PSI - Issue 64

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

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on one-third scale beams comprising three different types of specimens: conventional steel RC, SMA RC and SMA RC with pre-tensioning. Results showed that SMA RC beams demonstrated strong re-centering capability and significant enhancement in crack recovery capacity, in comparison to steel reinforced beams. Hosseini et al. (2015) employed superelastic Cu-based SMAs and ECC to strengthen RC bridge columns. The proposed design of the column comprised a prefabricated tube made with ECC and longitudinal and transverse rebars, where Cu-SMA rebars replaced the longitudinal steel rebars in the plastic hinge region of the column. The partial and the complete replacement of steel rebars with Cu-based SMA rebars reduced the permanent deformations of the column by up to 33% and 90%, respectively. However, the complete replacement of steel with SMA rebars resulted in a lower strength, stiffness, and energy absorption capacity than conventional RC columns. Saiidi and Jordan (2019) investigated the feasibility of splicing 30 mm superelastic Cu-Al-Mn bars to form a longer bar for application as reinforcement in plastic hinges to provide re-centering for bridge columns subjected to strong earthquake loading as a viable substitute to expensive Ni-Ti SMAs, showing promising results. During the last decade, Iron-based (Fe-based) SMAs with lower production costs have also received much consideration for RC reinforcement, exploiting though their strong shape memory effect (SME) as they possess a negligible superelastic behavior (Heredia Rosa et al., 2019). Schranz et al. (2019) presented the potential of new strengthening methods in RC structures using ‘memory-steel’ (Fe-based) SMAs. Applications included the mechanic fixation of memory-steel strips to concrete. Activation of the shape-memory effect and hence pre-stressing was accomplished either by resistive or infrared heating. Successful reduction of existing crack width was demonstrated and monitored with gauges. In a second application category, memory-steel bars were used to strengthen a cantilever slab of a residential building in the negative bending moment area. The shape-memory effect was activated by infrared heating of the bar over its unbonded length. The presented examples demonstrated the successful application of new effective strengthening methods for the retrofitting of existing concrete structures using SMA memory-steel products. To enhance the applicability of Fe-based SMAs in superelastic applications, studies focused on the improvement of their superelasticity with thermomechanical treatment (Mohri el al., 2022) and grain refinement (Khodaverdi et al., 2022) showing promising results for their future deployment. 3. Retrofit case study A RC residential building situated in Athens, Greece was selected as a case study to evaluate the potential of SMAs as an alternative solution to conventional steel reinforcement. It was constructed in the ’70s, empirically, without conducting static calculations. It is consisted of a basement and two upper floors having a plan view of 8.00 x 13.20 m (Fig. 1a). A characteristic of this building is the absence of RC walls which makes it more vulnerable in addition to its empirical construction. All columns have dimensions 300 x 300 mm and have 4 steel bars of Ø14mm considering a yield strength of 400 MPa as longitudinal reinforcement and stirrups of Ø8mm/25cm as shear reinforcement while the beams are 200 mm wide (b), and 550 mm deep (h). The building was modeled with SeismoStruct finite element commercial software (2021) (Fig. 1b). The mechanical properties of concrete were taken as follows: the mean compressive strength 20 MPa, the mean tensile strength 1.6 MPa, the modulus of elasticity 21 GPa and the material specific weight 24 kN/m 3 . The retrofit scheme regarded the jacketing of the ground floor’s external columns with C30/37 concrete with 4 ribbed bars of Ø16mm as longitudinal reinforcement and steel stirrups of Ø10mm/10cm as shear reinforcement embedded (Fig. 1c). Three types of superelastic SMAs were considered: Ni-Ti, Cu-Al-Mn and Cu-Al-Be. The transformation stresses of Ni-Ti were graphically defined from stress-strain curves presented in Fang et al. (2015) who conducted uniaxial tensile tests on Ø16 mm Ni-Ti bars. Cu-Al-Mn alloy’s transformation stresses were graphically defined from stress strain curves presented in Gencturk et al. (2014) who conducted experiments on Ø16mm bars with almost single crystal structure. Regarding Cu-Al-Be alloys, monocrystalline Cu-Al-Be SMA wires were found to be superior compared with Ni-Ti in superelastic capacity, showing great superelastic strain of up to 23% and cold-temperature performance and to have comparable performance in terms of fatigue, training effect and energy dissipation (Qiu and Zhu, 2014). Experimental data from experiments conducted by Qiu and Zhu (2014) on monocrystalline Cu-Al Be SMA wires were employed, as there were not available on the literature experimental data on Ø16 mm bars, to the best of the author’s knowledge. As it has been reported that bars exhibit lower stress values than the corresponding ones of wires (DesRoches et al., 2004), a 20% reduction was considered on the graphically defined