Hygrothermal properties of the historical blue bricks from Linqing:A comprehensive measurement and material characterization

Blue brick is a traditional material used in China’s historical architecture, which is subjected to different degrees of moisture-related degradation after experiencing hundreds of years of exposure to the natural environment. It is of great significance to study the hygrothermal properties of the historical blue bricks for the conservation and restoration of these heritage structures. This study carried out a comprehensive measurement and characterization of the historical blue bricks from Linqing, which are extensively utilized in many World Cultural Heritage Sites in China. The thermal properties and pore structure of the bricks are first investigated. Sorption isotherm and pressure plate tests are conducted to obtain the equilibrium moisture content in the hygroscopic and overhygroscopic ranges, respectively. The moisture transport characteristics are further assessed by water vapor transmission test, capillary water absorption test, and drying test. A material characterization is further conducted to acquire the moisture storage and moisture transport functions of the bricks. The results show that the historical blue bricks exhibit a large discrepancy in the porosity, thermal properties, moisture storage and transport properties due to the difference in their pore structures. By reproducing the drying and capillary water absorption processes by HAM simulations, the derived liquid water conductivity functions and vapor diffusivity functions of the blue bricks are further calibrated. The complete hygrothermal properties of the historical blue bricks achieved in this research contribute to a solid material data input for heritage protection and renovation.