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Ground Penetrating Radar Technique and it's Application in Non-Destructive Testing of Reinforced ConcreteYonghui Zhao, Jiansheng Wu, Jialin Wang, and Minghao Wan
Key Laboratory of Marine Geology, Ministry of Education.
1239 Siping Road, Shanghai, China.
Non-destructive testing technique of reinforced concrete has been a difficult question in civil and geo-technical engineering. Ground-penetrating-radar technique (GPR for short) has been introduced in the testing field in this paper. It can evaluate accurately the interior structure and quality of concrete, and true locate the unexposed concrete objects, such as bomb shelter etc. The practical engineering applications effects of the technique are also recommended and show that GPR is a better method of non-destructive testing of reinforced concrete.
Keywords: Reinforced concrete, Non-destructive testing, Ground-penetrating-radar, Imaging section.
Non-destructive testing is one of the most important means which supervisor, diagnose and evaluate the quality of the reinforced concrete. The testing results will be important gist of preservation and maintenance of the concrete structure. In view of the complicated property of the reinforced concrete and the limitation of testing site environment, some routine testing method such as, for example, ultrasonic testing, pulse-echo testing and infrared ray testing, can't be put to use or obtain scheduled testing purpose. Ground penetrating radar (or GPR for short) is non-destructive testing technique of non-metallic structure and it has been widely applied in engineering and environment surveys. GPR system has been introduced in China since 1990, especially in Shanghai, GPR is being taken into account and accepted by geotechnical engineering field and testing departments. Now in Shanghai the GPR technique has been used in many applications such as, for example, the measurement of the thickness of road surfaces, the internal lining of pipelines and tunnels, the study of geological formations field. It is particularly effective in the study of non-electroconducting materials and for detecting the presence of metal objects inside these materials such as reinforced concrete. GPR technique can be used to carry out imaging reinforced concrete structure and make out assessment to the quality of concrete structure.
GPR is the general term applied to techniques that employ radio waves, typically in the 10 to 2000 MHz frequency range, to map structure and features buried in the ground (or in man-made structures). GPR makes use of electromagnetic waves generated into the surface of the object studied by means of an antenna moving along the surface. A common instrumentation configuration consists of an antenna connected via a signal/power cable to a computer-based system control unit. Surveys are conducted by towing an antenna across a surface as it is repeatedly transmitting radar pulses into the subsurface. Whenever a radar pulse strikes a boundary interface of contrasting dielectric, a portion of the pulse is reflected back to the surface and a receiving antenna. Subsurface profiles will be generated by simultaneously towing the antenna across the surface and displaying the resulting echoes of individual pulses as a composite image displays on the control unit's monitor. The operator may opt to record an image for later analysis.
The propagation speed, surveying depth and resolution of GPR depend on the physical characteristics of the material in question. Generally speaking, the propagation speed of the wave is influenced by the dielectric constant and the magnetic susceptibility of the material. The maximum depth reached by the radar impulse depends on both the wave frequency and the electric resistivity of the material. Higher sounding frequency means better spatial resolution, but with more significant electromagnetic wave attenuation in the environment, resulting in lower sounding depth; and vice versa-lower frequency may lead to larger penetrating depth at the sacrifice of poorer resolution. Moreover, lower frequency produces larger initial insensitivity area (blind zone) of a GPR. A table below shows relationship between resolution, blind zone and sounding depth with reference to the antenna used.
|Blind zone, m||0.06||0.1-0.2||0.25-0.5||0.5-1.0|
In non-destructive testing of the reinforced concrete it is necessary for higher resolution, but the testing depth is often lower. So the higher frequency antenna will be often used in the non-destructive testing field.
GPR data were collected using pulseEKKO-IV and pulseEKKO-1000 systems that were manufactured by Sensor & Software Inc. (Canada). Equipment components include a system unit enclosure case (25 x 16 x 16cm), a set of shielded antenna having 25-1200 MHz center frequency and 20 m antenna control/power/data optical fibers. The antenna was connected to the GPR system unit via the fiber, allowing the antenna to be towed by a small simple tractor-trailer across the test plots.
The software of the GPR includes several methods of processing of GPR signal: inverse filtration, correlation and wavelet-correlation methods, and least mean square method with regularization and auto-correlation processing. Three-dimension image of objects is obtained by migration. Methods of signals require calibrating signals, measured in advance. Correlation and auto-correlation processing enable to compare measured reflected signals and calibrating signals, reflected from various objects, and choose the best suite waveform. Auto-correlation processing is most effective when the shape of the reflected signals differs greatly from the calibrating signals. This processing finds correlation between reflected signals, measured in closely spaced positions of antenna.
GPR reflections were interpreted as a raster image formed by adjacent echoes. Each echoes was displayed as a vertical line extending downward, where the length of the line was proportional to displayed penetration depth. The reflected intensity along the line was denoted by monochrome or 256 grey or color scale.
Depths to individual reflectors were determined by ground truths, matching known depths of medium horizon obtained from the medium descriptions to the reflectors. An overall dielectric was then calculated for the site, and used for subsequent depth calculations. Initial calibration studies and multiple ground truths allowed pattern recognition skills to be identification of the various subsurface features displayed in the GPR image.
The reflected waves had different features with the different objects. In non-destructive testing of reinforced concrete we should pay more attention to the following reflected wave features.
