Timber | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Timber

Timber used for construction activities is derived from the trees that are generally of two types.
(1) Exogenous trees
(2) Endogenous trees

Exogenous trees 

• These are the types of trees that grow in bulk by increasing in the outward direction.
• These trees consist of circular rings along their horizontal section, which are added up each year; hence, they are termed annual rings and are used to ascertain the age of the tree.
• The timber used in engineering activities is mostly derived from these trees only.

These trees are further of two types.
(A) Conifer Trees
(B) Deciduous Trees

Conifer trees:–

• These are the types of trees that never shed their leaves until new ones are grown; hence, they are also termed evergreen trees.
• These trees bear cone-shaped fruit and hence are known as conifer trees.
• These trees pass distinct annual rings and indistinct Medullary rays.
• The wood obtained from these trees is soft, light, weak, resinous, and light in color. Hence, these trees are also termed softwood trees.
• Example— Chir, Deodar, Pine, Spruce

 Deciduous trees: -

• These trees are also termed as broad-leaved trees.
• They shed their leaves in autumn, which again grow back in spring.
• These trees possess indistinct annual rings but distinct medullary rays.
• The wood obtained from these trees is hard, strong, heavy, non-resinous & dark in color; hence these trees are also termed hardwood trees.
• The timber used in engineering activities is mostly derived from these trees only.
• Example:– Sal, Teak, Oak, Babul.

Endogenous Trees

• These are the trees that grow in bulk in an inward direction and consist of fibrous mass throughout their longitudinal section; hence, they have limited engineering application.
• Example:– Bamboo, Cane, Palm.

Structure Of Tree

Pith (Medulla):–

• The innermost central portion of the tree is termed a pith.
• It entirely consists of cellulose that nourishes the tree at a young age.
• It is the darkest portion of the tree, which signifies that it does not play any active part in the growth of the tree.
• As the tree grows old, its pith ceases to be active and represents the decay of wood.

Heart Wood:–

• The inner annual rings surrounding the pith constitute the heartwood.
• Its color is also comparatively dark, which represents the dead portion of the tree i.e., it
   also does not take any active part in the growth of the tree but imparts strength & rigidity to it. 

• From an engineering point of view, heartwood is most suitable for construction activities.

Sap Wood (Alburnum):

• The outer annual rings in between the heartwood & cambium layer constitute the sap wood.

• This portion of the tree takes an active part in its growth, and its color is comparatively light than that of heartwood & pith.

Cambium layer:—

• The cambium layer consists of sap that is still not being converted into sapwood. Hence, it represents the future growth of the tree.

• If, for any reason, the bark of the tree is removed, exposing the cambium layer, the cells of the wood cease to be active, which finally results in the death of the tree.

Medullary Rays:–

• These are the radiate fibers that extend from the pith to the cambium layer and hold the annual rings of heartwood and sapwood together in position, thereby imparting strength to the timber seen in a transverse direction.

Bark:–

• It is the protective covering provided around the cambium layer in order to safeguard the future growth of the tree.

Processing of Timber:–

The processing of timber is carried out in four distinct operations.

• Felling of Trees.
• Seasoning of timber
• Conversion of timber
• Preservation of timber

Felling of Trees:–

• Felling of the tree should be done when it attains sufficient maturity. If a mature tree is cut, it will yield softwood, and if an over-mature tree is cut, it will yield decayed wood. 
• In either case, wood can not be used for engineering activity.
• The optimum age for the felling of trees varies between 50 to 100 years.
• The trees should be cut such that the maximum proportion of wood is obtained from it. For which the cuts are made at the section just above the ground.

• The felling of the trees should be carried out in the season when the movement of the sap is minimal.
• Hence, it is generally avoided in autumn and spring when the sap is in vigorous motion.
• In hilly areas, cutting of the trees is done in mid-summers as rainfall occurs in winter, and in plain areas, cutting of the trees is done in mid-winters as in summer, excessive loss of moisture may result in cracking.

Seasoning of Timber:–  

• Newly felled trees contain approximately 50% of the water by weight. Hence, it is dried before being used as construction material. This process of drying the timber to make it suitable for engineering purposes is termed seasoning.
• This water in timber is generally present in two forms– (i) In the form of sap (ii) In the form of moisture.
• This water is either present in the voids or in cell walls. The former water is termed as free water and the latter is termed as bound water.

• During the seasoning of the timber, free water is removed & the point at which it is completely removed is termed the fiber saturation point.
• Seasoning of the timber can be carried out naturally and artificially.

Methods of Artificial Seasoning:– 
(i) Boiling
(ii) Electrical seasoning
(iii) Kiln seasoning
(iv) Water seasoning
(v) Chemical seasoning

Boiling:  

• It is the process in which logs of suitable size are immersed in water, the temperature of which is raised up to the boiling point that is maintained for the next 8-4 hrs.
• Logs are removed from the water & are seasoned naturally  
• Due to the temperature of the log, the rate of evaporation drastically reduces the seasoning period.
• It is one of the quickest methods available for seasoning but is comparatively more costly.

Electrical Seasoning:

• It is the method of seasoning in which alternating current is passed through the timber section, which resists its movement as it is a bad conductor of electricity, due to which heat is developed, which raises the temperature of the timber section, thereby reducing the duration of seasoning.
• More costly.

