Electromagnetic Image Reconstruction for Damage Detection

Advanced composite materials are being used for seismic retrofit of reinforced concrete (RC) columns designed and constructed by old specifications. The writers are currently developing an electromagnetic (EM) imaging technology for assessing damage of the columns covered by such jackets, specifically debonding at the adhesive bonding layer between the jacket and the column. This technology takes advantage of the difference in electrical properties (including conductivity and dielectric constant) between the adhesive epoxy (under a good bonding condition) and air (in case of debonding). In this paper, an inversion method is presented that can efficiently and accurately reconstruct electrical property images (profiles) from continuous sinusoidal EM waves of different frequencies launched toward and reflected from the layered medium, such as the composite-adhesive-concrete medium. This inversion method consists of a forward model and an iterative optimization process. By comparing the wave signals sent to and reflected from the medium, the reflection coefficient of each layer of the medium can be measured and the electrical property profile (EM image) can be inverted. Using the inverted electrical property, the forward model computes the reflection coefficient. The Jacobian matrix method is applied to modify the inverted profile interactively to minimize the error between the measured and calculated reflection coefficient. A constrained optimization technique is introduced in order to achieve maximum stability of the inverted profile against noise. At the present stage, simulation study is carried out in which the electric property image for the bonding layer is satisfactorily reconstructed and debonding successfully detected using the proposed inversion method.