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Presence of chloride ions is the main cause responsible for the corrosion of steel rebars and other components in concrete structures. Detecting the presence and amount of chloride ions in concrete, however, is a challenging task. In this paper, we present our development on the use of synthetic aperture radar (SAR) imaging for the remote detection of chloride ion content in concrete. SAR imaging is a remote sensing technique capable of performing noncontact subsurface inspection of dielectric materials like Portland cement concrete. Such techniques can be applied to field inspection of concrete structures and laboratory material characterization. The objective of this paper is to demonstrate the application of SAR imaging on characterizing chloride content inside oven-dried concrete specimens, using a 10-GHz central frequency SAR imaging sensor. Twelve concrete specimens (0.3x0.3x0.05 m3 ) with a 0.45 water-to-cement ratio were manufactured in six groups of different chloride contents (0%, 2%, 4%, 6%, 8%, and 10% of cement weight). Four SAR image-based parameters were developed from each SAR image of concrete specimens for chloride, including integrated amplitude Iint, average maximum amplitude Imax, critical contour area Ac, and average Gaussian curvature of critical contour Kavg. A parametric analysis on the combined use of different numbers of SAR image parameters was carried out to determine the optimal application of SAR image parameters on the chloride detection problem in this paper. From our result, it is found that, for the purpose of chloride detection inside concrete specimens using single SAR parameter, integrated amplitude Iint. When using two SAR image parameters, combination of integrated amplitude Iint and critical contour area Ac shows the best performance among other two-parameter combinations. When using three SAR image parameters, combination of integrated amplitude, critical contour area, and average maximum amplitude shows the best performance among other three-parameter combinations. Furthermore, it is interesting to report that combination of three SAR image parametersprovides the overall best performance among all other combinations for the chloride detection problem in oven-dried concrete specimens.
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