Kiln seasoning:

• It is the method of seasoning in which a timber section is placed in an air-tight chamber, and fully saturated air is forced into it at the temperature of 35⁰C to 40⁰C.
• The temperature inside the kiln is then raised, depending on which heating of the timber section takes place w/o evaporation.
• The relative humidity of the air is then reduced, which carries out the uniform evaporation of the timber.
• Water seasoning:
  It is the method of seasoning in which timber section of suitable sizes is immersed completely in water & are subjected to the stream of flowing water.
• The larger portion of the section is placed on the U side in order to create turbulence and carry out the removal of sap.
• After which, it seasoned naturally.
• Water that replaces sap is removed comparatively faster, thereby bringing down the seasoning duration.

Chemical seasoning:

• It is a method of treating wood with chemicals to speed up the drying process and improve the quality of the wood.

Conversion of Timber

• The process of giving the desired shape & size to the timber section is referred as the processing of timber.
• Processing of timber can be carried out by any of the following methods:–

Ordinary Sawing

• It is the method of sawing in which soccers are made tangential to the annual rings.
• It is the most general, economical & easiest method of sawing.
• The timber section obtained by this method is liable to twist & warp due to unequal shrinkage of sapwood & heartwood.
• The wastage of timber in this method of sawing is minimal; hence this method is found to be more economical than other methods of sawing.

Quarter Sawing

• It is the method of sawing in which the cuts are made at right angles to each other.
• This method is generally adopted for sections having indistinct medullary rays.
• Sections obtained by this method are liable to bend in the transverse direction.

Tangential Sawing

• It is the method of sawing in which the cuts are made tangential to the annual rings, which meet each other at right angles.
• This method is also adopted for the sections that have indistinct medullary rays.
• Sections obtained by this method of sawing are weakest among all as, in this case, medullary rays are also cut, which can hold the annual rings together in position.

Radial sawing

• It is the method of sawing in which the cuts are made parallel to the medullary rays in a radial direction that imparts a decorative effect to it.
• Sections obtained by this method are strongest among all but wastage in this case is comparatively more.

Preservation of Timber

• Preservation of the timber is carried out in order to increase its life, make it more durable & more resistant to the attack of insects, fungi & termites.

Preservation of the timber can be carried out by only of the following methods:–

 (i) As Cu Treatment :– (As₂ DB O₅. 2H₂O, CusO₄ . 5H₂O, CuSO₄, K₂Cr₂O₇ / Na₂ Cr₂O₇)

• It is the method of preservation in which the timber surface is coated with the solution prepared by the addition of arsenic pentaoxide, copper sulphate, potassium dichromate & sodium dichromate in water.
• This process increases the resistance of the timber section against the attack of white ants.

(ii) Application of Chemical Salts:–

• Chemical salts like copper Sulphate, Zinc Chloride, mercury chloride & sodium fluoride increase the durability of timber sections by making them more resistant to cracking, shrinkage & warping.

(iii) Coal Tar:-

• In this method of preservation, the timber surface is coated with hot tar which makes it resistant against fire.
• The process of application of coal tar on timber section is known as taring.
• The application of tar gives an unpleasant smell and reduces its aesthetic importance.

(iv) Application of creosote oil:–

• Creosote oil is obtained from the distillation of tar.
• Application of creosote oil on timber section almost doubles its life.
• The method of application of creosote oil on the timber section is known as creosoting, in which logs of timber are placed in an air-tight chamber, and creosote oil at the pressure of 0.7 to 1 N/mm² and temperature of 50o^oC is pumped into the chamber, that results in uniform application & penetration of the oil over the surface of the timber section.

(v) Application of Oil paints & Solignum Paints:–

• Oil paints & solignum paints increase the resistance of the timber against the penetration of water.

Defects in Timber

(1) Defects due to conversion.
(2) Defect due to attack of fungi.
(3) Defects due to natural forces.
(4) Defects due to seasoning.

Defects due to conversion:

CHIP MARK:–

• This defect in timber is identified by the mark of the chip over the surface of the finished timber section that is left by the parts of the planning machine.

(b) Diagonal Grain:–

• This defect is identified by the diagonal mark over the straight-grained structure over the timber surface, which is left due to improper sawing.

(c) Torn Grain:–

• This defect is identified as a small depression over the finished surface of the timber section that is formed due to the falling of tools during the conversion.

(d) Wane:–

• This defect is identified by the presence of an original rounded surface over the finished section of the timber surface.

Defects due to attack of fungi: -

• The timber section is attacked by fungi if & only if the moisture content in it is greater than 20% and there is the availability of air surrounding that section.

Defects due to fungi in timber contain.

(a) Blue stain:–

• A certain type of fungi carries out the decomposition of the timber section, leaving behind blue spots and resulting in the disease termed blue stain.

(b) Sap stain: -

• There are certain type of fungi that do not carries out the decomposition of the entire wood but feeds on its sap. Resulting in a change of color, and leading to the disease termed as sap stain.

(c) Brown Rot: -

• It is the disease of the timber that is caused by the removal of the cellulose from it by a certain type of fungi that leads to the development of reddish-brown stains over its surface as the disease is termed brown rot.

(d) White Rot:- 

• This disease is just opposite to that of brown rot in which lignin is removed by a particular class of fungi.

(e) Dry Rot:–

• This distance is found when there is no free circulation of air around the timber section, which results in the growth of certain types of fungi that carry out the decomposition of the wood & convert it into dry powdered form.

(f) Wet Rot:–

• This disease is found when the timber section is subjected to alternate wetting & drying, due to which a particular type of fungi grows, which carries out the chemical decomposition of the timber section, resulting in the development of grayish spots over the timber section.

(2) Defects due to Natural forces:–

• Defects due to natural forces are of different types of shakes.

Shakes: The cracking of the timber section that results in the separation of the fibers either partially or fully is termed as shake.

Different types of shakes:–