[M-IS09] Weathering and conservation of cultural heritage and geosites

[MIS09-03] Changes in pore-size distribution of rocks induced by thermal cycles of low temperature

*藁谷 哲也¹ (1.日本大学大学院理工学研究科)

キーワード：熱風化、熱破砕、アコースティックエミッション、マイクロクラック、細孔分布

Many case studies have analyzed the relationship between thermo–mechanical fracture and temperature change induced by the insolation of rocks. However, extracting only thermal effects and discussing fracture in field observations are difficult. Therefore, laboratory experiments in which thermal cycles are applied to rocks are considered to be effective methods for simplifying and analyzing the relationship between fracture and thermal cycles. In particular, this approach is considered effective in measuring the generation of the acoustic emission (AE) and changes in pores to capture the initial stage of microscopic phenomena such as thermal fatigue fracture. Therefore, we experimentally investigated the fracture of rock specimens based on changes in pore-size distribution, ultrasonic propagation velocity, and the occurrence of AE. For the experiment, we used three rock types—granite and marble with a porosity of 2% or less and sandstone with a porosity of approximately 14%, which are often used as stone materials. Thereafter, we repeatedly subjected the prepared specimens from the rock types to temperature changes from 4℃ to 84℃ using a temperature-controlled chamber at a temperature change rate of ±2℃/min, which is the threshold for thermal shock fracture. Consequently, the P-wave velocity decreased by approximately 25%, 7.2%, and 2.5% for the granite, marble, and sandstone, respectively. Furthermore, for granite, the pore-size distribution decreased by approximately 6% in the diameter range of 0.8–0.5 μm and increased by approximately 5% in the diameter range of 0.4–0.1 μm. For the marble, the pore-size distribution decreased by approximately 16% in the diameter range of 0.5–0.1 μm and increased by approximately 16% in the diameter range of 6–0.65 μm. For the sandstone, the variation in pore size was negligible. Moreover, the occurrence of AE was detected in all rock types, but we found that its magnitude for the granite was the largest, followed by the marble and sandstone, respectively. Therefore, a low-temperature thermal cycle causes thermo–mechanical fracture inside rocks.