Classification of Timber

Classification of Timber on the following basis:

On the basis of Durability (IS 3629)

Class I : Natural durable heartwood timbers having average life of 120 months and over (high durability).
Class II : Natural durable heartwood timber having average life of 60 months and over but less than 120 months (moderate durability).
Class III : Timbers having average life less than 60 months (low durability).

On the basis of Treatability (IS 3629)

Class a : Heartwood is easily treatable.
Class b : Heartwood is treatable but complete penetration of preservative are not always obtained (when least dimension is more than 60 mm).
Class c : Heartwood is only partially treatable.
Class d : Heartwood is refractory to treatment.
Class e : Heartwood is very refractory to treatment and, penetration of preservative being practically nil even from the end.

On the basis of Seasoning (IS 1141)

Class A : Highly refractory, slow drying and difficult to season, free from cracking and splitting.
Class B : Moderately refractory, may be seasoned, free from surface and end cracking within reasonable short period, given a little protection against rapid drying conditions.
Class C : Non-Refractory, may be rapidly seasoned, free from surface and end cracking even in the open air and sun. If not rapidly dried, then they develop blue stain and mould on the surface.

On the basis of Modulus of Elasticity (E) and Extreme and Fibre Stress in Bending and Tension (f_b) (IS 3629)

Group A : E above 12.6 × 103 N/mm² and fb above 18 N/mm².
Group B : E above 9.8 × 103 N/mm² and upto 12.6 × 103 N/mm² and fb above 12 N/mm² and upto 18 N/mm²
Group C : E above 5.6 × 103 N/mm² and upto 9.8 × 103 N/mm² and fb above 8.5 N/mm² and upto 12 N/mm².

PROPERTIES OF TIMBER

• Specific gravity of timbers is 1.54.
• The compressive strength of timber is highest parallel to the grains (30 – 77.5 MPa).
• Tensile strength of timber is 2 – 4 times its compressive strength parallel to the grain. (80 – 130 MPa).
• Shrinkage in timber:
  – Shrinkage parallel to the grain is 0.1 – 0.3%, in radial direction it is 3 – 6% and in tangential direction it is 7 – 12%.
  – The radial shrinkage is approximately 60% of tangential shrinkage and volumetric shrinkage is 60% more than tangential shrinkage.
• Swelling of timber is 0.1 – 0.8% in longitudinal direction, around 5% in radial direction and 6 – 12% in tangential direction.
• Heat conductivity is 1.8 times higher along the fibers than across the fibers.
• Wood is not affected by weak alkali solution but affected by acidic medium if pH is less than 4.
• Along the fiber, wood has lower shear strength in the range of 6.5 – 14.5 N/mm² but across the grain it is about 3 – 4 times higher.

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