materials. Wood is used as structural elements in buildings, widely for doors, windows and partitions and find large use through secondary wood products like plywood, particle boards and laminated boards, etc. Both hard as well as soft woods are in use. Wood also forms an integral part of any interior of building and with the development in wood technology effective utilization of timber is essential. Therefore, in this unit we shall concentrate on Timber. 79
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Objectives After studying this unit, you should be able to •
explain definition, properties and drawbacks of timber,
•
elaborate classification and uses of timber,
•
know the characteristics of good timber,
•
know the defects in timber alongwith extensive details concerning the causes for the defects,
•
explain the objectives and methods of seasoning, and
•
understand the methods of stacking and preservation of timber.
4.2 DEFINITION OF TIMBER Wood suitable for building or other engineering purposes is called timber. When wood forms part of a living tree it is called standing timber whereas it is called rough timber when the tree has been felled. The wood is called converted timber when it has been sawn to various market forms such as beams, battens and planks etc. Properties Timber or wood, as building material, possesses a number of valuable properties such as : (i)
Low heat conductivity,
(ii)
Amenability to mechanical working,
(iii) Small bulk density, and (iv) Relatively high strength, etc. Drawbacks Timber has its own drawbacks such as : (i)
Susceptibility to decay and inflammability,
(ii)
Fluctuations in properties due to changes in moisture content,
(iii) Variations in strength in length and across fibres, etc. These shortcomings require careful consideration while making use of it.
4.3 USES OF TIMBER Timber is mainly used for following categories of works : (i)
It is used for construction purposes including building construction, houseposts, beams, rafters, bridges, piles, poles and railway sleepers etc.
(ii)
It is used for furniture and cabinet making.
(iii) It is used for light packing cases. 80
(iv) It is also used for heavy packing cases such as machinery and similar stores. (v)
Timber
It is used for manufacturing agricultural implements and tool handles.
(vi) It is used for making turnery articles and toys etc. (vii) It is used for manufacturing veneers and plywoods.
4.4 CLASSIFICATION OF TREES Trees are classified according to their mode of growth as shown below.
4.4.1 Endogenous Trees These trees are the ones that grow inwards in a longitudinal fibrous mass. Examples of such trees are banana, bamboo, palm, cane etc. Even though the “stem” of trees of this class is light and tough yet it is too flexible and slender to furnish material suitable for engineering works, with the exception of bamboo.
4.4.2 Exogenous Trees These trees are those that grow outwards by the addition of one concentric ring every year. These rings are known as annual rings. Since one ring is added to the tree every year so the number of annual rings in the stem of a tree indicates its age in years. The timber obtained from this class of trees is extensively used in engineering works. Conifers These are also known as ever-green trees and leaves of these trees do not fall till new ones are grown. As these trees bear cone-shaped fruits, they are given the name conifers. These trees yield soft woods. Deciduous The trees are also known as broad leaf trees and leaves of these trees fall in autumn and new ones appear in spring season. Timber for Engineering purposes is mostly derived from deciduous trees. These trees yield hard wood. Hard Wood A hard wood possesses the following characteristics : (a)
The wood is comparatively heavier and is darker in colour. 81
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(b)
The annual rings are not distinct.
(c)
It contains a large percentage of acid.
(d)
It is hard and difficult to work upon.
(e)
It resists shearing stresses.
(f)
It is close-grained and strong.
(g)
It is non resinous.
Shisham, Sal, Teak, Oak, Mahagony and Babul are examples of hard wood. Soft Wood A soft wood possesses the following characteristics. (i)
It is light in weight and colour.
(ii)
Annual rings are very distinct.
(iii) It is comparatively weaker and splits easily. (iv) It has straight fibres. (v)
It is resinous, i.e. contains resins and turpentine. It has a peculiar fragrance.
(vi) It is strong for resisting tensile forces. (vii) It is weak in directions across the grains. (viii) Its texture is soft and regular. Spruce, Deodar, Chir, Kail and Walnut etc., are examples of soft wood.
4.5 GROWTH OF A TREE The roots of the tree suck a solution of salts from the soil in spring season. These salts are food for the tree and roots transmit the same through the trunk of tree to its branches and leaves. This solution of salts looses some of the moisture because of evaporation and absorbs carbon dioxide from the air. This action in the presence of sun makes the solution a bit viscous. This transformed viscous solution is known as sap. This viscous sap descends below the bark and leaves a thick layer in autumn. Layer of sap left below the bark gets transformed to wood and is known as cambium layer. A fresh layer is thus added on the outside of the tree every year forming a new annual ring. The new ring represents a year’s growth of tree. Medullary rays carry the sap from below the bark to the interior thereby nourishing the tree.
4.6 STRUCTURE OF A TREE Now, after the classifications, we should know the structure of a tree, how it is formed and the in its formation. A tree basically consists of three parts, viz, trunk, crown and roots. From the visibility aspects, the structure of a tree can be divided into two categories : 82
(i)
Macrostructure
(ii)
Microstructure
Timber
4.6.1 Macrostructure The structure of wood visible to the naked eye or at a small magnification is called macrostructure. Figure 4.1 shows the macrostructure of exogenous tree.
Figure 4.1 Cross-section of an Exogenous Tree
These parts of the cross-section are described below : Pith or Medulla It is the first formed portion of the stem of tree and it consists entirely of cellular tissues. The pith contains a large amount of fluid and nourishes the plant when the plant is young. It dies up and decays when the plant becomes old and sap is then transmitted by the woody fibres deposited round the pith. Annual Rings The rings of woody fibre arranged in concentric circle around the pith are known as annual rings because one such ring is added every year. Heart Wood Inner most rings surrounding the pith constitute the heart wood. This wood is darker in colour, stronger, more compact and durable. Sap Wood The outer annual rings between heart wood and cambium layer of the tree constitute the sap wood which transmits the sap from roots to branches. Compared with heart wood, sap wood is lighter in colour, weaker and more liable to decay. Sap wood is also known as alburnum. Cambium Layer Outermost ring between the bark and sap wood which is not yet converted into wood is known as the cambium layer. Inner Bark The inner skin or layer covering the cambium layer is known as inner bark. It gives protection to cambium layer from any injury. Outer Bark
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The outer skin or cover of the tree is known as outer bark. It is the outermost protective layer and it sometimes contains cracks and fissures. It consists of cells of woody fibre and is also known as cortex. Medullary Rays These are thin horizontal veins radiating from the pith towards the bark. They carry sap from outside to the inner parts of tree and nourish it. They keep the annual rings tightly gripped together.
4.6.2 Microstructure The structure of wood apparent only at great magnifications is called microstructure. When studied under a microscope, it becomes evident that wood consists of living and dead cells of various sizes and shapes.
4.7 CHARACTERISTICS OF A GOOD TIMBER Following are the characteristics or qualities of a good timber : (i)
It should be free from sap and be from heart of a sound tree.
(ii)
It should have straight and close fibres.
(iii) It should give a clear ringing sound when struck. Dull heavy sound is a sign of internal decay. (iv) It should be of uniform dark colour. Light colour usually indicates timber with low strength. (v)
It should have regular annual rings.
(vi) Timbers with narrow annual rings are generally the strongest. (vii) Freshly cut surface should give sweet smell. (viii) It should have bright and smooth surface when planed. Dull appearance is a sign of defective timber. (ix) Teeth of saw should not get clogged while sawing. (x)
Out of same variety of timber, darker and heavier pieces are stronger.
(xi) It should be free from dead knots, from too many knots, shakes or other defects. (xii) It should have firm adhesion of fibers and compact medullary rays. (xiii) A good timber should be durable. It should be capable of resisting the actions of fungi, insects, chemicals etc. (xiv) A good timber should be capable of retaining its shape during conversion or seasoning. It should not bow or warp or split. (xv) A good timber should be capable of offering resistance to shocks due to vibrations.
4.8 DEFECTS IN TIMBER You have understood the tree, its classification and structure. Now, let us study the defects in timber. 84
As human body has many defects, timber also have defects. These are grouped into the following five categories : (i)
Defects due to conversion
(ii)
Defects due to fungi
Timber
(iii) Defects due to insects (iv) Defects due to natural forces (v)
Defects due to seasoning
Now, let us know various types of defects under each category : Defects Due to Conversion During the process of converting timber to commercial form, the following defects may occur : (i)
Chip mark
(ii)
Diagonal grain
(iii) Torn grain (iv) Wane Chip Mark This defect is indicated by the marks or signs placed by chips on the finished surface of timber. They may also be formed by the parts of a planing machine. Diagonal Grain This defect is formed due to improper sawing of timber. It is indicated by diagonal mark on straight grained surface of timber. Torn Grain This defect is caused when a small depression is formed on the finished surface of timber by falling of a tool or so. Wane This defect is denoted by the presence of original rounded surface on the manufactured piece of timber. Defects Due to Fungi Fungi are minute microscopic plant organisms. They attack timber only when the following two conditions are satisfied simultaneously. (i)
The moisture content of timber is above 12-15%.
