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The fracture process of rock via heat is important as the most universal form of weathering. The process can be considered to have two phases: thermal fatigue fracture and thermal shock fracture. However, the boundaries of this process greatly vary from 2 °C/min to 44 °C/min depending on the study. For example, Yamaguchi & Miyazaki (1970) reported from experiments that rock specimens did not break because of thermal shock when the heating rate was ≤ 200 °C/h (3.3 °C/min). However, Richter & Simmons (1974) reported that when the heating rate exceeded 2 °C/min and the maximum temperature was higher than 350 °C, cracks formed in a rock specimen and permanent deformation occurred. Thus, the threshold of the rate of temperature change (RTC) at which thermal shock fracturing occurs varies among studies. However, in many cases, the threshold value is set at 2 °C/ min (Matsuoka et al. 2017). At such a threshold, thermal 180shock fracturing of stone is likely to occur outdoors due to solar radiation.

Minerals have various coefficients of thermal expansion, therefore, heating leads to the formation of thermal stresses in a polycrystalline mineral assemblage. The thermal stress is a result of the anisotropy in the thermal expansion properties of different minerals. As a result, microcracks initiate at the mineral grain boundaries. To capture microcrack occurrence along such grain boundaries, the use of acoustic emission (AE) technology in geotechnical engineering has been developed during recent years. When a material is subjected to a stress and cracks develop, a transient elastic wave is produced by a sudden redistribution of stress in the material. This phenomenon of transient elastic wave generation is termed acoustic emission.

The AE technique is effective in that it is possible to nondestructively investigate the progress of stone degradation. However, there are few measurement cases in the field, and monitoring is an issue. To monitor crack growth in a brittle material, therefore, an AE technique that picks up the elastic wave is among the unique technologies as a nondestructive technique. Accordingly, herein, we estimate thermally induced weathering of stone via nondestructive monitoring of AE concurrently generated with microcrack formation.