Timber

All about timber as a construction material, from conversion of wood, seasoning, defects, deterioration and preservation, and wood products

1. TIMBER

2. CONTENTS Conversion of Wood Seasoning of Wood Deterioration and preservation of timber Special Timber products

3. Conversion of wood Wood conversion is a phrase used in reference to turning a log into a pile of boards, planks. This is done using a sawmill. Before the wood is actually used in construction, certain operations such as felling of trees, sawing them to logs , poles and beams etc. become essential.

4. How is wood converted FELLING ; This is a process of cutting down individual trees. The person cutting trees is a feller. There are two factors considered before felling: Maturity. Trees become mature only after certain years when there is a good balance established between there heartwood and sapwood. In mature trees growth rate is reduced considerably and they have achieved height and girth of standard characteristic for their species. only such trees should be felled for conversion. Young trees are immature and give mostly sapwood and old trees give poor quality as the heartwood starts decaying with age.

5. Cont’n Time of felling. Spring and summer are always bad seasons for felling trees because spring is a season for active growth where cells are rich in sap and summer is bad because the trees loss sap at a much faster rate during this period after felling. This fat drying is likely to cause development of a large of shrinkage defects. The best time for felling is autumn and beginning of winter. There is no growth of the tissues in living cells and also the shrinking defects are minimal with favorable working conditions. Branches and bark are removed

6. Cont ’ n It is important to take into account the taper (the difference in size between the two ends of the logs)the size of the taper affects how a log needs to be cut in order for the best use to be made of its timber. The size of the planks that are needed should also be put into consideration. Rot or cracks in the log and the roundness of the log.

7. Sawing of timber Sawn timber is timber that is cut from logs into different shapes and sizes. Sawn timber is generally cut into varying rectangular widths and lengths, but may also be wedge shaped. After their felling trees are usually cut diametrically in convenient lengths. These are called logs. Collectively called round timber . The logs are first air seasoned for some period and then further cut into transportable parts. The main methods of sawing of logs are; Through and through sawing and Selective sawing

8. Types of cuts/boards Tangential boards/cuts (crown, plain or flat sawn) are used extensively for beams and joists.  They are stronger when placed correctly edge up with the load in the tangential axis . These type of boards suffer from 'cupping' if not carefully converted,  seasoned, and stored properly.  Annual growth rings form an angle less than 45 degrees.   

9. Cont’N Radial boards (radial, figured or quarter sawn) are typically cut on 'the quarter'  and produce a pattern of the medullary rays especially in quartered oak.  Such timber is expensive due to the multiple cuts required to convert this board.   The radial face of the board is slightly stronger and stiffer than the tangentially face but the cross section and condition of the timber has more effect on strength.   Annual growth rings form an angle greater than 45 degrees .

10. Through and through sawing In this method, the log is cut lengthwise into pieces of suitable dimensions, one after the other, without giving it any rotating during sawing. All the cut surfaces are thus essentially parallel. Timber cut using this method is used for garden furniture , fencing posts, and for making pallets .

11. Cont’n ADVANTAGES This method is very cheap method because there is no need to double the log as compared to quarter method. Plain method is a very fast method compared to the tangential method. Very little waste produced since the cuts are parallel . DISADVANTAGES Planks contain hardwood and sapwood. Planks are not very strong. Planks are prone to warp, split and twist. No distinctive grain pattern

12. Selective sawing In this method, the log is rotated in different directions after one or more successive cuts in a particular direction. There are two methods in selective sawing namely; Quarter Sawing and Tangential

13. Quarter Sawing In this method, the log is cut into quarters and it produces radial boards which are cut in a such away that each board has one edge from the center of the log and the other edge from the peripheral part of the log. The process consists of; Cutting the log into two halves by a central cut Each of the two halves is given one cut again. This results in four parts of the log. Boards are then cut from each of the four parts by parallel cuts. Heartwood and sapwood can be separated using this method and it is used on hard woods. Timber cut this way is used for floorboards and furniture

