TIMBER

Introduction Oldest material used in construction Timber, word derived from timbrian which means to build. Denotes wood suitable for building works or engineering purposes. Different terms Converted timber-sawn and cut into suitable sizes Rough timber-obtained after felling a tree Standing timber-timber contained in a living tree

Properties of timber Amenability to mechanical working Small bulk density High strength Drawbacks of timber as engineering material Susceptibility to decay Fluctuations in properties due to change in moisture content Variations in strength in length and across fibre

CLASSIFICATION OF TREES Exogenous Trees : Conifers & Deciduous increase in bulk by growing outward distinct consecutive rings – annular rings most suitable for engineering purpose eg. Teak, deodar, sal Endogenous Trees grows inwards fiborous mass is in their longitudinal section very limited engineering application eg. Bamboo, cane, palm

STRUCTURE OF A TREE Consist of three parts : trunk, crown, roots Trunk : to support the crown and to supply nutrients from roots to leaves Roots: to implant the trees in the soil Macrostructure: Structure of wood visible to naked eye or at small magnification Microstructure: Structure of wood visible at great magnification

CROSS SECTION OF AN EXOGENOUS TREE

Pith innermost central portion core of the tree consist of cellular tissues nourishes the plant in young age Heart Wood inner annular rings surrounding the pith dark in colour imparts rigidity to the tree provides strong, compact and durable timber for engineering purposes Sap Wood outer annual rings between heart wood and cambium layer light in colour indicate recent growth, contains sap active part in growth of tree

Cambium layer thin layer of sap between sapwood and inner bark sap not been converted into sap wood Inner Bark inner skin which covers cambium layer give protection to cambium layer Outer Bark outer skin of tree dry and dead layer which prevents from external weathering Medullary rays thin radial fibers extending from pith to cambium layer hold together the annular rings of heart wood and sap wood

BAMBOO

VARIETIES OF TIMBER

BAMBOO

BANYAN

SIMUL

BIJASAL (VENTEAK)

COCONUT

TAMARIND

HOPEA

SEASONING OF TIMBER Fresh timber – contains 50% of dry weight as water – has to be dried The process of drying timber is known as seasoning of timber – getting rid of sap wood and moisture content Timber becomes lighter and less bulky Strength get increased Seasoning – controlled conditions – uniform rate Irregular seasoning – shrinkage

OBJECTIVES OF SEASONING OF TIMBER To impart hardness, stiffness, strength. To make timber safe from fungi, insects. To make timber easily workable. To decrease the weight of timber. To maintain shape and size when used. To make timber suitable for receiving paints, varnishes etc. To reduce tendency to crack, shrink and warp

METHODS OF SEASONING Natural seasoning / Air seasoning Artificial seasoning

Natural seasoning Timber is cut into suitable sections of planks. A platform is constructed 30 cm above GL. Timber stacked horizontally or vertically. The layers are separated by dry pieces of wood (spacers) of thickness 25 mm to 35 mm. Spacers may be kept at 450-600 mm. The stack is protected from rain & sunlight.

Natural seasoning

Advantages of natural seasoning Does not required any skilled supervision. Moisture content of wood can be brought down to about 10-20 %. Economical. Cheap and simple.

Disadvantages Less control over air. Non uniform drying Fungi and insects attack during seasoning. Time consuming

Artificial seasoning Water seasoning Boiling Electrical seasoning Chemical seasoning Kiln seasoning

Water seasoning Logs of wood are immersed in running water of stream for 2-4 weeks and dried in open air.

Boiling Timber is boiled in water for 3-4 hours, taken out and dried slowly. Period of seasoning is less More expensive. Less strength.

Electrical seasoning Passing high frequency AC through the timber. The timber gets heated and dries out. Results in uniform seasoning Very rapid method Capital cost of equipment is more

Chemical seasoning Green timber is soaked in saturated solution of suitable salt then removed and seasoned. Interior surface of timber dries in advance of exterior Less chance of external cracks.

Kiln seasoning Drying of timber is carried out inside an airtight chamber or oven. Heated air of temp 35-38oC is forced inside the chamber. Temperature maintained till a desired degree of moisture content is attained.

Preservation of timber The main principle in preservation of timber is application of a suitable preservative on the surface of the timber Commonly used preservatives are oil paints, coal tar, creosote oil, solignum paints, ascu and chemical salts Objectives of Preservation of timber 1. to increase the life of timber structures 2. to make timber structure durable 3. to protect timber from attack of fungi, insects etc. .

REQUIREMENTS OF GOOD PRESERVATIVES Capable of covering large area with small quantity Cheap and easily available Durable, not affected by heat, light etc. Free from unpleasant smell Non – inflammable Efficient in killing fungi, insects etc. Safe and harmless for persons and animals Not affect strength characteristics of timber High resistance to moisture and dampness

Methods of preservation Brushing Charring Dipping and steeping Hot and cold open tank treatment Injecting under pressure Spraying Fire proofing of timber

Brushing Preservative solution applied over timber using good quality brushes Simplest form, adopted for seasoned timber Pre-requisite : cracks should be filled up before brushing Several coats of preservative may be applied at proper intervals to ensure efficient absorption of preservatives

Charring Adopted for lower ends of timber posts before embedding in ground Kept wet for ½ an hour Burnt over fire (15 mm deep) – layer of coal is formed Cooled with water

Dipping and steeping Timber soaked or dipped in a solution of preservative for a short period ( few hours to days to week) Ensures better penetration of preservatives

Hot and cold open tank treatment Timber is submerged in a tank of preservative solution for few hours at 850 – 950 C Tank is then allowed to cool down gradually with the timber still soaked inside the tank Effective in giving protection to sap wood

Injecting under pressure Preservative is injected into the timber Mainly adopted in creosoting Most effective method of preserving timber Requires special treatment plant Specially suited for treating non durable timber (prone to attack of fungi and insects)

Spraying Preservative is filled in spraying pistol and is then applied on timber surface under pressure Pistol works under compressed air Effective and superior than brushing

Types of preservatives Ascu Chemical salt treatment : Zncl, NaF etc… Coal tar Creosote oil Oil paints Solignum paints

Ascu treatment Ascu – developed by Forest Research Institute, Dehradun Ascu – mixture of hydrated arsenic pentoxide, copper sulphate and potassium or sodium dichromate in 1:3:4 proportion Ascu – available in powder form 6 parts of ascu is mixed in 100 parts of water to form a solution Spray the solution on the timber surface

Chemical salts treatment Solution of different salts are prepared and is then applied on timber surface Zinc chloride Sodium fluoride Magnesium silico – fluoride Copper sulphate Odourless and non inflammable

Coal tar Timber - coated with hot coal tar with the help of a brush ( coal tar becomes workable when heated) Process is called as tarring Unpleasant smell and appearance Makes timber unsuitable for painting but makes it fire resistant

Creosoting Creosote oil – dark brown liquid Obtained by distillation of tar Well seasoned timber dried for 24 hrs Kept in airtight chamber and air removed Creosote oil pumped at 7 to 10 kg/cm2 at 50°C till full saturation at 480 C

Oil paints Coated with 2-3 coats of oil paint Pre-requisite : timber should be well seasoned or else the sap would get confined and causes decay Makes it durable

Solignum paints Preserve the timber from white ants, as they are highly toxic in nature; lead free glossy paints Can be mixed with colour pigments and can be applied on timber in hot state with the help of a brush Timber surface may be given desired colour and appearance