Natural Materials (Timber)
Natural Materials (Timber)
What is difference between wood and timber?
Wood is obtained from cutting of trees. And timber
is obtained from conversion(treatment) of wood.Different components of timber.
Components of
timber- outer bark, inner bark, sap wood, heart wood, pitch, madullary rays,
etc.
Properties
of good timber
Strength: It
s have good strength, i.e. compressive strength, tensile strength, shear
strength, etc. It able to take self weight and external load.
Appearance:
It is good in appearance. It have good finishing.
Economy: It
is easily available hence economical.
Recycle: It
is recycled easily when we obtain scrap from old structure.
It is durable. Also it is easily available. After
proper treatment it give maximum chemical resistance.
Different defects in
timber
I.
Defects occurred due to natural forces:
1.
Knots:
It is base or roots of small branches of trees. The formation of hard rings at
the base branches is called as konts (fig.1).
2.
Heart shakes: This defects caused due to shrinkage of heartwood (fig.2).
3.
Twisted fiber: This defects caused due to constant bending in on e
direction because of wind action and fibers are twisted. The wood having this
defects is unsuitable for the squaring purpose hence this type of wood is used
for only pole or fuel purpose (fig.3).
4.
Cup and ring shake: When any wind force like cyclone is passes through trees
then this type of defects is produced as shown in (fig.4).
5.
Radial shakes: This defect is due to quick drying the outside portion than
the interior at the time of seasoning. The cracks are fine, irregular and
numerous.
II.
Defects occurred due to conversion:
i.
Wane:
This defect is denoted by the presence of original rounded surface on the
manufactured pieces of timber.
ii.
Chip mark:
This defect is indicated by the marks placed by chips on the finished face of
timber.
iii.
Diagonal grain: This defects produced straight lines of grains due to
improper sawing of timber.
iv.
Torn grain:
This defect is a small depression caused by falling of tool.
III.
Defects due to insects:
- This defects are raised due to
attack of insects like battles, marine bores and termites.
- Battles make tunnel in wood and
convert wood or timber in to fine powder.
- Marine borers are generally found
in the salty water. They do not convert wood in to fine powder but make bore
tunnel in one direction for shelter.
- Termites are popularly known as
white ants. They live in colony and eat wood.
IV.
Defects due to fungi:
Fungi
are the microscopic plant organisms. Fungi attack the timber only when the
moisture content in timber is above 20% and presence of air and warm condition
for their growth.
V.
Defects due to seasoning:
Seasoning of timber
Seasoning of timber
Seasoning
of timber is the process of removing moisture content from timber or wood.
This
are formed due to improper seasoning of timber.
i. Bow defects: during
seasoning timber bend in longer or longitudinal direction which see like bow as
shown in fig.
ii. Cup defects: during seasoning timber bends in shorter or
with direction which see like cup as shown in fig.
iii. Twist defects: In
this type during seasoning timber twist as shown in fig.
Why seasoning of timber is necessary?
It
is necessary to following purposes-
- To reduce weight.
- To easy conversion.
- To make structurally stable.
- To increase decay resistance.
- To increase strength, hardness and
toughness.
Types
of seasoning
1.Natural seasoning:
1.Natural seasoning:
- a) water seasoning: In this method logs of wood are completely submerged under water in ponds, completely out of contacts air for period of 2 to 4 weeks.
- b) air drying: it is very cheap method but time consuming method. It require form 3 month to 3 year. For this timber should be placed in such a way that air will circulate and dried completely.
2.Artificial
seasoning:
- a) kiln seasoning: in this method moisture content from timber is removed by means of heating timber in kiln. For this purpose very high temperature capacity ovens are used. In this oven wood is placed and heated to remove moisture content.
- b) chemical seasoning: the timber is dipped in creosote and dried in a cylinder by boiling in a vacuum. The vapours of water and creosote escape and are condensed and subsequently separated. Surface of wood dried rapidly and becomes protected from insects and fungi.
- c) electric seasoning: in this method high frequency alternative current is used for drying. It is passed through timber which is bad conductor and gets heated. Because uniform heat moisture is driven out. It is very fast process but extremely costly.
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