A COMPLETE MANUAL ON CIVIL ENGINEERING MATERIALS

Chapter: 7

TIMBER (4 marks)
7.1) Introduction:
Wood is a hard solid substance found inside the bark of tree. Wood which is useful in structural
and other engineering purpose is called timber. Timber is an important civil engineering
material, as it possesses unique properties over other materials. Timber is highly workable, can
resist shock, have good-looking texture, easily available and possesses high salvage value.
Timber is obtained from trees. Timber denotes structural wood. Wood suitable for building or
other engineering purpose is called timber. A standing living tree is known as standing timber.
When tree has been cut and its stem and branches are roughly converted into pieces of suitable
length. It is known as rough timber. When roughly converted timber is further sawn and
converted in to commercialized the planks, battens, posts, beam etc it is called converted timber
From the pre historic times, timber has been used in a variety of ways in building, bridge, tunnel
construction and many other engineering works. Even today there are certain works, where
timber is considered as the most ideal material.

7.2) Growth and Structure of Tree:

Growth:
In spring, the tree sucks its food from soil through its roots in forms of sap. The sap is then raised by
cells and fed to the branches and leaves of tree. The sap consists of moisture and minerals, like
phosphates and nitrates. In addition of sap, tree also requires some organic foods in form of
proteins, fats and carbohydrates. The sap losses moisture through leaves and absorbs carbon
dioxide from air and gets denser. The process is called photosynthesis. The thickness of sap
descends in autumn and gets deposited under the bark in form of a thin layer know as cambium
layer. Cambium layer hardens and adds a layer of wood to the outside of trees every year in form of
concentric rings called annual rings. The new ring represents a year's growth of tree.

Structure of tree:
On examining the cross-section of trunk of an exogenous tree, we see the different parts as
shown in the figure.

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1) Pith or Medulla:-
It is the inner most central zone core of the tree. It is sometimes known as heart also. The size and
shape of pith varies for different types of trees. It consists entirely of cellular tissues when the plant is
young, contain large amount of fluid and nourishes the plant. When the plant becomes old, the pith dries
up and decays. The sap is then transmitted by wooden fibers deposited round the pith.
2) Heart wood:-
It is dark coloured portion of the tree surrounding the pith. It is almost dead portion of tree and
does not take active part in its growth. It is this portion of tree which provides strongest timber for various
engineering purpose.
3) Sap wood or Alburnum:-
Outer annual rings of the tree constitute the sap wood which transmits the sap from roots to
branches. Compared with heart wood, sap wood is lighter in colour, weaker and more liable to decay. The
wood is recent growth and thus contains a lot of sap. This is the active part of wood and thus helps in
growth of the tree.
4) Cambium layer:-
Outer most one ring between the sap wood and inner bark is known as cambium layer. It is a sap
which has yet not been converted into sap wood. If the bark is removed the cambium layer gets exposed
and the cells stop to be active which results in the death of tree.
5) Inner bark or Bast:-
The thin layer, covering the cambium layer, is known as inner bark. This layer protects cambium
layer from outside injuries.
6) Outer bark or Cortex:-
It is the outer most protective layer which may have fissures and cracks. It consists of wood fibre.
In course of time older layers split and scale off.

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7) Medullary rays:-
These are thin radial fibers, extending from cambium layer right up to pith. These rays help in
holding together annular rings of both heart as well as sap wood. They may be continuous but mostly they
are broken. In some varieties of trees, they are not very distinct.
8) Annual Ring:-
These consist of cellular tissue and woody fibres arranged in distinct concentric circles round the
pith. Annual rings are generally formed in every year, due to the sap deposition below bark. So, number of
annual rings represents the age of the tree.

