MICROWAVE SUB-SURFACE IMAGING TECHNOLOGY FOR DAMAGE DETECTION OF CONCRETE STRUCTURES Yoo Jin Kim 1 , Student Member ASCE Luis Jofre 2 , Franco De Flaviis 3 , and Maria Q. Feng 4 , Associate Member ASCE ABSTRACT Invisible damage such as voids and cracks inside concrete and debonding between rebars and concrete caused by corrosions and earthquakes are of significant concerns. In this study, a microwave sub-surface imaging technology using a bi-focusing operator has been developed in order to detect such internal voids/objects and to quantitatively assess their dimensions. 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 the transmitting and receiving fields. An imaging algorithm using a numerical focusing operator was developed, which allows the recovery of a 2-dimensional object from its scattered field. For the experimental verification, a prototype planar antenna array consisting of 64 transmitting and 64 receiving antennas was fabricated and tested on a concrete block. Internal and invisible voids in the block were successfully detected. The sub-surface imaging technology developed in this research for quantitative condition assessment of concrete structures will lead to improvement of the effectiveness and efficiency of the current visual-inspection-based maintenance. Keywords: Microwave, Sub-Surface Imaging, Concrete Structure, Damage Detection, NDE INTRODUCTION Nondestructive assessment of concrete structures, currently, heavily relies on visual inspections, which apparently have some limitations. Majority of highway bridges are concrete bridges, and such invisible damage as voids and cracks inside concrete and debonding between rebars and concrete caused by corrosions and earthquakes are of significant concerns. In their previous studies (Kim et al., 2001a; Feng et al., 2000, 2002), the authors developed microwave surface imaging technology for assessing bonding condition of concrete structures jacketed with fiber reinforced polymer (FRP) composite materials. Using this technology, the location and the area of the voids were successfully detected. Accurate information about the void’s depth, however, cannot be obtained easily because the correlation of the depth and the 1 Dept. of Civil & Environmental Eng., University of California, Irvine, CA 92697. E-mail: yook@uci.edu 2 Dept. of Signal Theory & Comm., Technical University of Catalonia, Barcelona, Spain. E-mail: luis.jofre@upc.es 3 Dept. of Electrical & Computer Eng., University of California, Irvine, CA 92697. E-mail: franco@uci.edu 4 Dept. of Civil & Environmental Eng., University of California, Irvine, CA 92697. E-mail: mfeng@uci.edu