## 3 Result and Discussion
In the adhesion force-displacement curves, the ascending section is followed by a descending one. Due to the sand addition, the resin-reinforcement contact surface in block 2 is rougher than in block 1. The maximum pull-out strength of block 2 is higher than block 1 (i.e., 18.79 kN vs. 14.2 kN), demonstrating an increase in the bond strength of approximately 32%. In block 1, the failure mode is sliding between CFRP reinforcement and the sealing material without cracks along the anchor length. Conversely, block 2 experiences failure due to concrete crushing surrounded by etching, without evidence of sliding on the reinforcement. The shift in the failure mode suggests that the resin with silica sand filler provides better mechanical interlocking and stress distribution, reducing the likelihood of premature debonding and enhancing the overall structural integrity. The increased pull-out force and the reduced slip at the maximum load indicate a more robust and reliable bond between the CFRP and the concrete, which is critical for the long-term performance of repaired structures. The bond-slip behavior of CFRP and the residual strength of concrete are critically dependent on the mode of failure and the failure status. Initially, the bond between CFRP and concrete is dominated by the adhesion between the epoxy resin and the concrete, exhibiting a linear bond-slip relationship [35]. As the loading increases, micro-cracks form, introducing non-linearity due to enhanced mechanical interlocking from surface \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **F Max** & \multirow{2}{*}{**S Max Average (mm)**} & \multirow{2}{*}{**\(\sigma\) Max Average (MPa)**} & \multirow{2}{*}{**Pu (kN)**} \\  & **Average (kN)** & & & \\ \hline block 1 & 14.21 & 0.38 & 7.11 & 15.61 \\ \hline block 2 & 18.79 & 0.23 & 9.40 & 16.97 \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Direct pull-out results. Figure 10: Bond stress vs. loaded-end slip relations of tested specimens: (**a**) block 1 and (**b**) block 2. Figure 9: Resin–CFRP rupture type. roughness [36]. In the macro-cracking phase, bond stress significantly decreases, with failure modes including concrete shear failure (block 2) characterized by a sudden drop in the bond stress and interface debonding, marked by a gradual stress reduction (block 1). Surface treatment techniques and consistent quality control are essential to achieve the desired roughness levels and maximize the bond performance [37]. The roughness of both CFRP bars and the grooves significantly influences the bond behavior in NSM-CFRP systems by enhancing mechanical interlocking and adhesion [37]. Increased roughness at the CFRP-resin interface improves mechanical interlocking and frictional resistance, reducing slip and enhancing the load transfer efficiency [38]. Similarly, a rougher groove-resin interface increases mechanical interlocking and the adhesive bond strength between the epoxy resin and concrete, preventing debonding and ensuring secure anchorage [39]. The combined roughness of the CFRP bars and grooves creates a synergistic effect, optimizing the load distribution, reducing the risk of localized failures, and enhancing the long-term performance and durability of the NSM-CFRP system [40]. [PERSON] et al. [41] demonstrated that the highest shear stress values, around 18 MPa, occur with roughened surfaces, leading to concrete failure, whereas smoother surfaces result in bar-concrete interface failure with lower maximum shear stress of around 4 MPa. According to Equation (3), in this study, \(\alpha\) is 0.25 for block 2 and 0.23 for block 1. The predicted pullout force values (Pu) compared to the experimental pullout force (fmax) are in Table 13. The comparison between the experimental and theoretical pull-out forces confirmed the validity of the model for predicting the bond strength of the NSM CFRP system, highlighting the effectiveness of the silica-filled epoxy resin in enhancing the bond performance.