7.3) Classification of Tree:

Based on the mode of growth, trees are classified into two categories:

a) Endogenous:-
In the endogenous tree, plant grows by the addition of new cells only at the end of the previous
year's plant height. Such a tree grows upwards without becoming thicker and broader. They remain thin
and slender. Examples are Bamboos, Banana, palm etc. Those timbers are practically useless as structural
timber.
b) Exogenous:-
In the exogenous tree, the plant grows by the addition of a new layer of cell all around these
grown in previous year. This process naturally results in increase in the height as well as diameter of
the plant. These trees grow outwards by the addition of one concentric ring every year. These rings
are known as annual rings. Since one ring is added to the tree every year so the number of annual
rings in the stem of a tree indicates its age in years. It is timber obtained from this class of tree that
is extensively used in engineering works. eg. Sal, Deodar, Teak etc.
It is also classified as
1) Conifer or Evergreen tree 2) Deciduous tree.
7.4) Characteristics of Good Timber:
A good timber should hold following properties:

i) It should have straight and compacted fiber.

ii) It should be of uniform color.
iii) A freshly cut surface should give a sweet smell.
iv) It should be sonorous when struck i.e. it should give clear ringing sound when struck.
v) It should be relatively heavier.
vi) A good timber should be free from shakes, dead knots, cracks and ruptures in it.
vii) When planed, its surface should present a firm bright appearance with silky luster.
viii) It should not clog the teeth of saw while sawing.
ix) It should have firm adhesion of fibers and compacted medullary rays.

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x) It should well seasoned to resist the action of fungi, insects and other physical environment.
xi) It should relatively be fire resistive.
xii) It should be hard strong durable as well as workable

7.5) Defects of Timber:

A defect refers to natural or artificial undesirable character which lowers the strength and quality of
timber. In all kinds of timber, several natural defects occur. There are caused by the nature of the soil upon
which the tree grew and also by the changes in nature to which it was subjected while growing. The defects
that usually occur in the timber may be classified in to two categories as follows.
1) Defects that develop during growth of tree.
2) Defects that develop after felling the tree.

1) Defects that develop during growth of tree.

i) Shakes:-
This is most serious type of defect in timber. These are sort of cracks which partly or completely
separate the fibres of wood. Shakes may be of several types.
a) Star shake:- These are radial cracks or splits that extend from bark towards the sap
wood. Temperature difference during different season is generally the cause for their development.
b) Heart shakes: These are wide splits running right through the heart wood of the tree.
These splits radiate from the pith running towards the sap wood. These are caused by the shrinkage
of interior parts or by decay because of accumulated mixture. This defect usually occurs in over
matured trees.
c) Cup shakes: This defect develop curved split between successive annual rings. The split
does not run for the full circumference of the annual rings.
d) Rings shakes:- When cup shake defects runs for full circumference of the annual ring, it
is called ring shake. It is more serious than cup shake.
Both cup and ring shakes defect usually develops due to unequal growth. Another
possible reason for their development may be contraction of timber under atmospheric changes
together with the twisting action of strong winds.
e) Radial shakes:- They are similar to star shakes. They are numerous, fine and irregular.
They usually occur when felled tree is exposed to sun for seasoning. The crack run for a short distance
from bark towards the center and then follows the course of an annual ring and ultimately goes towards the
pith.
ii) Rind galls:-
It is a curved swelling found on the body of the tree. They are usually caused by the growth of
layers over the wounds left after the branches have been cut off in an irregular and improper manner.

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iii) Twisted fibres:-

They caused by twisting at young trees constantly in one direction under the action of strong
prevalent winds. Timber with twisted fibres is unsuitable for sawing. The timber having this defect is
mostly used for posts and poles in an unsown condition.
iv) Upset or rupture:- This defect is due to crushing of fibres which are deformed as a result of the
injury thus caused. These are due to unskillful felling and also the result of violent wind. This defect indicates
change in direction of wooden fibres.
v) Knots:- Knots are generally developed at the base of branches out off from the tree. The wood
fiber, from where branch has been removed receives nourishment from the stem for a pretty long time. This
phenomenon ultimately results in the formation of dark and hard rings known as knots. As knot break the
continuity of the wooden fibers. They form a source of weakness. The amount of weakness caused by the
knot depends upon the position, size and degree of grain destruction around it.
vi) Wind cracks:- When wood is exposed to atmospheric agencies, its exterior surface shrinks. Such
shrinkage results into cracks known as wind cracks.
vii) Coarse grain: - The tree growing very fast has their annual rings quite widened. The timber
having widened annual rings is known as coarse grained timber. Such timber posses less strength.
viii) Druxiness: - This is the defect in which white decayed spots of timber remain concealed
under healthy wood. This defect is probably caused due to access of fungi.

