Influence of surface checks on wood moisture content during wetting and re-drying

Abstract

Wood decks are popular outdoor wood products in Japan, and their durability is the most important aspect for their long-term use. To prevent decay, wood members should be maintained at a low moisture condition. However, surface checks occurring during long-term use may make the moisture content inside the wood members high, causing decay. In this study, water absorption and drying tests of flat grain redwood (Sequoia sempervirens) and Japanese cedar (Cryptomeria japonica) specimens with or without a sawed slit of 10 mm or 20 mm depth were carried out. Sectional moisture content distributions of the specimens were examined in detail with an X-ray densitometry method. Consequently, the influence of slit depth of an artificial surface check on the water absorption and drying processes was revealed. Moisture content mappings at 1 mm resolution around the slit showed that the water absorption region along the tangential direction from the slit was approximately 1 mm wide after 24 h, regardless of the species and slit depth. On the other hand, high moisture content after the drying period was observed only in a specimen which had a 20 mm depth slit. For the 20-mm depth slit specimens, the moisture content at the bottom of the slit remained more than 30% after drying for 8 h, which is a moisture level sufficiently high to cause decay. The results of the tests indicate that checks that reach 20 mm in depth from the surface may increase the risk of decay.