(ii)
There is presence of air and warmth for the growth of fungi.
If any of the above condition is absent, decay of wood due to fungi would not occur. Hence, dry wood having moisture content less than 12-14 % will remain sound for centuries. Similarly, wood submerged in water will not be attacked by fungi because of absence of air. Following defects are caused in timber by fungi. (i)
Blue stain
(ii)
Brown rot
(iii) Dry rot (iv) Heart rot (v)
Sap stain
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(vi) Wet rot (vii) White rot Blue Stain Sap of wood is stained to bluish colour by the action of certain type of fungi. Brown Rot The term rot is used to indicate decay or disease of timber. Certain types of fungi remove cellulose compounds from wood and hence, wood assumes the brown colour. This is known as brown rot. Dry Rot Certain types of fungi feed on wood and during feeding, they attack on wood and convert it into dry powder form. This is known as dry rot. Following are to be noted : (i)
Dry rot occurs at places where there is no free circulation of air such as improperly ventilated basements, rooms, etc.
(ii)
Unseasoned soft woods and sap wood are easily attacked by dry rot.
(iii) If timber is not properly stored after being felled down, it is liable for the attack of dry rot. (iv) It is not necessary to have damp conditions for the development of dry rot. (v)
Dry rot is also caused by charring, painting and tarring the unseasoned timber.
Heart Rot This is formed when a branch has come out of a tree. In such a case, heart wood is exposed to the attack of atmospheric agents. Ultimately, the tree becomes weak and it gives out hollow sound when struck with a hammer. Sap Stain Certain types of fungi do not bring about the complete decay of timber. But they feed on cell contents of sap wood. In doing so, sap wood loses its colour. This is known as sap stain and it generally occurs where moisture content goes beyond 20% or so. Wet Rot Some varieties of fungi cause chemical decomposition of wood of the timber and in doing so they convert timber into a greyish brown powder. This is known as wet rot. The important facts to be ed are : (i)
Alternate dry and wet conditions favour the development of wet rot.
(ii)
If unseasoned or improperly seasoned timbers are exposed to rain and wind, they become easily liable for the attack of wet rot.
(iii) To prevent wet rot, well-seasoned timber should be used for exterior work or for underground work and it should be covered by tar or paint for protection against moisture. White Rot 86
This defect is just opposite of brown rot. In this case, certain types of fungi attack lignin of wood and wood assumes the appearance of a white mass consisting of cellulose compounds.
Timber
Defects Due to Insects Following are the insects which are usually responsible for the decay of timber : (i)
Beetles
(ii)
Marine borers
(iii) Termites Beetles These are small insects and they cause rapid decay of timber. They form pin-holes of size about 2 mm diameter in wood. They attack the sap wood of all species of hard wood. Tunnels are formed in all directions in sap wood by the larvae of these beetles. The timber is converted into fine flour-like powder. They usually do not disturb the outer shell or cover. Hence, timber piece attacked by beetles may look sound till it completely fails. Marine Borers These are generally found in salty water. Most of the varieties of marine borers do not feed on wood. But they make holes or bore tunnels in wood for taking shelter. The diameter and length of these holes may go as high as 25 mm and 60 mm respectively. The wood attacked by marine borers loses colour and strength. It may be noted that no timber is completely immune from the attack of marine borers. Termites These are popularly known as ‘white ants’ and they are found in abundance in tropical and sub-tropical countries. These insects live in a colony and they are very fast in eating away the wood from core of the cross section. They make tunnels inside the timber in different directions and usually do not disturb the outer shell or cover. Hence, timber piece attacked by termites may look sound till it completely fails. Very few good timbers such as teak, sal, etc., can resist the attack of white ants. Such timbers have certain chemicals in their composition and the smell of these chemicals is not favourable for termites. Defects Due to Natural Force The main natural forces responsible for causing defects in timber are two, namely (i)
Abnormal growth, and
(ii)
Rupture of tissues.
Following defects are caused by these forces : (i)
Burls
(ii)
Chemical stain
(iii) Coarse grain (iv) Dead wood 87
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(v)
Knots
(vi) Shakes (vii) Twisted fibres (viii) Upsets (ix) Water stain (x)
Wind cracks
Burls These are also known as ‘excrescences’ and they are particularly formed when a tree has received shock or injury in its young age. Due to such injury, the growth of tree is completely upset and irregular projections appear on the body of the timber. Chemical Stain Wood is sometimes discoloured by the chemical action caused to it by some external agency. This is known as chemical stain. Coarse Grain If a tree grows rapidly, annual rings are widened. It is known as coarse grained timber and such timber possesses less strength. Dead Wood Timber which is obtained from dead standing trees contains dead wood. It is indicated by light weight and reddish colour. Knots These are bases of branches or limbs which are broken or cut off from the tree. The portion from which the branch is removed receives nourishment from the stem for a pretty long time and it ultimately results in the formation of dark, hard rings which are known as knots. As continuity of wood fibres is broken by knots, they form a source of weakness. Figure 4.2 shows a typical knot.
Figure 4.2 : Typical Knot
Knots are classified on the basis of their size and form. Tables 4.1 and 4.2 show the classification of knots on the basis of their size and form and quality. Table 4.1 : Classification of Knots on the Basis of their Size
88
Sl. No.
Type of Knot
1
Pin knot
2
Small knot
Size Diameter upto 6.5 mm Diameter between 6.5 mm and 20 mm
3
Medium knot
4
Large knot
Diameter between 20 mm and 40 mm
Timber
Diameter greater than 40 mm
Table 4.2 : Classification of Knots on the Basis of Form and Quality Sl. No.
Type of Knot
Remarks
1
Dead knot
The fibres of knot are not properly interconnected with those of surrounding wood. Hence, it can be easily separated out from the body of wood. It is not safe to use wood with such a knot for engineering purposes.
2
Decayed knot
It is also known as an unsound knot and it is formed by the action of fungi on wood.
3
Live knot
It is also known as a sound knot. It is free from decay and cracks. It is thoroughly fixed in wood and hence, it cannot be separated out from the body of wood. Presence of such knots makes wood difficult to plane. However, wood containing such knots can be used for engineering purposes.
4
Loose knot
It indicates preliminary stage of dead knot. The fibres of this knot are not firmly held in the surrounding wood.
5
Round knot
Cross-section of this type of knot is either round or oval. It is obtained by cutting the knot at right angles to its long axis.
6
Tight knot
It indicates preliminary stage of live knot. The fibres of knots are firmly held in the surrounding wood.
Rind Galls Rind means bark and gall indicates abnormal growth. Hence, peculiar curved swellings found on the body of a tree are known as Rind Galls as shown in Figure 4.3. They develop at points from where branches are improperly cut off or removed.
Figure 4.3 : Rind Galls
Shakes These are cracks which partly or completely separate the fibres of wood. Following are the different varieties of shakes : Cup Shakes These are caused by the rupture of tissue in a circular direction as shown in Figure 4.4. It is a curved crack and it separates partly one
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annual ring from the other. It develops due to non-uniform growth. It may not prove to be harmful, if it covers only a portion of ring.
Figure 4.4 : Cup Shakes
Heart Shakes These cracks occur in the centre of cross-section of tree and they extend from pith to sap wood in the direction of medullary rays as shown in Figure 4.5. These cracks occur due to shrinkage of interior part of tree which is approaching maturity. Heart shakes divide the tree cross-section into two to four parts.
Figure 4.5 : Heart Shakes
Ring Shakes When cup shakes cover the entire ring, they are known as ring shakes as shown in Figure 4.6.
Figure 4.6 : Ring Shakes
Star Shakes These are cracks which extend from bark towards the sap wood. They are usually confined upto the plane of sapwood. They are wider on the 90
outside ends and narrower on the inside ends as shown in Figure 4.7. They are usually formed due to extreme heat or frost.
Timber
Figure 4.7 : Star Shakes
Radial Shakes These are similar to star shakes. But they are fine, irregular and numerous. They usually occur when the tree is exposed to sun for seasoning after being felled down. They run for a short distance from bark towards the centre, then follow direction of annual ring and ultimately run towards pith. Figure 4.8 shows radial shakes.
Figure 4.8 : Radial Shakes
Twisted Fibres These are also known as wandering hearts and they are caused by twisting of young trees by fast blowing wind. The fibres of wood are twisted in one direction as shown in Figure 4.9. Timber with twisted fibres is unsuitable for sawing. It can, however, be used for posts and poles in an unsawn condition.
Figure 4.9 : Twisted Fibres
Upsets
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These are also known as ruptures and they indicate wood fibres which are injured by crushing or compression. Figure 4.10 shows a timber piece with this defect. Upsets are mainly due to improper felling of tree and exposure of tree in its young age to fast blowing wind.
