Engineering Materials WEEN 334 Timber Cross section of a tree Classification of Timber • Mode of Growth a) Exogens (Exogenous)- these trees grow outwards » Conifererrous Trees » Decidous Trees b) Endogens (Endogenous)- These trees grows inwards Classification of Timber • Modulus of Elasticity • Group A: E=12.5 GPa • Group B: E=9.8GPa to 12.5GPa • Group C: E=5.6 GPa to 9.8GPa • Durability • High durability • Moderate durability • Low durability Classification of Timber • Seasoning Characteristics • High refractory (Class A • Moderately refractory (Class B • Non refractory (Class C) • Grading • Select Grade • Grade I • Grade II Identification of Timber • General Properties • Color- a darker color in wood indicates greater durability • Odor-it is present only on freshly cut trees • Hardness- is the ability of wood to withstand indentations caused by harder bodies • Density- denser woods are generally stronger • Grain- depending on the actual alignment the grain may be straight, spiral, interlocked, wavy or irregular • Texture- in hardwoods, the texture depends upon the size and distribution vessels and rays. In softwoods, it is determined by the size and distribution of tracheid. Structure of softwoods • Tracheids- these are hollow, needle shaped units closely packed together so that a cross section through them resembles a honeycomb. • Storage tissues (Rays and wood parenchyma cells-these are too small to be seen individually and are not of much value in wood identification. • Resin Canals- these are cavities in the wood lined by the parenchyma cells • Growth Rings- these are normally distinct in softwoods. Structure of Hardwoods • Pores or vessels- these are tubular dements running in a vertical direction and serve to conduct water and nutrients • Inclusions- the pores or vessels are normally open to enable conduction but many times particularly in the heartwood, they become filled with inclusions of various types • Parenchyma or soft tissues- these are storage tissues and consists of small, thin walled, rectangular or brick-shaped cells. They are vertically aligned. • Rays- are groups of horizontally aligned parenchyma cells running in a radial direction and their function is horizontal conduction and storage. • Fibres- these are vertically aligned thick walled elements with pointed tapering ends which makes up the bulk of the weight of the wood. • Intercellular cells- these are long tubular cavities seen in the horizontal or vertical directions. Felling and Conversion of Timber • Felling and Transportation of Timber • Felling is done by a clean saw cut as close to the base as possible. • Transportation of logs to saw mills depends on the infrastructure available. In Kenya logs are transport by road. In areas with large rivers such as the Amazon, logs are floated down the river. For long distance hauling, railway transport is preferred. • Conversion (Sawing) • There are 2 main Methods of sawing 1. Ordinary or flat sawing 2. Quarter sawing – Others include i. Tangential sawing ii. Radial sawing iii. Combination sawing Conversion of Timber • Some advantages of Plain sawing • Shrinks and swells less in thickness • Surface appearance less affected by round or oval knots compared to effects of spike knots in quarter sawn boards • (boards with round or oval knots not as weak as boards with spike knots) • Shakes and pitch pockets when present extends through fewer boards • Figure patterns resulting from annual rings and some other types of figure brought out more conspicuously • Is less susceptible to collapse in drying • Costs less as it is easy to obtain Conversion of Timber • Some advantages of Quarter sawing • Shrinks and swells less in width • Cups, surface-checks and splits less in seasoning and in use. • Raised grain caused by separation in annual rings does not become as pronounced • Figure patterns resulting from pronounced rays, interlocked and wavy grain are brought out more conspicously • Does not allow liquids to pass through readily in some species • Holds paint better in some species • Sapwood appears in boards at the edges and the width of such sapwood is limited by the width of the log Sawing of Timber Moisture of Timber • Moisture Content- it is one of the most important characteristics of timber that affects its use and its site performance. • Wood is hygroscopic i.e it picks up or gives up moisture relative to the humidity and temperature of the surrounding atmosphere. Changes in MC causes – changes in strength e.g. Bending strength can increase by 50% in going from green to a moisture content (MC) found in wood members in a residential structure. – Changes in size- shrinks as it dries and swells as it picks moisture with concomitant warpage potential. Moisture of Timber • Moisture content of wood is defined as the weight of water in wood expressed as a fraction, usually a percentage, of the weight of ovendry wood. • Weight, shrinkage, strength, and other properties depend upon the moisture content of wood • In trees, moisture content can range from about 30% to more than 200% of the weight of wood substance. • In softwoods, the moisture content of sapwood is usually greater than that of heartwood. • In