Characterization of flaws embedded in externally bonded CFRP on concrete beams by infrared thermography and shearography

Carbon fiber-reinforced polymer (CFRP) has become an important material to rehabilitate deteriorating concrete structures. The quality of the bond between the externally-bonded CFRP and concrete elements is crucial to durability and structural integrity of the rehabilitated concrete structure. However, flaws between CFRP and concrete interfaces can reduce the effective contact area significantly and therefore, the overall bond strength at these interfaces. Such flaws are not readily noticed by naked eyes, but can be detected and quantified non-destructively and effectively by using full-field and non-contact infrared thermography (IRT) and laser shearography. Flaw and sound areas presented within the FRP-concrete interfaces manifest very different thermal decay and mechanical fingerprints. In this paper, we report a study which tested, with IRT and shearography, the sizes and shapes of 17 round-shaped and artificial flaws embedded in 6 CFRP-concrete specimens; and then compared the results with the actual flaw sizes and shapes. In general, the results show that IRT underestimated the actual flaw sizes by 6.1% on average, whilst shearography overestimated the actual flaw sizes by 9.4% on average. These results demonstrated that both IRT and shearography are promising non-destructive evaluation techniques that can be used to define flaw boundaries and determine flaw sizes within the CFRP-concrete composites.