Active Microwave Thermography for Evaluation of FRP-Rehabilitated Cement-Based Structures

Fiber reinforced polymer (FRP) composite materials have become important and widely accepted for the rehabilitation of deteriorating concrete structures. Among numerous nondestructive testing and evaluation techniques for FRP-rehabilitated cement-based structures, active microwave thermography (AMT) is an integrated technique that utilizes a microwave based heat excitation and subsequent thermal monitoring. AMT has shown promise as a nondestructive testing and evaluation technique for the infrastructure and aerospace industries. In this paper, representative simulated and measured results for an AMT inspection of a cement-based material rehabilitated with carbon fiber reinforced polymer (CFRP) cornposites are presented. Specifically, the thermal contrast and signal-to-noise ratio are provided and discussed as a function of fiber orientation, frequency, and power level. It has been shown that in the case of unidirectional fibers, when the polarization of the incident electromagnetic energy is perpendicular to the fiber direction, a shorter illumination time is required for defect detection as compared to when the incident energy is polarized parallel to the fiber direction. In addition, the saturation time is independent of polarization, so perpendicular polarization is preferred for the inspection of unidirectional FRP-rehabilitated cement-based structures.