BASICS OF CIVIL

ENGINEERING

CIVIL ENGINEERING MATERIALS

TIMBER IRON

STEEL

TIMBER

Oldest form of materials used in construction

Also used for other needs of human beings

Timber is obtained from trees

World timber is derived from the English world timbrian which means to build

Timber means wood that is good for building or carpentary which is having a circumference of 600mm

Converted timber means timber which is sawn into suitable commercial sizes

Rough timber means timber obtained after felling a tree

Standing timber means timber contained in a living tree

CHARACTERISTICS OF GOOD TIMBER• Uniform colour• A Freshly Cut Surface should give a sweet smell• Regular annual rings• Straight and close fibers• Heavy in weight.• Durable• Possesses the property of elasticity• Fire resistant• Hard• Should be strong for working as a structural

member

USES OF TIMBER

Used in the form of piles, posts, beams, lintels, door window frames

Used for flooring, ceiling, paneling, and construction of partition walls

Used for formwork of concrete, centering for arch work, scaffolding, transmission pole and fencing

Wagon and coach building, marine installations and bridges

THE POWER OF TREE

It precipitates rainfall and lowers temperature

It prevents depletion of ozone layer It prevents soil erosion It purifies the air by producing oxygen .

It is estimated that a hectare of tree will produce about 10 tonnes of oxygen per day which is sufficient for 45 persons to live for one year

CLASSIFICATION OF TREE

TREES ARE CLASSIFIED AS ABOVE

Exogenous trees grow outwards and increase in bulk by forming concentric rings from the Centre. Outside rings are called annual rings as it grows per year

Exogenous trees are again classified into conifers and deciduous

Conifers are evergreen trees the leaves of which do not fall till a new ones are grown

They produce cone shaped fruits they are called conifers

They show distinct annual rings

• Deciduous trees- leaves fall in autumn and new ones appear in spring season

• Timber for engineering derive these type of trees

• These trees yield hard wood • They do not show distinct annual rings

example ,teak, jackwood, rosewood etc• Endogenous trees grows inward and fibrous

mass is seen in their longitudinal section– Very limited engineering use example

bamboo,cane,palm

 

CROSS- SECTION OF EXOGENOUS TREE

PARTS OF A TREE

1. Pith- innermost central portion of the tree is called pith. It contains cellular tissues and nourishes the plant in its younger age and when the plant becomes aged it dies

2. Heart wood –inner angular rings surrounding the pith constitutes the heart wood , usually dark in colour, it indicates dead portion of the tree and does not take part any role in the growth of tree

3. Sap wood –outer annual rings surrounding the pith constitutes the sap wood , it is light in colour and weight, it indicates recent growth , it takes active part in the growth of tree

4. Cambium layer-thin layer of sap between sap wood and inner bark are known as cambium layer. It indicates sap which are not converted into sap wood

5. Inner bark –inner layer covering the cambium layer is known as inner bark

6. Outer bark-outer cover of the tree is known as outer bark

7. Medullary layer- thin radial fibers extending from the pith to cambium layer is known as medullary rays

PROCESSING OF TIMBER

Felling of tree Seasoning of timber Conversion of timber Preservation of Timber

SEASONING OF TIMBER

• When a tree is cut , it contains about 50 % more of its weight as water . removal of this water is known as seasoning

• The timber is to be dried for attaining this • Seasoned timber should be kept away from

moisture content and from humidity

OBJECTIVES OF SEASONING

1. To allow the timber to burn if used as a fuel

2. To decrease the weight

3. To impart hardness ,stiffness, strength and better electrical resistance

4. To increase the resistance power of timber

5. To maintain shape and size

6. To make it easily workable and to facilitate operations during conversion

7. To make it fit for receiving treatment

8. To make it safe against attack of fungi

9. To make it easy for gluing

10. To reduce the tendency of timber to crack, shrink and warp

Methods of Seasoning of timber

Natural seasoning

Artificial seasoning

NATURAL SEASONING(AIR)

Timber is cut into planks or scantling They are stacked horizontally or vertically The ground for stack is to be leveled Brick support is constructed and over

which it is to be kept The timber logs are arranged in layers on

above the other as shown in figure. The timber of equal size should be kept in each stack

Each layer should be separated by spacers . Size of spacers should be 35mm*25mm to 50mm*35mm

The distance between the spacers depends on the size of the wood

The length of stack depends on the length of timber pieces

Stack to be protected from fast flowing wind, rain , extreme heat or sun . So it should be covered

NATURAL SEASONING

ADVANTAGES

The moisture content of wood can be brought down to about 10 to 20%.

