Figure 10: Lack of effective protection leads to serious deterioration. a. Head of the Statue b. Marble railing of Qing Dynasty monument
It is absolutely necessary to fill cracks and crystal gaps with suitable materials to slow down the deterioration process. We also suggest physical protecting methods be taken to prevent the statue from the weathering factors as sudden rain, sun light, ice and acid rain, etc.
Acknowledgements
This research was financially supported by the Honorary Chairman Soong Ching-ling Mausoleum Of The P. R. C. and by National Natural Science Foundation of China (No 51978472).
References
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Ma Hong-lin, Xiang Jian-kai, Zhang Gang, Ma Tao, Yan Bei, Wu Chong-ke, Li Zhan. 2018. The use of ultrasonic CT to detecting defects in timber structures of historic building. Science of Conservation and Archeology, Vol 30, No 6. pp74–81.
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179
ACOUSTIC EMISSION BEHAVIOR OF ROCKS SUBJECTED TO TEMPERATURE CHANGES
Tetsuya Waragai1, Takato Takemura2
IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.
– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –
VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.
1 Department of Geography, Nihon University, Sakurajyosui Setagaya Tokyo 156-8550, Japan
2 Department of Earth and Environmental Sciences, Nihon University, Sakurajyosui Setagaya Tokyo 156-8550, Japan
Abstract
Increasing temperatures associated with global warming are an imminent threat to European countries, where many historical items or edifices composed of stone are important to their cultural heritage. Repetitive cycles of heating and cooling by solar radiation generate large thermal stresses and increase the possibility of microcrack formation in stone and subsequent weathering. However, there are many unsolved questions regarding the relation among the rate of temperature change (RTC), microcrack detection and extension, and weathering processes. Accordingly, herein, we estimated thermally induced weathering of stone via nondestructive monitoring of acoustic emission (AE) concurrently generated with microcrack formation. Rock types that have frequently been used for stone items or edifices important to cultural heritage are granite, marble, and sandstone. The strain and AE of specimens composed of these rock types were measured in a temperature-controlled chamber programed with a heating–cooling range of 4–84 °C and RTC of ±2 °C/min. As a result, we confirmed strain changes and detected the AE amplitude in the specimens associated with temperature changes. This AE signal is considered to correspond to stress waves when microcracks form at grain boundaries. Microcrack formation in the stone and deterioration may be estimated using a system for monitoring strain and AE.
Keywords: thermal weathering, acoustic emission, microcrack, cultural stone
Introduction
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.
Description of rock specimens
Rock types that have frequently been used for stone items or edifices important to cultural heritage are granite, marble, and sandstone. We selected these three rock types as test rock.
The first rock selected is a granite collected in Inada, Japan, and which is used for buildings and tombstones. The second is a marble (Bianco Carrara) from Italy used for sculptures and building decor. The third is a sandstone from Cambodia used for the historical temples of Angkor, a World Heritage Site.
These selected rocks have different characteristics as follows. The granite selected has a polymineralic structure and is mainly composed of quartz, plagioclase, microcline, biotite, and amphibole. Although the average mineral size is approximately 2 mm, some quartz and plagioclase have
