Investigation of the effect of design parameters on the behavior of RC beams reinforced with shape memory alloys

Considerable age of numerous concrete structures and due to some reasons like changes in design philosophy, increase in applied loads, etc., have made strengthening and maintenance compulsory. Shear failure in reinforced concrete beams is frequently sudden and brittle. For this reason, efforts are made to avoid this type of failure by strengthening them, especially in structures that were made with engineering mistakes and damaged structures. Steel, fiber-reinforced polymers, and carbon fiber-reinforced polymers are used as conventional solutions, but these methods have some drawbacks. For instance, prestressing them is hardly applicable, and the prestressing force decreases over time. Therefore, nowadays, as an alternative, shape memory alloys (SMAs) are investigated as new strengthening methods owing to their unique features. Shape memory alloys are novel and smart material groups that have been considered in civil engineering for many purposes, including active and passive control of structures, dampers, and strengthening of structures like reinforced concrete structures and bridges, etc., due to unique features such as pseudo-elasticity and shape memory effect. They have the particular property of returning to their initial shape by heating which is called the shape memory effect. If the SMAs prevented from returning to their initial shape by using mechanical fixation, a prestress force develops owning to the shape memory effect property. NiTi or Nitinol has been used for damping applications in civil engineering, and it has been investigated in the literature. Iron-based shape memory alloys (Fe-SMAs) have attracted much attention in civil engineering applications due to their shape memory effect. Particularly for strengthening applications, iron-based shape memory alloys have some benefits such as wide transformation hysteresis, high elastic modulus, and lower cost compared to conventional NiTi alloys. The advantage of shape memory alloys over fiber-reinforced polymer is that they can be prestressed more easily than FRP, and the prestressing force will not reduce over time. In addition, it does not require any mechanical and hydraulic jacks. Prestressing these materials has some advantages in strengthening. For example, cracks and deformations can be reduced or at least prevented from further growing, and the stresses in internal stirrups are reduced. The usage of prestressing for shear strengthening is rare because it is very complex from a practical standpoint. This study aims to assess the behavior of RC beams strengthened in shear with iron-based shape memory alloys. For this purpose, based on experiments in the literature, T-beams with 5.2-meter long are investigated numerically by using finite-element analysis software, ABAQUS. Three-dimensional finite element models were developed using the concrete damage plasticity and were verified with experimental results. Comparison between the results from the FE models and experimental test results confirmed the accuracy of the proposed models. Furthermore, the effects of parameters such as shape memory alloy diameters, prestressing force, and shotcrete thickness on beams' shear behavior are also investigated. The results of the analysis indicate a notable increase in the final shear strength of the strengthened beams and a reduction in stirrups' stresses. The prestressing ability of shape memory alloys delays the yielding of stirrups and the appearance of shear cracks and reduces the thickness of the cracks.