Fig: Star Shake Fig: Heart Shake Fig: Cup and Ring Shake

..

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2) Defects that develop after felling the tree

The defects that may develop after felling the tree and also during conversion and seasoning are
following:

i) Bow:- When planks of converted timber shrink and bend in covered from, in the direction of
length, the defect so indicated is called bow.
ii) Cup:- This defect is indicated when wooden planks bent in covered from in transverse direction.
iii) Twist:- A plank which has distorted spirally along its length, the defect so formed is called twist.
iv) Case hardening:- The upper exposed surface of the timber dries at a very fast rate and as
such it shrinks and is subjected to compressive stress. The interior surface of timber, not
being exposed, dries very slowly and thus is under tension. This defect in which timber is
subjected to stress and strain due to unequal shrinkage of internal and external surface is known
as case hardening.
v) Honey combing defect:- Various radial as well as circular cracks develop in the internal
portion of the timber due to stress developed during drying. The timber thus suffers honey
combed defect.
vi) Check:- A crack separating wood fiber is known as check. Check don't extend from one end to the
other end.
vii) Split:- When check extends from one end to the other , it is known as split

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7.6) Seasoning of Timber:

The process of drying timber or removing moisture or sap present in a freshly felled timber under more or
less controlled condition in order to prevent it from possible shrinkage, uneven cracking and decay is
called seasoning of timber. Wood has the ability to absorb or give off moisture until its moisture content is
in equilibrium with surrounding environment. So seasoning of timber is done to bring the moisture content
that will match the average humidity of the working environment.

Generally the raw wood contains 100-200 % of moisture content. This moisture content is brought to about
10-12% after seasoning. So the loss of that large amount of water content should be carried out much
carefully.

Following are the advantages of seasoning:

i) It decreases the weight of timber and makes lighter. It increases the strength and makes stable. It
ii) improves the working quality of timber.
iii) iv) v) It reduces the possibility of attack by fungi and insects. It provides dimensional stability of timber.
vi) It enables timber to be easily planed, polished and preserved. It increases the strength up to 20-
vii) 30%.

Methods of seasoning

Seasoning of timber may be carried out two ways.

1. Natural seasoning
2. Artificial seasoning

1. Natural seasoning
Natural seasoning is also of two types. These are

i) Air drying / seasoning

In this method of seasoning, timber is stacked in a dry place at about one foot above the floor level as
shown in the figure (i). Each piece of timber is spaced few centimeters apart to allow free circulation of
air. These stacks should be protected from rain water and direct sunlight. It is economical method and
possesses less chance of damage. It does not require skilled manpower. But seasoning by this method takes
more time ranging from two to four years and covers more space.

ii) Water seasoning

In this process the log wood is kept completely immersed in running stream of water. By this process the
sap, sugar etc are leached out of wood and replaced by water. Then the log is kept out of the water and
placed in air. Stagnant water can be used if running stream is not available. This is the quick process but
elasticity and strength of timber are decreased by this process. Boiling of timber in water is another rather
quick process.

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2. Artificial seasoning
Artificial seasoning are the quick process of seasoning and moisture content can be controlled.

i) Kiln seasoning
The popular method of artificial seasoning is Kiln seasoning. In this process timber is seasoned under
controlled temperature and humidity condition with proper air circulation and ventilation system as shown
in the figure (ii). Timber is stacked properly in the field with open spaces for air circulation. The kiln is
heated for some time. The temperature is maintained slightly higher than atmospheric temperature to avoid
cracking or splitting of timber which becomes certain if temperature is suddenly raised. Once the timber has
been at lower heat and good humidity for some time, the temperature of kiln is raised,
humidity is reduced and air circulation is made faster. Hence, loss of moisture takes place

The rise in temperature should be such that the timber retains original strength and elastic properties.
Humidity controls the warping and cracks of timber. Circulated air causes the uniform drying of timber
and ventilation reduces the excessive heat. Due to uniform drying in controlled way, any moisture content
can be obtained from this method. But timber obtained is of inferior quality than that from natural
seasoning. This process is quick but costly. It requires skilled manpower. It takes four to five days for
completing in normal condition.