Figure 4.10 : Upsets
Water Stain Wood is sometimes discoloured when it comes into with water. This is known as water stain and this defect is usually found in converted timber. Wind Cracks If wood is exposed to atmospheric agencies, its exterior surface shrinks. Such a shrinkage results in cracks as shown in Figure 4.11. These are known as wind cracks.
Figure 4.11 : Wind Cracks
Defects Due to Seasoning Following defects occur in seasoning process of wood. (i)
Bow
(ii)
Case-hardening
(iii) Check (iv) Collapse (v)
Cup
(vi) Honey-combing (vii) Radial shakes (viii) Split (ix) Twist 92
(x)
Timber
Warp
Bow The defect is indicated by the curvature formed in the direction of length of timber as shown in Figure 4.12.
Figure 4.12 : Bow
Case-hardening The exposed surface of timber dries very rapidly. It, therefore, shrinks and is under compression. The interior surface which has not completely dried is under tension. This defect is known as case-hardening and it usually occurs in timbers which are placed at the bottom during seasoning. Check A check is a crack which separates fibres of wood. It does not extend from one end to the other. Collapse Due to uneven shrinkage, wood sometimes flattens during drying. This is known as collapse. Cup This defect is indicated by the curvature formed in the transverse direction of timber as shown in Figure 4.13.
Figure 4.13 : Cup
Honey-combing Due to stress developed during drying, various radial and circular cracks develop in the interior portion of timber. Timber thus assumes honey-comb texture and the defect so developed is known as honey-combing. Radial Shakes Radial shakes are explained earlier. Split When a crack extends from one end to the other, it is known as a split. 93
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Twist When a piece of timber has spirally distorted along its length, it is known as twist. It is shown in Figure 4.14.
Figure 4.14 : Twist
Warp When a piece of timber has twisted out of shape, it is said to have warped.
4.9 DECAY OF TIMBER Timber is said to be decayed when it is so deteriorated that it loses its value as an engineering material. Various defects in timber have been mentioned earlier. When these defects are in excess, timber decays and such timber is not used for engineering purpose. Following are the various causes or situations which favour the early decay of timber : (i) Alternate dry and wet conditions. (ii) Bad storage or stacking of timber. (iii) Fungi which are responsible for developing diseases in timber such as blue stain, brown rot, dry rot, heart rot, sap stain, wet and white rot. (iv) Improper seasoning. (v) Insects such as beetles, marine borers, termites, etc. (vi) Keeping timber in with damp wall, damp earth, etc. (vii) Shocks or impacts received during young age from natural forces such as fast blowing wind, etc. (viii) Use of timber without taking out sap wood from its structure. (ix) Using seasoned timber without applying suitably preservative on its surface. (x) Using unseasoned wood with the application of protective coat of paint or tar.
4.10 FELLING OF TREES A tree should always be felled only after it has fully matured but before the heart wood starts deteriorating. If felling is delayed then decay would set in the heart wood which is the best and the most important part of a tree. Early felling would give lesser quantity of timber which has not yet developed full strength. 94
The lower we go the more is the timber that the trunk of tree yields as such it would be wise to cut the tree from a place a little below the ground level but higher up than the roots.
4.10.1
Timber
Process of Felling
(i)
In the process of felling we make a deep cut with axe at the lowest possible point of the trunk.
(ii)
The trunk may be then sawn to a point beyond the centre of gravity of the trunk.
(iii) This cut should be made on the side opposite to that on which it is intended to be felled. (iv) A cut is then made on a side opposite to the one on which first cut was made. (v)
Top of tree is then tied with ropes on all the four diametrically opposite sides. The rope on the side, the tree is to be felled is pulled and the one on the opposite side is loosened slowly.
(vi) The tree would break at the level of cuts and it should be allowed to fall gently otherwise it is likely to get damaged. (vii) After felling, its branches are chopped off and the log is cut to the needed sizes. It should be protected against rapid drying particularly at the ends. Its bark should be removed and the log sawn at the earliest.
4.11 SEASONING OF TIMBER The art of seasoning is to extract the moisture under controlled conditions as nearly as possible at a uniform rate from all parts of timber and to leave the remaining moisture that cannot be extracted, uniformly distributed throughout the mass. Irregular drying will cause irregular shrinkage resulting in the setting up of internal stresses between the fibres. When these stresses become strong enough to overcome the cohesion of the fibres then the timber warps and shakes are formed.
4.11.1
Objects of Seasoning
(i)
Seasoning makes timber resistant to decay.
(ii)
Seasoning makes timber lighter.
(iii) It becomes easier to paint and polish seasoned timber. (iv) It is easier to treat seasoned timber with preservatives. (v)
Seasoned timber becomes stronger and more stable.
(vi) Seasoning stops shrinkage of timber on drying. (vii) Seasoned timber has better electrical resistance.
4.11.2
Methods of Seasoning
As per the recommendations of IS : 1141-1973, the seasoning methods should be classified as : (i)
Natural seasoning or Air Seasoning
(ii)
Artificial seasoning or Kiln Seasoning
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Natural Seasoning or Air Seasoning The log is converted by sawing it into battens and planks etc. as soon as possible after felling of tree. These are then stacked on a well drained place in the shade. Care should be taken to ensure free circulation of fresh air all around each piece while stacking. The stacking should be done on masonry or concrete s a few centimeters above the ground. The pattern of stacking is as shown in Figure 4.15. Care should be taken not to expose the freshly converted timber stacked for seasoning to severe winds or to sun. This process of seasoning timber is the best as it gives very strong and durable timber, but it is extremely slow. It takes more than six months for timber to season in moderate climates.
Figure 4.15 : Stacks for Natural Seasoning
Kiln Seasoning or Artificial Seasoning This method of seasoning speeds up the seasoning process. This method of seasoning is a must for large scale production of seasoned timber. Kiln seasoning is done in a chamber equipped with arrangements for heating and humidifying the air to required conditions of relative humidity and temperature and for its circulation across the timber stacked in the chamber for seasoning. Usually, it is steam that is used for heating and humidifying the air in the kiln. The seasoning of the timber is started at a comparatively lower temperature and high humidity. As the timber dries, these conditions are gradually altered until at the end of the seasoning. The temperature of the air inside the chamber is fairly high and the humidity is low. The kiln charge is allowed to cool inside the kiln to within 15 to 20o C of the outside temperature before removal. Seasoning of timber by this method takes about four to five days under normal conditions. Relative merits and demerits of the two methods of seasoning are given in Table 4.3. Table 4.3 : Comparison of Air Seasoning and Kiln Seasoning Air Seasoning
96
Kiln Seasoning
It is a slow process
It is a quick process
It is a simple and economical
It is quite technical and expensive
It is difficult to reduce moisture content
Moisture content can be reduced to
below 15 to 18%.
any desired level.
Air seasoned timber is more amenable to attacks of insects and fungi.
Kiln seasoned timber is less amenable to attacks of insects and fungi.
It requires more stacking space
It requires less stacking space
It gives stronger timber
A little weaker timber is obtained
Timber
4.12 STACKING OF TIMBER Timber, before seasoning, should be stacked in yards free from weeds and debris. The yard should have big shady trees to protect the timber from direct sun. Ends of logs should be protected against splitting by applying anti-splitting compositions and stacked on foundations in closed stacks in one or more layers. Stacks should be protected against direct sun by providing a covering if needed.
4.12.1
Methods of Stacking
(i)
One and Nine method
(ii)
Close crib method
(iii) Open crib method One and Nine Method This method of stacking timbers is best suited for moderately heavy coniferous timbers in hot climate and for heavy timbers in moist climates. Stacking pattern by this method is shown in Figure 4.16.
Figure 4.16 : One and Nine Method
Close Crib Method In this method, reduced air circulation slows down the pace of seasoning. This method is recommended for staking heavy structured timbers like sal in hot and dry localities. The stacking pattern is shown in Figure 4.17.
Figure 4.17 : Close Crib Method
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Open Crib Method This method is a modification of the close crib method and because of more air circulation taking place it is more akin to the one and nine method in its effects. The stacking pattern is shown is Figure 4.18.
Figure 4.18 : Open Crib Method
4.12.2
98
Precautions to be Taken in Stacking Timber
•
Stacks of not more than 100 sleepers are recommended to be made
•
Poles are stacked either in closed heaps or with crossers. If stacked in closed heaps, then there should be alternate layers of butt ends and of top ends so that the two ends of the stack are level. Poles themselves could be used as crossers, which should not be spaced more than three metres.
•
Fence posts should be stacked in open crib fashion in which successive layers of posts are at right angles to each other and there is a gap of about 8 cm between adjacent posts in the same layer. Centre to centre distance between crossers should not exceed 1.5 m and the height of stack should not exceed 3 metres.
