Defects of timber | Prevention - learncivil


DEFECTS IN TIMBER

The defects occurring in the timber are grouped into the following five divisions:

1. Defects due to fungi
2. Defects due to insects.
3. Defects due to seasoning.
4. Defects due to natural forces.
5. Defects due to conversion


1) Defects due to fungi: 

Defects of timber


The fungi are minute microscopic plane organisms.

Following defects are caused in the timber by the fungi:
  • Dry rot
  • Wet Rot
  • Brown rot
  • White rot
  • Heart Rot
  • Sap strain
  • Blue stain


  • Dry rot:

The term rot is used to indicate decay or disease of timber. The fungi of certain types feed on Wood and during feeding, they attack on wood and convert it into dry powder form. This is known as the dry rot.
The dry rot occurs at places where there is no free circulation of air. The fungus rapidly dies on exposure to air or sunlight. The unseasoned softwoods and sapwood are easily attacked by dry rot.
The dry rot may be prevented by using well-seasoned timber free from sap.


When part of the timber is seriously affected by dry rot, the damaged portion may be completely Removed and the remaining unaffected portion should be painted with a solution of copper sulphate.

  • Wet rot:

When timber is subjected to alternate wet and dry conditions, decomposition of its tissue Takes place and the timber is said to have been attacked by wet rot. The attacked part of timber gets Reduced to a grayish brown powder Standing tree or timber exposed to rain is subjected to attack

Wet rot can be avoided by the use of well seasoned timber treated with preservatives.


  • Brown rot:

The fungi of certain types remove cellulose compounds from wood and hence the wood comes brown color. This is known as the brown rot.



  • White rot:

The fungi of certain types of attack wood and the wood become a white mass consisting of cellulose compounds.

  • Heart rot: 


This is formed when the heartwood is exposed to the attack of atmospheric agents. Due to this, the tree becomes weak and it gives out hollow sound when struck with a hammer.
Sap stain: The fungi of certain types do not bring about the complete decay of timber. But they feed on cell contents of sapwood and the sapwood loses its color. This is known as the sap stain and it
generally occurs when moisture content goes beyond 25 percent or so.


  • Blue stain:

The sap of wood is stained to bluish color by the action of certain types of fungi.

2) Defects due to insects:

Following are the insects which cause the decay of timber:
Defects of timber


  • Termites.
  • Beetles.
  • Marine Borers

  • Termites:


These are popularly known as the white ants and they are found in abundance in tropical and sub-tropical countries. These insects live in a colony and they are very fast in eating away the
wood from the core of the cross-section. They make tunnels inside the timber in different directions and usually do not disturb the outer shell or cover.

  • Beetles:

These are small insects and they cause rapid decay of timber. They form pin-holes of size about 2mm diameter in wood. They attack the sapwood of all species of hardwoods.


  • Marine borers:

These are generally found in the salty water. Most of the varieties of marine borers do not feed on wood. But they make holes or bore tunnels in wood for taking shelter


3) Defects due to seasoning:


Following defects occur in the seasoning process of wood:
  • Bow
  • Cup
  • Case-hardening
  • Check
  • Collapse
  • Honey-combing Radial slakes
  • Split
  • Twist
  • Warp
Defects of timber


  • Cup:

This defect is indicated by the curvature formed in the transverse directions
transverse direction of timber.

  • Case-hardening:

The exposed surface of timber dries very rapidly. It therefore shrinks
The interior surface which has not completely dried is under tension
case-hardening and it usually occurs in timbers which are placed at the bottom during seasoning. 


  • Check:

A check is a crack which separates fibres of wood. It does not extend from one end to the
Checks
Surface checks: Shallow cracks extending along the grain on the face or cage.

  • Collapse:

Due to uneven shrinkage, the wood sometimes flatters during drying. This is known as the
collapse

  • Honey-combing:

Due to stresses developed during drying, the various radial and circular cracks developed in
the interior portion of timber. The timber thus assumes the honey-comb texture and the defect so
developed is known as the honey-combing.