II) Chemical seasoning

Chemical seasoning also known as salt seasoning is the process at which the timber is immersed in
soluble salt(eg; NaCl, sodium nitrate, urea etc) and seasoned in an ordinary way. Essential quality of salt
solution is that it reduces vapor pressure on surface of application. At air exposing, due to low vapor
pressure, sap is moved outwards from inner portion and dried faster. Thus, it enables the timber to dry from
inside.

III) Electrical seasoning

In electrical seasoning, the timber to be seasoned is passed through a induction coil due to which
moisture is reduced constantly. It is a highly costly process.

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7.6) Deterioration and Preservation of Timber:

Deterioration of timber depends on the various factors:

Moisture
Imperfect seasoning
Alternation of dry and wet states
Vegetation growth
Attack of fungi ,insects etc
Bad storage or stacking of timber
Decay can be due to dry rot or wet rot

Preservation of timber indicates the art of treating the timber with some chemicals so as to increase its life.
Timber has to be protected from the attack of insects and internal decay due to dry and wet rot. Proper
precaution should be taken to protect timber. Otherwise, life and strength of timber decreases gradually.

Following are the methods employed to preserve the timber.

i) Charring: Lower ends of the posts that are to be embedded in ground are generally charred to
prevent the attack of insects. It is done by quenching the ends of posts in water after they are
charred on wood fire to a depth of 1.5 cm.
ii) Tarring: Tarring is the coating of timber with tar or tar mixed with pitch. Embedded portion of
timber, fence post, ends of door and window frames, sides of door and window frames that
comes in contact with wall are usually tarred.
iii) Painting: Paints are also the good preservatives of wood as they form a thin layer in the
surface of timber, which acts as barrier for defective environmental factors. Painting should
only be done for well-seasoned timber otherwise it will block the pores of wood and cause
internal decay of timber.
iv) Creosoting: Creosote is a dark brown oily liquid. A well-seasoned timber is dried and placed
in an airtig2ht chamber first. Then the air is exhausted and creosote oil is pumped in pressure
of 9Kg/cm at 500 C to saturate the timber with oil. This oil preserves the timber from rot and
attack of insect, creosoting in timber produces undesirable color and smell.
v) Ascue treatment: Ascue is the preservative found in the form of powder. This powder is
soluble in water. 6% solution of ascue is prepared in water and is sprayed on the timber. Wood
treated by this method can be painted, varnished and polished. Fire resisting power is increased.
Abel's process: In this process, the surface of the timber is first painted with dilute solution of
vi) Sodium Silicate and with a cream like paste of slaked fat lime and finally with a concentrated
solution of soda. It improves the fire resistance of timber.
Fire proofing of timber: Wood cannot be made fire proof, but some chemical treatment may
vii) reduce the fire attack towards the timber. Soaking timber in ammonium sulphate, ammonium
chloride, Zinc chloride etc improves the fire resisting capacity of timber.

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7.8) Commercial product of Timber:

Timber may be found in the form of logs, beams, planks, struts and other wood based products. We discuss
here about some wood based products.

1. Veneers
Veneers are the thin sheets or slices of wood of superior quality, having thickness varying from 0.4 mm to
6 mm or more. These are obtained by cutting with rotatory saw or in a rotatory peeling machine. Veneers
are glued or cemented with other veneers. Veneers may be cemented in layers with the grain of successive
layers crossing each other so as to neutralize shrinkage and decrease the danger of cracking. It may also be
used to prepare decorative surface over the inferior quality wood.

2. Plywood
It is an assembled product made up of veneers or plies and adhesive. Odd number of plies or veneers are
glued together under pressure with grain alternately in opposite direction. The graining of plies give
strength and dimensional stability to the sheets. Plywood is generally manufactured with three plies, five
plies or seven plies and so on. Similarly the thickness of the ply may vary. Plywoods are used in furniture,
partitions, ceiling, decorative purpose and packing purposes. It is also employed in the construction of
aircraft due to its light weight. It is also used in shuttering and shoring purpose. It is found in wide size
ranging from 900x900 mm to 2400x1200 mm.

