Various materials and techniques have been developed to strengthen damaged or deteriorated civil structures. Fiber reinforced polymer (FRP) sheets and plates are the most widely used strengthening materials (Triantafillou and Plevris
1992; Elsanadedy et al.
2013; Loreto et al.
2013). The most common strengthening techniques used for deteriorated structures are external bonding reinforcement (EBR) and near-surfaced-mounted (NSM) methods (Kim
2015). The EBR technique is a strengthening method that improves structural performance by attaching steel or FRP plates on the surface of a structure. However, the strengthening material in the EBR method is exposed, making it vulnerable to damage due to fire or corrosion. Furthermore, different thermal coefficients of the concrete and the material can cause debonding, leading to a premature failure of either the epoxy resin matrix or the strengthening material (Park
2003; Kim
2005; Barros et al.
2006; Casadei et al.
2006; Choi et al.
2010; Hong et al.
2014; Bilotta et al.
2015; Coelho et al.
2015). NSM technology was proposed to overcome such problems of the EBR method. NSM involves inserting strengthening materials such as FRP bars, FRP strips, or steel strands into the grooves of concrete cover which needs to be strengthened, and then bonding the strengthening material with a cement mortar or epoxy resin matrix as a filler inside a groove (De Lorenzis et al.
2002; El-Hacha and Rizkalla
2004; El-Hacha and Gaafar
2011; El-Hacha and Soudki
2013). Recently, various researches on the strengthening civil structures using FRP materials for NSM methods have been conducted (El-Hacha and Rizkalla
2004; Gaafar and El-Hacha
2008; El-Hacha and Gaafar
2011; El-Hacha and Soudki
2013). Even though researches have not found a way to completely solve the premature failure of the strengthening material in the EBR method, the considerable improvement of the flexural behavior of reinforced concrete (RC) beams strengthened by the NSM method compared to the RC beams strengthened by the EBR method has been demonstrated. Prestressing strengthening methods (PSMs) are used for concrete which needs to improve its flexural behavior in terms of serviceability and deflection (Yang et al.
2009). For decades, PSMs have been performed by fastening the post-tensioned material on the tensile side of a concrete beam (Rosenboom and Rizkalla
2006). Compressive stresses at the bottom of the concrete member and tensile stresses at the top of the concrete member are produced by the prestressing force transmitted from the prestressed strengthening material, which is known as the cambering effect (El-Hacha et al.
2001). The RC beam strengthened by the prestressed strengthening material has various advantages, such as reducing both deflection and crack width, increasing load-carrying capacity, and resistance of fatigue failure (El-Hacha et al.
2001; Woo et al.
2008). Among the PSMs, the NSM technique using prestressed FRP material, referred to as prestressed FRP NSM, has been studied by many researchers failure (El-Hacha et al.
2001; Nordin
2003; Hong et al.
2006; Woo et al.
2008). It has been reported that the prestressed FRP NSM technique increases cracking and yielding loads and reduces deflection at mid-span and crack width for the strengthened RC beams. However, this technique faces the critical problem of reduced ductility of the strengthened RC beam (Badawi and Soudki
2009). The ductility of the strengthened RC beam is reduced with higher prestressing level of the prestressed FRP material, which finally causes failure at a smaller deflection. The reason for this is that a large part of the strain capacity of the FRP reinforcement is already used during prestressing. The deformability of the RC beam strengthened by prestressed FRP material is greatly reduced, resulting in all specimens being strengthened by the prestressed FRP material failing by rupture of the FRP material (Rezazadeh et al.
2015; Kara et al.
2016). The decreased ductility of the strengthened RC beam is because of increased tensile reinforcement and the prestressing effect, which leads to less energy dissipation. Furthermore, in the prestressed FRP NSM technique, it is difficult to assign and secure the FRP material in the groove and to apply a pre-tension force in the groove (Shahverdi et al.
2016). This is the reason why the prestressed FRP NSM technique is not widely used in practice.
The NSM technique using shape-memory alloys (SMAs) can be an alternative way to solve the abovementioned problems of the prestressed FRP NSM technique. SMAs are very widely known unique materials which have the ability to return to a pre-defined (memorized) shape when heated to a certain temperature, which is called the shape-memory effect (SME). An NiTi-based SMA (NiTi-SMA) having the SME and super elasticity is used in various fields such as the aerospace and medical industries (Miller and Lagoudas
2000). However, it is not feasible to use the NiTi-SMA as the strengthening material for retrofitting civil structures because of its high production cost. Fe-based SMAs (Fe-SMAs) were developed by Sato et al. in 1982, and their wide shape-memory capacity was subsequently studied by many researchers (Sato et al.
1982; Kajiwara et al.
2001; Farjami et al.
2004; Dong et al.
2009; Lee et al.
2013). Since the 2000s, Fe-SMA material to strengthen civil structures has been researched (Abdulridha et al.
2013; Cladera et al.
2014; Czaderski et al.
2014; Shahverdi et al.
2016). Deformation recovery of an embedded Fe-SMA strip in concrete is restrained, resulting in compressive force—namely prestressing force.
This paper presents a study on the feasibility of Fe-SMA strips as an NSM strengthening material for civil structures. The purpose of this paper is to acquire greater flexural capacity and ductility of RC beams by employing the Fe-SMA NSM technique. The flexural behavior of seven RC beams was investigated with four-point bending tests. The type of reinforcement material, the number of Fe-SMA strips, and the pre-straining level of the Fe-SMA strips were considered as experimental variables.