Active Retrofit of Shear Critical RC Components Using Self-Prestressing Iron-Based Shape Memory Alloys

Abstract

With growing traffic demands and structural degradation accelerated by climate change, there is a critical need for continued advancement of concrete repair and strengthening technologies to enable extended bridge service life. Specifically, transitioning to cost-effective reinforced concrete (RC) retrofit strategies that further enhance durability under service loading conditions and minimize damage development under extreme hazard conditions, which are more probable to occur in longer-lasting concrete structures, are key elements of next-generation concrete bridges. This thesis explores the use of iron-based shape memory alloys (Fe-SMAs) as an active shear retrofit strategy for RC components. SMAs are a class of smart materials with the unique property of the shape memory effect, allowing them to fully recover plastic deformations when subsequently heated. By installing pre-strained Fe-SMA strips and activating the shape memory effect, an active pressure can be introduced to help close cracks and apply a confining stress in the concrete. The primary objective is to evaluate the performance and practicality of Fe-SMA as an active shear strengthening technique in comparison to traditional methods such as external steel reinforcement and fibre-reinforced polymer (FRP) composites. The experimental phase of this study involved conducting push-off tests on RC specimens retrofitted with pre-deformed Fe-SMA strips. The goal was to assess the efficiency of active shear retrofitting. Additionally, finite element analysis (FEA) simulations were employed to model Fe SMA-retrofitted RC structures and investigate their behaviour under shear loading conditions. Key findings indicate that Fe-SMA retrofits, through transverse prestressing as part of the active retrofit strategy, contributed to notable improvements in the stiffness and strength of damaged RC components. Similar to passive retrofit methods, increased shear capacity was observed with higher levels of transverse reinforcement. Notably, combinations of substantial shear reinforcement ratios and elevated transverse prestressing provided the most significant gains in shear strength. Furthermore, the addition of prestressed Fe-SMA retrofits was found to effectively reduce shear crack widths and mitigate the progression of subsequent shear crack width growth. This study not only demonstrates the potential of Fe-SMA as a promising solution for active shear strengthening but also contributes to the development of future field-scale tests. The presence of FeSMA in damaged structures offers the prospect of multiple improvements, marking a significant advancement in the field of shear retrofitting