A testing the GPR on crack of the concrete
There appears seepage phenomenon of groundwater in concrete soleplate of the Shanghai Jinwantan Square. The thickness of the soleplate is 2.1 m, all the concrete is simultaneous paved and the total area is 8000 m2. Because of the temperature changing during the solidification, parts of the concrete soleplate come into being small cracks. It is necessary to make sure the outstretched depth and the crack extend direction in concrete soleplate.
We used a pulse EKKO-1000 GPR from Sensors & software Inc., for non-destructive testing. The antenna had center frequency of 900 MHz. All of the data were acquired using common-offset reflection profiling method. We completed a series of single line tests to optimize acquisition parameters, and used these results to design 3-D surveys. The step size data is 0.01 m and the transmitter-receiver separation data is 0.17 m. Figure 1 is one of the GPR testing images. We analyze the GPR reflection image to make sure the position and outstretched depth of the cracks.
|Fig 1: the GPR testing image of the cracks.|
In figure 1 the inphase axes of the reflected wave are unbalanced on the horizontal position of the 0.38 m and 0.78 m, which shows that the two parts of the concrete existed cracks. In the vertical direction the unbalance didn't disappear until 40ns(~2.0m depth), which shows that the cracks had extended to the bottom of the concrete soleplate. Figure 2 is the 3-D representation image acquired by synthetically analysis with other testing line results.
|Fig 2: the 3-D representation image of the cracks.|
A testing the GPR on the structure of the reinforced concrete
The building of Pudong Development Bank was built in 1923 and there enchases quintessential wall painting on the concrete wall. The wall painting is regarded as cultural relic. But we found the many cracks on the wall surface, which could affect the wall painting. It is necessary to maintain and reinforce the concrete wall. So we must make sure the detail information of the concrete structure such as, for example, rebar distribution and the thickness of the rebar shelter layer.
We used a pulse EKKO-1000 GPR from Sensors & software Inc., for non-destructive testing. The antenna had center frequency of 1200 MHz. We designed the testing lines on the wall both horizontal and vertical direction. Figure 3 and Figure 4 is respective GPR testing image of horizontal and vertical direction.
|Fig 3: the GPR testing image of horizontal direction testing line.|
Concrete could be regarded as isotropy medium in reinforced concrete structure, but the rebars would be regarded as abnormal objects. Radar wave would be shapely reflected at the interface between rebar and concrete because there existed strong abnormality between the two mediums. The reflected waveform from rebar was hyperbola in GPR image. According to the features, We analyzed the GPR reflection image to make sure the position of the rebars and the thickness of the rebar shelter layer. The following results were derived.
|Fig 4: the GPR testing image of vertical direction testing line.|
Synthetically analyzing GPR images of the other testing lines, we acquired the 3-D representation image of the reinforced concrete wall structure. Figure 5 truly described the distribution of the rebar in the reinforced concrete wall.
|Fig 5: the 3-D representation image of the reinforced concrete wall.|
A detecting the GPR on the bomb shelter
We were successful in locating unexposed bomb shelter using 200MHz antenna and a PulseEKKO IV instrument(Figure 6). Both sides and top surface of the bomb shelter was built by reinforced concrete, and it was covered with symmetrical clay. There will produce strong radar reflection wave at the interface between the bomb shelter and clay because of their dielectric properties difference. In GPR image (Figure 6) we can distinctly identify the reflection waves of both top and bottom of the bomb shelter. We analyzed the GPR profile and made sure the accurate position and dimension of the bomb shelter (Figure 7).
|Fig 6: the GPR detecting image of the bomb shelter .|
|Fig 7: the section image of the bomb shelter.|
A testing the GPR on quality of road surface
It is a difficult problem to accurately testing the quality of the concrete road surface. We attempted to use GPR to check up the quality of road surface and obtained better effect. Figure 8 and 9 were the two typical GPR images of road surface testing using 1200MHz antenna and a PulseEKKO 1000 instrument. On condition that the concrete of the road surface was symmetrical and there was tight fitting at the interface between concrete and roadbed, in GPR image reflection waveform was consecutive and balanced (Figure 8); on the contrary, the reflection waveform would be inconsecutive and unbalanced (Figure 9), the abnormity at area 1 indicated that there was not tight fitting at the interface, and the abnormity at area 2 indicated that concrete may be asymmetrical or exist cracks.
|Fig 8: the GPR testing image of the road surface (normal condition).|
|Fig 9: the GPR testing image of the road surface (abnormal condition).|
Ground penetrating radar has widely used in geophysical surveys. GPR was primarily focused on mapping structures in the ground; more recently GPR has been used in non-destructive testing of the non-metallic structures. The application in non-destructive testing of reinforced concrete is a new task and study field. The practical engineering applications effects of the technique show that GPR is a better method of non-destructive testing of reinforced concrete. In china, especially in Shanghai GPR was being more applied in civil and Geo-technique engineering construction. It will be a considerable market for GPR manufacturer.
This project is supported by China Education Development Foundation and Shuguang Project of Shanghai Education Commission (project serial number: 99SG09). Cooperative arrangements with Dr. Xiongyao Xie studying in Department of civil engineering, Weifang Jiang working in 9th design Academy of China Shipping and Chun Tan studying Department of marine geology and geophysics, are gratefully acknowledged.
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