Coupled frequency-moisture effects in dielectric modeling of polyurethane grouting materials

Polyurethane (PU) grouting materials’ dielectric constant is susceptible to moisture and the testing frequency of the instrument. To establish a dielectric model of PU grouting materials based on frequency and moisture effect analysis, a vector network analyzer equipped with an open-ended coaxial probe was used to conduct frequency sweep measurements of the dielectric constant for specimens of different densities within the 0.5 GHz to 6 GHz frequency range. Firstly, the dielectric properties tests of dry specimens were conducted. Based on the influence of frequency on the dielectric constant, the dielectric constant values of the PU matrix and pores were predicted using the fitting relationship between the dielectric constant and density. By introducing the Maxwell-Garnett multiphase mixing model, a dielectric model of dry PU grouting materials considering the frequency effect was constructed and validated, with the mean relative error (MRE) of less than 0.6%. Secondly, to reveal the impact of moisture on the dielectric constant, water absorption tests and dielectric property tests of moist specimens were conducted. According to the influence of water absorption on the dielectric constant and the established dielectric model for dry PU grouting materials, a comprehensive dielectric model considering frequency and moisture effects was constructed and validated with the MRE of less than 8%, demonstrating good applicability for the detection and evaluation of grouting effects.