hardwoods, the difference in moisture content between heartwood and sapwood depends on the species. Green Wood and Fiber Saturation Point • Moisture can exist in wood as – liquid water (free water) or water vapor in cell lumens and cavities – water held chemically (bound water) within cell walls. • Green wood is often defined as freshly sawn wood in which the cell walls are completely saturated with water; however, green wood usually contains additional water in the lumens. • Fiber Saturation Point is the moisture content at which both the cell lumens and cell walls are completely saturated with water= the maximum possible moisture content • Specific gravity is the major determinant of maximum moisture content. • Lumen volume decreases as specific gravity increases, so maximum moisture content also decreases as specific gravity increases because there is less room available for free water. Equilibrium Moisture Content • The moisture content of wood below the fiber saturation point is a function of both relative humidity and temperature of the surrounding air. • Equilibrium moisture content (EMC) is defined as that moisture content at which the wood is neither gaining nor losing moisture Seasoning of Timber • Seasoning is the controlled process of reducing the moisture content of timber so as to make it suitable for intended use • Objectives of Seasoning • To reduce movement of timber, to reduce the tendency to split • Make it immune to Fungal and insect attacks • To increase strength, durability, workability and resilience • Make the timber receptive to finish like preservatives, paints and varnish • Reduce weight and minimize cost of transport • Reduce Distortion and warping • Make timber burn more readily if used as fuel Seasoning of Timber % Moisture Content 22-20 Situation of use Limit of air seasoned timber 20 Limit for the occurrence of dry rot 16 Outdoor furniture 12-14 Occasionally heated areas 11-13 Heated areas 9-11 Very heated areas Seasoning of timber • Principles of seasoning – Factors that control seasoning 1. Relative humidity 2. Rate of air circulation 3. Temperature • Methods of seasoning 1) Natural/air seasoning 2) Artificial seasoning i. Kiln seasoning ii. Chemical seasoning iii. Electric seasoning 3) Water seasoning Natural seasoning • Timber is dried by direct action of air, wind and sun. • Timber is roughly sawn to size and stacked in open air using spacers called stickers • Vertical spacing is achieved by using timber battens (25mm) of the same species. The piling sticks should be spaced close enough to prevent bowing (600-900mm) but to allow free flow of air • The final stack should be protected from the direct influence of the elements • The ends of the beams should ideally be painted to prevent splitting. • Generally, natural seasoning requires a few months up to a year. Natural seasoning • Advantages – Cheap – Small labour cost once stack is made – Environmentally friendly • Disadvantages – Very slow – Large area of space required for a lot of timber – Only dries timber to approximately 20% MC therefore leaving it open to some insect and fungal attacks – Timber so seasoned is only suitable for outdoor joinery – Splitting is common at the end of the logs causing wastage. Kiln seasoning • There are two main types of kilns used in artificial seasoning – Compartmental kilns – Progressive kilns • Both methods rely on a controlled environment to dry out the timber and require the following factors – Forced air circulation (fans, blowers etc) – Humidity control (steam jets) • The amount and duration of air, heat, and humidity depends on – Species – Size – Quantity • As a general rule, the atmosphere in the kiln will at first be cool and moist. The temperature is gradually increased and the humidity decreased until the required MC is attained. Compartmental Kilns • This is a single enclosed container or building • The timber is stacked in the same manner as in natural seasoning • Whole stack is seasoned using a program of settings (temperature and humidity) until the whole stack is reduced to the required MC. Progressive Kilns • A progressive kiln has the stack on trolleys that progressively travel through a sequence of chambers • Each chamber has varying conditions that change the MC of the timber as it passes through • Advantage of this system is the availability of a continuous flow of seasoned timber. Kiln seasoning • Advantages – Faster due to higher temperatures, controlled ventilation and air circulation – Achieves lower MC – Defects associated with drying can be controlled – Allows more precise rates of drying for various timber species and thickness of boards • Disadvantages – Expensive – Requires skilled labour – Uses a lot of energy. Chemical seasoning • In chemical seasoning, carbon dioxide, ammonium carbonate or urea are used as agents for seasoning. • They are applied in dry state • These chemicals are more hygroscopic than timber. They absorb the moisture from the outer layers of the logs. This creates a vapour pressure gradient and the moisture is gradually drawn from the inner layers of timber. • Advantage is that it ensures uniform seasoning. Electric seasoning • In this method an