The natural durability of wood has been thought to mainly depend on bioactive extractives and the water exclusion ability. To clarify the contribution of each factor to wood durability, above-ground exposure tests were conducted on Japanese and European wood species. A double-layer test was carried out using heartwood specimens of four wood species, i.e., Japanese cedar, Japanese cypress, Scots pine, and Norway spruce in Akita Prefecture (40°N, 140°E) starting in June 2020. The moisture contents of the specimens were recorded over time using an electrical resistance moisture meter and a dielectric constant moisture meter. Damages (mainly decay) on the four sides of double-layered members were measured by visual inspection and palpation every 6 months after exposure. The water absorption and drying characteristics of the specimens were examined using floating and submersion tests (CN/TS 16818) as well as a water absorption test (JIS Z 2101). After 3 years of exposure, all four species showed a mean rating of decay of approximately 1 (onset decay), with Scots pine and Norway spruce exhibiting a slightly faster degradation followed by Japanese cedar and Japanese cypress. The moisture contents of Scots pine and Norway spruce remained around 20% during the double-layer test, while those of Japanese cedar and Japanese cypress were around 30% to 40% for most of the exposure period. Norway spruce, which had the lowest durability class, was shown to degrade considerably more slowly than Japanese cedar sapwood (as a reference specimen), which had a similar low durability class. This is probably due to the low moisture content maintained during exposure. Japanese cedar and Japanese cypress were classified as “moderately durable” even if their high water absorption and the moisture content remained at more than 30% during exposure, suggesting that the effect of bioactive heartwood extractives was more significant than that of an adequate water exclusion efficacy. It may be concluded that the combined influence of bioactive extractives and water exclusion ability for the durability of wood varies between the Japanese and European wood species examined.

Download document (461 kb)
free for the members of IRG. Available if purchased.

Purchase this document