Determination of critical contour area in SAR images of concrete for subsurface moisture sensing

Moisture content inside concrete affects the properties of Portland cement concrete structures. It also indicates the likelihood of structural damages (e.g., freeze-thaw, steel corrosion) in reinforced and prestressed concrete structures. While several laboratory techniques (intrusive/destructive) are available for moisture determination, it is a challenging task to quantify the moisture content of concrete in real structures without using intrusive methods/techniques. In this paper, an SAR (synthetic aperture radar) image analysis approach using critical contour area for subsurface moisture sensing inside concrete specimens is proposed. Concrete specimens cast and conditioned in laboratory environments were air-dried for three months and continuously monitored for their moisture variation by SAR imaging. Dimensions of concrete specimens were 0.3 m-by-0.3 m-by-0.05 m. A 10.5-GHz center frequency radar system with 1.5 GHz bandwidth was used to generate SAR images of the concretes at various moisture contents from 0% to 3.85% (by mass). All SAR images of concrete specimens in this paper were collected inside an anechoic chamber. To determine the critical contour area (Ac) of an SAR image for subsurface moisture sensing inside concrete, a selection criterion is proposed and applied to SAR images of concrete specimens at different moisture contents (ψ). A critical SAR amplitude is chosen at the highest coefficient of determination (R-square, R2) for all values of moisture content. From our SAR imaging result, it was found that critical contour area increases nonlinearly with a linear increase of moisture content of concrete specimens.