# Flexural Behavior of an RC Beam Externally Strengthened with a Steel- and CFRP-Based Method
## 2 Materials and Methods
### Specimen Design  
There were six full-scale beam specimens in total. For the reinforced concrete girder part, all six specimens were designed with identical cross-sections and steel reinforcements according to Chinese Code JTG3362-2018 [44]. The details of the RC beam are displayed in Figure 2. The beam length \(L\) and the beam height \(h\) were 6000 mm and 500 mm, respectively. The strength grade of the concrete was selected as C25. Two steel bars with a diameter of 16 mm and a strength grade of HRB335 were used as the tensile reinforcement in the bottom of the beam, while four steel bars with a diameter of 10 mm and a strength grade of HPB235 were placed as the compressive reinforcement in the top of the beam. The thickness of the steel reinforcement protective layer was 30 mm on both sides. Additionally, the stirrup was set at a space of 150 mm by using a steel bar with a diameter of 8 mm and a strength grade of HPB235. Figure 1: Concept of the proposed method. With the exception of the standard RC beam, the specimens were retrofitted using one of the single or multiple methods (see Table 1). According to the retrofitting method, the six specimens were named as L1, L2-A, L3-C, L4-St-A, L5-C-St-A, and L6-C-Ss-A. In the nomenclature, L and the following number represent the specimen number. The capital letters A, C, and S stand for the retrofitting materials, namely the angle steel, the carbon fiber, reinforced polymer (CFRP) plate, and the steel plate, respectively. The lowercase letters to and s denote the layout arrangement of the steel plate, which may be either continuous in the total span or separated along the beam. It should be noted that the order of the letters represents the retrofitting sequence. Figure 3 plots the actual methods adopted in the test. Specim L2-A was retrofitted using the angle steel with a cross-section at L50\(\times\)5. The bottom surface of the retrofitted beam was roughened; then, the angle steels were bonded to the bottom corners of the beam using epoxy adhesive. To anchor the angle steel, two 80 mm wide hooping steel plates were placed near the ends of the beam with a space of 250 mm, and four identical hoops were set near the pure bending section of the beam. Specim L3-C was strengthened with a 1.4 mm thick and 200 mm wide CFRP plate along the longitudinal direction of the specimen. Specim L4-St-A was initially bonded with a 4 mm thick and 200 mm wide steel plate. After the epoxy adhesive achieved the required strength, the angle steel was fixed to the bottom corners, making it identical to Specim L2-A. \begin{table}
\begin{tabular}{c c c c} \hline
**Specimen No.** & **Retrofitting Method** & **Retrofitting Material** & **Layout of Steel Plate** \\ \hline L1 & - & - & - \\ L2-A & Steel-based & Angle steel & - \\ L3-C & FRP-based & CFRP plate & - \\ L4-St-A & Steel-based & Steel plate/angle steel & Continuous \\ L5-C-St-A & Steel- and FRP-based & CFRP plate/steel plate/ & Continuous \\  & & angle steel & \\ L6-C-Ss-A & Steel- and FRP-based & CFRP plate/steel plate/ & \\  & & angle steel & \\ \hline \end{tabular} Note: The order of the retrofitting material represents the actual installation sequence.  
\end{table} 
Table 1: Details of specimens. Figure 2: Details for standard RC beam (unit: mm): (a) dimensions and reinforcement; (b) section A-A. To combine the steel- and FRP-based methods, Specimens L5-C-St-A and L6-C-Ss-A were first strengthened by the CFRP plate, which was the same as that used in L3-C. Then, the steel plate and the angle steel were bonded in sequence to the same location in Specim L4-St-A. The main difference between Specimens L5-C-St-A and L6-C-Ss-A was the layout of the steel plate. A continuous steel plate was bonded to the CFRP plate along the total longitudinal direction at the bottom of Specim L5-C-St-A, while the steel plate in Specim L6-C-Ss-A was set separately, with a space of 250 mm. Regarding the strength improvement, the length, width, and thickness of the separated steel plate were 170 mm, 50 mm, and 5 mm, respectively.