Figure 1. GPIR bridge deck inspection concept.
During FY93, we continued with development and experimental evaluation of components and system concepts aimed at improving ground-penetrating imaging radar (GPIR) for nondestructive evaluation of bridge decks and other high-value concrete structures. We developed and implemented a laboratory test bed, including features to facilitate component testing, antenna system configuration evaluation, and collection of experimental data from realistic test objects. In addition, we developed pulse generators and antennas for evaluation and use in antenna configuration studies. This project was part of a cooperative effort with the Computational Electronics and Electromagnetics and Remote Imaging and Signal Engineering Thrust Areas, which contributed signal- and image-processing algorithm and software development and modeling support.
Inspection of high-value structures, like bridges, is an application of Ground Penetrating Radar (GPR) technology, which is growing in importance. In a typical inspection application, inspectors use GPR to locate structural components, like reinforcing bars embedded in concrete, to avoid weakening the structure while collecting core samples for detailed inspection. Advanced GPR, integrated with imaging technologies for use as a nondestructive evaluation (NDE) tool, can provide the capability to quickly locate and characterize construction flaws and wear- or age-induced damage in these structures without the need for destructive techniques like coring. The bridge deck and its wearing surface are the most vulnerable parts of a bridge to damage from routine service, and they are particularly well suited for inspection using a vehicle-mounted inspection system.
More than 40% of the 578,000 highway bridges in the U.S. are either structurally deficient or functionally obsolete [1]. These conditions can limit bridge utility and, if they are not properly monitored and maintained, pose a safety threat to bridge users. An advanced GPR-based inspection system has the potential of addressing critical national and international needs for reliable, cost-effective NDE of bridges.
In an advanced bridge deck inspection system, like the one shown in Fig. 1, a mobile Ground Penetrating Imaging Radar (GPIR) gathers data for high-resolution image reconstruction of embedded defects and features. High-quality images, processed in centrally located computing centers, allow visualization of internal structure, permitting evaluation of deck conditions from data that are usually obtainable only by destructive means.
In this project, our goal is to demonstrate enhanced GPIR performance by the application of advanced hardware and software. Among the desired enhancements are: traffic-lane-wide coverage enabled by new transmitters, antennas and large-aperture receiving arrays; and high-speed data acquisition and accompanying inspection-vehicle speed made feasible by multiple receiver channels using state-of-the-art data transmission and storage equipment. In addition, down-range and cross-range resolutions can be improved by increasing transmitted pulse bandwidth and applying synthetic-aperture radar data-processing techniques.
Progress
During FY93, our efforts were aimed at experimental validation of system-level concepts developed in FY92 [2]. We established a laboratory enabling test and evaluation of GPIR components and system configurations, and development of experimental data to use for validation of computer simulations (performed through the Computational Electronics and Electromagnetics Thrust Area) and evaluation of imaging- and signal-processing algorithms and techniques (developed by the Remote Imaging and Signal Engineering Thrust Area). We designed, developed, and characterized ultra-wide-bandwidth (UWB) antennas and pulse generators and used them in the experimental test bed to generate data for image processing. We also tested available off-the-shelf antennas and pulse generators to provide benchmarks against which we could evaluate our own designs, and to determine their viability as possible components in an advanced GPIR.
GPIR Laboratory and Test Bed
The GPIR laboratory is equipped with both standard and special test equipment needed to support UWB radar experimentation and data acquisition. Standard test equipment includes oscilloscopes, impulse generators, UWB microwave amplifiers, power supplies, motor controllers, and meters. Specialized equipment, developed under the ongoing program, includes impulse generators and transmitting and receiving antennas designed specifically for the inspection application. Data-acquisition and control hardware used in experiments are under the control of a dedicated instrument-control computer.
A test bed, designed to provide a realistic test object during early development, is set up in the laboratory. Shown in Fig. 2 before concrete was poured, the test bed is a concrete slab (2m x 2m x 0.3m) containing reinforcing bars (rebars), both fixed and removable, and other objects designed to provide a means for evaluating data acquisition and imaging performance. Concrete poured in the test bed was prepared and handled in accordance with specifications used by the California Department of Transportation for concrete bridge decks. The test bed is equipped with motor-driven slide mechanisms designed to move antennas accurately and repeatably over the slab, simulating both the motion of an inspection vehicle over a roadbed or bridge deck and a fixed linear array of receiving antennas mounted on the vehicle. A typical experi-mental setup in the GPIR laboratory is shown in Fig. 3.
