Semiempirical electromagnetic modeling of crack detection and sizing in cement-based materials using near-field microwave methods

Detection and characterization of cracks in cement-based materials is an integral part of damage evaluation for health monitoring of civil structures. Microwave signals are able to penetrate inside of dielectric materials (e.g., cement-based materials) and are sensitive to local, physical, geometrical, and dielectric variations in a structure. This makes microwave nondestructive testing and evaluation (NDT&E) techniques suitable for inspection and health monitoring of civil structures. Near-field microwave NDT&E techniques offer the added advantage of providing high spatial resolution, requiring simple hardware that may be portable, low power, fast, real time, and robust. Additionally, these techniques are noncontact and one-sided. Besides the need for robust detection, electromagnetic modeling of a microwave probe response to a crack is also an important issue. Such a model can be used to obtain optimal measurement parameters and serve as the foundation for extracting important crack information such as its width and depth. In this paper, the utility of open-ended rectangular waveguide probes for detecting surface-breaking cracks in cement-based materials is discussed. Subsequently, the development of a semiempirical model capable of simulating the crack response is presented. The model described here translates the magnitude and phase of the reflection coefficient as a function of scanning distance into the complex reflection plane and takes advantage of the common shape of these signals for predicting a similar signal from an unknown crack. Finally, this empirical model is used to estimate crack dimensions from a set of measurements.