Moisture state and bending properties of Japanese larch (Larix kaempferi) boxed-heart timbers dried by high-temperature kiln-drying and stored under different conditions

The high-temperature setting (HTS) method is a kiln-drying method in which boxed-heart (BH) timbers are rapidly dried at high temperatures and low relative humidity. We investigated the moisture state and mechanical properties of Japanese larch (Larix kaempferi) BH timbers dried using this method and the effects of storage conditions on these properties. The timbers were dried under four different conditions: 18-h HTS kiln-drying, 48-h HTS kiln-drying, medium-temperature kiln-drying, and air-seasoning, and then stored for one year under three different conditions, namely outdoor storage exposed to the weather, outdoor roofed storage, and indoor storage. After any of the storage treatments, the moisture gradient from the surface to the middle in the HTS-dried timbers was larger than that in the air-seasoned timbers and similar to or larger than that in the medium-temperature-dried timbers. The gradient in the HTS-dried timbers after outdoor roofed storage or indoor storage was smaller than that after outdoor storage exposed to the weather. The mechanical properties did not differ significantly among the timbers after different storage conditions for all drying conditions. The modulus of elasticity of the HTS-dried timbers was comparable to that of the medium-temperature-dried and air-seasoned timbers. The bending strength of the timbers dried by 18-h HTS drying was lower than that of the timbers dried by air-seasoning, whereas comparable to or higher than that of the timbers dried by medium-temperature drying. The strength of the timbers dried by 48-h HTS drying was comparable to or lower than that of the timbers subjected to medium-temperature drying.