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Sensing & Measurement

Remote nondestructive testing of concrete structures

The condition of structures incorporating fiber-reinforced polymer composites can be visualized using distant inverse synthetic aperture radar measurements integrated with tomographic image reconstruction.
23 September 2007, SPIE Newsroom. DOI: 10.1117/2.1200709.0862

The use of fiber-reinforced polymer (FRP) composite jacketing systems to strengthen, repair and retrofit deteriorated concrete columns and bridge piers is gaining increased acceptance. Their success at securing the integrity and extending the service life of various structures ensures that FRP strengthening techniques will experience steady growth worldwide.

The nondestructive assessment of the condition of FRP-wrapped concrete structures represents a crucial safety requirement. For instance, the failure mechanism of FRP-wrapped concrete structures may involve brittleness with little deformation before ultimate failure. The presence of defects and damage may also speed up the failure process, hence the need for prognostic inspection methods. However, core conditions and possible existing defects cannot be fully assessed without the physical removal of the wrapping system, unless the structure has already experienced substantial damage. Another issue is that partial or complete removal of the wrapping may also increase the risk of structural collapse. For these reasons, there is an urgent need to develop a field-applicable nondestructive testing (NDT) technique for the in-depth inspection of FRP concrete systems.

Figure 1. Far-field inverse-synthetic-aperture-radar measurement scheme.

In our work, we investigate the mechanical behavior of FRP-concrete systems and their NDT using carbon-fiber-reinforced polymer (CFRP) and glass-fiber-reinforced polymer (GFRP) composites.1–4 We have developed a far-field airborne-radar (FAR) NDT technique that performs far-field inverse-synthetic-aperture-radar (ISAR) measurements and tomographic image reconstruction to assess their in-depth condition. This novel technique also allows NDT inspections to be conducted at distances greater than 10m from the structure, while providing in-depth profiles for visualization.

The FAR NDT technique (see Figure 1) utilizes far-field circular radar measurements and ISAR data processing to numerically improve/focus the resolution of the collected measurements. This represents an improvement over the mechanical focusing schemes used in most near-field microwave and radar NDT techniques. The presence of defects in the near-surface region is detected by the scattering signals reconstructed into images with the help of backprojection algorithms. In our implementation, the radar operates in monostatic mode, i.e., a common antenna is used for both transmitting and receiving. It collects the far-field response of the target structure at each frequency and at each azimuth angle. Reconstructed images can then be rendered with partial input of the total measurements, enabling flexibility of inspection at various levels of detail.

As part of the initial concept study, radar measurements were carried out on laboratory specimens in the 8-18GHz frequency range at the Compact Radar Cross Section facility of the MIT Lincoln laboratory. Collected ISAR measurements were then processed using a fast backprojection algorithm to generate the spatial profile of the target structures. Figure 2 shows GFRP-concrete specimens with a near-surface cubic anomaly (a) and with a near-surface delamination anomaly (b) as well as their reconstructed images (c, d). In the images, the location of the specimen is indicated by white dashed lines. Anomalies and defects were also detected by scattering signals in the resulting images. These results demonstrate that our approach can indeed detect concentrated anomalies. The images shown in Figure 2 were generated from a far-field distance exceeding 10m and show that the NDT assessment of GFRP-wrapped concrete infrastructure components is indeed possible.4,5 Further development is planned for the design and fabrication of a portable device to explore the performance of our technique in field applications.

Figure 2. Concrete specimens retrofitted with glass-fiber-reinforced polymers with different anomalies (a,b) and their respective images (c, d).

To summarize, we have developed a FAR technique consisting of far-field ISAR measurements followed by tomographic image processing for the nondestructive testing of FRP-wrapped concrete structures. The promising radar measurements recently achieved in our laboratory in the frequency range of 8–18GHz demonstrate its great potential for field applications.