Wood has a serious handicap as structural materials that it is vulnerable to decay by fungi. A weather exposed bridge has a possibility of an abrupt fall down, and wooden houses with heavily decayed structural members are easily destroyed by large earthquakes. To avoid such damages, a new predicting method on the decay progress in timber structures, and also its strength losing behavior along with time, is required. We are now preparing a computer simulation method that enables above mentioned prediction, but basic data on wood decay process is lacking. So, we carried out forced decay tests on small prismatic specimens with three anisotropic directions L, R and T in the length direction to clarify the decay developing velocity in the wood. Spruce was used as specimen, and both white-rot and brown-rot fungi were applied. At each projected term, decay progress was stopped, and the precise three dimensional density distributions in the specimen was measured using X-ray CT equipment. As a result, the velocity of decay progress in longitudinal direction was the highest by the both used fungi. The variability of decay progress in the specimen was observed on the CT images, especially by white-rot fungi. This variability was seemed to be caused by the moisture contents distribution. In the radial and tangential direction specimens, the apparent weight loss was sporadically occurred by brown-rot fungi, that is in one third of the specimens per each exposure time group. From CT images, many cracks and shrinkage were seen in the vicinity of the cultural media by the dried tested pieces.

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