Tropical In-Ground Durability of CCB-treated Keruing and other Structural Sarawak Timbers after 8 – 10 Years Exposure
A H H Wong, Ling Wang Choon
The heartwood of 12 mainly low-durability native timber species of Sarawak [4 Dipterocarpus (Keruing) species, Anisoptera grossivenia (Mersawa kunyit), Canarium apertum (Seladah), Dacryodes incurvata (Seladah laut), Dryobalanops oblongifolia (Kapur kelansau), Mesua macrantha (Mergasing), Podocarpus micropedunculatus (Kayu cina), Shorea compressa (Engkabang langgai), and Pterospermum javanicum (Bayur)] from various treatability classes, were pressure treated to refusal in 10% CCB preservative solution. Twenty replicated heartwood stakes of 19 x 19 x 457 (long) mm were planted to a depth of 230 mm in the ground and evaluated after the first 8 – 10 years exposure while the test continues. The field test site had high clay content, and despite the high annual rainfall (>3000 mm), samples with CCB dry salt retention of over 30 kg/m3 still remained sound after exposure (using an ASTM1758 10-point rating scale) period of up to 10 years, stakes with retention <25 kg/m3 had trace (rating: 9) or moderate (rating: 7) decay and termite attacks, while those with subsequently lower retentions failed within the first 6 years. Within an 8 – 10 years exposure evaluated at 6 monthly intervals, many stakes with only trace (rating: 9) or moderate (rating: 7) decay in the previous 6 months failed (rating: 0) abruptly within the next 6 months of evaluation instead of showing progressively severe biodegradation (i.e. ratings 4 or 7). Overall this first update of the field trial suggests for particular treatable structural species, CCB treated timbers exposed to ground contact (particularly wet clayey soils) could have a service life of at least 10 years when adequately treated to least 300 litre/m3 to achieve at least 30 kg/m3 retention.