In the Great East Japan Earthquake 2011, a large number of buildings collapsed and the strength of ceiling boards using plaster finishing was occurred as serious problems as a possible problem. That is why, since the Great East Japan Earthquake, the owners of old buildings are concerned about the deterioration of the building components. Therefore the number of owners seeking to strengthen their building's components have been increasing day by day. Moreover, in addition to preserving these historical buildings, it is also necessary to secure the safety from the standpoint of cultural and historical values. Therefore, we focused on these wood lath and plaster ceiling that's used as a target of evaluation for saving the historical building. In this study, an experimental study of inspection of an existing wood lath and plaster ceilings, we evaluate the strength and other properties of plaster, and finally conduct the objectives to prolong the duration of life with safety conditions.
Plaster materials have frequently been used in old building's ceilings and walls. In general, the role of plaster ceilings are to transfer stress buffering between the lath and plaster. Therefore, it is important for the lath and plaster to be strongly adhered. This portion has been called Plaster Key in general and its role is to evaluate the mechanical properties of the plaster ceiling and wall. In Japan, this type of ceiling using plaster and mortar has been used from the early 1900s, and over a century has passed since the lath and plaster ceilings were developed, and at the same time the plaster has frequently been damaged in earthquakes.
The two specimens used in experiments were cut from a renovation ceiling of Morioka bank (built in 1911). One of the ceilings has been constructed at the same time as the construction of the bank at 1911. The other ceiling has been constructed when taller buildings were being constructed in the 1930s. The experiments are the evaluation of wood lath specification, image analysis from the attic and hammering test for the plaster surface finishing.
The space of the wood lath of 1911 was more than 10mm length, and plaster keys were formed solidly. In addition, the timber around the lath were stable in both thickness and width. On the other hand, the specimen of the 1930s was two times as big as the 1911 specimen. Additionally, the gaps of the wood lath were less than 10mm wide. Therefore, the plaster ceiling was heavy and there were significant influences to the plaster layer exfoliation. Image analysis can be evaluated in a non-destructive manner the health of the traditional member by photos and sighting survey from the attic. Therefore, this method is particularly effective in the traditional member which is difficult to change the design and material. The strength of the ceiling at 1911 had a locally small part of plaster surfaces and was shown that the relative strength was smaller than average strength. The peak value of the strength of the 1911 was smaller than that of the 1930s. Thus, this means that the plaster of the ceiling had already been fragile, and when the peeling and falling of the plaster layer, the broken unit and particles were assumed to become small pieces. On the other hand, the hammering test results of the specimens from the 1930s were stable and small variability. The reason is that the thick plaster layer and high rigidity. Our results showed that non-destructive soundness evaluation of wood lath and plaster is reliable.