14. CONT’N Tangential sawing In this method, the cut is made tangential to the annual rings and requires a lot of turning of the log. Heartwood and sapwood are separated using this method Timber from this method is used for furniture and veneers Two varieties are; Slash cut . (crown, plain or flat sawn) are used extensively for beams and joists.  These are the boards that have been cut in a direction tangential to the annual rings. In such boards, the annual rings make an angle of less than 45 with a face. They give a plain appearance. They are stronger when placed correctly edge up with the load in the tangential axis.   These type of boards suffer from 'cupping' if not carefully converted,  seasoned, and stored properly.  Annual growth rings form an angle less than 45

15. Cont ’ Rift cut (vertical or edge grain ). These are boards in which the annual rings make an angle of more than 45 to the face. Such boards are obtained making parallel cuts through the central parts of the log traversing through the annual rings. Quality floor boards are prepared from rift sawn timber because it wears well and shrinks less.

16. ADVANTAGES The planks are very strong The planks have an attractive grain pattern (Flame figure) Heartwood and sapwood are separated DISADVANTAGES The planks are prone to swelling and shrinking Costly and slow Lots of waste

17. REASONS FOR CONVERSION OF WOOD To reduce the wood to workable size Small planks are easier to dry out than large logs

18. DRYING AND SEASONING OF WOOD

19. Seasoning This is the process of removing the moisture content from wood to minimize structural problems when used in construction or to provide less smoke and more uniform combustion when used as firewood. Seasoning is relevant as it improves certain properties of timber that make it of purpose in different construction work for example durability, strength, hardness and weight reduction.

20. Objects of seasoning Reduction in weight- freshly cut logs which may have 50 to 100% moisture content. Increase in strength- there is higher quantity of wood tissue in dry wood than in green wood hence the lower the moisture content the higher the strength. Improvement in workability. Green timber is more difficult to work compared to seasoned timber. Freedom from shrinkage defects- timber if not properly seasoned will shrink and wrap. Longer life of durability- less insect and fungal attack due to reduced moisture.

21. Methods of seasoning There are mainly two types of seasoning used while drying timber. Natural seasoning; - Air seasoning -Water Seasoning Artificial seasoning

22. Air Seasoning This is a traditional method for drying wood and is also the longest Stacks should be protected from rain, direct wind and direct scorching heat and free from ground level debris and built on dry ground . Natural Seasoning

23. Preparation for Natural Seasoning Stack ground . It should be leveled, free from debris and on a dry land. It may take a few centimeters from the ground level Stack Pillars . These are constructed at regular intervals out of bricks or masonry or concrete and may be of 30-50cm height from the ground level. The top surfaces should be flat and level with each other. Stack proper . These are made of sawn timber shapes(sleepers, planks, poles). One stack should have timber of one shape and same length and width.

24. Water seasoning This gives good results with logs of freshly cut trees. Logs are placed in running water and the sap from the cells is washed out. Timber logs can also be placed in stagnant water if running water is not available. Natural seasoning

25. Advantages of Natural seasoning It is highly economical that is air and water are readily available. It requires little supervision. It is applicable to thicker timber parts as well as thin sections

26. Disadvantages of Natural seasoning It is a very slow process because it is dependable on nature. It keeps valuable land and timber blocked for longer periods. Moisture content cannot be brought below a certain limit. Seasoning is not always uniform in all sections of timber.

27. Artificial Seasoning This is where timber is subjected to drying by use of artificial means. Its dived into several methods; Kiln Seasoning Microwave Seasoning Others include; Chemical Seasoning Seasoning by boiling Electrical Seasoning

28. Kiln Seasoning The is the most common and effective commercial process for drying wood and accelerates the process of removing moisture through the use of external energy. The drying takes 2-3days Kiln seasoning is divided into 2 methods Progressive Compartmental

29. Progressive method In the progressive kiln, timber enters at one end and moves progressively through the kiln much as a car moves through a tunnel. Progressive kilns may be further subdivided into natural draft kilns in which heated air is allowed to rise through the material by natural convection, and forced draft kilns in which fans are employed to force the air through the wood. A progressive kiln has the stack on trolleys that ‘progressively’ travel through chambers that change the conditions as it travels through the varying atmospheres. 