3.Lamin Board
Lamin boards are made up of inner core of narrow timber laminas of 3 to 7 mm width. The core in both
side is covered with one or two veneers whose grains usually run at right angles to the core strips. Core
strips also are glued to form a wooden slab. The thickness of such board are found to be 12 mm to 25 mm.
Width of such boards are found up to 1.5 m and length from 2.5 m to 3 m. These boards are used to
construct partition wall, packing cases, ceiling, furniture and doors etc.

4. Bock boards
A block board is constructed in the same way as the lamin board. In block boards instead of strips used in
lamin boards, blocks of length not exceeding 25 mm and thickness of block being from 5 to 8 mm are
used. These blocks are glued together to form a flat core. This core is then covered with glued veneers in
both sides and pressed to form block boards. The direction of grain of core is made perpendicular to the
direction of grain of veneers. Block boards are cheaper than lamin boards. These are used in bus bodies,
partition wall and doors etc.

5. Batten board
Batten boards are similar to block boards. The core of batten board consists of closed grained battens
(blocks) not exceeding 80 mm and glued together to form a flat slab. This core then is glued and covered
with veneers. Then it is pressed to form batten boards. It is used in door panels, partition walls ceilings,
packing cases etc.

6. Hard board
These boards are hard pressed and hence are compact, strong and durable. They impart internal
appearance and finish to a structure. They are least affected by change in temperature and humidity of
surroundings. They are impregnated with oil when properties of high non-absorbent nature is required.

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7. Fibre board
These are rigid boards and also known as pressed wood or reconstructed wood. They are made from a
mass of felled wood or other vegetable fibres. They may be either homogeneous or laminated. They vary in
thickness from 3mm to 12mm. They are available in lengths varying from 3m to 4.5m, in widths varying
from 1.2m to 1.8m.

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Physical and Mechanical properties of Timber (not in syllabus)

Physical properties

i) colour and odour :- Most trees are characterized with a typical colour and odour. So the colour
and odour of trees varies from one to another.
ii) Specific gravity:- wood is a very light material, its specific gravity being always less than
one. Hence wood floats in water. Wood show good deals of variation in their specific gravity.
Some varieties may be as light as 0.3 where as in other varieties of timber the specific gravity
may approach to 0.9. This depends on their structure and presence of porous in them. The heart
wood is heavier than sapwood in the same tree.
iii) Moisture Content: - All wood are porous to some extent. They gain moisture from the
atmosphere depending upon moisture content of their cell vis-à-vis the humidity of the
atmosphere. Wood may absorb moisture more than 2 to 2.5 times then owns weight. Hence
moisture content value of natural green wood may sometimes be as high as 250%. Moisture
content of 12 to 15 % of dry weight of air seasoned dry wood is considered quite safe for timber
in any consideration.
iv) Grain: - By grain, it is understood as the arrangement and direction of growth of the wood. The
fibers growing parallel to the length of the tree trunk is called straight grain. The fiber may be
very tightly and closely packed. In both case, the strength of the wood may be high without any
type of defects.
v) Shrinkage and Swelling:- The newly cut wood lose moisture when subject to drying naturally
or artificially. On drying, the wood undergoes shrinkage. Similarly drying and getting rain
soaked or wetted may undergo considerable swelling . Shrinkage and swelling is related to
the behavior of cell wall of wood tissue towards water. Shrinkage in longitudinal direction is
least (0.1 to 0.5 %), where as it is highest (7 to 15 % ) in direction tangential to cell wall.

Mechanical Properties

The most important fact about strength of timber is that it is not same in all direction. This is because wood is
an isotropic material (having different structure in different direction). Hence, the strength of wood is
determined with reference to the direction of grain of the wood under load.

i) Compressive strength:- Timber from most of the tree is amazingly strong under compression
load 500 to 700 kg / cm2. It is however to be noted that other things being same, the
compressive strength parallel to grain is always less than that determined at right angles to the
grain in the same type of wood.
ii) Tensile strength:- Wood is very strong to tensile force acting parallel to grain but very weak
when these force are made to act perpendicular to the grain. Thus, the tensile strength of some
wood range from 500 - 2000kg/cm2 parallel to the grain. Whereas same values lie between 10 -
100 kg/cm2 for the same varieties when tested perpendicular to the grain.

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