1. Introduction
Concrete structures are widely used in civil and architectural engineering. It is generally known that the major durability problem for RC structures under severe environments is the corrosion of the reinforcing rebars [
1]. Due to the corrosion of reinforcing steel, many structures were weakened in terms of serviceability and safety and thus had to be repaired or strengthened.
In terms of structural performance, the corrosion of steel bars in concrete has a significant effect on the strength and stiffness of the structures [
2]. The corrosion of reinforcing rebars results in a reduction in their cross-sectional areas, as well as a decrease in their strength, ductility, and other mechanical properties [
3]. Corrosion products can generate expansive forces that alter the stress state in concrete and result in cracks and damage to beams. In addition, the corrosion of reinforcing rebars can also weaken the bond strength between the corroded rebars and the surrounding concrete. In particular, after the formation of cracks in concrete the ability of concrete to restrain reinforcing bars is reduced. Furthermore, the corrosion of reinforcing rebars can weaken the mechanical interaction between the deformed ribs of the rebars and the concrete. The aforementioned factors can have a significant impact on the flexural capacity and failure mode of the RC beams that have undergone corrosion [
4,
5,
6].
It is necessary for the RC structures with corroded reinforcing rebars to be repaired or strengthened to restore the bearing capacity of the structure. The widely used strengthening methods for RC members include section enlargement, steel bonding, the external steel clad method, and CFRP strengthening. FRP composites have been successfully used to improve the strength and ductility of reinforced concrete members that have been severely damaged under different scenarios, such as earthquakes, fire, etc. [
7]. Van Cao et al. [
8] investigated the behavior of fire-exposed reinforced concrete slabs with FRP retrofitting. With the advantages of being lightweight and having convenient construction and good durability, it is an ideal method to strengthen the corroded reinforced concrete beam with a carbon fiber-reinforced polymer (CFRP) [
9]. Therefore, many researchers focus on the flexural performance of corroded RC beams strengthened with CFRP.
Since the beginning of the 21st century, CFRPs have been more and more widely used in the rehabilitation of corroded RC structures. A pilot study carried out by Soudki and Sherwood [
10] investigated the feasibility of bending corroded RC beams strengthened with CFRPs. They showed that CFRP sheets can restore the integrity and improve the capacity of the specimens; ten beams with three different corrosion levels (5%, 10%, and 15%), including six beams wrapped with CFRP sheets, were tested. The results showed the CFRP sheets can effectively restore the capacity of corrosion damaged concrete beams and increase the stiffness of the beams. Wang et al. [
11] carried out an experimental and analytical program which consisted of twenty-four 200 mm × 350 mm × 3500 mm beams. The findings demonstrated that the strength and failure mechanisms of the retrofitted RC beams could be influenced by several key factors, including the level of corrosion in the reinforcing rebars, the water–cement ratio of the concrete, and the arrangement and number of the FRP patches. Kutarba [
12] constructed thirty RC specimens to study the use of CFRP sheet repair as a rehabilitation technique for corroded RC beams. Following the repair and strengthening of twenty-six corroded beams using three distinct CFRP schemes, eight beams were load tested; at the same time, the remaining beams were subjected to the post-repair corrosion. The results showed that the application of CFRP significantly restored the bearing capacity of the specimens and provided a protective barrier capacity for the system to reduce the rate of secondary corrosion activity. Maaddawy et al. [
13,
14] designed an experiment which included eleven corroded beams; six of them were repaired with CFRP laminates, which extended the service life of the corroded RC beams. The research presented a new mathematical model for predicting the inelastic flexural response of corroded RC beams repaired with FRP laminates. The results showed that CFRP repair increased the ultimate strengths of the corroded beams but significantly reduced the deflection capacity at all levels of corrosion damage.
Many factors that affect the flexural performance of CFRP-strengthened corroded RC beams have been investigated by many scholars around the world. These factors include the corrosion of reinforcing rebars [
15]; the mechanical behavior of the corroded reinforcement [
16]; the quantity of CFRP sheets [
17]; anchoring [
18]; strengthening schemes [
15]; the bond behavior of corroded reinforcing rebars in relation to the concrete interface [
16]; initial load; and damaged concrete cover [
19,
20,
21,
22,
23]. Kashi et al. [
24] investigated the effect of marine environmental conditions on the durability of RC corroded columns strengthened with FRP sheets. Some researchers also investigated the structure performance of AFRP-strengthened corroded RC beams [
25,
26]. Gotame. M et al. [
27] investigated the non-linear finite element analyses that had been conducted to assess the flexural behavior of corrosion-damaged RC beams strengthened with externally bonded FRP. Zheng, A et al. [
28] studied the shear behavior of reinforced concrete (RC) beams with corrosion-damaged stirrups strengthened using a fiber-reinforced polymer (FRP) and a grid-reinforced engineered cementitious composite (ECC) matrix. Yang J. et al. [
29] investigated the feasibility of using externally bonded FRP laminates combined with U-jackets, applied directly and without repairing the deteriorated concrete cover, to strengthen beams with corroded reinforcing rebars. The shear span-to-depth ratio can significantly influence the behavior of an RC beam shear strengthened with EBR–FRP composites and can even determine the shear failure mode of RC beams [
30]. Chen et al. [
31] proposed ultra-high-performance concrete (UHPC) combined with fiber-reinforced polymer (FRP) composites for the shear strengthening of corroded reinforced concrete beams.
In summary, domestic and foreign scholars have conducted many studies on the flexural behavior of corroded concrete beams strengthened with CFRP sheets. However, very limited information is available on the calculation method for the flexural capacity of corroded concrete beams strengthened with CFRP sheets. The present study reports the experimental findings on the flexural behavior of RC beams that underwent corrosion and were subsequently strengthened with CFRP sheets. The experimental study included eleven RC beams that were subjected to accelerated corrosion of the longitudinal reinforcing rebars at various levels. Following the corrosion, all the beams were strengthened by the bonding of one layer of CFRP sheets to the tension side to restore the strength loss due to the corrosion. Based on the regression analysis of the test results, a calculation approach was introduced for determining the flexural bearing capacity of the corroded RC beams strengthened with CFRP in this study.