Retrofit of Reinforced Concrete Beams using Externally Bonded and Unbonded Fiber Metal Laminate

This research investigates the flexural behavior of reinforced concrete (RC) beams retrofitted with fiber metal laminate (FML), an advanced hybrid material composed of alternating layers of metal and fiber-reinforced polymer (FRP) composites bonded through a thermoplastic or thermoset polymeric matrix. While FRP composites are commonly used for structural retrofits, their brittle failure mode, due to the linear elastic behavior of the fibers that cannot deform plastically, limits their effectiveness in applications requiring ductility. To address the drawbacks associated with FRP, this project proposes FML as a potential alternative.

Flexural testing was conducted on seven RC beams with different configurations of FML and FRP under four-point bending. The goal of the project was finding an ideal retrofit for the RC beam that increased the peak load without a sacrificing the ductility. The beams, which were simply supported, were subjected to two point loads in order to assess their complete load-deformation behavior. Displacements and applied loads were measured at the midspan, and strain data wasrecorded along the length of the retrofits. Four beams were retrofitted with FML, two with FRP, and one served as a control specimen that did not have a retrofit. In order to prevent a premature debonding failure between the RC beam and retrofit, this study also explored different bonding methods: hybrid bonding and unbonded anchorage configurations. Four of the retrofitted beams had a hybrid bonded anchorage configuration and two had an unbonded anchorage configuration.

Analytical modeling was performed to predict the behavior of RC beams with various retrofit configurations and bonding types. The modeling procedure for fully bonded retrofits followed the prescribed method in ACI 440.2R-17 that assumes full strain compatibility between the RC beam and retrofit. Due to the lack of strain compatibility for unbonded retorifts, an analytical procedure was developed to generate the moment-curvature response and is reported in Appendix D. The modeling techniques accurately predicted the load-deformation behavior observed in the experiments.

The results indicated that FML is an appropriate retrofit material for RC beams, with beam behavior highly dependent on the fiber orientation within the FML. RC Beams retrofitted with fully bonded, unidirectional fibers experienced the highest strength gains but exhibited decreased ductility. In contrast, beams retrofitted with fully bonded, off-axis fibers showed moderate strength gains without a reduction in ductility. Unbonded retrofits were effective in increasing both the strength and ductility of the beams, displaying performance similar to the fully bonded retrofits fiber orientation.

This study demonstrates the potential of FML as a retrofit material that offers a balance between strength enhancement and ductility. The main findings highlights the significance of fiber orientation and bonding methods in optimizing the performanae of RC beam retrofits.