Notes to tropical woods datasheet
Notes to the properties of tropical timbers for outdoor use different from Teak and Suar wood.
Durability
The natural durability of timber may be defined as “the inherent resistance of timber to attack by wood destroying organisms such as wood decaying fungi and wood destroying insects”. Natural durability only refers to the heartwood of the tree.
In the following table results of the time taken to reach mean visual rating of 7 (Moderate decay or termite attack):
| CLASS | SERVICE LIFE |
|---|---|
| Very durable | >10 yrs |
| Durable | 5 – 10 yrs |
| Moderately durable | 2 – 5 yrs |
| Not durable | <2 yrs |
Natural durability of timber is determined using a “graveyard” test. In this test, wood stakes 50mm x 50mm in section are inserted in the ground so that half of the length of the stake is buried. The average time taken for these stakes to fail is used to assign a natural durability rating.
Movement
The method used in the determination of the movement of different timber species is similar to the procedure described in “The Movement of Timbers – Technical Note No. 38, Princess Risborough Laboratory, BRE, UK, 1976”. However, the constant humidity – temperature settings required for the two exposure tests were adjusted to 30°C/95% relative humidity with equilibrium moisture content (EMC) of 20%, and 30°C/60% relative humidity with EMC of 12% respectively.
A total of 20 pieces of quarter-sawn and flat-sawn representative samples were used to measure the radial and tangential movements of each species of timber.
These were obtained from 30mm boards which had been air seasoned for three to six months to attain the air-dry EMC of approximately 14 to 19%. Each sample was then fine-dressed to a thickness of 6mm and cut to a standard length of 50mm and width of 150 mm.
For the purpose of describing the movement of timbers for decking, the following classification was used:
| MOVEMENT RATING | TANGENTIAL MOVEMENT |
|---|---|
| Type I | < 1,5% |
| Type II | 1,5 to 2,0% |
| Type III | 2,1 to 2,5% |
| Type IV | 2,6 to 3,0% |
| Type V | > 3,1% |
Strength classification
The strength properties have been determined using the test procedure described in ASTM designation: D143-52 developed by the American Society for Testing Materials.
| STRENGTH GROUP | VERY STRONG | STRONG | MODERATELY STRONG | WEAK |
|---|---|---|---|---|
| Bending and tension parallel to grain | 20.69 | 17.24 | 12.41 | 7.59 |
| Compression parallel to grain | 17.24 | 13.79 | 9.65 | 6.55 |
| Compression perpendicular to grain | 1.72 | 1.03 | 0.69 | 0.41 |
| Shear parallel to grain | 2.76 | 2.07 | 1.38 | 1.38 |
| Modulus of elasticity mean-minimum | 13,800-8,600 | 11,000-6,200 | 9,000-5,200 | 5,700-3,000 |