# Flexural Behavior of Reinforced Concrete Beams Retrofitted with Modularized Steel Plates
## 5 Test Results and Discussion  
### Cracking Behavior and Modes of Failure  
Table 5 shows the crack pattern at maximum load for each specimen tested. For the D100-A2 and D100-B2 specimens with variable thicknesses of the bottom plate, it has been confirmed that the crushing of compression zone widens as the thickness of the bottom plate increases. As the number of vertical grid increased, the crushing and spalling of compression zone were more localized.  
The loading was terminated when the load was reduced 80% after the maximum load was reached. All the specimens showed flexural behavior as shown in Figure 11, cracks started appearing in the compression zone. As the load increased, the cracks of the compression zone became wider, and some spalling was observed on the mortar grouted on the bottom plate. The spalling of the mortar was observed differently according to the vertical grid. More spalling occurred in the D100-B0 specimen without the vertical grid. Finally, as the concrete crushing and spalling increased, the load gradually decreased. Local buckling of the Z-shape side plates and bolt fracture were not observed in all retrofitted specimens as shown in Figures 12 and 13. It is concluded that sliding was not observed at the end of the beam because the steel plates and concrete were perfectly bonded.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **Crack Propagation** & **Observed Damage** \\ \hline \hline \multirow{4}{*}{Bottom plate thickness} & D100-A2 & \multirow{4}{*}{-Concrete crushing at the top of the concrete -As the bottom plate thickness increases, cracks propagate widely} \\  & D100-B2 & & \\ \hline \hline \end{tabular}
\end{table}
Table 5: Crack propagation examples.  
The Load-displacement curve is shown in Figure 14. Specimens retrofitted with modularized steel plates showed greater stiffness and maximum load than RC specimen. Retrofitted specimens showed similar initial stiffness. Remarkable differences were not observed in terms of the number of vertical grid between D100-B2 and D100-B4. The maximum load for D100-B2 was 602 kN and that for D100-B4 was 598 kN, but the maximum load for D100-B0 was 514 kN, which was increased by approximately 1.17 times depending on the presence of vertical grid. This is to achieve the restraining effect of reinforcing parts on the mortar injected into reinforcing areas, and the flexural strength due to reinforcement is improved depending on whether or not it is installed.  
Figure 11: The shape of the test specimen after a flexural test.  
Figure 12: The side plate and bottom plate after a flexural test.  
Figure 13: Load-Deflection Relationship.  
The maximum load increased as the thickness of bottom plate increased. A comparison of the load-displacement curves of the two specimens confirmed that the maximum load for the D100-A2 specimen was 508.48 kN, the maximum load for the D100-B2 specimen was 602.62 kN, and that the flexural performance was approximately 1.19 times higher for the D100-B2 specimen.  
The test results are summarized in Table 6. The nominal flexural strength \(P_{n}\) is calculated from plastic stress distribution method suggested by AISC 360-10 [15]. The yield load \(P_{y}\) defines a point as the yield load where a line parallel to the line connecting the origin to the maximum load meets the load-displacement curve as shown in Figure 15. The proposed retrofitting method has shown an increase in the strength, with increasing thicknesses of bottom plates and the number of vertical grids. The experiment resulted in an average increase of 3.2 times the maximum load and an average increase of 1.3 times the maximum displacement.