Timber, Wood Based Products PDF

ROYAL UNIVERSITY OF BHUTAN COLLEGE OF SCIENCE AND TECHNOLOGY PHUNTSHOLING: BHUTAN Timber, Wood based products 8/28/2024 1 Timber Wood: is a hard and fibrous substance which forms a major part of the trunk...

ROYAL UNIVERSITY OF BHUTAN COLLEGE OF SCIENCE AND TECHNOLOGY PHUNTSHOLING: BHUTAN Timber, Wood based products 8/28/2024 1 Timber Wood: is a hard and fibrous substance which forms a major part of the trunk and branches of a tree. Timber: A wood which retains its natural physical structure and chemical composition and suitable for various engineering works. Lumber: Wood material that has been roughly sawn and cut into lengths. 8/28/2024 2 Introduction to TREES Trees are composed of roots, the trunk and the crown. In autumn, the sap descends and deposits in the form of a layer below the bark. This layer known as cambium layers hardens and form concentric rings. Cells formed in cambium layers are commonly referred as fibers and are cemented into groups by lignin. Arrangement of cells and fibers are the causes of grains of the wood. 8/28/2024 3 Introduction to TREES 8/28/2024 4  Introduction to TREES Rapidly growing trees with wide annual rings produce coarse grained wood. Slower growth with narrow rings produce fine grained wood. Wood is said to be straight-grained when elements are straight and run parallel to the pith. Wood is said to be cross-grained when the elements do not run parallel to the axis 8/28/2024 5 8/28/2024 6 Classification of TREES Trees are classified according to the mode of growth: a. Endogenous Trees: Grow by addition of new material inwards, e.g. Palm, Bamboo, etc. b. Exogenous Trees: Grow by the addition of new wood outwards, immediately under the bark. They are subdivided as conifers and deciduous. 8/28/2024 7 8/28/2024 8 Difference between Soft wood and Hard wood S. Property Soft Wood Hard Wood No. 1 Colour Lighter Darker 2 Growth Faster Slower 3 Weight Lighter Heavier 4 Density Low High 5 Annual Ring Distinct Indistinct 6 Strength Strong along Strong along and the grain across the grain 7 Conversion Easy Difficult 8/28/2024 9 Classification of Timber The classification is done according to IS: 399. a. On the basis of its Position Standing Timber – Living tree Rough Timber – Forms a part of felled tree Converted Timber – Are logs of timber sawn into planks, post, etc. 8/28/2024 10 Classification of Timber b. On the basis of Modulus of Elasticity Group A – Modulus of elasticity in bending above 12.5kN/mm2 Group B –Modulus of elasticity in bending above 9.8kN/mm2 and below 12.5kN/mm2 Group C –Modulus of elasticity in bending above 5.6kN/mm2 and below 9.8kN/mm2 8/28/2024 11  Classification of Timber c. On the basis of Seasoning Characteristics Timbers are classified upon their behaviour to cracking and splitting during normal air-seasoning. Highly refractory (Class A) are slow and difficult to season and free from defects. Moderately refractory (Class B) can be seasoned and free from surface defects. Non-refractory (Class C) can be rapidly seasoned and free from defects. 8/28/2024 12 Classification of Timber d. On the basis of Treatability. Is based upon the resistance offered by the heartwood to preservatives. Easily treatable. Treatable but complete preservation not easily obtained. Only partially treatable. Refractory to treatment. Very refractory to treatment. 8/28/2024 13 Characteristics of good timber Following are the important characteristics of good timber: a. Narrow annual rings b. Compact medullary rays c. Dark colour d. Uniform texture e. Sweet smell and a shining fresh cut surface f. Sonorous sound produced when struck g. Free from defects h. Heavy weight i. No woolliness at fresh cut surface 8/28/2024 14 1.4 Properties of wood a. Physical Properties b. Mechanical Properties Density and Specific Compressive Strength Weight Tensile Strength Bulk Density Moisture Movement Bending strength Shrinkage Stiffness Swelling Toughness Heat Conductivity Cleavability Sound Conductivity Hardness Resistance to action of Acids and 8/28/2024 Alkalis 15  a. Physical Properties i. Density and Specific Weight Mechanical property of clear wood is directly related to its density which is directly related to specific gravity. Percentage of moisture in wood has a very large effect upon the specific weight. True specific gravity of all species are approximately averages to 1.54 Apparent specific gravity and specific weight varies with density of wood. 8/28/2024 16  a. Physical Properties ii. Bulk Density Depends on volume of pores and moisture content of the wood. Bulk density value is used to determine the quality factor. Quality factor is the ratio of compressive strength to the bulk density. iii. Moisture Content Due to the movement of moisture, timber can shrink as well as swell. Moisture movement is different in all directions. 