Microwave reflection properties of early-age concrete specimens: Sensitivity analysis

The reflection properties of early-age concrete specimens have been investigated with an open-ended waveguide radiator at R-band (frequency range of 1.7 GHz - 2.6 GHz). The complex dielectric permittivity of a concrete specimen is determined using measured reflection coefficient and simulation with CST Microwave studio. The numerical investigation into sensitivity of the reflection coefficient to the changes of the radiator-specimen arrangement is performed. These changes are selected due to their potential effect on measurement accuracy. They include a small gap between the waveguide aperture and the surface of the specimen and a shift of the aperture with respect to the center of the specimen. The sensitivity of reflection coefficient to the changes of specimen size has also been investigated. It is shown that the sensitivity of reflection coefficient to the selected values of gap (0-0.5 mm) is ~0.01 mm^-1. The influence of the shift of the aperture within surface of the specimen was negligible at wide range of frequencies while small changes are observed at lower and higher frequencies at a relatively small shift (25 mm) along both x-axis and y-axis. It is found that the change of size of an early-age cubic concrete specimen has negligible effect on reflection coefficient if the radiator is located at the center of specimen side which size is larger than 150 mm.