Comparison of Synthetic Aperture Radar and Impact-Echo Imaging for Detecting Delamination in Concrete

In this paper we evaluate the utility of microwave and mechanical wave nondestructive testing techniques to detect delamination in reinforced concrete bridge deck mock-up samples. The mechanical wave tests comprise air-coupled impact-echo measurements, while the microwave measurements comprise three-dimensional synthetic aperture radar imaging using wideband reflectometery in the frequency range of 1-4 GHz. The results of these investigations are presented in terms of images that are generated from these data. Based on a comparison of the results, we show that the two methods are complementary, in that provide distinct capabilities for defect detection. More specifically, the former approach is unable to detect depth of a delaminated region, while the latter may provide this information. Therefore, the two methods may be used in a complementary fashion (i.e., data fusion) to give more comprehensive information about the 3D location of delamination

Nondestructive Evaluation of Mechanically Stabilized Earth Walls with Frequency-Modulated Continuous Wave (FM-CW) Radar

Effective techniques for a nondestructive evaluation of mechanically stabilized earth (MSE) walls during normal operation or immediately after an earthquake event are yet to be developed. MSE walls often have a rough surface finishing for the purpose of decoration and are reinforced with both horizontal and vertical steel bars. Two wide-band microwave inspection approaches were investigated for detecting and evaluating characteristics of materials behind an MSE wall section. The first approach used spot measurements with a wide-band frequency-modulated continuous wave (FM-CW) radar system. While effective to penetrate through mortar blocks in the laboratory, the radar system was found to have limited applications in MSE walls due to its thick layer, material heterogeneity, surface roughness, and the presence of steel bars. The second approach took wide-band measurements on a two-dimensional (2D) grid and produced three-dimensional (3D) images using a synthetic aperture radar algorithm. Imaging allows for signal averaging and relatively easy distinction of localized features such as steel bars from undesired flaws. Two-dimensional slice images at the location of the anomalies were produced. Several different anomalies placed behind the wall can be detected, demonstrating the effectiveness of the imaging technique as a potential approach for back-fill soil inspection (e.g., moisture, void) behind an MSE wall

Novel Reflectometer for Millimeter Wave 3D Holographic Imaging

Small, portable, and low-cost millimeter wave imaging systems are desired in many nondestructive testing and imaging applications, as these system are capable of producing high-resolution images of interior of composite structures. Typically, systems capable of producing holographic three-dimensional images incorporate expensive and bulky wideband heterodyne coherent reflectometers or vector network analyzers. This paper presents a novel wideband, small, and low-cost reflectometer capable of producing holographic three-dimensional millimeter wave images