electric current is passed through the timber logs. Water Seasoning • In this method, timber logs are kept immersed whole in a flowing stream of water. • The sap present in the timber is washed out. • The logs are then taken our and air dried to remove the free water Seasoning characteristics of timber • Class A (Highly refractory wood)- these timbers are slow and difficult to dry if the final product is to be free from cracks and splits • Class B (Moderately refractory woods)- these timbers shows a moderate tendency to crack and split during seasoning • Class C (non refractory woods)- these timbers can be rapidly seasoned to be free of defects. Finding the MC • A moisture meter is most commonly used to establish the MC of a particular batch of timber • These meters are usually attached to 2 probes which send an electrical signal through the wood. • Water is a conductor of electricity and the more water present, the higher the conductivity. • Manual method of establishing the MC is to remove random samples from the stack. Each sample is weighed, then placed in an oven until the moisture has evaporated. The sampled is weighed again. • The % MC is calculated from the two weights as Wet weight-dry weight X 100 =%MC Dry weight Seasoning and Shrinkage • Seasoning causes positive changes such as increase in strength but also distortion and shrinkage. • The greatest amount of shrinkage takes place tangentially along the grain with little loss over the radial direction and along the length of the board. • Because of these varying shrinkage rates, tangential boards tend to cup because of the geometry of the annual rings. • Outer rings are longer than inner rings and therefore shrinks more Physical properties of Timber • Specific Gravity (SG): – Generally, specific gravity (SG) and the major strength properties of wood are directly related. SG for the usually used structural species ranges from roughly 0.30 to 0.90. Higher allowable design values are assigned to those pieces having narrower growth rings (more rings per mm) or more dense latewood per growth ring and, hence, higher SG • Moisture Content (MC) and Shrinkage: – wood’s reaction to moisture provides more problems than any other factor in its use. – Wood is hygroscopic, which causes changes in strength; – E.g bending strength can increase by about 50% in going from green to a moisture content (MC) found in wood members in a residential structure Physical properties of Timber • Moisture Content (MC) and Shrinkage: – Wood shrinks as it dries, or swells as it picks up moisture, The swelling and shrinkage processes are reversible and approximately linear between fiber saturation point and 0% MC. – Wood decay or fungal stain does not occur when the MC is below 20%. – There is no practical way to prevent moisture change in wood; – Most wood finishes and coatings only slow the process down. Thus, vapor barriers, adequate ventilation, exclusion of water from wood, or preservative treatment are absolutely essential in wood construction. Physical properties of Timber • Thermal Properties/Temperature Effects: • Although wood is an excellent heat insulator, its strength and other properties are affected adversely by exposure for extended periods to temperatures above about 40°C. • The combination of high relative humidity or MC and high temperatures, as in unventilated attic areas, can have serious effects on roof sheathing materials and structural elements over and above the potential for attack by decay organisms. • At temperatures above 100°C, wood takes on a thermoplastic behavior. • This characteristic, which is rarely encountered in normal construction, is an advantage in the manufacture of some reconstituted board products, where high temperatures and pressures are utilized. Physical properties of Timber • Environmentally friendly • Strong and lightweight • Safe-Timber has low toxicity and therefore requires no special safety precautions to work with it • Easy to install • Durable • Comfortable • Flexible Mechanical Properties • Tensile strength-Timber is stronger in tension along the grain • Compressive Strength- the strength along the grain is important for columns, props and posts. • Shear Strength- Shear strength is important in the case of beams and slabs • Brittleness is used to describe the property of suffering little deformations before breaking. • Cleavability- High resistance for cleavage is important for nailing and screwing while low spliting strenghth is important for wood used as firewood • Hardness- is important when timber is used as paving blocks, floor bearings and other similar purposes. Defects in Timber • Knots- are dark, hard pieces occurring as signs of branches broken or cut off. They are classified according to – Size (pin, small, medium and large) – Form (round, oval, splay and spike) – Quality (live, sound, tight and dead) – Quality (loose, knot hole, decayed knot) Timber defects-shakes • Shakes- or cracks caused by rupture of tissues resulting in partial or complete seperation of the fibers along the grain – Star shakes – Heart shakes – Cups – Ring – Radial Timber defects • Rind Galls-these are typical curved swelling formed upon the trunk of