8/28/2024 17 a. Physical Properties iii. Moisture Content Moisture movement is more when cut parallel to the annual ring and less when cut at right angle. 8/28/2024 18  a. Physical Properties iv. Shrinkage It is the reduction in linear and volumetric dimensions when dried. Shrinkage occurs due to the evaporation of hygroscopic water/ moisture and not capillary moisture. There are three forms of shrinkage according to the method of conversion. Radial Shrinkage Tangential Shrinkage Longitudinal Shrinkage  Radial is 60% of tangential and longitudinal is negligible. 8/28/2024 19  a. Physical Properties iv. Shrinkage 8/28/2024 20  a. Physical Properties v. Swelling It is the increase in the linear and volumetric dimensions. Swelling is 7% higher in tangential direction then in radial direction. vi. Heat Conductivity Heat conductivity is quite low. It is 1.8times greater along the fiber then across. As bulk density of wood increases, the air entrapped and moisture content decreases, resulting in greater heat conductivity. 8/28/2024 21 a. Physical Properties vii. Sound Conductivity Velocity of sound in wood is 2-17 times greater than that in air. Wood has high sound conductivity. viii.Resistance to action of Acids and Alkalis Not affected by weak alkali solutions but decays in acidic medium. 8/28/2024 22 b. Mechanical Properties i. Compressive Strength Wood is strongest when subjected to compressive force parallel to the axis of growth. Compressive strength across/perpendicular to the fibre is much lower than that parallel to the fibre. Failure in compression parallel to the grain may occur due to collapsing or through lateral bending of the cells and fibres. 8/28/2024 23  b. Mechanical Properties ii. Tensile Strength Wood when subjected to tensile forces acting parallel to the grain it is found to have greater strength. Tensile strength parallel to the grain is 2-4 times greater than the compressive strength. Across grain, the tensile strength of wood is low. Failure along the fibre occurs by transverse rupture of elements. Failure in tension across grain occurs through separation of the cells and fibres in longitudinal planes. 8/28/2024 24  b. Mechanical Properties iii. Bending Strength Wood will withstand static bending. Initial failure is indicated by a wrinkling of overstressed compression fibres. Final failure occurs when the ultimate tensile strength is reached accompanied by snapping or breaking of individual fibres. iv. Shearing Strength Wood has low shearing strength along the fibres. Resistance of wood to cutting across the fibre is 3-4 times greater than that along the fibres. 8/28/2024 25 b. Mechanical Properties v. Stiffness It is the inability of a wood to move easily. Denser wood and well seasoned timber is more stiff than a green timber. vi. Toughness A wood which has a large capacity to resist shock or blows is called tough. Green timber is tougher than seasoned timber. Toughness is best measured by the energy of the blow required to rupture a beam in transverse impact. 8/28/2024 26 b. Mechanical Properties vii. Cleavability It is the measure of the ease with which wood may split. viii. Hardness It is defined and measured as resistance to indentation and resistance to scratching. 8/28/2024 27  1.5 Testing of Timber The evaluation of basic properties of timber has been done on the basis of tests carried out on small clear specimens of timber. i. Specific Gravity Test Since the mechanical property of seasoned timber vary with moisture content, therefore the moisture content and specific gravity of the specimen must be known. 8/28/2024 28 1.5 Testing of Timber ii. Volume Shrinkage Test V1 V 2 Wr  Wo Vol.Shrinkage  x100 MoistureContent  x100 V1 Wo iii. Radial and Tangential Shrinkage test l1  lr Wr  Wo R. / T.Shrinkage  x100 MoistureContent  x100 l1 Wo iv. Static Bending Strength Test Deflection is measured at suitable load intervals. Failure is recorded according to its appearance and development of cracks. 