It does not require skilled supervision For thicker sections, artifical seasoning is

uneconomical where natural seasoning can be adopted.

Cheap and simple

DISADVANTAGES

Less control over air The drying of different surfaces will not

be even or uniform Chance of attack by fungi and insects

during seasoning.

ARTIFICIAL SEASONING

To make the drying more uniform and even, by controlled conditions, artificial seasoning is adopted

It reduces the time for seasoning shrinkage, cracking and warping are minimized in artificial seasoning.

The wood with desired moisture content may be obtained by this.

ARTIFICIAL SEASONING

The various methods used are, Boiling Chemical Seasoning Electrical seasoning Kiln seasoning Water seasoning

1. BOILING

Immersed in water and then boiled for about 3 to 4 hours

Dried very slowly under a shed The periods of seasoning and

shrinkage are reduced In place of boiling steam is also

used but expensive

2. CHEMICAL SEASONING(SALT)

Immersed in solution of suitable salt Taken out and seasoned in the ordinary

way The interior surface dries in advance the

chances of formation of external cracks reduced

3.ELECTRICAL SEASONING

High frequency AC current is used The resistance increases as the wood

becomes more and more dry This is the most rapid method of

seasoning Initial maintenance cost is very high

4. KILN SEASONING

In this seasoning is carried out in a chamber Timber is arranged inside the chamber with space for

circulation of air Air which is fully saturated with moisture at 350 C to

380C admitted in to the chamber by some arrangement The forced air is allowed to circulate round the timber

as air is fully saturated with moisture the evaporation from the surface is prevented and the heat gradually reaches inside the timber pieces

The relative humidity is gradually reduced The temperature is then raised and maintained till the

desired degree of moisture content is reached

KILN SEASONING

WATER SEASONING

Timber is cut into small pieces They are immersed in water ,

preferably in running water Thicker end is pointing upstream Timber is taken out after 2 to 4

weeks and allowed to become dry

ADVANTAGES

Handling and transportation cost of wood can be reduced

Improve the Engineering properties of wood

Dimensional stability of finished products can be maintained

Eases the workability of timber Before the application of paints,

varnishes, preservatives ,glues etc. seasoning of wood is a prerequistite.

Sl no

item Natural seasoning

Artficial seasoning

1 Moisture content

Dificult to reduce below 15 to 18%

Can be reduced to any level

2 Nature Simple and economical

Expensive and technical

3 Quality of timber

More liable to attack by insects and fungi

Kiln seasoned timber are less liable for attacks

4 space Requires more space for stacking

Less space

5 Speed slow quick6 strength Stronger weaker

TIMBER DEFECTS

• Defects due to conversion• Defects due to fungi• Defects due to insects • Defects due to natural forces • Defects due to seasoning

DEFECTS IN TIMBERDEFECTS DUE TO CONVERSION

While converting in to commercial forms the following defects may occur

Chip mark- marking due to chips on the finished surface Diagonal grain-due to improper sawing of timber,

diagonal mark on the straight grain Torn grain-This defect is caused when small depression

is formed on the finished surface of the timber by falling a tool

wane – original rounded surface on the manufactured piece of timber

 

2)DEFECTS DUE TO FUNGI

Fungi are minute microscopic plant they attack only when certain conditions are satisfied

Moisture content of the timber above 20%

There is presence of warmth and air for the growth of fungi

Rot – physically weak Blue stain , brown rot, Dry rot, heart rot . Sap stain ,wet rot ,white rot

3. DEFECTS DUE TO INSECTS Beetles – insects having beating mouth, Due to

this timber is converted into fine flour like powder.

Marine borers- insects that bore the wood, found in salt water, timber loses colour and strength due to this.

Termites- these are also white ants, live in colonies and eat way the timber in different ways.

