## 4 Experimental Results and Analysis  
### Crack Propagation and Failure Modes  
Figure 7 presents the experimental results for all specimens, encompassing crack propagation and the observed failure mode. Subsequent to the testing phase, the crack propagation in specimens retrofitted with modularized steel plates was inspected by detaching the steel plates. For the MSP-L specimen, due to safety considerations, steel plate removal was not feasible. Hence, its crack propagation remains undocumented. The non-retrofitted specimen, labeled CB, manifested initial flexural cracks centrally, which subsequently evolved into shear cracks extending from the supports to the load application points. The propagation of these shear cracks culminated in concrete spalling in the compression zone, resulting in brittle failure.  
In comparison, specimens retrofitted using modularized steel plates demonstrated shear cracks similar to those in CB. However, these specimens experienced more frequent flexural cracks within their tension zones, narrower shear crack widths, and an abundance of minute cracks. The MSP-NC specimen, which lacked bolted connections between its steel plates, displayed significant concrete spalling within its tension zone compared to its bolt-connected counterpart, MSP-C. The failure mode of MSP-C was predominantly concrete crushing at the point of load application. In contrast, MSP-NC exhibited intensified cracking at its interface with the steel plate, indicating a lack of cohesive behavior. The absence of connections between the steel plates in MSP-NC resulted in a deviation in its flexural behavior compared to the RC beam. This led to the steel plates in MSP-NC bearing less tensile force than those in MSP-C. Figure 8 supports this, showing that the Z-shaped side plates in MSP-C underwent more pronounced deformation than those in MSP-NC, with clear signs of debonding evident in MSP-NC. Upon analyzing the behavior of modularized steel plates with different unit dimensions, as illustrated in Figure 8b,c, both MSP-C and MSP-L lacked interconnections. As shown in Figure 5b, while MSP-C was subjected to load within a single module, MSP-L experienced loading at the terminal points of the module. The concentrated loading at these terminal points exposed MSP-L to forces that promoted debonding. The lack of connections between modules led to almost complete separation of MSP-L after testing. This highlights the importance of module dimensions based on the load borne by the RC beam and the mechanism of load transfer.  
The effectiveness of retrofitting methods that attach reinforcements to pre-existing structures is largely influenced by the bonding strength between the core material and the added reinforcement. Figure 9 provides a detailed view of the bond status post testing between the RC beam and steel plate. Within an individual module, the steel plate maintained a strong bond with the existing RC beam on both its sides and bottom. Additionally, there were no signs of bolt loosening, joint shearing, or any form of buckling in the steel plates.  
Figure 6: Locations of the measurement sites (mm).  
Figure 7: Crack patterns for (**a**) CB, (**b**) MSP-NC, and (**c**) MSP-C.