Direct and semi-direct ultrasonic testing for quality control of FRC-concrete adhesion

Externally bonded Fiber Reinforced Composite (FRC) materials, such as Fiber Reinforced Polymers (FRP) and Fiber Reinforced Cementitious Matrix (FRCM), have been increasingly used over the last decades for retrofitting existing structural elements due to their excellent mechanical and physical properties. However, the effectiveness of the reinforcement depends on the quality of the adhesion between the reinforcement and its substrate. The present paper reports the results of an experimental campaign in which the Ultrasonic Pulse Velocity (UPV) testing has been applied to detect the presence of adhesion defects at the interface FRC-concrete. UPV has been carried out on concrete specimens strengthened with different types of FRC. Some inclusions have been specifically placed at the interface between the reinforcement and the concrete to simulate adhesion defects. UPV has been applied both in the direct and in the semi-direct mode, to simulate different realistic on-site situations, including the possible inaccessibility of some parts of the element to check. Results show that UPV, applied by means of both direct and semi-direct modes, allows to identify the presence of adhesion defects, and evoke the possibility of discriminating between different defects.