  • Radial shakes:

These are radial cracks.

  • Split:

When a check extends from one end to the other, it is known as a split.

  • Twist: When a piece of timber has spirally distorted along its length, it is known as a twist. 



  • Warp: When a piece of timber has twisted out of shape, it is said to be warped


4) Defects due to natural forces:

Defects of timber


Following defects are caused by natural forces:
  • Burls
  • Callus
  • Chemical stain
  • Coarse Grain
  • Deadwood
  • Druxiness
  • Foxiness
  • Knots
  • Rind galls
  • Shakes
  • Twisted fibers
  • Upsets
  • Water strain
  • Wind cracks


  • Burls:
These formed when a tree has received shock or injury, the growth of tree is completely upset
and irregular projections appear on the body of timber.

  • Callus:
It indicates soft tissue or skin which covers the wound of a tree.

  • Chemical strain:
The wood is sometimes discolored by the chemical action caused with it by some external
agency. This is known as the chemical strain.

  • Coarse grain:
If a tree grows rapidly, the annual rings are widened. It is known as the coarse-grained timber
and such timber possesses less strength.

  • Deadwood:
The timber which is obtained from dead standing trees contains dead wood. It is indicated by lightweight and reddish color.

  • Druxiness:
This defect is indicated by white decayed spots that are concealed by healthy wood. They
are probably formed for the access of fungi.

  • Foxiness:
This defect is indicated by red or yellow tinge in wood or reddish brown strain or spots round
the pith of tree discoloring the timber. It is caused either due to poor ventilation during storage or
by the commencement of decay due to over-maturity or due to the growth of the tree in a marshy soil.

  • Knots:
    Defects of timber
These are the bases of branches or limbs which are broken or cut off from the tree. The portion from which the branch is removed receives nourishment from the stem for a pretty long time and it ultimately results in the formation of dark hard rings which are known as the knots. As continuity of wood fibers is broken by knots, they form a source of weakness.

  • Rind galls:
    Defects of timber
The rind means bark and gall indicates abnormal growth. It is an irregular growth cause the fibers on wound fell after the branches have been cut off in an irregular manner.

  • Shakes:
These are cracks which partly or completely separate the fibers of the wood. The following are the different varieties of shakes.
Cup shakes
Heart shake
Ring shake
Star shake
Radial shake


5) Defects due to conversion

During the process of converting timber to the commercial form, the following defects may occur:
  • Chip mark.
  • Diagonal mark.
  • Torn mark.
  • Wane.
    Defects of timber

  • Chip mark:
This defect is indicated by the marks or signs placed by chips on the finished surface of timber. They may also be formed by the parts of a planning machine.

  • Diagonal grain:
This defect is formed due to improper sawing of timber. It is indicated by a diagonal mark on the straight grained surface of timber.

  • Torn grain:
This defect is caused when a small depression is formed on the finished surface of timber by
falling of a tool or so.

  • Wane:
This defect is denoted by the presence of original rounded surface on the manufactured piece
of timber.
Also read: Classification of timbers

DECAY OF TIMBER
The timber is said to be decayed when it is so deteriorated that is, loses its value as an
engineering material.

Following are the various causes or situations which favor the early decay of timber:

1. Alternate dry and wet conditions.
2. Bad storage or stacking of timber.
3. Fungi which are responsible for developing diseases in timber such as blue stain, brown rot, dry rot, heart rot, sap stain, wet rot, and white rot.
4. Improper seasoning.
5. Insects such as beetles, marine borers, termites, etc.
6. Keeping timber in contact with damp wall, damp earth, etc.
7. Shocks or impacts received during young age from natural forces such as fast blowing wind, etc.
8. Use of timber without taking out sapwood from its structure.
9. Using seasoned timber without applying a suitable preservative on its surface.
10. Using unseasoned wood with the application of a protective coat of paint or tar.

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