8/28/2024 29 1.5 Testing of Timber v. Impact Bending Strength Test Deflection due to weight of hammer is measured at the centre of the specimen. The hammer is dropped from different heights. vi. Compressive Strength Test Parallel to Grain Perpendicular to Grain vii. Tensile Strength test Parallel to Grain Perpendicular to Grain 8/28/2024 30 1.5 Testing of Timber viii.Cleavage Strength Test (Parallel to Grain) ix. Brittleness Test x. Torsional Strength Test xi. Moisture Content test 8/28/2024 31  1.6 Seasoning of Timber 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. Can also be defined as the process of drying the wood to a moisture content approximately equal to the average humidity of the surrounding. Rapid seasoning of timber must be avoided as it causes hardening and thus increases resistance to penetration of preservatives. 8/28/2024 32  1.6 Seasoning of Timber Some objectives of seasoning are as follows: Reduce shrinkage and warping after placement in structure Increase strength, durability and workability Reduce its tendency to split and decay Make it suitable for painting Reduce its weight 8/28/2024 33  1.6 Seasoning of Timber There are two methods of seasoning i.e., natural and artificial. i. Natural or Air Seasoning Log of wood sawn into planks of convenient sizes or timber are stacked in cross-wise direction in alternate layer under covered shed. Rate of drying is slow. Used extensively in drying large size structural timber. Single stack may be of height 3-4m. 8/28/2024 34  1.6 Seasoning of Timber ii. Artificial Seasoning Prevalent methods of artificial seasoning are as follows: a. Water Seasoning: Logs of wood kept completely immersed in water with larger ends pointing upstream. Consequently, the sap, sugar and gum are leached out and replaced by water. Logs are then kept in air to dry but this reduces the elastic property and strength. 8/28/2024 35 1.6 Seasoning of Timber 8/28/2024 36 1.6 Seasoning of Timber b. Boiling: Exposing to hot water or steam spray is a very quick method but an expensive one. Boiling in water for 3-4 hours removes the sap and then made to dry. Such timbers dry quickly but this method can not be done on a large scale. c. Kiln Seasoning: Adopted for rapid seasoning of timber on large scale to any moisture content. The scantlings are arranged for free circulation of heated air with some moisture/ super heated steam. The circulating air takes up moisture from the wood and ultimately seasons it. 8/28/2024 37 1.6 Seasoning of Timber c. Kiln Seasoning: For successful kiln seasoning, the timber should be brought to a high temperature as it will stand without injury before drying is began; otherwise the moisture in the hot outer fibre of wood will tend to flow towards the cooler interior. Loss of strength is less then 10% plus the wood is thoroughly and evenly dried. The two types of Kiln seasoning are as follows: 8/28/2024 38 1.6 Seasoning of Timber 1. Progressive: Timber enters from one end and travels on a trolley through chambers of various air conditions to progressively dry the wood. 8/28/2024 39  1.6 Seasoning of Timber 2. Compartment: This Kiln is a single enclosed container or compartment. The timbers are stacked in the same manner as air seasoning. The whole stack is seasoned till the required moisture content is reached. 8/28/2024 40 1.6 Seasoning of Timber d. Chemical or Salt Seasoning: Aqueous solutions of certain chemicals have lower vapour pressure than pure water. If outer layers of timber are treated with such chemicals the vapour pressure will reduce. Interior contains no salts therefore retains its original vapour pressure and then tends to dry as rapidly as possible. e. Electric Seasoning: Logs are placed in such a way that their two ends touches the electrodes. Current is passed through, being a bad conductor it will resist the flow of current. This will generate heat which results in drying. 8/28/2024 41 1.6 Seasoning of Timber f. Mc. Neill’s Process: Timber is stacked (1/3rd the capacity) and containing products of combustion of fuels in the fire place. Times required for complete seasoning is 15-60days. This method is the most expensive of all the methods. 