CLT is a large-scale wooden board which consists of several layers of sawn lumber stacked in altering directions. Therefore, a lot of large cross sections appear on all narrow faces of CLT. As water penetrates easily into timber from cross section, wooden material whose cross section is exposed to water in in a usage environment is susceptible to deterioration by organisms. Wood deterioration organisms, such as fungus also easily penetrates timber from the cross section. The large cross sections of CLT would be weakness against deterioration by organisms. Therefore, the cross sections of CLT must be considered when evaluating its durability of CLT. To develop the accelerated decay test for CLT regarding with the cross sections, we examined the acceleration of deterioration by contacting untreated CLT samples of the dimension 150×150×150 mm with fungus (Fomitopsis palustris) grown in agar medium in plastic container for 3 and 6 months. As the results, the mass loss of the samples attached with two containers (samples sandwiched between two containers) was higher than of with one container. The mass loss of the samples exposed for 3 months with two containers was lower than that of the samples exposed for 6 months with one container. The method with two containers is considered to be more suitable for the accelerating decay test of CLT.

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