Monument Future

173

ULTRASONIC TESTING OF THE DOLOMITE MARBLE STATUE OF SOONG CHING-LING WITH RESPECT TO THE DEPTH OF CRACKS AND DETERIORATION STATE

Honglin Ma¹, Shibing Dai², Zhou Yue-e³, Bin Qian⁴, Zhong Tang², Gang Zhang¹, Jian-kai Xiang¹, Gang Zhen¹

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 Key Scientific Research Base of Conservation on Stone and Brick Materials, National Cultural Heritage Administration (Shaanxi Provincial Institute for the Conservation of Cultural Heritage), No. 35 Kejiyilu, 710075 Xian, China, 362203704@qq.com

2 Architectural Conservation Laboratory CAUP Tongji University, No. 1239 Siping Road, 200092 Shanghai, China, daishibing@tongji.edu.cn

3 Shanghai Bauwin Construction Consultant Ltd., No.168 Ancheng Road, Jiading District, 201805 Shanghai, China

4 Honorary Chairman Soong Ching-ling Mausoleum Of The P. R. C., No. 680 Yaohong Road, 201103 Shanghai, China, milanqian@126.com

Abstract

The marble statue of Soong Ching-ling stands in the memorial square of Soong Ching-ling cemetery in Shanghai, China. The statue has been inaugurated in 1984 and is a Chinese National Monument. Since 2014 many micro cracks appeared on the surface, especially on the head of the statue. To evaluate the deterioration 174condition of the statue and the depth of the cracks, non-destructive ultrasonic technology was applied. The deterioration state was tested by the USCT (Ultrasonic Computed Tomography) method and the depths of surface cracks were determined.

Based on the USCT, there were no penetrative severe cracks. The depth of the superficial cracks on top of the head was not more than 50 mm. However, a clearly deteriorated, shell-like zone with a thickness from 10 to 50 mm was found around the head. Fifteen micro cracks were detected and the depths of those cracks ranged from 0 to 68 mm. The results provided fundamental information to work out a preservation concept.

Keywords: Soong Ching-ling statue, dolomite marble, ultrasonic detection, USCT, cracks

Introduction

Madam Soong Ching-ling (June 1893–May 1981) was the wife of Mr. Sun Yat-sen, the founder of the Republic of China. She had been honorary Chairlady of the Peoples’ Republic of China.

The marble statue of Soong Ching-ling stands in the memorial square of Soong Ching-ling cemetery in Shanghai, China (Fig. 1). The statue was inaugurated in 1984. It stands 2.52 m high on top of a granite basement, which is 1.1 m high above the ground level.

The statue is composed of 4 pieces of Fangshan Hanbaiyu, a valuable and famous dolomitic marble from Fangshan, Beijing.

Mineralogically it consists of approximately 92–97 % dolomite(CaMg(CO₃)₂), minor amounts of quartz and muscovite have also been identified.

Since 2014 many micro cracks appeared on the surface, especially on the head of the statue (Fig. 2).

Inspection under the in-situ microscope shows different stages of disintegration of the crystal fabric on the surface.

Figure 1: Marble statue of Ms. Song in Shanghai.

Figure 2: Progressive crack on the head.

A comprehensive conservation and compatible maintence concept is needed to check the weathering process. But first of all, the deterioration condition of the statue and the depth of the cracks had to be evaluated. Non-destructive ultrasonic technology was applied. The deterioration condition was tested by USCT method and the depths of surface cracks were determined.

Principle and method of ultrasonic testing
Ultrasonic wave and natural stone

Ultrasonic waves are mechanical waves which can spread in solid, liquid and gas mediums. They may be differently attenuated when propagating in different mediums, and also have different velocities in different mediums. The velocity and the attenuation are the two most important parameters in ultrasonic testing.

For the ultrasonic testing of stone, the favourable frequency range is 20 kHz–1000 kHz. The velocity and attenuation of ultrasonic waves in stone depend among other factors on the density, water content and cracks. The amplitude and velocity of the first wave received are positively correlated to the mechanical strength of the stone, and the mechanical strength directly responds to the weathering condition of the stone.

Therefore ultrasonic testing is an appropriate method to detect the position and trend of weathered zones and cracks inside a stone.

There are many situations for cracks, crazings, and splits on the surface and in the interior of stone sculptures. Ultrasonic waves may go directly through a crack if the fracture surfaces are still in contact with each other with little effect on the velocity but with a distinct attenuation of the amplitude. For the situation that the fracture surfaces are completely apart from each other, the waves bypass the crack and the transit time increases. Due to the open split that runs through the stone, the wave will not be received on the other side.

For evaluating the stone weathering level, we use the P-wave velocity V normalized with respect to the velocity of an unweathered sample V₀ as shown in table 1, which is normally used by conservators. For the Fangshan Hanbaiyu marble V₀ is about 4,500 m/s.