30. Cont’n

31. Compartmental method A compartment kiln is a single enclosed container or building, etc. The timber is stacked as described above and the whole stack is seasoned using a program of settings until the whole stack is reduced to the MC required. Compartment kilns differ from progressive kilns in that the timber is loaded into the kiln and remains in place throughout the drying process. Compartment kilns are usually smaller than progressive kilns, and because of their construction the temperature and humidity conditions within them can be closely controlled.

32. Cont’n

33. Solar Kiln This method combines the speed of kiln seasoning with the low energy of air drying. Solar kilns have single-thickness windows on the south side of the structure that work as collectors to trap the sun’s energy. Heat collectors, made from black metal are attached near the top of the window sashes. Various methods force the heated air to circulate through the kiln to dry the wood. Some solar kilns have insulation to retain heat at night

34. Microwave Seasoning Microwave seasoning uses pulsed energy directed into timbers to drive out moisture in a manner that will not cause seasoning degrade. This method also provides advantages such as high speed and high quality and is well suited for seasoning lumber, blocks, veneer, chips, paper and wood-based composite materials. Areas in the wood with the most moisture absorb the most energy resulting in even temperature during the drying process and a uniform moisture content. These factors enhance quality and reduce timber checking and warping.

35. Other methods of artificial seasoning Chemical /Salt Seasoning : In this method, timber to be seasoned is treated with a chemical solution like sodium chloride, nitrate or urea, the purpose of the solution is that it reduces the vapour pressure on the surface of application. Chemical seasoning enables timber to dry fast from inside. It is advantageous in that it prevents risk of cracking of the outer shell

36. Cont’N Seasoning by boiling: T his is a quick method of removing sap from within the cells, the timber to be seasoned is immersed in water and water is heated to the boiling point. It is kept boiling for 4 to 5 hours. The sap is washed out by this process and on placing such treated timber in air, it dries quickly but at the cost of strength and elasticity of fibers, so it is rarely used .

37. Cont’N Electrical Seasoning : It is of theoretical importance only. Dry wood is a non-conductor of electricity but when an Alternating current is passed through a green piece of timber, heat generated is enough to dry out moisture of the cells which do conduct some electricity, in this way, the timber pieces are dried quickly. The technique is costly in that it requires high electricity consumption and also causes damage to the cell walls

38. ADVANTAGES OF ARTIFICIAL SEASONING The quality of seasoned timber is highly satisfactory DISADVANTAGES It’s a costly method because it involves expensive equipment. It needs expert handling for good quality seasoning.

39. DETERIORATION AND PRESERVATION

40. DETERIORATION AND PRESERVATION OF TIMBER. Deterioration of timber Timber is subjected to deterioration from many causes and its strength eventually becomes so weakened that it must be removed and replaced. The chief factors of this deterioration are decay, mechanical abrasion, fires etc .

41. Causes of decays Insects A few insects are known to actually spoil huge quantities of timber within a short time. Termites are the common insects that attack timber/wood Other insects which attack timber are: pole-borers and marine borers.

42. Cont’n Fungi Decay of timber is attributed to small plants and organisms that live on and spread within the timber. A special type of organism called fungus starts growing on the timber, especially when it is placed on a moist ground or is used in a damp situation.

43. The three main types of fungi that affect timber The white rot : this type of fungus destroys the lignin part of the cell by consuming it and leaving behind the fluffy light colored cellulose part in rotten form . The brown rot : unlike white rot, it eats up and destroys the cellulose part of the cell tissue leaving behind a brownish lignin part in rotten form.

