Enhanced Infrared Thermography for Characterizing Delaminations in FRP–Concrete Systems

Delaminations in the concrete substrate, fiber-reinforced polymer (FRP) laminate, and FRP–concrete interface are common failure initiation sites in FRP–concrete systems. This paper uses active infrared thermography to detect and estimate the location, area, and depth of such delaminations in both bare concrete and FRP–concrete systems. An algorithm previously developed to detect air voids in concrete is adapted for the present purpose. The proposed strategy, involving the use of longer heating pulses to identify defects in the concrete substrate and shorter pulses to detect defects in the FRP laminate, appears to work well. The area of substrate voids is determined reasonably accurately in all cases except when the depth of the void exceeds the void size by more than 50%, thus surpassing threshold void detection criteria in the literature. In case of interlaminar or interfacial voids, the void area is predicted with high accuracy. A time-domain-based approach that relies on the time of maximum thermal contrast to determine the void depth is proposed. It requires knowledge of the thermal properties of the concrete/FRP–concrete system. A one-dimensional inverse heat-transfer model that accurately determines the thermal properties in minimal time is implemented. This considerably simplifies the use of the algorithm under field conditions. The algorithm is tested for a variety of void configurations and performs satisfactorily. It is further tested for different types of externally bonded FRP and under different ambient conditions. It has also been shown to be capable of detecting cracks in the concrete substrate induced by rebar corrosion.