Determination of Water Vapor Transmission Properties of Sandstones in the Yungang Grottoes

Abstract

ABSTRACT The Yungang Grottoes, one of China’s most notable cultural heritage sites, are located in sandstone strata. The main causes of deterioration observed in the field are associated with moisture action, which greatly affects the long-term performance of the material. In particular, for immovable cultural heritage that is exposed to natural climatic fluctuations throughout the year, interaction with water vapor cannot be avoided. To obtain an insight into the water vapor transmission properties of local sandstone, a series of laboratory tests were conducted using the wet-cup method. The correlation of water vapor transmission with environmental relative humidity and pore structure was examined. The results indicate that as the relative humidity increased, the diffusion coefficient increased considerably due to the contribution of capillary condensate flow. In addition, a positive relationship was found between the diffusion coefficient and open porosity. The water vapor diffusion in sandstone occurred in transitory mode and the diffusion coefficient was estimated from the average pore radius. The calculated values were two orders of magnitude larger than those measured in the tests, reflecting the complex microstructural features concerning diffusion paths and area. The weathering processes constantly alter the sandstone’s diffusion parameters, causing various damages in the Yungang Grottoes.