Deterioration mechanism and hygrothermal condition of tuff stones used for exteriors at former Koshien Hotel

Two types of tuffs, Nikka stone ( pumiceous lapilli tuff) and Tatsuyama stone ( rhyolitic lapilli tuff), were used as the exterior materials in the former Koshien Hotel, a modern building, which continues to serve as a building for the Faculty of Architecture. These tuff stones exist in similar environments, but deteriorate differently: the Nikka stone has large spalling, and the Tatsuyama stone has thin multiple scaling. We hypothesized that this difference in deterioration is due to the differences in the wet area, which are in turn caused by differences in the hygrothermal properties of the stones and surrounding environment. We determined the moisture and temperature conditions in the stone by performing numerical calculations, considering the stone's material properties and the microenvironment, and investigated the correspondence between the hygrothermal condition and deterioration. We performed a one-dimensional hygrothermal calculation, where the physical stone material properties measured and estimated experimentally were used as the physical properties and the actual condition as the boundary condition. The conditions under the effects of low and heavy rain were subsequently compared. The results demonstrated that the differences in the heat and moisture transfer properties led to differences in the depth of water penetration in the stone and wetting and drying speeds. The wet area of the stone almost corresponded to the deterioration area. As the Nikka stone has faster water penetration during rain and takes a longer time to dry, it becomes significantly damaged once the stone starts to freeze. The Tatsuyama stone is wet only on the surface during rain and dries rapidly thereafter. The repetition of wetting and drying can result in multiple scaling. In addition, we demonstrated the influences of rain intensity and frequency on the deterioration of the stones.(c) 2023 Consiglio Nazionale delle Ricerche (CNR). Published by Elsevier Masson SAS. All rights reserved.