# Flexural Behavior of an RC Beam Externally Strengthened with a Steel- and CFRP-Based Method
## 1 Introduction  
At present, as the national economy heavily depends on transportation, highway bridges play an increasingly pivotal role in not only economic construction but also urban life. According to the 2020 report published by the Ministry of Transport of China, there are 912,800 bridges throughout the country, with a total length of 66,285,534 m [1]. With the passage of time, the effects of damage, the environment, and defects gradually result in the degradation of bridges' structural bearing capacity and durability [2; 3]. Therefore, bridge rehabilitation is one of the major challenges in terms of service reliability. As the number of reinforced concrete beam bridges is the largest among all the bridge types, most researchers have focused on strengthening methods for these bridges. Considering the corresponding working conditions, the flexural capacity is the key aspect that it is necessary to improve. The bonding steel plate method is one of the earliest proposed methods [4; 5; 6]. [PERSON] et al. [7] and [PERSON] et al. [8] used steel plates to replace steel bars in a reinforced concrete beam structure. Concerning the bonding between the concrete and angle steel, an epoxy adhesive was used as the structural glue. The external retrofitting configuration tends to tightly connect with the original girder. However, it should be noted that the hardening process of the epoxy adhesive and the heaviness of the steel material can significantly increase the construction complexity [9; 10; 11]. In recent years, angle steel has been widely used to strengthen beams, columns, and joints. [PERSON] et al. [12]used circular steel bars and spiral steel bars to strengthen columns, and the corresponding cracking load, the ultimate load, and the energy absorption capacity were increased by up to 162%, 112%, and 190%, respectively [12]. [PERSON] et al. [13] adopted retrofitting angle steel as the connecting component in beam-column joints. [PERSON] et al. [14] conducted experiments on concrete beams bonded with steel plates under impact loads. It was found that the loss failure of the concrete could be relieved by an increase in the plate thickness, with the residual displacement reducing from 29.8 mm to 21.0 mm. The steel-based method dramatically enhances the flexibility of the reinforced concrete (RC) beam; however, it also increases the construction complexity because of the additional weight and connection configurations required. Therefore, fiber-reinforced polymer (FRP) has been widely used as an alternative to steel plates in retrofitting RC structures owing to its relatively light weight and high tensile strength [15; 16; 17]. The improvement ratio directly depends on both the volume of the FRP and its type [18; 19; 20; 21; 22; 23]. However, with an increase in the total thickness of the FRP laminate or plate, the debonding failure caused by bond slip replaces the material failure [24]. Correspondingly, only up to 30-50% of the FRP tensile strength can be utilized [25; 26; 27; 28], which significantly limits the increased range. To solve this problem, the prestressed FRP strengthening method was proposed, in which debonding failure is avoided through the installation of a special end-anchorage device [29; 30; 31; 32]. [PERSON] et al. [33], [PERSON] et al. [34], and [PERSON] et al. [35] pointed out that prestension can effectively utilize the outstanding tensile strength of FRP laminates. However, [PERSON] et al. [36] found that this type of reinforcement potentially leads to failure at the end of the composite plates owing to the stress concentration. Considering that the fracture strain of the FRP material is around 1.5% [37], the brittle material tends to result in a sudden collapse in structures that are only strengthened by the FRP-based method. According to 61 related samples summarized by [PERSON] et al. [38], the prestressed FRP retrofitting method further reduces the deformation capacity of FRP by 30.0%.