Nondestructive evaluation of air voids in concrete structures using microwave radar technique

Abstract

Nondestructive inner inspection techniques are essential to certify the safety or health of concrete structures, and one of the important issues in these inspections is the detectability of thin air voids or cracks. In this study, we propose a novel method for discriminating the variety and estimating the thickness of air voids based on the microwave radar technique. Specifically, to distinguish air voids from other objects, the phase shift of the microwaves was analysed using time-frequency analysis. In addition, based on a simple parallel incidence model, microwave reflection and refraction analysis were conducted on homogeneous layers, and a particular relation was calculated between the reflectance and the air void thickness. The experimental performance of the functions to distinguish air voids was good, however, quantitative thickness estimation failed. Therefore, we introduced a novel sophisticated propagation path model based on ray-traced propagation, which provides a couple of propagation paths. Additionally, the attenuation feature of microwaves was also considered. A quantitative thickness estimation method was re-applied to the experimentally observed waveforms. The estimation accuracy improved by approximately 54%, and the validity of quantitative thickness estimation was confirmed.