Microwave near-field nondestructive detection and characterization of disbonds in concrete structures using fuzzy logic techniques

This paper presents a near-field microwave nondestructive testing technique for disbond/crack detection and evaluation in a concrete structure backed by an infinite half space of any material. A model describing the interaction of waves radiated out from an open-ended rectangular waveguide, in the near-field, with any layered medium will be utilized. The theoretical model calculates the effective reflection coefficient of the structure, at the aperture of the waveguide, as a function of the frequency of operation, the thickness and dielectric properties of the layers of the structures, including the standoff distance. The frequency of operation and standoff distance (the measurement parameters) can be optimized to achieve maximum sensitivity to the presence of the disbond. The presence of a disbond in a structure is viewed as an additional layer and will change the properties of the effective reflection coefficient (phase and magnitude). This change will depend on the thickness and location of the disbond. This fact will be used to investigate the potential of utilizing multiple frequency measurements to obtain disbond location and thickness information. A fuzzy logic model relating the phase of reflection coefficient, frequency of operation, and standoff distance to the disbond thickness and depth was generated and utilized.