8/28/2024 42  1.7 Preservation of Timber (IS 401) Well seasoned timber kept in dry area, immersed in water or buried underground, can often last for centuries. However, unprotected timber can decay easily due to water, fungi, bacteria, insects, etc. In order to give an extra layer of protection to an already well seasoned timber, preservatives are used. Preservative treatment does not improve the properties of timber. It creates an unfavourable conditions for fungi, water absorption, decay and other undesirable conditions. Some methods to prevent from fungus are as follows: 8/28/2024 43 1.7 Preservation of Timber (IS 401) Type 1-Oil Type Preservatives Applied over outside of exposed timber. Gives Unpleasant smell. Not suitable for timber which is to be painted. E.g. Creosote, carbolinium, etc. With or without admixture like petroleum or suitable oil. 8/28/2024 44  1.7 Preservation of Timber (IS 401) Type 2- Organic Solvent Preservatives These are insoluble in water. Consists of toxic chemical compounds like benzene-hexa-chloride, dichlorodiphenyl trichloro- ethane (DDT) and copper naphthenate. These chemicals are dissolved in solvents like naphta or petroleum products such as kerosene, spirit, etc. Treated timber can be painted, waxed or polished. Acetic Anhydride treatment: Used for protection of veneers, plywood and light lumbers against decay and attack by insects. 8/28/2024 45  1.7 Preservation of Timber (IS 401) Type 3- Water soluble preservatives Are odourless organic/ inorganic salts adopted for inside locations only. If applied outside, the salts can be leached by rain. E.g. Zinc chloride, boric acid(borax), etc. 8/28/2024 46 1.7 Preservation of Timber (IS 401) Various Treatment Processes i. Surface application: Done by dipping, spraying or brushing the preservative on a debarked timber. At least two coats should be applied. The second and subsequent coats should be applied only after the first one has dried. Mostly done on site. ii. Soaking treatment: Debarked timber is submerged in the preservative solution till the required absorption is obtained. 8/28/2024 47 1.7 Preservation of Timber (IS 401) iii. Hot and Cold process: Ensures sterilization against fungi and insects. Two baths are used. The first containing water where hot treatment is given to submerged timber. The second (cold bath) containing preservatives. 8/28/2024 48 1.7 Preservation of Timber (IS 401) iv. Boucherie Process: This process is similar to water seasoning. The log of wood is attached to the hose pipe which is connected to the reservoir containing preservative. Due to hydrostatic pressure, preservatives replaces the sap in wood. This treatment is stopped when the concentration of preservative at the lower end of the log is the same as that in the reservoir. 8/28/2024 49 1.8 Defects in Timber Defects principally occur during growing period, conversion and seasoning process. Defects are due to irregularities in the character of grains/rupture of tissues. Defects affect the quality and reduce the quantity of useful wood. Defects can be classified as: i. Defects due to Abnormal Growth. a. Checks: Are longitudinal cracks which are usually normal to the annual rings. It adversely affect the durability as it readily admits moisture and air. 8/28/2024 50  1.8 Defects in Timber i. Defects due to Abnormal Growth. a. Checks 8/28/2024 51 1.8 Defects in Timber b. Shakes: Longitudinal separation between annual rings. It reduces allowable shear strength. The three types of shakes are as follows: Heart Shake: Occurs due to shrinkage in heart wood due to over maturity. Cracks starts from the pith and runs towards the sapwood. Wider at center and diminishes outwards. Cup Shake: These appear as curved split which partly or wholly separate annual rings from one another. Occurs due to excessive frost action on sap. 8/28/2024 52 1.8 Defects in Timber Star Shake: These are radial splits or cracks and are wide at circumference and diminishes towards the center. Caused due to severe frost and fierce heat. 8/28/2024 53 1.8 Defects in Timber c. Rind gall: ‘Rind’ means bark and ‘Gall’ indicates abnormal growth. Is characterized by swelling caused by the growth of layer of sap over wounds after the branch has been improperly cut. The newly developed layers do not unite properly with the old rot, leaving cavities from which decay occurs. 8/28/2024 54 1.8 Defects in Timber d. Knots: Are the bases of twigs or branches buried by cambial activity. The root of branch embedded in the stem with the formation of annual rings at right angles to those of the stem. The knot interrupts the basic grain direction resulting in reduction of strength. It reduces workability and cleavability as fibres get curved. Knots can be classified on the basis of size, form, quality and occurrences. 