Water plays a significant role in the deterioration of reinforced concrete buildings; therefore, it is essential to evaluate the water content of the cover concrete. This study explores a novel non-destructive method for assessing the water content using sub-terahertz (sub-THz) waves. Among the four frequencies selected to evaluate the water content, an increase in reflectance was observed as the unit volume water content increased, and smaller data scatter was confirmed as the frequency increased. The derived empirical equation can classify the corrosion risk of the rebar environment based on the water content obtained using reflectance measurements. In other words, it can contribute to the diagnosis of the building integrity associated with rebar corrosion.

A feasibility study was conducted on the detection of corroded rebars inside concrete using sub-THz waves. In this study, spectral measurements were performed with a reflection-type system in the 20–50 GHz frequency range. Measurements were conducted on steel plates corroded in salt water and on concrete specimens containing these corroded plates. The results confirmed that reflectance decreases as corrosion progresses. Furthermore, it was demonstrated that the presence or absence of internal steel plates and corrosion can be detected up to a cover thickness of 20 mm. The frequency spectral peaks and their periodicity also provided a means to estimate the cover thickness.