•
Horizontal stacking of sawn timber is done on vertical pillars of treated timber, brick masonry or of cement concrete 30 cm square in section and 30 to 45 cm high. The pillars are spaced 1.2 m centre to centre along the length and the breadth of the stack. The length of material to be stacked decides the length of stacking unit. Long beams of cross-section 10cm × 10cm and above are placed on the foundation pillars to form a framework for stacking timber.
•
Scantlings and squares should be stacked with crossers 5 cm × 4 cm in section and spaced 2.5 m to 3 m apart. The ends should be protected with moisture proof coatings.
•
Planks should be stacked on level platform with crossers of uniform thickness and section, which should be in vertical alignment in a stack. Longer planks should form the bottom of the stack and the shorter one’s the top. Heavy wooden beams should be placed on the top to prevent top layers from warping. A gap of about 2.5 cm should be left between ading planks for free circulation of air in the centre of stack. The stack should be protected against rain and sun by providing a shed over it.
4.13 PRESERVATION OF TIMBER
Timber
Timber has to be protected from the attack of insects, e.g. white ants etc., and from internal decay due to dry and wet rots. Perfect seasoning is the most effective means of preservation. Timber should be so used that either it is wholly dry and well ventilated or is wholly under water. It will not decay when kept under water but it will become soft and weak. Proper damp proofing of the building and providing free circulation of air around the built in portions of timber are essential for the preservation of the timber used. However, when these conditions cannot be obtained then preservatives have to be applied for preservation. Timber should be well seasoned before the application of preservatives as otherwise the preservatives would block the pores of timber thereby causing its decay due to the entrapped moisture. Direct with lime mortar should be avoided while using preservative with masonry.
4.13.1
Methods of Preservation of Timber
Following are some of the common methods of preservation adopted (i)
Charring
(ii)
Tarring
(iii) Painting (iv) Creosoting (v)
Wolman salt
(vi) Ascu treatment (vii) Fire proofing of timber Charring Lower ends of the posts that are to be embedded in ground are generally charred with a view to prevent dry rot and attack of worms. It is done by quenching the ends of posts in water after they are charred on wood fire to a depth of 1.5 cm. Tarring It consists in coating with tar or tar mixed with pitch. Embedded portions of timber fence posts, ends of door and window frames, battens and beams built in wall are usually tarred. Tarring is not done in case of those portions of structural that are open to view, because of unsightly black colour. Painting A paint when applied to timber acts not only as a good preservative but also it enhances the appearance of the surface so treated. Only well seasoned timber should be painted as otherwise the moisture entrapped in the timber, because of the closing of timber bores by paint, would cause decay. Paints however, protect seasoned timber against moisture thereby prolonging its 99
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life, e.g. soligum paints have excellent preservative properties and protect timber against the attack of white ants. Creosoting Creosote oil is a dark brown thick oily liquid. Thoroughly seasoned timber dried for 24 hrs before its treatment is placed in an airtight chamber. After the air has been exhausted from this chamber, the creosote oil is then pumped in at a pressure of 9 kg/ cm2 at a temperature of 50o C so long as the timber is not fully saturated with oil. The oil preserves the timber from rot and from the attacks of white ant. Uses •
It is used in case of railway sleepers, piles and transmission poles.
Limitations •
Undesirable colour and smell, inability to take paint well and the tendency to stain plaster limit its use.
Wolman Salt This salt consists of creosote and sodium fluoride and is soluble in water. It is odourless and leaves no stain on wood. After treatment, timber could be painted or varnished. These salts destroy many kinds of fungi that cause timber to rot. This renders the timber extremely fire resistant too. Treatment of timber with zinc chloride, sodium fluoride, magnesium, silico fluoride or copper sulphate renders the timber immune from the attacks of fungi. The timber so treated is capable of being painted on drying. Ascu Treatment Ascu is available in the form of powder and is made up of three chemicals mixed in the ratios given below : (i)
1 part by weight of hydrated arsenic pentaoxide (As2 O5.2H2 O)
(ii)
3 parts by weight of blue vitriol (CuSO4 . 5H2 O)
(iii) 4 parts by weight of potassium dichromate (K2 Cr2 O7 . 2H2O) Six parts of this powder are mixed with 100 parts by weight of water. Ascu solution can be applied or sprayed in two coats. To achieve better results, timber may be soaked in the solution and impregnated with it under pressure. The timber should be allowed to dry for three to six weeks. This treatment renders timber immune to the attacks of white ant. Ascu treated timber may be painted, varnished, polished or waxed. The solution is odourless. Fire Proofing of Timber Timber cannot be made completely fireproof, however, by treating as below it can be made fire resistant to a sufficient extent. 100
Soaking timber in ammonium sulphate, ammonium chloride, ammonia phosphate, sodium arsenate, zinc chloride etc. or spraying on timber, a
solution of sodium silicate, potassium silicate or ammonia phosphate etc. imparts fire resisting properties.
Timber
Abel’s methods of fire proofing timber is painting the surface first with a dilute solution of sodium silicate (Na2 SiO3) then with a cream like paste of slaked fat time and in the end with a concentrated solution of silicate of soda.
4.14 MEASUREMENTS OF TIMBER Sawn timber shall be measured as follows : Length Rounded or damaged end portions of the sawn timber shall be excluded from the length which shall be measured in metres. Fractions of a meter shall be rounded of to the nearest lower 0.01m. Width and Thickness It shall be nearest at the narrowest section in centimetres and shall be rounded off to the nearest centimetres. Volume It shall be measured in cubic metres correct to three places of decimal based on accepted sizes. The volume of a log of wood is calculated by the quarter girth formula as given below : 4
⎛G⎞ V =⎜ ⎟ ×L ⎝4⎠
where
V = volume in m3, G = girth in m, and L = length in m.
Dimensions Sawn timber is generally available in the following lengths and cross-sections : Length 2 m; 2.5 m; 3 m and 3.5 m Cross- section 20 × 10 cm; 25 × 12.5 cm 20 × 12.5 cm; 25 × 15 cm and
20 × 15 cm; 30 × 15 cm
4.15 TESTS OF TIMBER The following tests are carried out to check up the physical properties of a test piece of timber.
4.15.1 Moisture Content Test To determine the moisture contents of a specimen, a test piece 5 cm × 5 cm × 2.5 cm is taken and weighed fresh. It is then dried in an oven at a temperature of 103 + 2o C. The weight of the specimen in the oven is regularly observed till the variation in the last two observations does not exceed 0.002 gm. The specimen is then considered to have dried.
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Let W1 be the weight of fresh sample and W0 the weight of the oven dry specimen. Moisture content is then calculated as below : Percentage of moisture content =
4.15.2
W1 − W0 × 100 . W0
Specific Gravity Test
The specimens for the test shall be 5 × 5 × 15 cm pieces and free from usual defects. The specimens shall be weighed (W1) usually green, correct to 0.001 gm and volume measured (V1) by immersion method correct to 0.01 cm3. The specimens are then end coated with paraffin wax by immersion and left to air season at room temperatures till moisture content of about 12% are reached. The weight (Wr) and volume (Vr) of the specimen are then noted by the immersion method when the moisture content is r percent. The specimens are then kept in an oven at a temperature of 103 + 2o C till the weight becomes constant. Weight (W0) and volume (V0) are noted then : Specific gravity at test
=
W1 V1
Standard specific gravity
=
W0 V1
Over dry specific gravity
=
W0 V0
Moisture content r percent =
Wr − W0 × 100 % V0
=
W1 − W0 × 100 % V0
or
SAQ 1
102
(a)
Differentiate between hard wood and soft wood. Could a soft wood be stronger than hard wood?
(b)
Describe the characteristics of a good timber.
(c)
Explain the precautions required to be taken for prevention of wet rot and dry rot.
(d)
Explain as to why timber used for structural purposes should be properly seasoned?
(e)
Enumerate various defects in timber.
4.16 SUMMARY
Timber
In this unit, you have gained adequate knowledge of Timber as one of the important construction materials. Wood is used as structural elements in buildings for various purposes. The timber used for engineering purposes should be free from defects as far as possible. Defects free timber is possible only when tree has been felled at proper time and with adequate precautions. It is also necessary that timber should be properly seasoned and necessary preservatives are used before it is used for engineering purposes. Thus, you can see that like any other construction materials, timbers are also very important construction materials which play an important role in construction industry.
4.17 ANSWERS TO SAQs Refer the relevant preceding text in the unit or other useful books on the topic listed in Section “Further Reading” to get the answers of the SAQs.
