Monument Future. Siegfried Siegesmund
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Название: Monument Future
Автор: Siegfried Siegesmund
Издательство: Автор
Жанр: Документальная литература
isbn: 9783963114229
isbn:
2 Architectural Conservation Laboratory CAUP Tongji University, No. 1239 Siping Road, 200092 Shanghai, China, [email protected]
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, [email protected]
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.
| Weathering Level | Vi / V0 |
| Not Weathered | ≥ 0.9 |
| Void ratio increased | 0.75–0.9 |
| Floor level of weathering | 0.75 |
| Slightly waethered | 0.5–0.75 |
| Moderately weathered | 0.25–0.5 |
| Severly weathered | ≤ 0.25 |
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.