Subsurface Moisture Characterization for Sustainable Concrete Structures using Imaging Radar

Subsurface moisture content in Portland cement concrete structures indicates the short-term (cement hydration) and long-term (durability) of these structures and can be used in the nondestructive evaluation (NDE) and structural health monitoring (SHM) of concrete structures. Subsurface moisture content of concrete structures provides insightful health-related information such as the potential of steel rebar corrosion and alkali-silica reaction (ASR) inside concrete. However, existing approaches for moisture measurement are either intrusive (e.g., embedded moisture sensors) or not applicable in the field (e.g., gamma-ray attenuation, neutron scattering). This paper presents a field-applicable, NDE technique for characterizing subsurface moisture content of concrete specimens, based on synthetic aperture radar (SAR) imaging. Laboratory concrete specimens made of three water-to-cement (w/c) ratios (0.4, 0.5, and 0.55) were cast and conditioned to create various moisture contents in the range of 0% (oven-dried) to 4% (saturated). Concrete specimens were inspected by a 10GHz SAR imaging sensor to generate radar images for predicting both the w/c ratio and moisture content of each specimen. From our experimental data, it is found that the amplitude of SAR images can be used to predict the moisture content of concrete specimens. It is also found that critical contour area (distribution of SAR amplitudes) of SAR images increases with the increase of moisture content, serving as a useful indicator for subsurface moisture distribution inside concrete. An empirical model capable of predicting both the w/c ratio and moisture content from an SAR image is proposed. Research approach is finally summarized in a procedure for future applications.