Numerical and Experimental Assessment of FRP-Concrete Bond System

Fiber reinforced polymer (FRP) composite systems are widely used to repair structurally deficient constructions thanks to their good corrosion resistance, light weight and high strength. The quality of the FRP-substrate interface bond is a crucial parameter affecting the performance of retrofitted structures. In this study, ultrasonic testing have been used to assess the quality of the bonding. In the case of FRP laminates adhesively bonded to concrete, high scattering attenuation occurs due to the presence of concrete heterogeneities. The substrate material behaves almost like a perfect absorber generating a considerable number of short-spaced echo peaks that make the defect echo not distinguishable. In order to avoid scattering, waves longer than the discontinuity have to be used, but this expedient makes bonding defects undetectable. The presented technique is based on the energy distribution measurement of ultrasonic signals by means of a statistical parameter, named Equivalent Time Length (ETL). A preliminary numerical study involving a 1-D system with a material discontinuity was performed. 2D finite element (FE) analyses were also performed. The experimental study involved laboratory FRP reinforcements bonded to concrete substrates with imposed well-known defects, and seismic retrofitted concrete walls. The experimental and the numerical findings showed that the ETL is sensitive to the presence of bonding defects in the sense that lower values mean higher reflection of wave energy (low quality of bonding) and higher values mean lower reflection and higher penetration through the bonding (good quality of bonding).