## 4 Conclusions  
The incorporation of silica sand into epoxy resin composites significantly improved the mechanical and adhesion properties, highlighting its potential for enhancing CFRP-concrete bonding in structural applications. The composite material comprises a matrix enveloping a filler, with each component fulfilling a distinct role in the material's mechanical behavior. The matrix acts as a binder, bearing all stresses, distributing forces, and safeguarding the reinforcement. Meanwhile, the filler introduces novel properties to the matrix. The interaction between the matrix and the filler, as demonstrated through the enhanced mechanical and adhesion properties, underscores the effectiveness of silica sand in the composite structure. The detailed analysis of these components reveals the following key findings:  
* The addition of silica sand to the epoxy resin resulted in a noticeable increase in the density of the composite material. This aligns with findings from other studies that highlight the impact of high-density fillers on composite materials, thereby enhancing their structural integrity and resistance to deformation. * Tensile tests showed that the optimal addition of silica sand (10%) to the epoxy resin significantly improved its tensile strength and elastic modulus. The tensile strength increased from 29.47 MPa for unfilled resin to 35.52 MPa, and the elastic modulus rose from 4.38 GPa to 5.83 GPa. These enhancements can be attributed to the effective mechanical interlocking and improved load transfer provided by the silica particles, which confirms previous research on mineral fillers in polymer composites. * Adhesion tests between CFRP and the resin composites revealed a substantial increase in the pull-out force, from 6.37 kN in the unfilled resin to 8.97 kN in the resin with 10% silica sand. This improvement highlights the effectiveness of silica fillers in enhancing the bond strength by increasing surface roughness and providing better mechanical interlocking, which is critical for the long-term durability of repaired structures. * This study identified that a 10% silica sand content provided the best balance between strength and ductility, improving the mechanical properties and the bond strength without significantly compromising the material's flexibility. Beyond this percentage, the material exhibited increased brittleness, which could reduce its effectiveness in certain applications. * The direct pull-out tests on concrete blocks demonstrated that the silica-filled resin significantly improved the bond strength between CFRP and concrete, increasing the maximum pull-out force from 14.21 kN to 18.79 kN. This finding supports the use of silica-filled epoxy resin as a cost-effective and efficient solution for enhancing the performance of CFRP-concrete bonds in structural applications. * The inclusion of silica sand shifted the failure mode from adhesive debonding to concrete crushing, indicating a stronger bond and better load distribution across the composite material. This shift is critical for ensuring the durability and reliability of reinforced concrete structures subjected to high loads. * The adopted analytical models align closely with the experimentally observed bond stress-slip relationship for CFRP lamella. While the innovative use of silica-filled epoxy resin composites offers a promising avenue for advancing the performance and sustainability of CFRP-concrete bonding systems in civil engineering, future research should explore:  
* The long-term durability of these composites under various environmental conditions, such as moisture, temperature fluctuations, and chemical exposure. * The optimization of filler particle size and distribution to further enhance the mechanical properties and adhesion strength. * The application of other mineral fillers and their potential synergistic effects with silica sand to develop more advanced composite materials.