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104
Timber
Structure 4.1
Introduction Objectives
4.2
Definition of Timber
4.3
Uses of Timber
4.4
Classification of Trees 4.4.1 4.4.2
Endogenous Trees Exogenous Trees
4.5
Growth of a Tree
4.6
Structure of a Tree 4.6.1 4.6.2
Macrostructure Microstructure
4.7
Characteristics of Good Timber
4.8
Defects in Timber
4.9
Decay of Timber
4.10 Felling of Trees 4.11 Seasoning of Timber 4.11.1 4.11.2
Objects of Seasoning Methods of Seasoning
4.12 Stacking of Timber 4.12.1 4.12.2
Methods of Stacking Precautions to be taken in Stacking Timber
4.13 Preservation of Timber 4.14 Measurements of Timber 4.15 Tests of Timber 4.15.1 4.15.2
Moisture Content Test Specific Gravity Test
4.16 Summary 4.17 Answers to SAQs
4.1 INTRODUCTION Timber is one of the important construction materials. Wood is used as structural elements in buildings, widely for doors, windows and partitions and find large use through secondary wood products like plywood, particle boards and laminated boards, etc. Both hard as well as soft woods are in use. Wood also forms an integral part of any interior of building and with the development in wood technology effective utilization of timber is essential. Therefore, in this unit we shall concentrate on Timber. 79
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Objectives After studying this unit, you should be able to •
explain definition, properties and drawbacks of timber,
•
elaborate classification and uses of timber,
•
know the characteristics of good timber,
•
know the defects in timber alongwith extensive details concerning the causes for the defects,
•
explain the objectives and methods of seasoning, and
•
understand the methods of stacking and preservation of timber.
4.2 DEFINITION OF TIMBER Wood suitable for building or other engineering purposes is called timber. When wood forms part of a living tree it is called standing timber whereas it is called rough timber when the tree has been felled. The wood is called converted timber when it has been sawn to various market forms such as beams, battens and planks etc. Properties Timber or wood, as building material, possesses a number of valuable properties such as : (i)
Low heat conductivity,
(ii)
Amenability to mechanical working,
(iii) Small bulk density, and (iv) Relatively high strength, etc. Drawbacks Timber has its own drawbacks such as : (i)
Susceptibility to decay and inflammability,
(ii)
Fluctuations in properties due to changes in moisture content,
(iii) Variations in strength in length and across fibres, etc. These shortcomings require careful consideration while making use of it.
4.3 USES OF TIMBER Timber is mainly used for following categories of works : (i)
It is used for construction purposes including building construction, houseposts, beams, rafters, bridges, piles, poles and railway sleepers etc.
(ii)
It is used for furniture and cabinet making.
(iii) It is used for light packing cases. 80
(iv) It is also used for heavy packing cases such as machinery and similar stores. (v)
Timber
It is used for manufacturing agricultural implements and tool handles.
(vi) It is used for making turnery articles and toys etc. (vii) It is used for manufacturing veneers and plywoods.
4.4 CLASSIFICATION OF TREES Trees are classified according to their mode of growth as shown below.
4.4.1 Endogenous Trees These trees are the ones that grow inwards in a longitudinal fibrous mass. Examples of such trees are banana, bamboo, palm, cane etc. Even though the “stem” of trees of this class is light and tough yet it is too flexible and slender to furnish material suitable for engineering works, with the exception of bamboo.
4.4.2 Exogenous Trees These trees are those that grow outwards by the addition of one concentric ring every year. These rings are known as annual rings. Since one ring is added to the tree every year so the number of annual rings in the stem of a tree indicates its age in years. The timber obtained from this class of trees is extensively used in engineering works. Conifers These are also known as ever-green trees and leaves of these trees do not fall till new ones are grown. As these trees bear cone-shaped fruits, they are given the name conifers. These trees yield soft woods. Deciduous The trees are also known as broad leaf trees and leaves of these trees fall in autumn and new ones appear in spring season. Timber for Engineering purposes is mostly derived from deciduous trees. These trees yield hard wood. Hard Wood A hard wood possesses the following characteristics : (a)
The wood is comparatively heavier and is darker in colour. 81
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(b)
The annual rings are not distinct.
(c)
It contains a large percentage of acid.
(d)
It is hard and difficult to work upon.
(e)
It resists shearing stresses.
(f)
It is close-grained and strong.
(g)
It is non resinous.
Shisham, Sal, Teak, Oak, Mahagony and Babul are examples of hard wood. Soft Wood A soft wood possesses the following characteristics. (i)
It is light in weight and colour.
(ii)
Annual rings are very distinct.
(iii) It is comparatively weaker and splits easily. (iv) It has straight fibres. (v)
It is resinous, i.e. contains resins and turpentine. It has a peculiar fragrance.
(vi) It is strong for resisting tensile forces. (vii) It is weak in directions across the grains. (viii) Its texture is soft and regular. Spruce, Deodar, Chir, Kail and Walnut etc., are examples of soft wood.
4.5 GROWTH OF A TREE The roots of the tree suck a solution of salts from the soil in spring season. These salts are food for the tree and roots transmit the same through the trunk of tree to its branches and leaves. This solution of salts looses some of the moisture because of evaporation and absorbs carbon dioxide from the air. This action in the presence of sun makes the solution a bit viscous. This transformed viscous solution is known as sap. This viscous sap descends below the bark and leaves a thick layer in autumn. Layer of sap left below the bark gets transformed to wood and is known as cambium layer. A fresh layer is thus added on the outside of the tree every year forming a new annual ring. The new ring represents a year’s growth of tree. Medullary rays carry the sap from below the bark to the interior thereby nourishing the tree.
4.6 STRUCTURE OF A TREE Now, after the classifications, we should know the structure of a tree, how it is formed and the in its formation. A tree basically consists of three parts, viz, trunk, crown and roots. From the visibility aspects, the structure of a tree can be divided into two categories : 82
(i)
Macrostructure
(ii)
Microstructure
Timber
4.6.1 Macrostructure The structure of wood visible to the naked eye or at a small magnification is called macrostructure. Figure 4.1 shows the macrostructure of exogenous tree.
Figure 4.1 Cross-section of an Exogenous Tree
These parts of the cross-section are described below : Pith or Medulla It is the first formed portion of the stem of tree and it consists entirely of cellular tissues. The pith contains a large amount of fluid and nourishes the plant when the plant is young. It dies up and decays when the plant becomes old and sap is then transmitted by the woody fibres deposited round the pith. Annual Rings The rings of woody fibre arranged in concentric circle around the pith are known as annual rings because one such ring is added every year. Heart Wood Inner most rings surrounding the pith constitute the heart wood. This wood is darker in colour, stronger, more compact and durable. Sap Wood The outer annual rings between heart wood and cambium layer of the tree constitute the sap wood which transmits the sap from roots to branches. Compared with heart wood, sap wood is lighter in colour, weaker and more liable to decay. Sap wood is also known as alburnum. Cambium Layer Outermost ring between the bark and sap wood which is not yet converted into wood is known as the cambium layer. Inner Bark The inner skin or layer covering the cambium layer is known as inner bark. It gives protection to cambium layer from any injury. Outer Bark
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The outer skin or cover of the tree is known as outer bark. It is the outermost protective layer and it sometimes contains cracks and fissures. It consists of cells of woody fibre and is also known as cortex. Medullary Rays These are thin horizontal veins radiating from the pith towards the bark. They carry sap from outside to the inner parts of tree and nourish it. They keep the annual rings tightly gripped together.
4.6.2 Microstructure The structure of wood apparent only at great magnifications is called microstructure. When studied under a microscope, it becomes evident that wood consists of living and dead cells of various sizes and shapes.
4.7 CHARACTERISTICS OF A GOOD TIMBER Following are the characteristics or qualities of a good timber : (i)
It should be free from sap and be from heart of a sound tree.
(ii)
It should have straight and close fibres.
(iii) It should give a clear ringing sound when struck. Dull heavy sound is a sign of internal decay. (iv) It should be of uniform dark colour. Light colour usually indicates timber with low strength. (v)
It should have regular annual rings.
(vi) Timbers with narrow annual rings are generally the strongest. (vii) Freshly cut surface should give sweet smell. (viii) It should have bright and smooth surface when planed. Dull appearance is a sign of defective timber. (ix) Teeth of saw should not get clogged while sawing. (x)
Out of same variety of timber, darker and heavier pieces are stronger.
(xi) It should be free from dead knots, from too many knots, shakes or other defects. (xii) It should have firm adhesion of fibers and compact medullary rays. (xiii) A good timber should be durable. It should be capable of resisting the actions of fungi, insects, chemicals etc. (xiv) A good timber should be capable of retaining its shape during conversion or seasoning. It should not bow or warp or split. (xv) A good timber should be capable of offering resistance to shocks due to vibrations.