8/28/2024 55 1.8 Defects in Timber e. End Splits: Caused by greater evaporation of sap at the end grains of log and can be reduced by painting. f. Twisted Fibres: Caused by wind constantly turning the trunk of young tree in one direction. Wood with twisted fibre is known as reaction wood. It is difficult to work with and on drying wood with twisted fibres suffer high shrinkage. 8/28/2024 56  1.8 Defects in Timber g. Upsets: Caused by crushing of fibres running transversely during the growth of the tree due to strong wind and unskilled felling consequently results in discontinuity of fibres. Twisted Fibres Upset Fibres 8/28/2024 57 1.8 Defects in Timber h. Foxiness: Sign of decay appearing in the form of yellow or red discolouration of over matured trees. i. Rupture: Caused due to injury or impact. j. Defects due to Conversion: Conversion defects are due to unsound practice in milling or attempts to economize during conversion of timber. k. Defects due to seasoning: Occurs due to the movement of moisture and change in moisture content. Excessive or uneven drying, exposure to wind and rain and poor stacking during seasoning can also produce distorted timber. Common types of seasoning defects are: 8/28/2024 58 1.8 Defects in Timber Checks: Longitudinal separation of fibres not extending throughout the timber. Splitting: Separation of fibres extending from one face to another. Warpage: Consists of cupping, twisting and bowing. 8/28/2024 59 1.9 Decay in Timber Timbers are adversely affected by fungi, insects, rodents, weather and chemical attack. Sapwood is more prone to decay than heartwood. i. Decay due to fungal and Bacterial Attack Wood is susceptible to attack by both bacteria and fungi. Bacteria are living organisms which do not cause any serious damage except for some discolouration. Bacteria and fungi attack in wet environment. Fungi is a system of plant organism which attack timber causing rot and decay. 8/28/2024 60 1.9 Decay in Timber Fungi produces spores which send out mycelia which intern destroys the wood tissue by secreting chemicals and enzymes. Basic requirement for fungi is moisture therefore, seasoned and converted timber does not develop fungal decay. Some fungi provides moisture by themselves and thrives even during fairly dry condition leading to dry rot. Controlling method: Timber should not be subjected to alternative wet and dry conditions. When unavoidable, should be treated with preservatives. Provide adequate ventilation. 8/28/2024 61 1.9 Decay in Timber ii. Damage due to insects: Insect attack can develop in growing trees, freshly felled trees, partly or recently seasoned timber. Most insects are killed when timber is seasoned with a exception to Powder-Post Beetles. Powder-post beetles remain active unless Kiln sterilized. Termites, beetles and ants are the insects which are responsible for damaging the timber. 8/28/2024 62 1.9 Decay in Timber Termites: Are small white ants with vast colonies and cast to carry on specialized function. They excavate the wood leaving the outer shell intact. Beetles: Small insects causing rapid decay by converting timber into fine powder. Powder-post beetles: These lay eggs in the pores and the larva that comes out eats through the wood, leaving fine powder. Even seasoned timbers containing sapwood is not immune. Controlling method: Can be done by applying insecticide in gas form. Commonly used is turpentine mixed with orthodichlorobenzene. 8/28/2024 63 1.9 Decay in Timber iii. Damage due to Rodents: They are capable of penetrating both wood and concrete. Controlling method: Close all opening or passage. 8/28/2024 64 1 Self study 1. Method of Conversion 2. Properties of commonly available timbers in Bhutan. 8/28/2024 65 2. Wood Products Wood based products have been developed to economize the use of timber, These products are manufactured under controlled conditions. Have desired shapes, dimensions, strength and durability. Raw materials for these products are wood waste and parts of tree which are not used for construction. Gluing material governs the durability. 