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TYPES OF TREES:Trees are classified into twogroups depending upongrowth pattern.(1) Endogenous trees:The trees which grow inwardsin longitudinal fibrous massare called endogenous trees.
(2) Exogenous tress :The trees which grow in out wardsacross horizontal section of stemare called exogenous trees.These trees are only fit forengineering construction.Exogenous trees are againsubdivided in to two types. 8
Conifers or Evergreen: They give soft wood. They have pointed leaves.Examples: Deodar Pine Chir Kail,etc 9
Deciduous: These have hard wood. These have broad leaves.Examples: Teak Sal Shisham, etc. 10
SEASONING OF TIMBER:As fresh timber which is obtainedfrom trees contains about 30 to 40% sap or moisture. This sap is veryharmful for the life of a timber.Therefore, it is necessary to removethat sap by applying some specialmethods. All those methods whichare used for removing the sap fromtimber are collectively termed asseasoning of timber. 11
Advantages of seasoned timber: It has reduced weight, It is strong and durable, It has resistance to decay or rot, It takes high polish, It is easier to work, Its life is more. 12
Types of Timber Seasoning:The main types of timber seasoning areas under.(1)Natural Seasoning,(2)Artificial Seasoning,(a) Kiln Seasoning,(b) Chemical Seasoning,(c) Electric Seasoning,(3) Water Seasoning, 13
(1) Natural Seasoning:In the air seasoning or natural seasoning ornatural drying, seasoning of timber, timberis dried by direct action of air, wind andsun. In this method, the timber logs arearranged one over the other, keeping somespace or distance between them for aircirculation of fresh air.Generally this type of seasoning requiresfew months to over a year, this is very slowprocess. 14
(2) ARTIFICIAL SEASONING(a) Kiln Seasoning,(b) Chemical Seasoning,(c) Electric Seasoning, Developed by:G.S.Solangi 16
(a) Kiln Seasoning:In kiln seasoning timber is placed in achamber with some special heatingarrangement.In this process one thing should bekept in mind that heating systemshould be under control, other wisetimber will be crack or wrap. The timerequired for this seasoning is 3 to 12days. This is quick process. 17
(b) Chemical Seasoning:In chemical seasoning carbondioxide, ammonium carbonate orurea are used as agents forseasoning, those are applied in drystate, the inter surface of timberdries first than outer side.This ensures uniform seasoning.The time required for thisseasoning is 30 to 40 days. 19
(c) Electric Seasoning:In this method electric currentis passed through the timberlogs. The time required for thisseasoning is 05 to 08 hours. 20
Electric Seasoning Developed by:G.S.Solangi 21
(3) Water Seasoning:In water seasoning, timber logs arekept immersed whole in the flowingwater. The sap present in timber iswashed away. After that logs aretaken out from water and are keptin open air, so water present intimber would be dried by air. Thetime required for this type ofseasoning is 2 to 4 weeks. 22
USES OF TIMEBR:Timber is used in:1.Building construction,2.Construction of house posts,3.Construction of beams,4.Construction of rafters,5.Construction of bridges,6.Construction of piles, poles and railway sleepers, Continued-------- Prepared by:G.S.Solangi
7. For furniture making,8. For light packing cases,9. For high packing cases (for machinery and similar stores),10. For manufacturing of agricultural implements,11. For making toys, etc,12. For manufacturing of veneers and ply woods. 24
VENEER:Thin sheet of uniform thickness ofwood is called veneer. PLYWOOD:Veneers used for making plywood areknown as plies and ply wood is madeby gluing together plies in oddnumbers. Gluing is done underpressure. 25
DEFECTS IN TIMBER:Most common defects in timber are:1. Heart Shakes2. Star Shakes3. Cup Shakes4. Radial Shakes5. Rind Galls6. Wind Cracks7. Knots8. Dead Wood 26
(1) Heart Shakes:These are splits occurring in the centre of thetree and running from the pith (inner mostpart) to wards the sap wood from themedullary (vascular tissues) rays.In some timbers, these splits are hardly visibleand in some timbers these are quitepermanent.Heart shakes are caused due to shrinkage ofinterior parts due to age. A heart shakestraight across the trunk is not a seriousdefect. 27
Medullary Rays:These are thin horizontal veinsradiating from the pith to wards thebark. These carry sap from outer sideto inner side.
(2) Star Shakes:These are splits which radiate fromthe centre of the timber or from thebark (outer side), running in theplanes of medullary rays.These occur due to severe frost orscorching heat of the sun. 30
(3) Cup Shakes:These are curved splits whichseparate the whole or part of oneannual ring from an other. Theseare caused due to the unequalgrowth of the timber. 32
(4) Radial Shakes:These are similar to the starshakes and occur in felled timberwhen exposed to the sun duringseasoning.Radial shakes are generallyirregular, fine and numerous.In this many splits are appeared. 34
DETERIORATION OF TIMBER (OR) DECAY OF TIMBER: 46
DETERIORATION OF TIMBER (OR)DECAY OF TIMBER:There are so many agencies whichmay cause decay of timber. But thereare three main harmful agencieswhich cause timber decay.
(1) Decay or Rot: Decay or rot of timber is the result of the activity of various bacterias and fungi, which utilize various portions of timber as food; they require both oxygen and excess moisture. Hence timber having moisture content below 25 % will not rot easily.
(2) Insects:There are so many insectswhich attack the wood, out ofthem termites (white ants) arethe main insects which are verydangerous for timber. Termitesof one class live under groundand eat wood and forms tubesor tunnels inside it.
(3) Fire: Fire is also the damaging factor for timber. As timber has tendency to burn, so fire can damage it easily.
TIMBER PRESERVATIVES:There are three main classes of timberpreservatives.(1) Oily substances insoluble in water(2) Water soluble salts
(1)Oily substances insoluble in water:Coal tar oil is the best known andwidely used preservative material ofthis class. It is obtained during thedestructive distillation of bituminouscoal. It is available in many grades andtypes. It has high degree of penetration.It has highly toxic effect to wooddestroying fungi.
(2) Water soluble salts:Zinc chloride is the most extensivelyused preservative of this type. It isclean and odourless.
SPECIAL PAINTS TO SAVE TIMBERFROM FIRE:Following paints are used to savetimber from fire. Diammonium phosphate, Mono ammonium phosphate, Mono magnesium phosphate, Phosphoric acid.
METHODS OF APPLYINGPRESERVATIVES:Before applying preservatives, thetimber should be completely seasoned.There are some important methods ofapplying timber preservatives which aregiven below.1.Painting and dipping method2.Pressure process or full cell process3.Empty cell process
(1) Painting and dipping method:This is the most common method inwhich the preservative material isapplied by means of a brush severaltimes. The timber is also immersed in atank full of liquid (preservative material).In both types the penetration hardlyexceeds 1/16’’. The duration ofimmersion and temperature of solutionis increased the penetration rate.
(2) Pressure process or full cell process:In this process, the timber is placed in an airtight chamber, from which air is with drawnby creating a vacuum. The cells are fullemptied to receive preservative material.After that preservative material is pumpedunder pressure of 100 to 200 psi and at atemperature of 120degreeF. As the timbercontains required quantity of preservative alow vacuum is maintained to remove excesspreservative. Such a timber is generallyused in case of piles in salt water andrailway sleepers.
(3) Empty cell process: This method is similar to the full cell process but initial vacuum is not to be maintained and no attempt is to be made to remove the air from cells. The preservative material is applied under pressure of 200 psi.
MucetDepartment of civil engineeringBY Mucetcivil.blogspot.com Developed by:Mohsin Mehmood Gondal k12-ce27 60