Monument Future - Siegfried Siegesmund

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6: Cracks in the marble of the Soong Ching-ling statue.

      The ultrasonic velocity in fresh marble similar to 177the material of the statue is about 4,500 m/s, and the measured velocities shown in the result image range between 1,000–4,500 m/s.

      According to the USCT results no penetrative severe cracks were found. The depth of the superficial cracks on the top of the head was not more than 50 mm. However, a clearly deteriorated zone with a thickness from 10 to 50 mm, caused by the disintegration of the crystal fabric, was found around the head.

       Conclusion

      According to the observation, almost all of the 77 cracks should be developed from the stone interlayers. The depth of 15 cracks has been detected and the results are between 0–68 mm. By USCT, a loosened zone with thickness up to 50 mm has also been found.

      For the reason of the statue has just been exposed to the natural environment for only 35 years, the marble should be in the early stage of deterioration, that is surface crystal fabric loosing and surface cracks developing.

      The main factors that cause crystal fabric loose on marble surface, development of spalls and cracks should be sharp drop in temperature caused by sudden rain in summer, repeated uneven expansion and contraction, scouring and dissolution of 178acid rain, ice splitting action and growth of lower organisms such as mosses and lichens, etc.

      If no effective measures are taken, the deterioration level of the statue may increase quickly, perhaps even seriously in the next 50 years producing a similar state as the marble railing of Tian’anmen-Qing Dynasty monument (Fig. 10).

      Figure 7 : Laboratory samples and corresponding USCT images. a. Ø 22 cm wood sample and corresponding USCT image. b. Ø 28 cm wood sample and corresponding USCT image. c. Ø 22 cm wood sample and the corresponding USCT image. d. Ø 55 cm limestone sample and corresponding USCT image.

      Figure 8: USCT testing section.

      Figure 9: USCT testing. a. USCT testing array b. USCT image

      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

      Bei Yan, Chongke Wu & Honglin Ma. 2018. Ultrasonic detection of contour node represented voids and cracks. Nondestructive testing and evaluation. http://www.tandfonline.com/loi/gnte20

      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.

      Bei Yan, Chongke Wu & Honglin Ma. 2017. Study on the method of nonmetallic defects based on ultrasonic tomography and morphology. 2017. 12th IEEE Conference on Industrial Electronics and Applications (ICIEA), pp1287–1292.

      Ma Honglin, Qi Yang, Ma Tao, Yang Junchang, Yan Min, Zhen Gang. 2015. Application of ultrasonic CT technique on weathering condition of stone sculptures of Qianling mausoleum. Science of conservation and archeology, Vol 27, Suppl. pp64–70.

      Ma Honglin, Ma Tao, Qi Yang & Yang Junchang. 2014. Research on ultrasonic detection of stone sculptures of Qian Mausoleum –Tang dynasty. Proceedings of the international conference on conservation of stone and earthen architectural heritage, Gungju. pp2530.

      179

       ACOUSTIC EMISSION BEHAVIOR OF ROCKS SUBJECTED TO TEMPERATURE CHANGES

      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.

      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

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