Debonding Analysis of Fibre Reinforced Polymer Anchors in Concrete via In-Situ X-Ray Microtomography Tests Coupled to Volume and Digital Image Correlation

Background Externally Bonded (EB) Fibre Reinforced Polymer (FRP) strengthening systems are effective solutions for enhancing structural members’ flexural behaviour. However, some issues are encountered near joints, where the stress transfer between concrete and FRP can be sub-optimal. Objective In this work, we aim at bringing understanding through a novel test set-up of the mechanical interaction between concrete and Carbon Fiber Reinforced Polymer (CFRP) anchorage systems in the framework of the flexural strengthening of reinforced concrete (RC) joints. Methods The investigation has been carried out on small scale beams with variable cross-sections representative of FRP strengthened joints and subjected to a bending loading. Material degradation and anchor pull-out are tracked thanks to 3D tomography and Digital Volume Correlation (DVC). Results The accurate reconstruction of the 3D kinematics in the anchorage region provides a better understanding of the physical phenomena involving CFRP strengthened joints and gives useful information for the development of dedicated numerical models. The knowledge of material evolution during the entire loading history leads to the identification of the relevant mechanisms responsible for reducing the structural stiffness up to failure. Conclusions Material bulk degradation in small-scale FRP-strengthened concrete joints can be reconstructed thanks to the proposed test set-up. The failure mode in the anchorage region is highly tridimensional and can be interpreted as a modified version of the typical conical failure observed in standard pull-out experiments. The main observation is that mixed-mode interfacial debonding is influenced by inclination and stiffness of the anchor. The analysis also gives the internal forces in concrete (compression) and in the anchor (tension). Thanks to the knowledge of this latter quantity and the anchor slip, the pull-out response has been determined.