# Flexural Behavior of an RC Beam Externally Strengthened with a Steel- and CFRP-Based Method
## 3 Results and Discussion
### Load-Deflection Relationship  
Figure 6 plots the relationship between the load and the midspan deflection. Generally, the curves of all the specimens can be divided into three stages, namely the elastic stage, the working stage with cracks, and the failure stage. In Specim L1, with the increase in load, the crack of the concrete and the yield of the steel bar caused two breaking points in sequence; this caused the slope of the curve to decrease, i.e., there was a change in the flexural stiffness. It can be concluded that the curve variation trends of the five retrofitted specimens were similar to that of the standard RC beam Specim L1. With respect to Specimens L2-A and L3-C, the load corresponding to the first breaking point was larger than that of Specim L1. As the angle steel yielded, there was an obvious second breaking point, which led to a secondary stiffness degradation in the failure stage. For Specim L3-C, the slope was relatively stable after the first breaking point. As a kind of elastic material, the CFRP plate could effectively limit crack development and further keep the slope stable after the initial crack. As there was a 4 mm thick steel plate added between the beam and the angle steel in Specim L4-St-A, the initial stiffness and the load-bearing capacity were significantly improved in comparison with Specim L2-A. This phenomenon demonstrated the feasibility of the collaborative work between the steel plate and the angle steel.  
For Specim L5-C-St-A, a 1.4 mm thick CFRP plate was added to the beam bottom over the inner side of the steel plate. It was found that its initial stiffness was nearly identical to that of Specim L4-St-A. It was inferred that the small change was caused by the gap in the adhesive layer and the CFRP plate, which could be considered as a relatively soft integrated unit. This delayed the deformation of the retrofitting material until the deflection increased to a certain extent. Therefore, in the early working stage, the CFRP plate even led to a small decrease in stiffness. After the crack appeared, the CFRP started to function with an increase in its strain. The curve of Specim L5-C-St-A remained parallel to that of Specim L4-St-A in the second stage and exceeded it in the failure stage. This phenomenon indicated that the CFRP plate in the collaborative method tended to improve the ultimate load-bearing capacity with respect to extreme loads, which could be considered as safety storage after the yield of the steel.  
In Specim L6-C-Ss-A, the continuous steel plate was replaced by a series of separated steel plates. Notably, the initial stiffness of Specim L6-C-Ss-A was less than that of Specimens L4-St-A and L5-C-St-A; however, it was similar to that of Specim L3-C. Furthermore, it was found that the improvement (221.27 kN) of the ultimate flexural capacity in Specim L6-C-Ss-A was slightly larger than the sum of the enhancements (212.16 kN) in Specimens L2-A and L3-C at the same deflection. It was inferred that the separated steel plates contributed little to the improvement of the load-bearing capacity. Given the crack distribution of Specim L6-C-Ss-A, the load could barely be transferred from the midspan to the beam ends. Therefore, the separated steel plate functioned more as an anchorage of the CFRP plate. Conceivably, a thicker CFRP plate would contribute to the flexural performance of the retrofitting system.