Shearography-based nondestructive testing of CFRP – Concrete bonding integrity

Externally bonded carbon fiber–reinforced polymer (CFRP) sheets are widely used to strengthen aging and deteriorated concrete structures, such as highway bridges, yet the long-term performance of such systems critically depends on the integrity of the CFRP–concrete bonding interface. However, early-stage debonding is often invisible and difficult to assess accurately and in a timely manner, making periodic inspections essential to ensure adequate bonding conditions. Existing inspection techniques are often labor-intensive, time-consuming, and economically impractical for large-scale applications. This paper presents a new nondestructive inspection method based on shearography, a laser-interferometric technique capable of full-field, non-contact measurement of surface deformation. First, a feasibility test was performed using a transparent plastic plate with paper inserts embedded in the adhesive layer to visualize deformation anomalies caused by simulated debonding. Subsequently, concrete cube specimens were prepared with controlled-defect inserts made of three materials: PTFE, silicone, and paper. CFRP sheets were bonded using epoxy, and thermal excitation was applied to generate deformation gradients across the bonded surfaces. The experimental results reveal abrupt changes in the butterfly-shaped patterns, corresponding to prefabricated defects in the phase maps. Additionally, differences in the observed butterfly-shaped patterns in the stripes suggest that defect characteristics influence these changes. These findings demonstrate the strong potential of shearography as a practical tool for evaluating the bond integrity of CFRP-concrete interfaces, thereby establishing a foundation for developing a shearography-based method for structural health monitoring of CFRP-strengthened concrete infrastructure. © 2026 Elsevier Ltd.