2 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 termsConverted timber-sawn and cut into suitable sizesRough timber-obtained after felling a treeStanding timber-timber contained in a living tree
3 Properties of timberAmenability to mechanical workingSmall bulk densityHigh strengthDrawbacks of timber as engineering materialSusceptibility to decayFluctuations in properties due to change in moisturecontentVariations in strength in length and across fibre
4 CLASSIFICATION OF TREES Exogenous Trees : Conifers & Deciduousincrease in bulk by growing outwarddistinct consecutive rings – annular ringsmost suitable for engineering purposeeg. Teak, deodar, salEndogenous Treesgrows inwardsfiborous mass is in their longitudinal sectionvery limited engineering applicationeg. Bamboo, cane, palm
6 STRUCTURE OF A TREE Consist of three parts : trunk, crown, roots Trunk : to support the crown and to supply nutrients from roots to leavesRoots: to implant the trees in the soilMacrostructure: Structure of wood visible to naked eye or at small magnificationMicrostructure: Structure of wood visible at great magnification
9 Pithinnermost central portioncore of the treeconsist of cellular tissuesnourishes the plant in young ageHeart Woodinner annular rings surrounding the pithdark in colourimparts rigidity to the treeprovides strong, compact and durable timber forengineering purposesSap Woodouter annual rings between heart wood and cambium layerlight in colourindicate recent growth, contains sapactive part in growth of tree
10 Cambium layerthin layer of sap between sapwood and inner barksap not been converted into sap woodInner Barkinner skin which covers cambium layergive protection to cambium layerOuter Barkouter skin of treedry and dead layer which prevents from external weatheringMedullary raysthin radial fibers extending from pith to cambium layerhold together the annular rings of heart wood and sap wood
20 SEASONING OF TIMBERFresh timber – contains 50% of dry weight as water – has to be driedThe process of drying timber is known as seasoning of timber – getting rid of sap wood and moisture contentTimber becomes lighter and less bulkyStrength get increasedSeasoning – controlled conditions – uniform rateIrregular seasoning – shrinkage
21 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
22 METHODS OF SEASONING Natural seasoning / Air seasoning Artificial seasoning
23 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 mm.The stack is protected from rain & sunlight.
26 Advantages of natural seasoning Does not required any skilled supervision.Moisture content of wood can be brought down to about %.Economical.Cheap and simple.
27 Disadvantages Less control over air. Non uniform drying Fungi and insects attack during seasoning.Time consuming
28 Artificial seasoning Water seasoning Boiling Electrical seasoning Chemical seasoningKiln seasoning
29 Water seasoningLogs of wood are immersed in running water of stream for 2-4 weeks and dried in open air.
30 BoilingTimber is boiled in water for 3-4 hours, taken out and dried slowly.Period of seasoning is lessMore expensive.Less strength.
31 Electrical seasoningPassing high frequency AC through the timber. The timber gets heated and dries out.Results in uniform seasoningVery rapid methodCapital cost of equipment is more
32 Chemical seasoningGreen timber is soaked in saturated solution of suitable salt then removed and seasoned.Interior surface of timber dries in advance of exteriorLess chance of external cracks.
33 Kiln seasoningDrying 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.
34 Preservation of timber The main principle in preservation of timber is application of a suitable preservative on the surface of the timberCommonly used preservatives are oil paints, coal tar, creosote oil, solignum paints, ascu and chemical saltsObjectives of Preservation of timber1. to increase the life of timber structures2. to make timber structure durable3. to protect timber from attack of fungi, insects etc..
35 REQUIREMENTS OF GOOD PRESERVATIVES Capable of covering large area with small quantityCheap and easily availableDurable, not affected by heat, light etc.Free from unpleasant smellNon – inflammableEfficient in killing fungi, insects etc.Safe and harmless for persons and animalsNot affect strength characteristics of timberHigh resistance to moisture and dampness
36 Methods of preservation BrushingCharringDipping and steepingHot and cold open tank treatmentInjecting under pressureSprayingFire proofing of timber
37 BrushingPreservative solution applied over timber using good quality brushesSimplest form, adopted for seasoned timberPre-requisite : cracks should be filled up before brushingSeveral coats of preservative may be applied at proper intervals to ensure efficient absorption of preservatives
38 CharringAdopted for lower ends of timber posts before embedding in groundKept wet for ½ an hourBurnt over fire (15 mm deep) – layer of coal is formedCooled with water
39 Dipping and steepingTimber soaked or dipped in a solution of preservative for a short period ( few hours to days to week)Ensures better penetration of preservatives
40 Hot and cold open tank treatment Timber is submerged in a tank of preservative solution for few hours at 850 – 950 CTank is then allowed to cool down gradually with the timber still soaked inside the tankEffective in giving protection to sap wood
41 Injecting under pressure Preservative is injected into the timberMainly adopted in creosotingMost effective method of preserving timberRequires special treatment plantSpecially suited for treating non durable timber (prone to attack of fungi and insects)
42 SprayingPreservative is filled in spraying pistol and is then applied on timber surface under pressurePistol works under compressed airEffective and superior than brushing
43 Types of preservatives AscuChemical salt treatment : Zncl, NaF etc…Coal tarCreosote oilOil paintsSolignum paints
44 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 proportionAscu – available in powder form6 parts of ascu is mixed in 100 parts of water to form a solutionSpray the solution on the timber surface
45 Chemical salts treatment Solution of different salts are prepared and is then applied on timber surfaceZinc chlorideSodium fluorideMagnesium silico – fluorideCopper sulphateOdourless and non inflammable
46 Coal tarTimber - coated with hot coal tar with the help of a brush ( coal tar becomes workable when heated)Process is called as tarringUnpleasant smell and appearanceMakes timber unsuitable for painting but makes it fire resistant
47 Creosoting Creosote oil – dark brown liquid Obtained by distillation of tarWell seasoned timber dried for 24 hrsKept in airtight chamber and air removedCreosote oil pumped at 7 to 10 kg/cm2 at 50°C till full saturation at 480 C
48 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 decayMakes it durable
49 Solignum paintsPreserve the timber from white ants, as they are highly toxic in nature; lead free glossy paintsCan be mixed with colour pigments and can be applied on timber in hot state with the help of a brushTimber surface may be given desired colour and appearance