The objective of this study was to develop a novel and rapid artificial drying schedule for a boards of Japanese cedar (Cryptomeria japonica). Boards of green wood were dried to a moisture content of 15% under 9 artificial drying conditions with a combination of three dry-bulb temperatures (50°C, 70°C and 90°C) and three wet-bulb depressions (5°C, 10°C and 20°C). We found that the condition of 90°C dry-bulb temperature and 20°C wet-bulb depression produced dried boards in the shortest period. The boards indicated no significant color change for practical use. In addition, the dried boards showed durability against brown-rot fungi Fomitopsis palustris and subterranean termites Coptotermes formosanus comparable to naturally dried heartwood boards. The relative contents of cadinols demonstrated negative correlation with weight loss in decay tests. These results may lead to improvement of drying schedules for Japanese cedar boards while retaining natural durability.

In this study, the effect of drying schedule on internal stresses of sugi (Cryptomeria japonica) pithless square timber was determined. The released strain distribution of the timber was measured using a slicing method. Square timbers with cross-sectional dimensions of 120 mm × 120 mm were dried under three different conditions. After drying, cross-cut specimens of 15 mm thickness were cut from the center of the timber and divided using four different methods. The released strain distribution was measured using images of the specimen surface before and after splitting. The surface layer of the timber showed a large positive released strain, indicating a large compressive stress. This tendency was remarkable at higher initial dry-bulb temperature. The released strain at the surface showed a strong positive correlation to the released strain gradient, indicating that the released strain at the surface could be used to estimate the released strain gradient. These findings were already known for boxed-heart square timber, and this study shows that the same is true for pithless square timber.

To evaluate the strength of structural members using a stress wave propagation method, it is necessary to determine a constant elastic modulus value, regardless of the sensor insertion position and distance between the sensors. Therefore, the elastic modulus values of sugi (Cryptomeria japonica), hinoki (Chamaecyparis obtusa), akamatsu (Pinus densiflora), and karamatsu (Larix kaempferi), which have been used for wooden civil engineering structures, were evaluated while varying the sensor insertion position and the distance between sensors. The elastic modulus was found to be larger when the sensor was inserted between the structural member cross sections, rather than between the surfaces. Moreover, a positive correlation between elastic modulus and distance between sensors was discovered. Subsequently, a correction formula was devised that ensures that the elastic modulus is constant, regardless of sensor insertion position and distance between sensors, with the correction formula calculated for each of the examined wood species.

In order to evaluate the bonding strength between cross layers of CLT, we carried out three shear tests: torsion test, block shear test according to JAS 3079, and block shear test loaded at 45 degrees to the fiber direction. As a result, adhesive types had no effect on shear strength, and air-dried density of specimens had positive correlation to shear strength in each test method. Shear strength derived from linear regression between air-dried density of specimens and shear strength in block shear tests were approximately 45%-57% of the torsional strength. However, those of the block shear test loaded at 45 degrees direction were approximately 90%-94% of the torsional strength. In addition, block shear test loaded at 45 degrees to the fiber direction was shown to be useful as a method for evaluating the bonding strength between cross layers of CLT, because rolling shear is avoided and it can be carried out with a general-purpose test device.