Construction materials - Timber

Construction materials - Timber 

 TIMBER

Living tree, yielding good timber is called standing timber. After felling and separating branches it is known as rough timber. When bark is removed and stem is roughly converted into pieces of suitable length it is known as log. After the log is seasoned and converted into commercial sizes like planks, battens, posts and beams, it is known as converted timber.

Classification

1. On the basis of mode of growth timber is classified as exogenous and endogenous. Exogenous trees grow outward adding a distinct ring every year while endogenous trees grow inwards. 

Example of exogenous trees are mahogony, sal, teak , babul while examples of endogenous trees are bamboo and cane. Exogenous trees are further classified as coniferous and deciduous. 

Coniferous trees have cone shape and their leaves do not fall till new ones grow. Deciduous trees have broad leaves and they fall in autumn and new ones appear in the spring.

In the cross section of deciduous trees the following components are clearly visible from inside to outside. Pith, heartwood, sapwood, cambium layer, inner bark, outer bark and medullary rays.

Sapwood contains annual rings, count of which gives the age of the tree.

2. Classification based on durability: Durability test is conducted by Forest Research Institute of India, Dehradun by burying   600 × 50 × 50 size specimen upto half their length and observing them over several years. Then timber is classified as

(a) High durability: Average life of more than 10 years

(b) Moderate durability: Average life 5 to 10 years

(c) Low durability: Average life is less than 5 years.

3. Classification based on grading: On the basis of strength, defects, etc. IS: 12326–1976 classifies timber as special grade, Grade – I and Grade – II.

4. Classification based on Availability:

X : Most common, 1415 m³ or more every year

Y : Common, 335 m³ to 1415 m³ per year

Z : Less common, less than 335 m³ every year.

Seasoning of timber Seasoning is the process of reducing moisture content in a freshly cut tree to the desired level. Seasoning makes timber more durable and stable. The various methods of seasoning used are:

Natural seasoning: Air seasoning or water seasoning.

Artificial seasoning: Boiling, kiln seasoning.

Chemical seasoning or electrical seasoning 

Conversion of timber Market names of converted timber are battens, plank, pole, scanting, beams, etc. They are available in different sizes and length. They are obtained by sawing logs. 

The various methods adopted for sawing are: ordinary, quarter, tangential and radial.

Defects in timber Defects may be due to natural forces, attack by fungi or insects or due to erroneous seasoning. The defects due to natural causes are knots, shakes, rind galls, upsets, twisted fibres, wind cracks, burls, dead wood, foxiness and stain.

Preservation of timber The widely used preservatives are coal tar, solignum paints, chemical salt, creosote and ASCU. ASCU is a special preservative developed by FRI, Dehradun.

Industrial timber Veneers, plywood, fiberboards, particle boards, block boards, hard boards and Glulam.

Indian timber trees Babul, Bamboo, Casurina, Deodar, Jack, Mango Mahogany, Rosewood, Teak, sandalwood, Sisso.