FUNDAMENTAL STUDY ON SURFACE EROSION OF WOOD FOR EXTERIORS CAUSED BY PHOTODEGRADATION

Wood used for exteriors, if left uncoated, experiences discoloration in the initial stage, and goes on to develop small checks on its surface. As exterior exposure progresses, the surface of the wood partially deteriorates due to weathering of the earlywood and latewood at the exposed surface and increased checking caused by differential shrinkage, resulting in an uneven wood surface. Hereinafter, this phenomenon is referred to as surface erosion. Wood that experiences surface erosion may develop various types of degradation in its surface layer. Surface coating of exterior wood will not lastingly protect it from surface erosion unless the wood is recoated in a timely manner as the coating wears down. On the other hand, although surface erosion is some of the well-known degradation phenomena of wood, few studies have attempted to quantify the degradation process. One of the reasons is that it is relatively difficult to quantify the depth of degradation of uneven wood surfaces. Another factor is that protective coating and recoating of exterior wood is commonly included in building planning in line with the original design concept. For this reason, building maintenance planning rarely envisages the possibility that exterior wood will be left in an unprotected state owing to protective coating degradation, with resulting progression of surface erosion. However, in actual buildings, recoating is often not performed in a timely manner. Therefore, in this study, we conducted an experimental verification of the process by which surface erosion and chipping progress on the surface layer of wood and the effect of various coatings to suppress such degradation. The following conclusions were reached. 1) Measurement of surface erosion using a laser displacement sensor on a rail confirmed that the surface of the wood, which had surface erosion, exhibited a very complicated pattern of height differences with a large number of depressions too narrow to be measured with a ruler or calipers.2) The depth of the surface erosion of the uncoated specimens increased linearly with age. Within the scope of this experiment, the degradation process was found to be the same for all types of wood, whether softwood (conifers) or hardwood (broad-leafed trees).3) When the difference in the depth of surface erosion of the uncoated test specimens was rearranged by the density of each tree species, the depth of surface erosion tended to grow shallower as the density became higher. The s