SEMINAR

Basic Civil Engineering

Decay of Timber1

Seasoning of Timber2

Advantages3

Overview

DECAY OF TIMBER

When timber loses its value as engineering material, it is said to be decayed.

The decayed timber cannot be used for engineering purposes.

Decay of timber

Situations favoring early decay of Timber

1) Alternate dry and wet conditions.2) Bad storage or stacking of timber.3) Fungus attack.4) Contact with damp surfaces.5) Insects.6) Improper seasoning.7) Shocks or impacts received during growth.8) Useage of timber without applying any

preservatives.9) Useage of timber without taking out sapwood

from its structure.10)Useage of unseasoned wood.

Seasoning of Timber• Fresh timber contains about 50% or more of its own dry

weight as water. The water is to be removed before it used for engineering purposes. In other words timber should be dried.

• The process of drying of timber is known as seasoning of Timber.

• The moisture content should be removed under controlled conditions at a uniform rate from all parts of timber.

• Seasoning of timber should be protected from rain and humidity.

If the drying is irregular, the shrinkage of timber will also be irregular and it will setup internal stress between fibers.

Seasoning of timber should be protected from rain and humidity.

OBJECTS OF SEASONING

1. To impart hardness, stiffness, strength and better electrical resistance to timber.

2. To maintain the shape and size of the components of the timber articles, which are expected to remain unchanged in form.

3. To decrease the weight of timber and their by to reduce the cost of transportation.

4. To make timber easily workable and to facilitate operations during conversions.

5. To make timber suitable for receiving treatment of paints, preservatives, varnishes etc.

6. To make timber safe from the act of fungi and insects.7. To make suitable for joinery works.8. To reduce the tendency of timber to crack, shrink and

warp.

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METHODS OF SEASONING

Natural seasoning

Artificial seasoning

NATURAL SEASONING

When seasoning of timber is carried out by natural air, it is called natural seasoning or air seasoning.

Procedure adopted for air seasoning

Timber is cut and sawn into suitable sections of planks or scantlings. The place where stack is to be constructed is cleared and leveled for good drainage. The timber pieces can either be stacked horizontally or vertically.The base platform of the stack may be kept 30cm above the ground level.The timber pieces are sorted out according to their sizes. They are stacked in layers one above the other.The layers are separated by dry pieces of wood of thickness 25mm to 35mm. These spacers may be kept at 450mm to 600mm.The stack is protected from rain, sunlight and heavy wind.

Advantages of natural seasoning

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

It does not require skilled supervision.

For thicker sections artificial seasoning is uneconomical where natural seasoning can be adopted.

This method is cheap and simple.

Disadvantages of natural seasoning

1.Less control over air

2.The drying of different surfaces may not be uniform

3.Chance of attack of 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 the artificial seasoning.

Various methods of artificial seasoning are,

1.Water seasoning2.Boiling3.Electrical seasoning4.Kiln seasoning

Water Seasoning

To remove the sapwood, the logs of wood are immersed in water, preferably in running water of stream and river from 2 to 4 weeks.Then it is dried under open air . Wrapping and cracking can be avoided but become brittle and their strength has also reduced.

Boiling

In this method the timber is boiled in water for about 3 to 4 hours. It is then taken out and slowly dried. Even though the period of seasoning is less it is an expensive method.

Timber may be exposed to hot stream also.

Electrical Seasoning

This is done by passing high frequency AC current through the timber. The timber gets heated and dries out.

This method results in uniform seasoning of wood but the capital cost of equipment needed is more.

It is a very rapid method. This method is used in the manufacture of plywood.

It is an established fact that a green timber offers less resistance to the flow of electric current as compared to a dry timber.

Chemical Seasoning

In this method, green timber is soaked in saturated solution of suitable salt and then removed and seasoned.

The interior moisture is drawn out by the saturated salt solution and the interior part dries before the outer part. Less chance of external cracks.

Kiln SeasoningIn this method timber is carried out inside an air tight chamber.

Various process in Kiln seasoning1) The timber is arranged inside the chamber such that

spaces are left for free air circulation.

2) Humid and heated air of temperature of about 350 C to 380 C is then forced inside a chamber by suitable arrangement.

3) It is allowed to circulate around the timber pieces.

4) Evaporation from the surface of timber pieces is prevented because of high humidity of air.

5) The heat gradually reaches inside the timber pieces. The relative humidity is now gradually reduced.

6) The temperature is then increased and maintained till the desired degree of moisture content is obtained.

NOTE

Depending upon the mode of construction and operation Kilns are divided as ,1. Stationary Kilns2. Progressive kilns

Advantages of Seasoning1) Handling and transportation cost of wood can be reduced.

2) Seasoned timbre shows good resistance to decay.

3) The timbre should be seasoned before the application of paints, varnishes, preservatives, glues etc.

4) Ease the workability of timbre.

5) Dimensional stability of furnished products can be maintained.

6) Improved the engineering properties of wood.