4)DEFECTS DUE TO NATURAL FORCES

Abnormal Growth Rupture of Tissues Most important is knot and shake

DEFECTS DUE TO NATURAL FORCES

Knots , shakes and rind gall(abnormal swelling)

Classification of knots(branches which are broken or cutoff from tree)

Dead knot- connected with surrounding wood- not safe to use for engineering purposes

decayed knot – due to fungi Live knot-sound knot- difficult to remove

- ,makes sawing difficult, can be used for Engineering purposes

Loose knot-preliminary stage of dead knot

Round knot-shape is round Tight knot-preliminary stage of live knot

2.Twisted fibers- happens due to twisting of young trees by fast blowing wind. - Fibres are twisted in one direction , not suitable

for engineering purposes.

3.Shakes –cracks which are partly or completely separate the fibers of wood

a) cup shakes

b) heart shakes

c) Ring shakes

d) Star shakes

e) Radial shakes

4.Water stain- wood is sometimes discoloured when it comes into contact with water, found in converted timber

5.Wind cracks- if wood is exposed into atmospheric agencies

-its exterior surface shrinks, results into cracks.

6. Rind galls- these develop at places from where branches are improperly cut.

-Swelling due to abnormal growth is the reason for this

-Timber at this part is very weak.

KNOT AND RIND GALL

SHAKES

RADIAL SHAKE AND TWISTED FIBRES

CUP SHAKE AND HEART SHAKE

SHAKES

DEFECTS DUE TO SEASONING

Defects due to seasoning 1)bow- curvature formed in the direction of

length 2)case hardening-exposed surface dries up

speedily and the inside will be under tension 3)check-crack which separates fibers of wood 4)collapse-uneven shrinkage the wood

sometimes flattens during drying. 5) cup- curvature is formed in the transverse

direction 6)Honey combing-due to stresses developed

during drying various radial and circular cracks develop in the interior portion of the timber

PRESERVATION OF TIMBER

Objective To increase the life To make it durable To protect the timber from attack

of destroying agents such as fungi and insects

TYPES OF PRESERVATIVES

1) Ascu treatment 2) Chemical salts 3) Coal tar 4) Creosote oil 5) Oil paints 6) Solignum paints

ASCU TREATMENT 1) Developed in forest research

institute Dehradun and consist of the following

Parts by weight of Arsenic pentoxide Parts by weight of copper sulphate The material available in powder form Six parts by weight of Ascu mixed in 100

parts by weight of water and applied in the wooden surface

2)CHEMICAL SALTS Water borne preservatives mostly salts

dissolved in water Commonly used are copper sulphate,

Mercuric chloride ,Sodium fluoride and Zinc chloride

COAL TAR

When coal tar is applied on the surface of wood it is called tarring

Unpleasant smell and appearance Adopted only for unimportant work Fire resistant

CREOSOTE OIL

Obtained by distillation of tar Process is as follows 1. Timber is thoroughly seasoned and

dried 2. It is then p laced in an air tight

chamber 3. Air is pumped out fro the chamber 4. Creosote oil is pumped in to the

chamber 5. After a period of about 1 or 2 hours

when timber is completely absorbed the oil it is to be taken out

OIL PAINTS

Timber surface is coated with 2 or 3 coats of oil paints

Oil paint preserve timber from moisture and make it durable

SOLIGNUM PAINTS These paints preserve timber from

white ants as they are toxic in nature Mixed with colour pigments and applied

with the help of a brush

METHOD OF PRESERVATION 1)Brushing- applying preservative by

using brush 2)Charring-kept wet for an half an hour

and then burning 3)Dipping and Steeping(Let sit in a

liquid to extract a flavor or to cleanse) 4) hot and cold open tank treatment 5) injecting under pressure 6) spraying

FACTORS AFFECTING STRENGTH OF TIMBER Abnormalities of growth Faults in seasoning Invasion of insects Irregularities of grain Moisture content Presence of knot, shakes etc Way in which the timber piece is cut

from the log

DECAY OF TIMBER-CAUSES Alternate wet and dry condition Bad storage and stacking of timber Attack of fungi Improper seasoning Insects beetles ,marine borers, termites Keeping in contact with damp wall,

damp earth etc

Use of timber without taking out sap wood from the structure

use of seasoned timber without using preservative

Use of unseasoned wood with the application of protective of paint or tar