PSI - Issue 64

Ali Saeedi et al. / Procedia Structural Integrity 64 (2024) 2044–2050 Ali Saeedi/ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Shape-memory alloys (SMAs), especially iron-based shape memory alloys (Fe-SMAs), have recently been utilized to strengthen concrete structures by exerting recovery forces (Czaderski et al. (2014), Shahverdi et al. (2018)). The behavior of SMAs is characterized primarily by two mechanical phenomena: the shape memory effect (SME) and the pseudo elasticity effect (PEE). In the case described by the SME, significant recovery forces can be generated through the pre-straining of SMAs followed by an increase in temperature. These induced recovery forces influence the mechanical properties of composites reinforced with SMAs (Saeedi et al. (2017)). Iron-based SMAs are expected to be highly applicable in civil structures due to their ability to produce substantial recovery stresses at a cost that is more affordable than the other types of SMAs (Schranz et al. (2021)). The Fe-SMA strips can be pre-stressed even when embedded directly into the concrete without a duct. The pre-stressing method involves pre-straining the SMA strip, embedding it into concrete, and then heating it through electrical resistive heating to trigger a phase transformation. This process allows the strip to develop full pre-stress as it cools back to ambient temperature (Czaderski et al. (2014). In another study, the use of ribbed Fe-SMA bars embedded in a shotcrete layer was considered to strengthen RC structures (Shahverdi et al. (2016)). The results indicated that pre-stressing with Fe-SMA bars significantly enhanced the serviceability stage, increasing the cracking load compared to the reference beam. In case of modeling, Abouali et al. (2019) employed a 3D nonlinear finite element model developed in ABAQUS to analyze the strengthening of RC beams in flexure using Fe-SMA reinforcements. The model's accuracy was verified using experimental results from previously published literature. The model was then employed to assess the impact of various design parameters on the performance of RC members reinforced with Fe-SMA. Activating the Fe-SMA embedded in concrete typically involves resistive heating through the application of electric current. Monitoring the temperature of the SMA and the surrounding concrete during this process is of high importance. This ensures that the temperature within the SMA reaches a level sufficient to generate the necessary activation force, while also preventing damage to the interface or the surrounding concrete. While several experimental investigations have explored the activation of Fe-SMA within concrete structures, there are a few modeling studies. The present study simulates the activation process of an Fe-SMA-reinforced concrete beam using multiphysics modeling. Different geometries of Fe-SMA reinforcements are considered for the modeling. The temperature distribution results are compared for these geometries. The effect of activation parameters on the temperature distribution have been also studied using the model. 2. Modelling Multiphysics modeling with Comsol software is employed to simulate the activation of Fe-SMA within concrete. This involves integrating electrical, structural, and heat transfer modules to accurately simulate the SMA activation process. The activation of Fe-SMAs embedded in concrete involves a complex multiphysics process that includes primary physical mechanisms: electrical current flow, heat transfer (conduction between SMA and concrete, and convection between concrete and surrounding air), continuum mechanics, and martensitic phase transformation. Upon activation, an electrical current is passed through the SMA wires, which heats the SMA due to its electrical resistivity. This heat triggers a reversible martensitic phase transformation, resulting in a shape-recovery phenomenon. For multiphysics simulations in COMSOL, two material components, concrete and Fe-SMA, were utilized. The material properties used for modeling these materials are outlined in Table 1. The shape of the SMA plays an important role in its activation; therefore, two different forms of SMA reinforcement, namely bars and strips, are utilized in the modeling, as shown in Fig.1. While the cross section area of the bar and strips are the same, their primitives are different. Various dimensions for both reinforcement were selected. The dimensions of the concrete beam are similar for both cases. The electric potential of 12V was applied to one end of Fe-SMA wire, while the other end was insulated.