PSI - Issue 64

Maria Ntina et al. / Procedia Structural Integrity 64 (2024) 2036–2043 Maria Ntina/ Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Reinforced Concrete (RC) structures represent the vast majority of the existing infrastructure. Over the years, they risk of experiencing deterioration of their seismic capacity due to crack formation that can induce corrosion related degradation of steel reinforcement. Moreover, designed according to capacity-based provisions, they are expected to undergo significant inelastic deformation, implying extensive damage and residual drifts of structural members in the case of serious earthquake events, often leading to large-scale costly interventions. To upgrade their performance and deal with the issue of significant residual deformation after earthquake action, a re-centering-based orientation has been introduced in current research efforts in structural retrofit employing smart materials such as Shape Memory Alloys (SMAs). SMAs have two distinct properties the superelastic and the shape memory effect that enable them to return to their undeformed shape without significant residual deformation upon unloading or heating respectively. They have been widely investigated due to their prominent features including the energy dissipation capacity, hysteretic damping, excellent fatigue, large strain and re-centering abilities with negligible residual deformations (DesRoches and Smith, 2004) that make them ideal candidates for structural applications. In this context, the present work addresses the deployment of SMAs in RC retrofit taking advantage of their superelastic effect. An existing RC building situated in Athens, Greece is retrofitted considering the jacketing of the existing columns with SMA reinforcement. To assess the effectiveness of this advanced material as an alternative solution to conventional steel, different types of SMAs are investigated performing pushover and non-linear dynamic time history analysis with SeismoStruct software (2021), comparing the results obtained with those of the initial unretrofitted structure and with those of a traditionally retrofitted structure with steel reinforcement. 2. Studies of SMA as a reinforcement Among the commercially available superelastic SMAs, Nickel-Titanium (Ni-Ti) SMAs have been broadly investigated for their superior superelasticity reaching up to 8% strain, damping capacity, durability, fatigue and corrosion resistance. They possess a high forward transformation (‘yield’) stress and a higher modulus of elasticity compared to other types of SMAs. These characteristics have attracted the interest of researchers conducting numerical and experimental studies and applications. Noguez and Saiidi (2012) conducted shake table tests on a bridge model made with six columns to study the effect of advanced materials on reducing the permanent deformation and damage caused by earthquake loads. One pair of columns was fabricated with superelastic Ni-Ti SMA reinforcement in the bottom plastic hinge along with Engineered Cementitious Composites (ECC), whereas the other two pairs of columns had post-tensioned steel rods, one with and the other without build-in elastomeric rubber pads. Results showed that the bridge pier with SMA-ECC and the one with both post-tensioned steel rods and elastomeric rubber pads experienced minimal damage and permanent deformation. Saha and Debbarma (2014) conducted cyclic and monotonic load tests of RC beams reinforced with conventional steel bars and treated beams, reinforced with both conventional steel and Ø 8 mm superelastic Ni-Ti SMA bars. Results revealed that there were negligible increments in deflection and the ratio of residual deflection to maximum deflection was almost zero in beams with SMA bars over the whole span, concluding that RC flexural members with SMA and steel provide a better choice for structures subjected to cyclic loads and earthquakes. Morcos and Palermo (2019) presented experimental results of a slender concrete shear wall with superelastic Ni-Ti SMA reinforcement. Testing illustrated the capacity of the wall to recover inelastic displacements to a larger extent in comparison to a companion conventional steel-reinforced wall. It also yielded at a displacement five times greater than the steel-reinforced wall, but at a slightly smaller corresponding lateral load due to the lower modulus of elasticity of the Ni-Ti SMA bars. A significant drawback of Ni-Ti SMAs is their high material cost and machining difficulty (Shrestha et al., 2013) that does not favor their use in large quantities and may not be practical for structural applications. For this reason, Copper-based (Cu-based) SMAs have received research attention. Cu-based alloys possess a superelasticity comparable to Ni-Ti SMAs but lower transformation stresses and a lower elastic modulus, they are cheaper and easier to manufacture (Medina et al., 2023). Shrestha et al. (2013) examined the potential of superelastic Cu-Al-Mn bars as partial replacements for conventional steel bars in RC beams, in order to reduce residual cracks in structures during and after intense earthquakes. They conducted four-point reverse cyclic bending