# Flexural Behavior of an RC Beam Externally Strengthened with a Steel- and CFRP-Based Method
## 3 Results and Discussion  
### Failure Modes  
Figure 5 plots the failure mode of the specimen, where the red and black lines denote the crack in the pure bending section and the shear bending section, respectively. As a standard RC beam, the damage pattern of Specimen L1 represented the typical bending failure. When the load rose to 34.33 kN, a fine vertical bending crack was initially observed near the midspan of the beam; the corresponding deflection of the crack was 2.51 mm. Then, a series of cracks soon developed in the pure bending section and near the loading points. The tensile steel bar yielded at a load amplitude of 68.86 kN. Finally, when the load reached 77.15 kN, the strain of the tensile steel bar increased abruptly and dramatically, with a deflection of 36.43 mm. When the corresponding maximum crack width was measured as 2.11 mm, the loading was stopped. Figure 5: _Cont._  
Specimens L2-A and L3-C were designed to investigate the strengthening effect of the single retrofitting method with the angle steel or the CFRP plate. The first crack of Specimen L2-A appeared with a width of 0.02 mm when the load reached 35.01 kN. The corresponding deflection was 2.26 mm. When the load was increased to 172.20 kN, the angle steel yielded, with the crack developing to 0.72 mm in width. The crack finally developed suddenly to 2.00 mm in width, which indicated the failure of the specimen. The corresponding ultimate load was 217.10 kN, and the deflection was 43.24 mm. The crack load of Specimen L3-C was 36.09 kN. When the load exceeded 118.11 kN, the tensile steel bar yielded with a noticeable sound inside the CFRP plate. The failure mode of the specimen was the serious crack development with a load of 170.61 kN. Notably, there was limited bond slip between the concrete and the CFRP plate. In contrast to the crack distribution in Specimens L1 and L2-A, the crack number in Specim L3-C decreased; this was due to the CFRP plate, which functioned as a kind of linear elastic material. The excessive stress in the beam could be redistributed to the CFRP plate; therefore, the average crack spacing could be effectively increased. Specimens L4-St-A, L5-C-St-A, and L6-C-Ss-A were designed to investigate the strengthening effect of the collaborative retrofitting method. With the 4 mm thick steel plate anchored by the angle steel, the crack loading of L4-St-A was improved to 43.51 kN. Meanwhile, the corresponding deflection was only 1.69 mm, with a maximum crack width of 0.02 mm. The location of the angle steel resulted in an early yield with a load of 294.25 kN. Then, the number and the average crack width significantly increased, with a deflection of 25.40 mm. Finally, when the load reached 320.00 kN, with a deflection of 41.55 mm, the specimen failed due to excessive crack development. Based on the configuration of Specim L4-St-A, a CFRP plate was directly bonded to the bottom of the beam in Specimen L5-C-St-A. As the three types of retrofitting materials could cooperate with each other, the crack load was increased to 63.46 kN. However, the corresponding deflection was 3.36 mm; this was caused by the delay in the adhesive layer and the CFRP plate. In contrast to the other retrofitted specimens, the inner steel bar yielded earlier than the external angle steel. Soon afterwards, there was a certain degree of buckling observed at the left end of the angle steel. When the load reached 340.57 kN, the specimen failed due to serious crack development, with a deflection of 43.20 mm. Notably, the crack distribution was more limited and was sparser than that in the other specimens. In addition, the crack in the pure bending section was relatively short. This phenomenon partially resulted from the effective control provided by the CFRP plate directly bonded to the bottom of the beam. In comparison with L3-C, the steel plate and the angle steel also contributed significantly. As there was no bond slip between the beam and the retrofitting units, the deflection resulted in a tensile deformation.