the tree by successive layers of the sapwood • Upsets- are clipping or buckling of fibers caused by crushing of fibers when the trunk is felled violently over a rocky surface. Timber defects • Twisted or spiral grain- fibers or wandering hearts are caused by the prevalent wind turning or twisting the trees at its young age constantly in one direction • Wind cracks- are shakes on the outside of a log due to the shrinkage of the exterior surface caused by the atmospheric influences Timber defects-due to seasoning • Bow- a curvature of the timber in the direction of its length. • Cup- a curvature in the transverse direction of the timber • Twist- a spiral distortion along the length of the timber • Spring- a curvature of the timber in its own plane Timber defects-due to seasoning • Case hardening- caused by unequal drying of the exterior surface under compression and the interior surface under tension due to rapid drying • Split- separation of the fibers along the grain and extends from one end of the plank to the other • Honey Combing- separation of the fibers in the interior due to drying stresses • Collapse- the cells of the timber are flattened due to excessive shrinkage. Defects due to manufacturing or use • Chipped or Torn grain- a defect caused by the breaking away of timber below the surface of the dressing by the action of the planning tool • Chip Mark- indentation on the finished surface of the timber caused by the chips or other small pieces of the timber being carried around on the planning knife edges • Wane- the original round surface of the tree that remains on the finished prank. • Boxed heart- the pith at the centre fully enclosed within one piece in the process of conversion Evaluation of defects • Knots- tend to weaken timber in tension but may improve the strength in compression • Checks, splits and shakes- these reduce shearing resistance of the timber • Compression wood- increases density and shrinkage along the grain and decreases shock resistance. • Tension Wood- it has high longitudinal shrinkage increasing its tendency to warp and split • Sap Rot- this results in decreased shock resistance and increased absorption • Slope or Grain- this lowers the tensile strength parallel to the grain and the modulus of rupture which increases in the case of radial slope of grain Evaluation of defects • Loose Grain- weaker than closed grain timber • Wane- this reduces the mechanical properties and volume • Worm hole deep large worm holes spoil the appearance and reduce the mechanical properties of timber • Pith Pockets-affects the appearance and mechanical properties • Boxed heart-in cut sizes of timber, this defect has little or no effect on quality • Discoloration of heartwood- has no effect on quality • Mould-spoils appearance of the wood. • Sap Stain does not affect the quality Decay of Timber • Decay or Rot- is usually caused by the presence of sap or by the alternate dry and wet conditions in timber – Forms of decay • • • • Wet rot Dry rot Fungal attack Insect attack- such as termites, beetles and borers Preservation of Timber • Three main methods of preservative application – Painting – Dipping – Pressure/Empty cell process • In painting and dipping preservatives rarely ingress beyond 1/16’’. Ingress is determined by duration of contact and temperature of preservative • In pressure/empty cell process, timber is placed in an air tight container and a vacuum. Preservative is then pumped in at 100-200psi. Very good penetration is achieved. • Timber treated in this manner is mostly used as piles in water or for railway sleepers. Name of Compound Use Advantages Polyborate Preservative. Guards internal framing against insect attack Self diffusing. Water soluble. colourless CCA (copper, chromium arsenic) Preservative. Guards timber used externally against fungal and insect attack. Water soluble in formulations. Fixes in wood. Broad spectrum biocide. Carbendazim Anti-Sapstain Low toxicity. Stable in aqueous formulations in which it forms a suspension. TBTO (tri-butyl tin oxide) Preservative. Guards above ground Colourless. Difficult to leach. timber (e.g. piles) against insect and fungal attack. Creosote Preservative. Guards timber used externally (e.g. railway sleepers) against insect and fungal attack. Broad spectrum biocide. Good use of an otherwise undesirable product. Preservation of timber • Characteristics of a good preservative – Should be toxic to insects and fungi – Should disperse readily into the timber – Should be cost effective – Should be easy to apply – Should form stable solutions in water (should not react with water) – Should be difficult to leach – Should not increase flammability of timber Availability and uses of timber • Advantages of timber – In terms of specific strength, it can be used for both load bearing and non load bearing structures – Can easily be converted to any size and shape – With the advent of superior adhesives, wastage in timber construction can be minimized – Has high resale and salvage value – It has better insulating properties than most other construction materials – When properly seasoned and preserved, it is highly durable – It is light weight – has good aesthetics