8/28/2024 66 2.1 Veneer (IS: 14315) Is the primary process in the manufacture of wood based products. Thickness varies from 0.4-0.6mm. Logs are kept in cold storage to avoid end splitting. The barks are then removed and cut into veneers. There are two types of veneer according to their cutting process. Rotary Veneer Sliced veneer Veneers are classified into two types or surfaces: 8/28/2024 67  2.1 Veneer (IS: 14315) SI No. Defect Categories Type of surface A B 1 Discolouration Nil 5% 2 Dote 50mm/m2 150mm/m2 3 Insect hole Scattered up to 12 Scattered up to holes/m2 24holes/m2 4 Joints One joint for every No restriction multiple of 200mm provided, 5 Knots(Dead) 2 no. up to 12 mm 4 no. up to 20mm dia/m2 dia/m2 6 Pin Knot (Dead) 2no./m2 6 no.m2 8/28/2024 68  2.1 Veneer (IS: 14315) SI No. Defect Categories Type of surface A B 7 Pin Knot (Live) No restriction No restriction 8 Knots(tight) 6 no. upto 25mm No restriction dia/m2 9 Patches 4 Patches/m2 Any number provided provided they are all they are all tight patches tight patches 10 Splits 2 splits, each not 3 splits, each not more more than 1mm than 4mm wide wide 11 Swirl Unlimited No restriction 8/28/2024 69 2.2 Plywood Layers or piles of veneers glued together in odd numbers (3,5,7,etc.) is known as plywood. Improves dimensional stability, stiffness and strength. Outer most veneers are called faces. Interior ply/piles having grain direction parallel to that of the faces are termed as core/centre. Ply/ piles having grain direction parpendicular to the faces are termed as cross bands. 8/28/2024 70 2.2 Plywood Classification of Plywood Plywood for general purposes should be of 2 grades depending upon the bond strength developed by the adhesive. a. Boiling Water Resistant (BWR) Grade and b. Moisture Resistant (MR) Grade Plywood is also classified based on appearance. a. Type AA b. Type AB c. Type BB This is based on the quality of the two surfaces. The better surface is known as ‘face’ and the opposite side is known as ‘back’. 8/28/2024 71 2.2 Plywood Plywood manufacturing process After cutting the veneer, they are dried to a particular moisture content depending on the type of resins used. After heating, it is cooled. The cooling section regulates ambient air over the veneer to reduce temperature. Veneer that did not achieve its desired moisture are made to redry. Veneers are sometimes glued at the edges to form larger sheets. This process is known as composing. 8/28/2024 72 2.2 Plywood Plywood manufacturing process After drying they are conveyed to a layup operation, where the resin is spread over the veneer. Generally, resins are spread on both sides of a ply of veneer, which is then places between two plies of veneer that are not coated with resins. This laid up veneers are sent to a hot press in which it is consolidated under heat and pressure. Hot pressing has two main objectives: To press the glue into a thin layer over each sheet and To activate thermosetting resins. 8/28/2024 73 2.2 Plywood Plywood manufacturing process Plywood are taken to finishing process to trim the edges. The face and back of the plywood may or may not be sanded smooth. 8/28/2024 74 2.2 Plywood Merits of Plywood It has good strength both along and across the grains. Since plywood is cross-grained construction, the tendency to shrink or swell is reduced. It has better splitting resistance due to their grain arrangement. High grade plywood is superior to most metals in strength-to-weight ratio. 8/28/2024 75 2.3 Block Boards (IS: 1659) Are also known as solid core ply boards. The core of board is made by strips of wood each not exceeding 25mm width, forming a slab, glued between at least two veneers. Veneers can b either rotary or sliced cut. Cross bands thickness varies between 1-3mm. 8/28/2024 76 2.3 Block Boards (IS: 1659) Face veneer thickness between 0.5-1.5mm. Direction of grain of core is perpendicular to the adjacent outer veneer. Grades and types of boards are: ICOM- Interior grade commercial type IDEC- Interior grade decorative type XCOM- Exterior grade commercial type XDEM- Exterior grade decorative type They are also sub-graded to: Grade 1- Exterior grade used for bus bodies, railways coaches, prefabricated houses, etc. Grade 2- Interior grade for furniture, partition, paneling, ceiling, etc. 8/28/2024 77 2.4 Batten Boards and Lamin Boards Batten boards have core made up of 80mm wide wood pieces, forming a slab and glued between at least two surface veneers. Lamin boards have core made up of wooden strips of less than 7mm in thickness. The grain of core is perpendicular to the outer adjacent veneers. 8/28/2024 78 2.5 Particle Boards ( IS: 3129, 12406, 3478) Also known as chip boards, chip core or composition board. Manufactured from particles of wood or other lignin cellulose materials which are agglomerated, formed and pressed together by the use of an organic binder in presence of heat and pressure. Manufactured from small timber pieces and wood waste. 8/28/2024 79