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Civil Engineering 3
Civil Engineering
University of Kerala
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CONVERSION OF TIMBER
The process of sawing and cutting of timber into suitable sections is known as the conversion of timber. There are four types of sawing of timber. 1 or bastard or flat or slab sawing 2 sawing 3 sawing 4 sawing In ordinary sawing the saw cuts are tangential to the annual rings. In quarter sawing the saw cuts are at right angles to each other. In radial sawing the saw cuts are made radially in a parallel direction to the medullary rays.
DEFECTS IN TIMBERS
The followings are the five main types of defects in timber:
- Defects due to Natural Forces
- Defects due to Attack by Insects
- Defects due to Fungi
- Defects due to Defective Seasoning
- Defects due to Defective Conversion
Defects in timber
Defects in Timber due to Natural Forces :-
Knots: Knots are the most common defects caused due to natural forces. During the growth of a tree, branches close to the ground or lower branches die. Bases of those branches remain in the tree as the trees grow. These bases may create imperfection known as knots.
Twist: Twist in timber rotates the ends of the timber in opposite directions. The main reason behind this defect is twisting of
the trees by the strong wind.
Shakes: Shakes are timber defects that occur around the annual ring or growth ring of a timber. In other words, cracks or splits in the woods are called shakes.
Defects in Timber due to Attack by Fungi:- Stain: When fungi feed only on sapwood, where the food materials are stored, it causes a stain. Heartwood doesn’t
Heart rot- heart wood is exposed to the attack of atmospheric agents; tree becomes weak and it gives out hollow sound when struck with a hammer Wet rot- some fungi cause chemical decomposition of wood; convert timber into greyish brown powder White rot- certain types of fungi attack lignin of wood; wood assumes appearance of white mass consisting of cellulose
compounds
Defect in Timber due to Defective Seasoning:- Faulty method of seasoning causes serious defects in woods. During seasoning of timber, exterior or surface layer of the timber dries before the interior surface. So, stress is developed due to the difference in shrinkage. In a perfect seasoning process, stress is kept minimum by controlling the shrinkage. Some of the defects resulting from defective seasoning are as follows:- Bow: Curvature formed in direction of the length of the timber is called bow. Cup: Curvature formed in the transverse direction of the timber is called a cup. Check: Check is a kind of crack that separates fibers, but it doesn’t extend from one end to another.
Split: Split is a special type of check that extends from one end to another. Honey Combing: Stress is developed in the heartwood during the drying process or seasoning. For these stresses, cracks are created
in the form of honeycomb texture.
Chip mark- indicated by marks or signs placed by chips on the finished surface of timber Diagonal grain- formed due to improper sawing of timber Torn grain- defect is caused when a small depression is formed on the finished surface of timber by falling of a tool or so Wane- denoted by presence of original rounded surface on the manufactured piece of timber
SEASONING OF TIMBER
Air drying timber stack. Wood drying (also seasoning lumber or wood seasoning) reduces the moisture content of wood before its use when the drying is done in a kiln. The product is known as kiln dried timber or lumber whereas air drying is the more traditional method. Methods of seasoning:-
NATURAL DRYING
Strength: Timber framings are strong in real structural terms
Esthetic appeal: timber is a beautiful material for
both internal and external appearance
QUALITIES OF GOOD TIMBER
Good timber should have the following qualities HARDNESS A good quality timber should be hard enough to resist deterioration. STRENGTH It should have sufficient strength to resist heavy structural loads. TOUGHNESS It should have enough toughness to resist shocks due to vibrations. It should not break in bending and should resist splitting. Timbers having narrow annual rings, are generally the strongest. ELASTICITY It should have the property of elasticity so as to regain its original shape after removal of loads. This is a very important property to be considered if the timber is used in making sport goods. DURABILITY It should be able to resist attacks of fungi and worms and also atmospheric effects for a longer period of time.
DEFECTS
Timber should be prepared from the heart of a sound tree and be free from sap, dead knots, shakes and other similar defects.
Qualities of good timber
FIBRES AND STRUCTURE It should have straight and closed fibers and compact medullary rays. It should give a clear ringing sound when struck. Dull heavy sound is an indication of internal decay. Its annual rings should be uniform in shape and colour. structure of a timber APPEARANCE AND COLOUR Freshly cut surface should give sweet smell and present shining surface. It should have dark colour, as light colored timbers are generally weak in strength. SHAPE AND WEIGHT It should retain its shape during the process of seasoning. Heavy timbers are always stronger than light weight timbers. WORKABILITY
The preservation of wood includes painting and other applied finishes. Timber preservation became necessary because naturally durable timbers became scarce, so ways of improving the durability of cheaper timbers were developed. Timber needs to be protected from: Insects and fungi Wear and tear Effects of weathering
PRESERVATIVE TYPES
The main forms of preservative are:
- Tar oil preservatives
- Water borne preservatives
- Solvent-based preservatives. These preservatives can protect the timber in two ways.
- They give physical protection by stopping fungi and insects getting at the wood.
- They give chemical protection, making the wood poisonous to insects and fungi. Paint, for example, gives excellent physical protection.
TAR OILS
- Normally dark liquids
- Made from coal-tar mixed with chemicals
- Suitable for outdoor use
- Black / Brown colour
- Example: creosote (ESB poles)
- Applied by brushing, spraying, dipping or pressure treatment
TAR OILS
Advantages Toxic to fungi and insects Easily absorbed by the wood Long lasting Inexpensive Disadvantages Strong smell Toxic to plants Difficult to paint over
WATER-BORNE PRESERVATIVES
Made from mixture of toxic salts(copper and zinc) and water. When applied to wood, water evaporates leaving toxic salts to act as preservatives
- Usually pressure impregnated
Safety
All preservatives must be handled with care.
- Read any instructions on the container carefully.
- At all times ensure it is safe for use in the room or house and that it will not harm people, animals or the environment in any way.
- When working with any chemicals you should wear all the appropriate protective clothing.
- Remember that safety and care of the environment also applies to the disposal of cartons and containers which hold the
preservatives.
BRUSHING AND SPRAYING
Preservatives can be applied using a brush or a spray gun. This method is the easiest to use and is not too costly. The preservative, however, doesn't penetrate deep into the wood and the treatment needs to be repeated regularly to keep
the wood protected.
IMMERSION
The wood is placed into a container of preservative.
The wood absorbs the preservative over a short time.
The immersion can be done for a short time (dipping) or for a longer period (steeping).
The immersion process penetrates better than brushing and can be even more effective if the preservative is heated.
PRESSURE TREATMENT
The most effective method of applying preservative is by forcing the preservative into the wood under pressure. This pressure can come from inside (vacuum) or outside (pressure). The timber is put into a treating cylinder.
- The air is sucked out of the cylinder and held for a few minutes to let all the air come out of the wood.
- The cylinder is then flooded with preservative while the vacuum is held. The vacuum is released allowing the preservative to take the place of the air in the wood.
- Then pressure is applied forcing the preservative into the wood.
- A final vacuum is applied for a short time to remove any excess
Civil Engineering 3
Course: Civil Engineering
University: University of Kerala
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