Defects in Timber and Preservation of defects | Heavy timber construction

Defects in Timber and Preservation of defects

Heavy timber construction

Heavy timber construction


There are various defects that are likely to occur in the timber may be grouped into the following three: Heavy timber construction

  1. Due to natural forces (Heavy timber construction)
  2. Due to defective seasoning and conversions.
  3. Due to attacks by fungi and insects.

Defects due to the Natural Forces:

The following defects are caused by natural forces:

(a) Knots

(b) Shakes

(c) Wind cracks

(d) Upsets


Knots (Heavy timber construction): When a tree grows, many of its branches fall and the stump of these branches in the trunk is covered. In the sawn (7nos) pieces of timber, the stump of fallen branches appears as knots. Knots are dark and hard pieces. Grains are distorted in this portion. If the knot is intact with the surrounding wood, it is called a live knot. If it is not held firmly it is a dead knot.

Wind Cracks (Heavy timber construction): These are the cracks on the outside of a log due to the shrinkage of the exterior surface.

Upsets (Heavy timber construction): This type of defect is due to excessive compression in the tree when it was young. Upset is an injury by crushing. This is also known as rupture.

Defects due to Defective Seasoning and Conversion:

If the timber seasoning is not uniform, The (timber) converted may warp and twist in various directions. Sometimes honey combining and even cracks appear. This type of defect is more susceptible in the case of kiln seasoning.

In this process of converting timber to commercial sizes and shapes are the following types of defects are likely to arise: chip marks, torn grain, etc.

Defects due to Fungi and Insects Attack:

Fungi are minute microscopic plant organisms. They grow in wood if the moisture content is more than 20°C and exposed to air. Due to the fungi attack rotting of wood, is taking place. Wood becomes weak and stains appear on it.

Beetles, marine borers, and termites (white ants) are the insects that eat wood and weaken the timber. Some types of woods like teak have chemicals in their compositions (natural resist power) and resist such attacks. Other types of timber woods are to be protected by chemical treatment.



Preservation of Timber 

Preservation of timber means protecting the timber from fungi and insects attack so that its life is increased. The Timber is to be seasoned well before the application of preservatives. The following are the widely used preservatives (Heavy timber construction):

    1. Tar
    2. Paints
    3. Chemical salt
    4. Creosote
    5. ASCO

Heavy timber construction

Tar: Hot coal tar is applied to timber with a brush. The coating of tar protects the timber from the attack of fungi and insects (Heavy timber construction). It is the cheapest way of protecting the timber. The main disadvantage of this method of preservation is that appearance is not good after the tar is applied it is not possible to apply other attractive paints. Hence tarring is made only for unimportant structures like fence poles.

Paints: Two to three coats of oil paints are applied on a clean surface of the wood. The paint protects the timber from moisture. The paint (enamel paint) is to be applied from time to time. Paint improves the appearance of the timber. Solignum paint is a special paint that protects the timber from the attack of termites.

Chemical salt: These preventative methods are the preservatives made by dissolving salts in water. The salts used are copper sulfate, masonry chloride, zinc chloride, and sodium fluoride. After treating the timber with these chemical salt paints and varnishes can be applied to get a good appearance.

Creosote: Creosote oil (Creosote is a category of carbonaceous chemicals) is obtained by distillation of coal tar. The seasoned timber is kept in an airtight chamber and the air is exhausted. After creosote oil is pumped into the chamber at a pressure of 0.8 – 1.0 N/mm2 at a temperature of 50°C. After that 1 to 2 hours, this timber is taken out of the storage chamber.

ASCO: This type of preservative is developed by the Forest Research Institute, Dehradun. It consists of 1 part by weight of hydrated arsenic pentoxide (As2O5, 2 H2O), 3 parts by weight of copper sulfate (CuSO4⋅5 H2O), and 4 parts by weight of potassium dichromate (K2Cr2O7) or sodium dichromate (Na2Cr2O7⋅2 H2O). This preservative is available in powder form (defects). By mixing 6 parts of this powder with 100 (hundred) parts of water, the solution is prepared. The solution is then sprayed over the surface of the timber. This treatment prevents the attack from termites. The surface may be painted to get the desired appearance.

Uses of Timber


Timber is used for the following works (Heavy timber construction):

  1. For heavy construction works like columns, trusses, piles.
  2. For the light, construction works like doors, windows, flooring, and roofing, etc.
  3. For other permanent works like railway sleepers, fencing poles, electric poles, and gates.
  4. For temporary timber works in construction like scaffolding (for masonry works, elevation work), centering, shoring and strutting, packing of materials.
  5. For decorative works like showcases and furniture.
  6. For body works of buses, lorries, trains, and boats
  7. For industrial uses like pulps (used in making papers), cardboards, wallpapers
  8. For making sports goods and musical instruments.