Development of electromagnetic imaging technology for damage detection.

摘要

Jacketing technology using fiber reinforced polymer (FRP) composites is being applied for seismic retrofit and rehabilitation of reinforced concrete (RC) structural members designed and constructed under older specifications. However, seismic damage of the structural members covered by the jackets remains a significant concerns, as cannot be visually detected and it may substantially weaken the structural performance. In this dissertation, an electromagnetic (EM) imaging technology is developed for detecting such internal and invisible damage in RC structures, especially those retrofitted with FRP composites.; The author has developed surface imaging technology using focused microwave for detecting debonding between the jacket and the column. This technology is based on the reflection analysis of a continuous EM wave sent toward and reflected from layered FRP-adhesive-concrete medium: Voids and debonding areas will generate air gaps which produce additional reflections of the EM wave. In this study, the computer simulation demonstrated the difficulty in detecting damage by using plane waves. In order to alleviate this difficulty, dielectric lenses were designed and fabricated, focusing the EM wave on the bonding interface. For experimental verification, three concrete columns were constructed and wrapped with glass-FRP jackets with various voids and debonding conditions artificially introduced in the bonding interface. Using the proposed surface imaging technology involving the designed lenses, the locations and areas of these voids and debonding areas were successfully detected by scanning the column surface.; Furthermore, in order to detect the internal voids/objects and to obtain information about the depth of a void or a debonding area, microwave sub-surface imaging technology using a bi-focusing operator has been developed. The proposed imaging system consists of several cylindrical or planar arrayed antennas for transmitting and receiving signals, and a numerical focusing operator is applied to the external signals both in transmitting and in receiving fields. An imaging algorithm using numerical focusing operator was developed and the numerical simulation demonstrated that a sub-surface image can be successfully reconstructed by using the proposed sub-surface imaging technology. For the experimental verification, a prototype antenna array was fabricated and tested on a concrete block. Internal and invisible voids in the block were successfully detected.