Depth Detection of Bond Defects in Multilayered Externally Bonded CFRP-to-Concrete Using Pulse Phase Thermography

Although the practice of repairing or strengthening concrete infrastructure with externally bonded carbon-fiber-reinforced polymer (CFRP) has become common, the development of robust guidelines and techniques for its quality assurance, inspection, and monitoring is lagging significantly. Whereas common inspection methods (e.g., visual inspection or acoustic sounding) are sufficient to broadly identify deboned regions, it is difficult to reliably and accurately define the boundaries of bond defects, as is often required by acceptance criteria. Infrared thermography is a more sophisticated alternative, but the results of its application are influenced by ambient environmental conditions and operator interpretation of the data. Further, for applications in which multiple layers of CFRP are required, none of these techniques are capable of fully characterizing existing bond defects, including the depth through the thickness. To this end, pulse phase thermography (PPT) has the potential to more reliably and accurately detect the location and size of bond defects. By observing images in the frequency domain, one is able to identify a specific “blind frequency” at which a certain defect is no longer visible, and thereby also predict the defect’s depth through the thickness. This paper presents a procedure using PPT to develop a calibrated equation based on blind frequency, specific to the strengthening system, which allows for one to predict the interface at which a bond defect is present for externally bonded CFRP-to-concrete systems. With the development of more robust techniques enhancing quality assurance and reliability, and the facilitation of regular monitoring and inspection, infrastructure owners will gain more confidence in this repair or strengthening technology, ultimately leading to its widespread acceptance and adoption.