44. Cont’n Mold it’s a cotton like growth of fungus spread over various parts on the surface of the board. It may not spoil the inner parts of the portions of the wood but its appearance on the surface is also quite displeasing The blue stain . It’s a fungus that lives on sugars and starching the cell tissue. Such a fungus causes staining or discoloration of the original board.

45. Preservation of Timber This is carried out mainly to prevent timber from decaying. The main objective of such a treatment is to ensure a longer, trouble free life of timber.

46. Types of methods used for preserving timber Application of some chemical substances (called the preservatives) on the surface of the timber. Injection of the preservatives into the body of timber. Construction of protective covering or shields around timber used in construction.

47. wood preserving chemicals The Oil-Soluble Salts Such compounds are soluble only in oils. The most commonly used wood preservative, coal tar creosote oil, Water soluble Salts Such salts make an easy solution with water. Among the water soluble salts are included zinc chloride, copper sulphate , sodium fluoride , sodium fluosilicate , sodium dinitrophenoxide and compounds of arsenic .

48. Cont’N Volatile Base Salts These make solutions with substances like petroleum. The creosote petroleum blends are typical examples of this category

49. Methods of application Preservatives are used by different methods depending upon the extent of preservation required. These methods include; Brush application D ipping O pen tank

50. Brush application In this method , timber is given one or two coats of the preservative with the help of a brush. This is used for painting coal tar at the ends of beams that are embedded in the walls or base of poles and posts that go to the ground with coal tar . The method is quite cheap at the same time, it is not very effective.

51. Dipping Method The timber part to be treated is made to dip in the preservative and kept immersed in it for various periods from a few hours to few days . It is used when organic preservative solvents are to be applied.

52. Open Tank Application I n this method , the timber is kept immersed in a suitable metallic tank of proper size till a proper saturation is obtained . The tank containing the timber is then heated at about 70 - 80 c for several hours this ensures deeper penetration of the preservative into the timber. After this, the timber is allowed to cool within the tank in the presence of preservative .

53. SPECIAL TIMBER PRODUCTS

54. Special Timber Products Veneers A veneer is essentially thin sheets of timber with a thickness varying between 0.4mm to 6mm. Its also sometimes called a ply. This is the basic component where most wood products are prepared from and easily.

55. Manufacture of Veneers They are obtained from a suitable type of timber logs using 3 methods Sawing Slicing Rotary cutting

56. Sawing method This is the earliest method and its done in this way; Firstly, the logs are cut into smaller units called flinches. Each flinch is then subsequently swan carefully to obtain veneers of required thickness

57. Slicing Method Horizontal slicing . Here the log is held in position by a mechanical system and the cutting edge is set into a horizontal motion Vertical slicing Here the log is held in position by a mechanical system and the cutting edge is set into a vertical motion

58. Rotary cutting Method Its a modern method by which quality of veneers can be cut quickly. A special wood log is kept in one position as it rotates about a sharp knife(cutter)

59. Uses of Veneers Veneers are mainly used in the manufacture of plywood, laminated board and batten boards . Used in interior decorations

60. PLYWOOD Plywood is a flat panel made by bonding together, under pressure, a number of thin layers of veneers(or plies).

61. Structure of plywood In the simplest type of plywood the board consist of 3 layers of plies arranged in a such a manner that; One ply that lies in the middle called the central ply . Two plies each lying on the either sides of the central play. The sides of the plier are called the free ply and the back ply.

62. Properties of Plywood It has a better resistance to shrinkage defects like swelling and warping . The plywood is free from splitting defects It has uniform mechanical properties in both the directions It has better strength than the ordinary wood of equal thickness It is light in weight and easy to handle and convenient for using.

63. Manufacture of Plywood Commercially manufacture of plywood involves the following stages Veneering Bonding Stacking Pressing

64. Veneering This involves obtaining thin sheets or plies or veneers from a selected type of timber. Veneers may be obtained by sawing, slicing, rotary cutting or peeling. The thickness of the veneers varies from 0.4mm to 6mm and veneers so obtained are sorted and dried to a moisture content level below 10%

65. Bonding This stage consists of applying adhesive or glue over the surface of the veneers Animal glues, vegetable glues and synthetic resin glues are variously used for bonding the plies together.

