Wood and wood products.

Presentation on theme: "Wood and wood products."— Presentation transcript:

1 Wood and wood products

2 Wood -Introduction Wood is a hard and fibrous substance forming the major part of the trunk and branches of a tree Does not age Natural and man made Natural – timber, lumber Manmade – composite wood materials in which natural wood is just a basic ingredient of a matrix or a laminate

3 Why is wood preferred over other building materials?
Easily available Easy to transport and handle More thermal insulation More sound absorption and electrical resistance Ideal to be used in sea water Good absorber of shocks – earthquake resistance Easily worked and repaired

4 Uses of wood In buildings as doors, windows, frames, partition walls, roof trusses and ceilings, form work

5 Classification of trees
Based on the mode of growth - Endogenous and Exogenous Endogenous trees – trees grow endward Exogenous trees – trees grow outward and are used for structural elements – conifers and deciduous Conifers – evergreen trees having pointed needed like leaves – light weight Deciduous – flat board leaves – heavy weight

6 Growth of trees During spring – sap (ساب) formation
During autumn – sap hardens and forms an outer ring – concentric rings/ annual rings – age of the log Timber should be felled as soon as it is matured. If it is felled when the sap is vigorous in movement, the timber decays. If the tree is cut young, it yields softwood and tends to decay

7 Structure of timber A tree can be divided into three parts – crown (branches and leaves), trunk and roots

8 Structure of timber In the cross section the main parts of a tree are bark, phloem, cambium, xylem, sapwood, heart wood, pith

9 Characteristics of a good timber
Strength, durability, finishing appearance – principal characteristics Narrow annual rings Compact medullary rays Dark colour Uniform texture Sweet smell and a shining fresh cut surface When struck a sonorous sound is produced Free from the defects in timber Heavy weight

10 Seasoning of timber Timber cut from freshly felled trees is too wet for normal use. Therefore it needs to be seasoned Objective of seasoning Reduce the shrinking and warping after placement in structure Increase strength, workability and durability Reduce its tendency to spilt and decay Make it suitable for painting Reduce its weight Seasoning is the process of reducing the moisture content of timber in order to prevent the timber from possible fermentation and making it suitable for use

11 Methods of seasoning Timber can be seasoned naturally or artificially
Natural or Air-Seasoning The log of wood is sawn into planks of convenient sizes and stacked under a covered shed in crosswise direction in alternate layers – to permit free circulation of air. The rate of drying is very slow. Air seasoning reduces the moisture content of the wood to 12-15%.

12 Methods of seasoning Artificial seasoning Water seasoning Boiling
Kiln seasoning 2. Boiling Boiling in water or exposing the wood to the action of steam spray is very quick but expensive. Water seasoning The logs of wood are kept immersed in running water. Consequently the sap, sugar and gum are leached out and are replaced by water. The logs are then kept out to dry. It is a quick process but the strength of the wood is reduced.

13 Methods of seasoning 3. Kiln seasoning
Rapid seasoning of timber on large scale to any moisture content. Free circulation of heated air with some moisture or superheated steam. This seasoning is the best because there is little loss in strength (less than 10%)

14 Defects in timber Defects may be naturally occurring or can be man-made. Natural defects can be due to many reasons such as environmental factors, growth patterns, soil composition, etc. Man-made defects can occur at many points ... from the felling of the tree, transport, storage, sawing, drying, etc Bow A curve along the face of a board that usually runs from end to end Cupping Warping along the face of a board across the width of the board. This defect is most common of plain-sawn lumber. Crook Warping along the edge from one end to the other. This is most common in wood that was cut from the centre of the tree near the pith

15 Defects in timber Checking
A crack in the wood structure of a piece, usually running lengthwise. Checks are usually restricted to the end of a board and do not penetrate as far as the opposite side of a piece of sawn timber. Split A longitudinal separation of the fibres which extends to the opposite face of a piece of sawn timber. Twist Warping in lumber where the ends twist in opposite directions.

16 Defects in timber Wane The presence of bark or absence of wood on corners of a piece of lumber Pitch An accumulation of resinous material on the surface or in pockets below the surface of wood. Also called gum or sap.

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18 Diseases of timber Dry Rot
Decomposition of felled timber caused by the action of various fungi. The fungi(الفطريات) reduces fibre to a fine powder. Occurs when the timber is not seasoned well and placed in a moist air having no free access of air. Remedy – when exposed to sunlight or air, the fungi dies. The best solution is to cut away the affected part and paint the remaining part. Wet Rot When subjected to alternate wet and dry conditions, decomposition takes place. Remedy – well seasoned timber is used with preservatives and paints

19 Particle Board/ Chip board Block board/ Lamin board
Wood products Many wood based products have been developed to economize on the use of timber. These wood products are manufactured under controlled conditions. Veneers Plywood Fibre board Particle Board/ Chip board Block board/ Lamin board

20 Wood products Veneers (القشرة)
Primary process in the manufacture is veneering which produced thin sheets of wood ( mm) known as veneers. Best example of tree for veneering is walnut. First the logs are kept in wet storage and softened by heating with hot water or steam and the bark is removed. The log is then cut into veneers. Used in the manufacture of plywood and other laminated boards

21 Wood products Used for partitions, ceilings, doors, concrete form works Plywood (الخشب الرقائقي) A wood panel glued under pressure from an odd number (3-13) of layer of veneers is known as plywood. When the piles are bonded with water soluble glues used for interior grade plywoods. When the piles are bonded with phenol formaldehyde adhesive, it is used for exterior grade plywoods, which is completely water proof.

22 Wood products Fibre boards
These boards are built up of felting (compressing (ضغط), heating and chemical treating (التعامل مع المواد الكيميائية) from wood or vegetable (wood waste, paper waste, agricultural waste). For better performance wood preservatives and other admixtures are added to the pulp. They are manufactured in varied densities – soft, medium and hard. Soft boards – walls and ceilings Medium boards – panelling, partition walls, doors and windows Hard boards have one surface smooth and the other one textured. Trade names – Masonite, Celotex, Essex boards

23 Wood products Particle boards or Chip boards
Manufactured from particles of wood which are agglomerated, formed and pressed together by the use of an organic binder together in the presence of heat, pressure or moisture. Eucalyptus, rubber wood, waste of saw mill. These provide dimensional stability, smooth uniform surface and no difficulty in nailing. Used as partitions, ceilings, floor slabs doors and windows, furniture, etc

24 Wood products Block board/ Lamin board
The core of block boards is made up of strips of wood each not exceeding 2.5cm in width, forming a slab, glued between at least two surface veneers. Extensively used for the construction of railway carriages, bodies of buses, marine and river crafts, partitions and furniture, etc Batten boards and Lamin boards are examples of block boards