We designed and characterized UWB antennas for transmitting and for use in receiving arrays. A slotline-bowtie antenna [3] designed for use as a transmitting antenna is shown in Fig. 4 set up in an anechoic chamber for antenna-pattern measurements. This antenna was designed to provide good antenna-pattern control and impulse response over a relatively wide band. Characterization measurements were made over a frequency range from 500MHz to 8GHz. The measurements showed that the antenna provided usable gain and a well-behaved radiation pattern over the full range. Antenna patterns measured for this antenna from 1GHz to 4GHz are shown in Fig. 5.
We also assembled and tested UWB pulse generators using step-recovery diodes, to evaluate their potential as impulse sources for a GPIR inspection system. Key requirements for the generator include peak power of 100W to a few kilowatts, pulse widths of 100 to 300ps, and pulse repetition frequencies (PRF) up to 5MHz. In the impulse generators we tested, avalanche transistor pulse generators were used to drive series stacks of step-recovery diodes.
Output pulses and spectra for both unipolar and bipolar impulse generators are shown in Figs. 6 and Fig. 7. The unipolar pulse shown in the figures has peak power of 30W, usable bandwidth of greater than 3GHz, and was operated at a PRF > 1MHz. The PRF was limited by the ability of the avalanche transistor to operate at high repetition rates. Further work is needed to complete the evaluation of this technique for GPIR impulse generation.
In experiments conducted in FY93, we collected data and reconstructed images of features embedded within a concrete test bed. In those experiments, we collected data using mono-static antennas driven by a low-power impulse generator (1W, peak). Data were acquired with the receiving antenna configured to receive the cross-polarized returns scattered from targets and clutter in the illuminated volume. Data recording used a sampling oscilloscope with a UWB microwave front-end preamplifier. Antennas were mounted 50mm above the concrete surface, and moved over the test bed while data were collected over a rectangular sampling grid at 12.7mm spatial intervals.
Figure 8 shows a sequence of two-dimensional images that are planar slices from a three-dimensional image reconstruction; the slices are parallel to the surface of the concrete. The depth calculated in the reconstruction is indicated for each frame. The sequence starts near the concrete surface (Frame A) and progresses through the volume.
Within the imaged volume are four cylindrical voids; these voids are in locations where removable rebars have been removed. The first two voids are parallel with each other, with top surfaces about 63mm below and nearly parallel with the concrete surface. One of the voids, at the top of the image, is only partially visible in Frames C through E; the other is visible in Frames B through E.
The third void is beneath (at a depth 94mm below the surface) and perpendicular to the first two, and it slopes slightly deeper into the concrete at the end near the top of the image. In Frame D, near the bottom of the frame, the third void first appears, and it is visible in Frames D through G. The difference in depth, within the image, from one end of the third void to the other, is approximately 18mm; its sloping orientation is evident in the image sequence.
A portion of the fourth void is visible in the lower right corner of Frames G and H of the sequence. This void starts at a depth 117mm and slopes deeper into the concrete and slightly toward the top of the frame. Because of the low output power of the impulse generator used in this experiment, scattered signal power was too low to permit detection of objects deeper than 125mm in the concrete.
Future Work
Our continuing technical efforts are aimed toward advancing radar data-acquisition hardware to improve our ability to recover data from deeper in concrete, while maintaining UWB. Improvements here will enable high-resolution imaging of objects that are buried more deeply in the material. Our efforts will focus on development and testing of higher power impulse generators and receiver front ends with improved noise performance and temporal gain control. We will continue with evaluation of antenna configurations that are better suited to the bridge-deck-inspection application.
Another important goal for the next fiscal year is development of a scaled, demonstration, prototype bridge-deck-inspection system for an external funding sponsor. That effort will require collaboration with industry to be successful.
Acknowledgements
We wish to thank D. Goodman, Remote Imaging and Sensor Engineering Thrust Area Leader, and J. DeFord, Computational Electronics and Electromagnetics Thrust Area Leader, for their support in supplying resources needed to develop and evaluate imaging techniques and perform electromagnetic modeling for this project.
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