4.8 DEFECTS IN TIMBER You have understood the tree, its classification and structure. Now, let us study the defects in timber. 84
As human body has many defects, timber also have defects. These are grouped into the following five categories : (i)
Defects due to conversion
(ii)
Defects due to fungi
Timber
(iii) Defects due to insects (iv) Defects due to natural forces (v)
Defects due to seasoning
Now, let us know various types of defects under each category : Defects Due to Conversion During the process of converting timber to commercial form, the following defects may occur : (i)
Chip mark
(ii)
Diagonal grain
(iii) Torn grain (iv) Wane Chip Mark This defect is indicated by the marks or signs placed by chips on the finished surface of timber. They may also be formed by the parts of a planing machine. Diagonal Grain This defect is formed due to improper sawing of timber. It is indicated by diagonal mark on straight grained surface of timber. Torn Grain This defect is caused when a small depression is formed on the finished surface of timber by falling of a tool or so. Wane This defect is denoted by the presence of original rounded surface on the manufactured piece of timber. Defects Due to Fungi Fungi are minute microscopic plant organisms. They attack timber only when the following two conditions are satisfied simultaneously. (i)
The moisture content of timber is above 12-15%.
(ii)
There is presence of air and warmth for the growth of fungi.
If any of the above condition is absent, decay of wood due to fungi would not occur. Hence, dry wood having moisture content less than 12-14 % will remain sound for centuries. Similarly, wood submerged in water will not be attacked by fungi because of absence of air. Following defects are caused in timber by fungi. (i)
Blue stain
(ii)
Brown rot
(iii) Dry rot (iv) Heart rot (v)
Sap stain
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(vi) Wet rot (vii) White rot Blue Stain Sap of wood is stained to bluish colour by the action of certain type of fungi. Brown Rot The term rot is used to indicate decay or disease of timber. Certain types of fungi remove cellulose compounds from wood and hence, wood assumes the brown colour. This is known as brown rot. Dry Rot Certain types of fungi feed on wood and during feeding, they attack on wood and convert it into dry powder form. This is known as dry rot. Following are to be noted : (i)
Dry rot occurs at places where there is no free circulation of air such as improperly ventilated basements, rooms, etc.
(ii)
Unseasoned soft woods and sap wood are easily attacked by dry rot.
(iii) If timber is not properly stored after being felled down, it is liable for the attack of dry rot. (iv) It is not necessary to have damp conditions for the development of dry rot. (v)
Dry rot is also caused by charring, painting and tarring the unseasoned timber.
Heart Rot This is formed when a branch has come out of a tree. In such a case, heart wood is exposed to the attack of atmospheric agents. Ultimately, the tree becomes weak and it gives out hollow sound when struck with a hammer. Sap Stain Certain types of fungi do not bring about the complete decay of timber. But they feed on cell contents of sap wood. In doing so, sap wood loses its colour. This is known as sap stain and it generally occurs where moisture content goes beyond 20% or so. Wet Rot Some varieties of fungi cause chemical decomposition of wood of the timber and in doing so they convert timber into a greyish brown powder. This is known as wet rot. The important facts to be ed are : (i)
Alternate dry and wet conditions favour the development of wet rot.
(ii)
If unseasoned or improperly seasoned timbers are exposed to rain and wind, they become easily liable for the attack of wet rot.
(iii) To prevent wet rot, well-seasoned timber should be used for exterior work or for underground work and it should be covered by tar or paint for protection against moisture. White Rot 86
This defect is just opposite of brown rot. In this case, certain types of fungi attack lignin of wood and wood assumes the appearance of a white mass consisting of cellulose compounds.
Timber
Defects Due to Insects Following are the insects which are usually responsible for the decay of timber : (i)
Beetles
(ii)
Marine borers
(iii) Termites Beetles These are small insects and they cause rapid decay of timber. They form pin-holes of size about 2 mm diameter in wood. They attack the sap wood of all species of hard wood. Tunnels are formed in all directions in sap wood by the larvae of these beetles. The timber is converted into fine flour-like powder. They usually do not disturb the outer shell or cover. Hence, timber piece attacked by beetles may look sound till it completely fails. Marine Borers These are generally found in salty water. Most of the varieties of marine borers do not feed on wood. But they make holes or bore tunnels in wood for taking shelter. The diameter and length of these holes may go as high as 25 mm and 60 mm respectively. The wood attacked by marine borers loses colour and strength. It may be noted that no timber is completely immune from the attack of marine borers. Termites These are popularly known as ‘white ants’ and they are found in abundance in tropical and sub-tropical countries. These insects live in a colony and they are very fast in eating away the wood from core of the cross section. They make tunnels inside the timber in different directions and usually do not disturb the outer shell or cover. Hence, timber piece attacked by termites may look sound till it completely fails. Very few good timbers such as teak, sal, etc., can resist the attack of white ants. Such timbers have certain chemicals in their composition and the smell of these chemicals is not favourable for termites. Defects Due to Natural Force The main natural forces responsible for causing defects in timber are two, namely (i)
Abnormal growth, and
(ii)
Rupture of tissues.
Following defects are caused by these forces : (i)
Burls
(ii)
Chemical stain
(iii) Coarse grain (iv) Dead wood 87
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(v)
Knots
(vi) Shakes (vii) Twisted fibres (viii) Upsets (ix) Water stain (x)
Wind cracks
Burls These are also known as ‘excrescences’ and they are particularly formed when a tree has received shock or injury in its young age. Due to such injury, the growth of tree is completely upset and irregular projections appear on the body of the timber. Chemical Stain Wood is sometimes discoloured by the chemical action caused to it by some external agency. This is known as chemical stain. Coarse Grain If a tree grows rapidly, annual rings are widened. It is known as coarse grained timber and such timber possesses less strength. Dead Wood Timber which is obtained from dead standing trees contains dead wood. It is indicated by light weight and reddish colour. Knots These are bases of branches or limbs which are broken or cut off from the tree. The portion from which the branch is removed receives nourishment from the stem for a pretty long time and it ultimately results in the formation of dark, hard rings which are known as knots. As continuity of wood fibres is broken by knots, they form a source of weakness. Figure 4.2 shows a typical knot.
Figure 4.2 : Typical Knot
Knots are classified on the basis of their size and form. Tables 4.1 and 4.2 show the classification of knots on the basis of their size and form and quality. Table 4.1 : Classification of Knots on the Basis of their Size
88
Sl. No.
Type of Knot
1
Pin knot
2
Small knot
Size Diameter upto 6.5 mm Diameter between 6.5 mm and 20 mm
3
Medium knot
4
Large knot
Diameter between 20 mm and 40 mm
Timber
Diameter greater than 40 mm
Table 4.2 : Classification of Knots on the Basis of Form and Quality Sl. No.
Type of Knot
Remarks
1
Dead knot
The fibres of knot are not properly interconnected with those of surrounding wood. Hence, it can be easily separated out from the body of wood. It is not safe to use wood with such a knot for engineering purposes.
2
Decayed knot
It is also known as an unsound knot and it is formed by the action of fungi on wood.
3
Live knot
It is also known as a sound knot. It is free from decay and cracks. It is thoroughly fixed in wood and hence, it cannot be separated out from the body of wood. Presence of such knots makes wood difficult to plane. However, wood containing such knots can be used for engineering purposes.
4
Loose knot
It indicates preliminary stage of dead knot. The fibres of this knot are not firmly held in the surrounding wood.
5
Round knot
Cross-section of this type of knot is either round or oval. It is obtained by cutting the knot at right angles to its long axis.
6
Tight knot
It indicates preliminary stage of live knot. The fibres of knots are firmly held in the surrounding wood.
Rind Galls Rind means bark and gall indicates abnormal growth. Hence, peculiar curved swellings found on the body of a tree are known as Rind Galls as shown in Figure 4.3. They develop at points from where branches are improperly cut off or removed.
Figure 4.3 : Rind Galls
Shakes These are cracks which partly or completely separate the fibres of wood. Following are the different varieties of shakes : Cup Shakes These are caused by the rupture of tissue in a circular direction as shown in Figure 4.4. It is a curved crack and it separates partly one
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annual ring from the other. It develops due to non-uniform growth. It may not prove to be harmful, if it covers only a portion of ring.
Figure 4.4 : Cup Shakes
Heart Shakes These cracks occur in the centre of cross-section of tree and they extend from pith to sap wood in the direction of medullary rays as shown in Figure 4.5. These cracks occur due to shrinkage of interior part of tree which is approaching maturity. Heart shakes divide the tree cross-section into two to four parts.
Figure 4.5 : Heart Shakes
Ring Shakes When cup shakes cover the entire ring, they are known as ring shakes as shown in Figure 4.6.
Figure 4.6 : Ring Shakes
Star Shakes These are cracks which extend from bark towards the sap wood. They are usually confined upto the plane of sapwood. They are wider on the 90
outside ends and narrower on the inside ends as shown in Figure 4.7. They are usually formed due to extreme heat or frost.
Timber
Figure 4.7 : Star Shakes
Radial Shakes These are similar to star shakes. But they are fine, irregular and numerous. They usually occur when the tree is exposed to sun for seasoning after being felled down. They run for a short distance from bark towards the centre, then follow direction of annual ring and ultimately run towards pith. Figure 4.8 shows radial shakes.