66. Stacking This consists of arranging the glued plies around a core ply in such a manner that grain in adjacent plies runs at right angles to each other.

67. Pressing . The stacked plies are either hot pressed or cold pressed depending upon the type of adhesive used in bonding. Pressing is in 2 ways; Cold pressing is applied when vegetable glues have been used

68. Cont’n Hot pressing is applied when synthetic resins have been used . In hot pressure, the material is raised to a temperature where glue is perfectly liquefied and then the pressure varying between 7-14kg/cm2 is applied through hydraulic presses.

69. Manufacture of plywood Processes

70. Advantages of plywood(Why plywood?) Dimensional stability Strength and stiffness/weight ratio Split resistance Panel shear or shear through thickness capacity Resistance to concentrated loads Resilience, impact and fatigue resistance Thermal insulation with low thermal mass Chemical resistance Sound reflectance Workability Aesthetics

71. Types of Plywood Exterior type This is where water proof glue is used for bonding at the manufacturing stage and such plywood is suitable for external applications in building construction e.g. doors, windows. Such plywood is obtained from pine, spruce, mahogany, Douglas fir etc.

72. Interior Type This uses common glues are used. Such plywood is mostly used in the interior where highly aesthetic finish is required. Its used in ceilings, wall lining, internal furniture, interior cladding etc.

73. General uses of plywood Plywood is extensively used in construction and furniture making such as doors, windows, tables, etc. It can be used as roof and wall materials. Plywood is used in sheathing and flooring. Used for wall and ceiling tile backings Used for making table reels and floor coverings Can be used for truck bodies, streamlined trains and boat interiors Shuttering boxes are made of shuttering plywood.

74. Laminated Timber This another type of processed wood that consists of suitably selected wood sheets that have been glued together in a such a way that grains of all the sheets are parallel longitudinally .

75. Laminated timber( cont’n ) It comes in a wide variety of shapes and appearances and those are; Industrial glade laminated timber . Here small voids are allowed but one face is made free of loose knots and holes. It is used in garages and industrial plants. The Architectural grade . This has a very smooth external surface in which all the knots have been removed and voids so created filled with clear wood tissue The premium grade . This is manufactured with great care and is used where highly appealing and smooth external finish is required.

76. Fiberboards These are made by pressing together fibrous materials such as fiber of wood, cane and even vegetable matter. The fiber mass is first heated and then pressed hard in that hot state. On cooling, a rigid material of light of light density and good utility is obtained. Its thickness varies between 3mm to 10mm

77. Uses of fiberboards They are used as; Insulating boards for both heat and sound insulation Partition wall panels Parts of flushed doors Finished flooring Low slope roofing

78. Impreg and Compreg Timbers This is a class of processed timber products where advanced technology is used. In this class, the veneers are treated w ith suitable types of resins. The type of resin used, the method of method of treatment and the type of raw material used gives rise to a variety of such treated timbers

79. Cont’n In impreg timber , the veneers are immersed in resins, commonly phenol formaldehyde at ordinary temperature and pressure. Once the tissue is saturated and the resin is set, the consolidated mass is cured at a raised temperature(150c). It’s then finally given proper finishing treatment and is marked under trade names.

80. Cont’n C ompreg timber . This a variety of impreg timber that is prepared by curing the resin impregnated timber at a high temperature and high pressure. This type of curing results into a timber of high density and strength that is quite durable.

81. Advantages of impreg and compreg timber Very beautiful in appearance Almost impervious outer surface Resistance to acids and solutions High strength, low shrinkage and much increased life(durability)

82. References Building materials by S.K.Duggal Wikipedia Materials for Construction and Civil Engineering by Mr.Clara Materials in Construction Technology by G.D.Taylor