Effect of Concrete Moisture on Radar Signal Amplitude

Abstract

The moisture content of concrete is a critical parameter for most of the physicochemical pathologies, such as steel reinforcement corrosion, alkali-aggregate reaction, and freezing-and-thawing cycles. Therefore, the detection of moisture is important for the diagnosis of concrete structures at early stages of deterioration. The present study was undertaken to assess the effect of the degree of saturation, water-cement ratio (w/c), and volumetric water content on the amplitude variation of direct and reflected radar waves. Four concretes with w/c of 0.5, 0.6, 0.7, and 0.78 were evaluated. Radar measurements were carried out on 72 samples saturated at 0, 20, 40, 60, 80, and 100%. Test results show that an increase in the degree of saturation leads to significant decrease in the amplitude of both direct and reflected waves. This is mainly attributed to the increase in complex permittivity related to the polarization and conduction mechanisms occurring in concrete. Generally, the radar measurements showed good repeatability, which was evaluated using the coefficient of variation, and the statistical dispersion of the direct wave amplitude was lower than that of the reflected wave. For dry and wet concrete, w/c did not appear as an influent parameter regarding the experimental results. A good correlation was found between the amplitude of direct and reflected waves for each degree of saturation. This suggests that the direct and reflected wave amplitudes provide similar information regarding the concrete moisture. Irrespectively of w/c and the depth of reflector, an empirical relationship between signal amplitude of direct wave and volumetric water content was established that can be used to estimate concrete moisture.