175Table 1: Normalized velocity ratios for the definition of stone weathering levels.

Crack depth detection

In case of open cracks the ultrasonic waves run from the emitting probe to the end of a crack, and then back to the receiving probe. Assuming that the crack is perpendicular to the surface and the ultrasonic waves propagate with constant velocity, the depth can be easily calculated.

We measure the ultrasonic transit times between points A and B for the path ACB and between points D and F for the path DCF, and also the distances AE and DE for mode A. For mode B, the respective transit times are for the paths ACB and ACE and the distances AD and DB as shown in Fig. 5. The choice of mode A or B depends on the field situation. The data is evaluated with respect to the crack depth by Equation 1 (mode A) or Equation 2 (mode B). Both equations can be deduced by geometrical reasoning from the sketches in Fig. 3.

Figure 3: Crack depth detection modes.

Ultrasonic CT method

The principle of ultrasonic CT is shown in Fig. 4. Abundant data of wave time are collected by fanshaped testing. S₁–S_n are the emitting points, R₁₁, R₁₂…R_ni, R_nj are the corresponding receiving points.

Hypothesize that there are N testing line in the section plane, and the section plane may be separated to M grids on request of calculating accuracy.

Equation 1: Crack depth equation of mode A T₁ – ultrasound wave time DCF; T₂ – ultrasound wave time ACB L₁ – distance DE; L₂ – distance AE H – height of the triangle, depth of the crack

Equation 2: Crack depth equation of mode B T₁ – ultrasound wave time ACE; T₂ – ultrasound wave time ACB L₁ – distance AD; L₂ – distance BD H – height of the triangle, depth of the crack

The result will be got by solving the matrix equation below:

Equation 3: USCT matrix equation l_ij – length of path i in unit j; S_j=1/ V_j – slowness of unit j; t_i – wave time of path i.

The velocity V_j of ultrasonic wave in each unit of the section is given by the reciprocal of each S_j.

Detecting of the statue
Detecting for the depth of the cracks

There were 77 micro cracks observed on the surface of the statue and 15 of them were chosen for testing with either mode A or B, depending on the position of the crack.

The ultrasonic device used was a Proceq PunditLab+, with the precision of 0.1 µs on wavetime reading and the probes were Proceq 40 17-B 54 KHz conical probes (Fig. 5), with the contact area of diameter 4 mm, that ensures the precision of the contact points and the accurcy of testing results. Fig. 6 shows examples of cracks detected. Table 2 shows the calculated depth results of fifteen cracks and the depth ranged from 0 to 68 mm.

176

Figure 4: Principle of ultrasonic CT.

USCT testing of the head

The authors have developed a USCT system that can be used for testing wood and stone structures. It comprises a Proceq Pundit Lab+ non-metal ultrasonic device, an amplifier between the receiving probe and the detector, a sensor diameter convertor, a multi sensor fixator and the USCT analysis software. We have 20 Sonotec L40 54 kHz sensors of diameter 50 mm, and the convertors transmit the diameter to 10 mm when contacted the tested object. That makes the coordinates of each contacting points more precise. Non couplant is used for testing thus avoiding the penetration of couplants into the object through open cracks.

Laboratory tests were made with several wood and limestone samples, and the USCT images correspond very well with the visible appearance of the samples commendably. Fig. 7 shows four of them. All the equipment can be packed into one suitcase and easily transported for on-site testing.

For the on-site testing of the statue, a section of the cranial region of the statue was chosen as shown in Fig. 8, and sixteen probes were used. Fig. 9 shows the USCT detecting array and USCT image.

Figure 5: Probe for crack depth detection.

Figure 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.

Table 2: Crack depth data.

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

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 a grain size > 5 mm. Meanwhile, the marble is practically monomineralic (calcite) metamorphic rock. The average size of the calcite is < 0.5 mm. Major minerals in the sandstone are quartz and albite; its average size is < 0.5 mm. Clinochlore and illite occur between the major minerals as a matrix of the sandstone. At the microscopic level, the granitic minerals have cleavage planes and previously formed intramineral microcracks. Regarding the marble and sandstone, the mineral cleavage planes and microcracks are obscure.

Regarding the physical and mechanical properties of samples, specific gravity ranges from 2.60 for the granite to 2.72 for the marble, and the porosity shows 0.64 % for the granite, 2.23 % for the marble, and 13.5 % for the sandstone. Mechanically, the granite is more brittle, and the granite and sandstone (9.4 MPa) have a higher tensile strength than that of the marble (6.4 MPa).

The P-wave velocity was determined for each specimen (50 mm in diameter and 100 mm in height) before testing using a TICO instrument (Proceq). The velocity shows 4,654 m/s for the granite, 4,410 m/s for the marble, and 3,092 m/s for the sandstone.