Microwave Far-Field Nondestructive Detection and Characterization of Disbonds in Concrete Structures

Abstract

Inspection of concrete structures is an important and challenging issue. Standard nondestructive testing methods are not always capable of testing these structures for the presence of disbonds. Near-field microwave nondestructive testing has shown great potential for this purpose. Although this method is very sensitive to the presence of thin dis bonds, it is also very sensitive to variations in the standoff distance and surface roughness. Concrete structures are generally large in size and have some surface roughness associated with them. This paper presents a far-field microwave nondestructive testing technique for disband detection and evaluation in a structure made of any number of layers backed by an infinite half space or by a conducting plate. A theoretical model describing the interaction of the waves in the far-field with the layered medium will be presented. The theoretical model calculates the effective reflection coefficient of the structure as a function of the frequency of operation and the thickness and dielectric properties of the layers of the structures. 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 was used to investigate the potential of utilizing multiple frequency measurements to obtain disbond location and thickness information theoretically and experimentally. Using multiple frequency measurements alleviated the problems associated with calibrating the measurement setup.