Self-prestressing Iron-based Shape Memory Alloy (Fe-SMA) Epoxy Composite for Active Reinforced Concrete Shear Strengthening

Abstract

In Canada, the rapid deterioration of aging reinforced concrete (RC) structures has become a continuing issue, with more than 40% of bridges being older than 40 years old and 38% being in poor and fair condition, necessitating billions for rehabilitation (Cusson & Isgor, 2004; Lafleur, 2023). The loss of the strength and stiffness in RC bridge structures can be attributed to age and exposure, and it has been exacerbated with the increase of freight weight, traffic, extreme freezing/thawing cycles, and climate change. A concerning RC failure mode is shear due to its brittle and abrupt nature. Hence, various shear-strengthening mechanisms have been developed. Most of these mechanisms involve fiberreinforced polymers (FRP) and are passive, acting after the structure experiences damage. Active (prestressing) mechanisms have gained notoriety due to their ability to act immediately after application, reducing crack widths and propagation. However, implementing shear prestressing is complex, often requiring expensive and impractical large jacking equipment. Smart materials such as iron-based shape memory alloys (Fe-SMAs) have the potential to be implemented in cost-efficient and simple shear strengthening and retrofitting techniques. Fe-SMAs present a thermomechanical property known as shape memory effect (SME) that allows the material to return to its undeformed shape after reaching an activation temperature, which can be done with resistive heating. If the material is restrained, the Fe-SMA has the capacity to self-prestress an element without the need of jacking tools. This project presents an experimental study on the shear strengthening feasibility and capacity of a near-surface bonded (NSB) active Fe-SMA epoxy composite. The composite consists of u-bent strips embedded into grooves filled with epoxy. After the epoxy cures, the Fe-SMA strips are heated to at least 180oC with an electric current to self-prestress the concrete. Three shear-critical RC beams were cast, with one beam being used as control, and the other two beams being shear strengthened. Two FeSMA ratios were assessed 0.05% and 0.1%. The strengthened beams exhibit about a 27% increase in strength, and the reduction of crack widths and stirrup stresses. The NSB Fe-SMA strips interrupt the formation and widening of diagonal cracks; however, increasing their ratio may not mean an increase in shear strength. A dense NSB Fe-SMA - concrete interface weakens the stirrup plane, creating horizontal cracks running along the top face of the beam (ends of the Fe-SMA u-wrapped strips) in the compression region and causing the separation of the side concrete cover. Additional insights on the active shear strengthening have been provided with two FEA parametric studies using Vector2 by evaluating prestress level and Fe-SMA ratio. This project assesses the shear strengthening effect of near-surface bonded (NSB) active Fe-SMA epoxy composites on the load-displacement response, crack widths, and reinforcement stresses on shear-critical RC specimens.