## 2 Materials and Methods  
### Beams' Characteristics  
The initial stage of the experimental study involved three shear-critical RC beams, each 1.2 m long, forming a rectangular cross-section. These beams were subjected to a four-point monotonic loading setup and had the following dimensions: a width of 100 mm, a height of 200 mm, and an effective depth of 175 mm, with a shear span of 500 mm, and a shear span-to-depth ratio (\(\alpha/d\)) of 2.86, respectively. Figure 1 provides a schematic representation of the geometrical details of the initial beams. Henceforth, the initial beams are notated as \"B\" in subsequent references for consistency. Further, the \"B\" beams were reinforced with identical deformed steel longitudinal tension and compression bars, featuring 14 mm-diameter tension bars (2\(\mathcal{O}\)14 at the bottom) and 10 mm-diameter compression bars (2\(\mathcal{O}\)10 at the top). The tensile yield strength of these bars was \(f_{yl}=550\) MPa. The reference beam \"B500\" had no transverse shear reinforcement in the shear spans, while the other two beams were equipped with mild steel closed stirrus of 5.5 mm in diameter with a yield tensile strength of \(f_{yv}=225\) MPa. The stirrups were spaced at 200 mm, measuring from the beam's mid-point for \"B200a\", and at 200 mm, measuring from the support in beam \"B200b\", respectively, as shown in Figure 1. Both beams numbered two stirrups in each shear span [126]; according to the two-branch crack development of shear propagation theory, \"B200a\" focused on limiting the growth of the inclined cracking to the upper mid of the shear span, while \"B200b\" attempted to intercept the propagation on the lower part of the shear span to avoid extensive cracking development and concrete spalling near the supports. The geometric and mechanical reinforcement ratios of the tension longitudinal bars \(\rho_{l}\) and \(\rho_{l}f_{yl}/f_{c}\) and the vertical stirrups \(\rho_{v}\) and \(\rho_{v}f_{yv}/f_{c}\) for each beam are provided in Table 1. As a part of the experimental design, the beams are targetally equipped with insufficient shear reinforcement relative to their provided bending reinforcement, leading to higher flexural strength than shear. Different configurations (number) of stirrups were used in the \"B\" elements to ensure that a range of shear strengths and deformation performances and varying degrees of cracking could be exhibited [2]. In progress to the \"B\" beams' failure, the current experimental project is focused on retrofitting and retesting the three heavily damaged beams. The retrofitting method for the shear-damaged RC beams induces thin U-shaped jackets designed to restore and potentially enhance the original shear strength and deformation capacity while improving the overall performance. The retrofit includes small-diameter (5.5 mm) mild steel reinforcement and a 25 mm-thick layer of cement-based mortar. The retrofitting process involves applying jacket reinforcement to the surface of the existing RC beams. A few dowels were utilized primarily to enhance the connection and facilitate force transfer between the old concrete and the new cementitious grout. Before implementing the reinforcement and constructing the jacket, careful surface preparation was conducted to establish a robust bond between the original concrete and the new retrofitting layer. This preparation included the utilization of a chisel and a hammer to remove damaged and spalled concrete sections. In continuation, a sharpener and an electric concrete breaker were used to clean and rough the concrete surface. Moreover, abrasive sandblasting was utilized to remove rubble, yielding a concrete substrate that exhibited appropriate smoothness, roughness, and uniformity, as depicted in Figure 2.