Electromagnetic image reconstruction for damage detection

Advanced composite materials are being used for seismic retrofit of reinfor ced concrete (RC) columns designed and constructed by old specifications. T he writers are currently developing an electromagnetic (EM) imaging technol ogy for assessing damage of the columns covered by such jackets, specifical ly debonding at the adhesive bonding layer between the jacket and the colum n. This technology takes advantage of the difference in electrical properti es (including conductivity and dielectric constant) between the adhesive ep oxy (under a good bonding condition) and air tin case of debonding). In thi s paper, an inversion method is presented that can efficiently and accurate ly reconstruct electrical property images (profiles) from continuous sinuso idal EM waves of different frequencies launched toward and reflected from t he layered medium, such as the composite-adhesive-concrete medium. This inv ersion method consists of a forward model and an iterative optimization pro cess. 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 inver ted electrical property, the forward model computes the reflection coeffici ent. The jacobian matrix method is applied to modify the inverted profile i nteractively to minimize the error between the measured and calculated refl ection coefficient. A constrained optimization technique is introduced in o rder to achieve maximum stability of the inverted profile against noise. At the present stage, simulation study is carried out in which the electric p roperty image for the bonding layer is Satisfactorily reconstructed and deb onding successfully detected using the proposed inversion method.