Figure 4.8 : Radial Shakes
Twisted Fibres These are also known as wandering hearts and they are caused by twisting of young trees by fast blowing wind. The fibres of wood are twisted in one direction as shown in Figure 4.9. Timber with twisted fibres is unsuitable for sawing. It can, however, be used for posts and poles in an unsawn condition.
Figure 4.9 : Twisted Fibres
Upsets
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These are also known as ruptures and they indicate wood fibres which are injured by crushing or compression. Figure 4.10 shows a timber piece with this defect. Upsets are mainly due to improper felling of tree and exposure of tree in its young age to fast blowing wind.
Figure 4.10 : Upsets
Water Stain Wood is sometimes discoloured when it comes into with water. This is known as water stain and this defect is usually found in converted timber. Wind Cracks If wood is exposed to atmospheric agencies, its exterior surface shrinks. Such a shrinkage results in cracks as shown in Figure 4.11. These are known as wind cracks.
Figure 4.11 : Wind Cracks
Defects Due to Seasoning Following defects occur in seasoning process of wood. (i)
Bow
(ii)
Case-hardening
(iii) Check (iv) Collapse (v)
Cup
(vi) Honey-combing (vii) Radial shakes (viii) Split (ix) Twist 92
(x)
Timber
Warp
Bow The defect is indicated by the curvature formed in the direction of length of timber as shown in Figure 4.12.
Figure 4.12 : Bow
Case-hardening The exposed surface of timber dries very rapidly. It, therefore, shrinks and is under compression. The interior surface which has not completely dried is under tension. This defect is known as case-hardening and it usually occurs in timbers which are placed at the bottom during seasoning. Check A check is a crack which separates fibres of wood. It does not extend from one end to the other. Collapse Due to uneven shrinkage, wood sometimes flattens during drying. This is known as collapse. Cup This defect is indicated by the curvature formed in the transverse direction of timber as shown in Figure 4.13.
Figure 4.13 : Cup
Honey-combing Due to stress developed during drying, various radial and circular cracks develop in the interior portion of timber. Timber thus assumes honey-comb texture and the defect so developed is known as honey-combing. Radial Shakes Radial shakes are explained earlier. Split When a crack extends from one end to the other, it is known as a split. 93
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Twist When a piece of timber has spirally distorted along its length, it is known as twist. It is shown in Figure 4.14.
Figure 4.14 : Twist
Warp When a piece of timber has twisted out of shape, it is said to have warped.
4.9 DECAY OF TIMBER Timber is said to be decayed when it is so deteriorated that it loses its value as an engineering material. Various defects in timber have been mentioned earlier. When these defects are in excess, timber decays and such timber is not used for engineering purpose. Following are the various causes or situations which favour the early decay of timber : (i) Alternate dry and wet conditions. (ii) Bad storage or stacking of timber. (iii) Fungi which are responsible for developing diseases in timber such as blue stain, brown rot, dry rot, heart rot, sap stain, wet and white rot. (iv) Improper seasoning. (v) Insects such as beetles, marine borers, termites, etc. (vi) Keeping timber in with damp wall, damp earth, etc. (vii) Shocks or impacts received during young age from natural forces such as fast blowing wind, etc. (viii) Use of timber without taking out sap wood from its structure. (ix) Using seasoned timber without applying suitably preservative on its surface. (x) Using unseasoned wood with the application of protective coat of paint or tar.
4.10 FELLING OF TREES A tree should always be felled only after it has fully matured but before the heart wood starts deteriorating. If felling is delayed then decay would set in the heart wood which is the best and the most important part of a tree. Early felling would give lesser quantity of timber which has not yet developed full strength. 94
The lower we go the more is the timber that the trunk of tree yields as such it would be wise to cut the tree from a place a little below the ground level but higher up than the roots.
4.10.1
Timber
Process of Felling
(i)
In the process of felling we make a deep cut with axe at the lowest possible point of the trunk.
(ii)
The trunk may be then sawn to a point beyond the centre of gravity of the trunk.
(iii) This cut should be made on the side opposite to that on which it is intended to be felled. (iv) A cut is then made on a side opposite to the one on which first cut was made. (v)
Top of tree is then tied with ropes on all the four diametrically opposite sides. The rope on the side, the tree is to be felled is pulled and the one on the opposite side is loosened slowly.
(vi) The tree would break at the level of cuts and it should be allowed to fall gently otherwise it is likely to get damaged. (vii) After felling, its branches are chopped off and the log is cut to the needed sizes. It should be protected against rapid drying particularly at the ends. Its bark should be removed and the log sawn at the earliest.
4.11 SEASONING OF TIMBER The art of seasoning is to extract the moisture under controlled conditions as nearly as possible at a uniform rate from all parts of timber and to leave the remaining moisture that cannot be extracted, uniformly distributed throughout the mass. Irregular drying will cause irregular shrinkage resulting in the setting up of internal stresses between the fibres. When these stresses become strong enough to overcome the cohesion of the fibres then the timber warps and shakes are formed.
4.11.1
Objects of Seasoning
(i)
Seasoning makes timber resistant to decay.
(ii)
Seasoning makes timber lighter.
(iii) It becomes easier to paint and polish seasoned timber. (iv) It is easier to treat seasoned timber with preservatives. (v)
Seasoned timber becomes stronger and more stable.
(vi) Seasoning stops shrinkage of timber on drying. (vii) Seasoned timber has better electrical resistance.
4.11.2
Methods of Seasoning
As per the recommendations of IS : 1141-1973, the seasoning methods should be classified as : (i)
Natural seasoning or Air Seasoning
(ii)
Artificial seasoning or Kiln Seasoning
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Natural Seasoning or Air Seasoning The log is converted by sawing it into battens and planks etc. as soon as possible after felling of tree. These are then stacked on a well drained place in the shade. Care should be taken to ensure free circulation of fresh air all around each piece while stacking. The stacking should be done on masonry or concrete s a few centimeters above the ground. The pattern of stacking is as shown in Figure 4.15. Care should be taken not to expose the freshly converted timber stacked for seasoning to severe winds or to sun. This process of seasoning timber is the best as it gives very strong and durable timber, but it is extremely slow. It takes more than six months for timber to season in moderate climates.
Figure 4.15 : Stacks for Natural Seasoning
Kiln Seasoning or Artificial Seasoning This method of seasoning speeds up the seasoning process. This method of seasoning is a must for large scale production of seasoned timber. Kiln seasoning is done in a chamber equipped with arrangements for heating and humidifying the air to required conditions of relative humidity and temperature and for its circulation across the timber stacked in the chamber for seasoning. Usually, it is steam that is used for heating and humidifying the air in the kiln. The seasoning of the timber is started at a comparatively lower temperature and high humidity. As the timber dries, these conditions are gradually altered until at the end of the seasoning. The temperature of the air inside the chamber is fairly high and the humidity is low. The kiln charge is allowed to cool inside the kiln to within 15 to 20o C of the outside temperature before removal. Seasoning of timber by this method takes about four to five days under normal conditions. Relative merits and demerits of the two methods of seasoning are given in Table 4.3. Table 4.3 : Comparison of Air Seasoning and Kiln Seasoning Air Seasoning
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Kiln Seasoning
It is a slow process
It is a quick process
It is a simple and economical
It is quite technical and expensive
It is difficult to reduce moisture content
Moisture content can be reduced to
below 15 to 18%.
any desired level.
Air seasoned timber is more amenable to attacks of insects and fungi.
Kiln seasoned timber is less amenable to attacks of insects and fungi.
It requires more stacking space
It requires less stacking space
It gives stronger timber
A little weaker timber is obtained
Timber
4.12 STACKING OF TIMBER Timber, before seasoning, should be stacked in yards free from weeds and debris. The yard should have big shady trees to protect the timber from direct sun. Ends of logs should be protected against splitting by applying anti-splitting compositions and stacked on foundations in closed stacks in one or more layers. Stacks should be protected against direct sun by providing a covering if needed.
4.12.1
Methods of Stacking
(i)
One and Nine method
(ii)
Close crib method
(iii) Open crib method One and Nine Method This method of stacking timbers is best suited for moderately heavy coniferous timbers in hot climate and for heavy timbers in moist climates. Stacking pattern by this method is shown in Figure 4.16.
Figure 4.16 : One and Nine Method
Close Crib Method In this method, reduced air circulation slows down the pace of seasoning. This method is recommended for staking heavy structured timbers like sal in hot and dry localities. The stacking pattern is shown in Figure 4.17.
Figure 4.17 : Close Crib Method
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Engineering Materials
Open Crib Method This method is a modification of the close crib method and because of more air circulation taking place it is more akin to the one and nine method in its effects. The stacking pattern is shown is Figure 4.18.
Figure 4.18 : Open Crib Method
4.12.2
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Precautions to be Taken in Stacking Timber
•
Stacks of not more than 100 sleepers are recommended to be made
•
Poles are stacked either in closed heaps or with crossers. If stacked in closed heaps, then there should be alternate layers of butt ends and of top ends so that the two ends of the stack are level. Poles themselves could be used as crossers, which should not be spaced more than three metres.
