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Pulse Phase Thermography Inspection of Bond Defects in Environmentally Conditioned FRP-to-Concrete Specimens



Externally bonded fiber-reinforced polymer (FRP) composite systems have emerged over the last few decades as an effective means for strengthening concrete structures. Typically these systems are installed by a process known as wet lay-up which involves the saturation of dry fibers with epoxy and applying them to the surface of a concrete member. This in-situ manual process is widely susceptible to a number of defects along the bondline due to entrapped air, moisture and other foreign inclusions. While current guidelines suggest acceptable defect sizes without the need for repair, significant research is still needed that examines the non-destructive evaluation (NDE) of such defects and the durability of FRP strengthened structures in extreme environments. Due to the speed and ease of application, acoustic sounding (hammer tapping) is among the most common inspection methods, but it is considered insufficient due to the limited information it provides and its accuracy depends on user interpretation. To this end, many have turned to Infrared Thermography (IRT) as an alternative whose use has been demonstrated by several researchers. However, its effectiveness is also affected by the operating environment. Occurrences such as surface reflection, humidity and non-uniform heating can lead to false readings that produce an inaccurate assessment of bond quality. Pulse Phase Thermography (PPT), a derivative technique, has been shown to overcome such issues by performing a Fourier Transform on the thermal data captured during the cooling period. PPT not only enables the removal of thermal noise by observing the phase values in the frequency domain, it also enjoys the benefit of rapid deployment by using shorter heating periods. This paper presents the results from a study involving the environmental conditioning of small scale, single-lap FRP-to-concrete pull-test specimens. Fifteen specimens were manufactured with and without pre-existing bond defects and were subjected to water submersion and freeze-thaw protocols. PPT inspection images were taken before and after the conditioning to observe the growth of defected areas as a result of conditioning. Also the tested performance of these specimens was examined to assess the criticality of discovered flaws. Finally, conclusions are made on the durability of the FRP-to-concrete interface when defects are initially present.

doi: 10.12783/SHM2015/55

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