## 1 Introduction  
Civil engineering structures, such as bridges, buildings, and high-rise structures, may suffer various defects in the design, production, and construction processes. Under long-term service conditions, they are faced with cracking, rustiness, and aging problems due to overload, corrosion, fatigue, etc. [1; 2; 3]. Under the action of fire, earthquake, wind, and other disasters, existing structures may also sustain damage and deformation [4; 5; 6; 7]. In the United States and Europe, where the infrastructures were constructed earlier, a large number of engineering structures have been suffering aging problems and require reinforcement and maintenance. Although the prosperous infrastructure construction in China started late, increasing demands for structural reinforcement are emerging, which contains great scientific research value and economic prospects. The beam, an essential and fundamental component of structures, plays a crucial role in achieving the spanning, bearing, and linking functions. Therefore, it is of considerable value to strengthen and repair the deteriorated beams in existing structures to ensure their bearing capacity and prolong their service life. Shape memory alloys (SMAs), as a new intelligent material, have been widely used in aerospace, robotics, medicine, and other fields, possessing two typical characteristics: shape memory effect (SME) and superelasticity (SE) [8; 9; 10; 11]. SMAs contain two different composition phases: austenite and martensite. The phase transformation between austenite and martensite, which is induced by changes in temperatures or stresses, is referred to as the martensite transformation [12; 13; 14]. The martensite transformation is a diffusionless and solid-state phase transition, in which the atoms move cooperatively. It differs from the plastic deformation of normal metal materials, which is due to the dislocations of crystals slip [15]. Thus, the martensite transformation is a reversible process, and the characteristics of SME and SE benefit from this peculiarity. In civil engineering, SE is mainly used in structural seismic resistance and energy dissipation. SMA can be manufactured into basic components (wire, bar, spring, etc.) and key components (dampers, supports, isolation devices, etc.) [16; 17; 18; 19]. SME can be used for structural reinforcement and enhancement. The recovery stress generated by activating SMA can introduce prestress based on increasing the stiffness of structures [20; 21; 22]. Retrospectively, the shape memory materials were first discovered in the gold-cadmium (Au-Cd) alloys by [PERSON] in 1932 [23]. Decades later, the reversible phase transformation of SME, governed by the thermoelastic martensite phase of Au-Cd alloys, was reported by [PERSON] and [PERSON] in 1949, and by [PERSON] and [PERSON] in 1951 [24; 25]. In the 1960s, [PERSON] et al. found the SME of nickel-titanium (Ni-Ti) alloys in shock absorption tests [26]. In the 1970s, [PERSON] et al. found that the shape memory effect of Cu-Al-Ni alloys was closely related to the thermoelastic martensite transformation [27]. In the early 1980s, [PERSON] et al. first studied the SME of Fe-Mn-Si alloys [28; 29]. NiTi-SMA and Fe-SMA are the two SMAs appropriately employed in civil engineering. The shape recovery ability of Fe-SMA is weaker than that of NiTi-SMA, but it also possesses high strength, excellent plasticity, and good forming properties [30; 31]. Furthermore, its lower production costs compared to NiTi-SMA make it more economical for large-scale application in the field of structural reinforcement [32]. After thermal activation, Fe-SMA can realize the active control of structural stiffness and deformation, as well as repair local damage and cracks of structures [33; 34]. Due to SME, no complex tension devices are required to achieve pretension, which eliminates the restriction on operating spaces and improves the convenience of construction [35].