•
Fence posts should be stacked in open crib fashion in which successive layers of posts are at right angles to each other and there is a gap of about 8 cm between adjacent posts in the same layer. Centre to centre distance between crossers should not exceed 1.5 m and the height of stack should not exceed 3 metres.
•
Horizontal stacking of sawn timber is done on vertical pillars of treated timber, brick masonry or of cement concrete 30 cm square in section and 30 to 45 cm high. The pillars are spaced 1.2 m centre to centre along the length and the breadth of the stack. The length of material to be stacked decides the length of stacking unit. Long beams of cross-section 10cm × 10cm and above are placed on the foundation pillars to form a framework for stacking timber.
•
Scantlings and squares should be stacked with crossers 5 cm × 4 cm in section and spaced 2.5 m to 3 m apart. The ends should be protected with moisture proof coatings.
•
Planks should be stacked on level platform with crossers of uniform thickness and section, which should be in vertical alignment in a stack. Longer planks should form the bottom of the stack and the shorter one’s the top. Heavy wooden beams should be placed on the top to prevent top layers from warping. A gap of about 2.5 cm should be left between ading planks for free circulation of air in the centre of stack. The stack should be protected against rain and sun by providing a shed over it.
4.13 PRESERVATION OF TIMBER
Timber
Timber has to be protected from the attack of insects, e.g. white ants etc., and from internal decay due to dry and wet rots. Perfect seasoning is the most effective means of preservation. Timber should be so used that either it is wholly dry and well ventilated or is wholly under water. It will not decay when kept under water but it will become soft and weak. Proper damp proofing of the building and providing free circulation of air around the built in portions of timber are essential for the preservation of the timber used. However, when these conditions cannot be obtained then preservatives have to be applied for preservation. Timber should be well seasoned before the application of preservatives as otherwise the preservatives would block the pores of timber thereby causing its decay due to the entrapped moisture. Direct with lime mortar should be avoided while using preservative with masonry.
4.13.1
Methods of Preservation of Timber
Following are some of the common methods of preservation adopted (i)
Charring
(ii)
Tarring
(iii) Painting (iv) Creosoting (v)
Wolman salt
(vi) Ascu treatment (vii) Fire proofing of timber Charring Lower ends of the posts that are to be embedded in ground are generally charred with a view to prevent dry rot and attack of worms. It is done by quenching the ends of posts in water after they are charred on wood fire to a depth of 1.5 cm. Tarring It consists in coating with tar or tar mixed with pitch. Embedded portions of timber fence posts, ends of door and window frames, battens and beams built in wall are usually tarred. Tarring is not done in case of those portions of structural that are open to view, because of unsightly black colour. Painting A paint when applied to timber acts not only as a good preservative but also it enhances the appearance of the surface so treated. Only well seasoned timber should be painted as otherwise the moisture entrapped in the timber, because of the closing of timber bores by paint, would cause decay. Paints however, protect seasoned timber against moisture thereby prolonging its 99
Engineering Materials
life, e.g. soligum paints have excellent preservative properties and protect timber against the attack of white ants. Creosoting Creosote oil is a dark brown thick oily liquid. Thoroughly seasoned timber dried for 24 hrs before its treatment is placed in an airtight chamber. After the air has been exhausted from this chamber, the creosote oil is then pumped in at a pressure of 9 kg/ cm2 at a temperature of 50o C so long as the timber is not fully saturated with oil. The oil preserves the timber from rot and from the attacks of white ant. Uses •
It is used in case of railway sleepers, piles and transmission poles.
Limitations •
Undesirable colour and smell, inability to take paint well and the tendency to stain plaster limit its use.
Wolman Salt This salt consists of creosote and sodium fluoride and is soluble in water. It is odourless and leaves no stain on wood. After treatment, timber could be painted or varnished. These salts destroy many kinds of fungi that cause timber to rot. This renders the timber extremely fire resistant too. Treatment of timber with zinc chloride, sodium fluoride, magnesium, silico fluoride or copper sulphate renders the timber immune from the attacks of fungi. The timber so treated is capable of being painted on drying. Ascu Treatment Ascu is available in the form of powder and is made up of three chemicals mixed in the ratios given below : (i)
1 part by weight of hydrated arsenic pentaoxide (As2 O5.2H2 O)
(ii)
3 parts by weight of blue vitriol (CuSO4 . 5H2 O)
(iii) 4 parts by weight of potassium dichromate (K2 Cr2 O7 . 2H2O) Six parts of this powder are mixed with 100 parts by weight of water. Ascu solution can be applied or sprayed in two coats. To achieve better results, timber may be soaked in the solution and impregnated with it under pressure. The timber should be allowed to dry for three to six weeks. This treatment renders timber immune to the attacks of white ant. Ascu treated timber may be painted, varnished, polished or waxed. The solution is odourless. Fire Proofing of Timber Timber cannot be made completely fireproof, however, by treating as below it can be made fire resistant to a sufficient extent. 100
Soaking timber in ammonium sulphate, ammonium chloride, ammonia phosphate, sodium arsenate, zinc chloride etc. or spraying on timber, a
solution of sodium silicate, potassium silicate or ammonia phosphate etc. imparts fire resisting properties.
Timber
Abel’s methods of fire proofing timber is painting the surface first with a dilute solution of sodium silicate (Na2 SiO3) then with a cream like paste of slaked fat time and in the end with a concentrated solution of silicate of soda.
4.14 MEASUREMENTS OF TIMBER Sawn timber shall be measured as follows : Length Rounded or damaged end portions of the sawn timber shall be excluded from the length which shall be measured in metres. Fractions of a meter shall be rounded of to the nearest lower 0.01m. Width and Thickness It shall be nearest at the narrowest section in centimetres and shall be rounded off to the nearest centimetres. Volume It shall be measured in cubic metres correct to three places of decimal based on accepted sizes. The volume of a log of wood is calculated by the quarter girth formula as given below : 4
⎛G⎞ V =⎜ ⎟ ×L ⎝4⎠
where
V = volume in m3, G = girth in m, and L = length in m.
Dimensions Sawn timber is generally available in the following lengths and cross-sections : Length 2 m; 2.5 m; 3 m and 3.5 m Cross- section 20 × 10 cm; 25 × 12.5 cm 20 × 12.5 cm; 25 × 15 cm and
20 × 15 cm; 30 × 15 cm
4.15 TESTS OF TIMBER The following tests are carried out to check up the physical properties of a test piece of timber.
4.15.1 Moisture Content Test To determine the moisture contents of a specimen, a test piece 5 cm × 5 cm × 2.5 cm is taken and weighed fresh. It is then dried in an oven at a temperature of 103 + 2o C. The weight of the specimen in the oven is regularly observed till the variation in the last two observations does not exceed 0.002 gm. The specimen is then considered to have dried.
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Let W1 be the weight of fresh sample and W0 the weight of the oven dry specimen. Moisture content is then calculated as below : Percentage of moisture content =
4.15.2
W1 − W0 × 100 . W0
Specific Gravity Test
The specimens for the test shall be 5 × 5 × 15 cm pieces and free from usual defects. The specimens shall be weighed (W1) usually green, correct to 0.001 gm and volume measured (V1) by immersion method correct to 0.01 cm3. The specimens are then end coated with paraffin wax by immersion and left to air season at room temperatures till moisture content of about 12% are reached. The weight (Wr) and volume (Vr) of the specimen are then noted by the immersion method when the moisture content is r percent. The specimens are then kept in an oven at a temperature of 103 + 2o C till the weight becomes constant. Weight (W0) and volume (V0) are noted then : Specific gravity at test
=
W1 V1
Standard specific gravity
=
W0 V1
Over dry specific gravity
=
W0 V0
Moisture content r percent =
Wr − W0 × 100 % V0
=
W1 − W0 × 100 % V0
or
SAQ 1
102
(a)
Differentiate between hard wood and soft wood. Could a soft wood be stronger than hard wood?
(b)
Describe the characteristics of a good timber.
(c)
Explain the precautions required to be taken for prevention of wet rot and dry rot.
(d)
Explain as to why timber used for structural purposes should be properly seasoned?
(e)
Enumerate various defects in timber.
4.16 SUMMARY
Timber
In this unit, you have gained adequate knowledge of Timber as one of the important construction materials. Wood is used as structural elements in buildings for various purposes. The timber used for engineering purposes should be free from defects as far as possible. Defects free timber is possible only when tree has been felled at proper time and with adequate precautions. It is also necessary that timber should be properly seasoned and necessary preservatives are used before it is used for engineering purposes. Thus, you can see that like any other construction materials, timbers are also very important construction materials which play an important role in construction industry.
4.17 ANSWERS TO SAQs Refer the relevant preceding text in the unit or other useful books on the topic listed in Section “Further Reading” to get the answers of the SAQs.
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