## 8 Practical Recommendations  
Based on the comprehensive review of hybrid fiber UHPC, the following practical recommendations are proposed to guide both research and application. 1. **Optimize the mixture and fiber distribution. ** Optimize the granular composition of the UHPC mixes to ensure a uniform distribution of the mixed fibers while maintaining self-compacting workability. It is recommended that various cements, mineral fillers, fine powders, and even nanomaterials be explored to improve the flow of the mix and the bond between the fibers and the matrix. A key goal is to reduce the gap between the tensile and compressive strength of UHPC. A balanced matrix can better utilize fiber reinforcement, maximizing the structural performance of the composite. 2. **Enhance fiber-matrix bonding. ** Focus on improving the bond properties and efficacy of each fiber type in hybrid combinations. This includes investigating new fiber materials and shapes (different polymers, carbon, glass, etc. ), as well as specialized fiber geometries or surface treatments (e.g., hooked ends, crimping) that anchor fibers more effectively. Optimizing fiber-matrix adhesion will lead to higher tensile strength and toughness. When designing hybrids, use multiple fiber types strategically: longer or thicker fibers (especially steel fibers) should primarily boost flexural and post-cracking load capacity, whereas shorter or thinner fibers (e.g., micro polypropylene or PE fibers) should be included to increase tensile strength, crack resistance, impact resistance, and fatigue performance. It is important to determine the optimal fiber content and the best long-to-short fiber ratio for each application. This is especially true for large structural elements under cyclic or impact loads, as fiber efficiency observed in small lab samples may vary at the structural scale. 3. **Ensure long-term durability. ** Conduct long-term durability studies for UHPC with hybrid fibers to fully assess its performance in harsh environments. Evaluate resistance of the material to deterioration mechanisms such as freeze-thaw cycles, chemical attack, penetration of chloride (from deicing salts or seawater), and exposure to sulfate. Ensuring that hybrid fiber UHPC can maintain its integrity under these conditions is critical for real-world use. Different fiber types have distinct effects on durability-for example, incorporating synthetic fibers or basalt fibers can enhance resistance to chloride ingress, sulfate attack, and even fire, more effectively than steel fibers alone. Therefore, further research and field trials should identify optimal fiber combinations for structures exposed to aggressive environments to leverage fibers that improve durability while still providing mechanical strength. 4. **Develop standardized design models. ** Update and refine the empirical formulas and constitutive models used for UHPC to account for hybrid fiber reinforcement. Existing predictive models are often based on specific test results and do not generalize well due to variations in test methods and equipment. Extensive testing and data collection should be performed to develop standardized models and design guidelines that reliably capture the behavior of hybrid fiber UHPC. These models should be verified across different mix compositions and loading conditions, ensuring engineers can predict performance without needing case-by-case experimentation. 5. **Study dynamic and high-temperature behavior. ** Address the current lack of data on the behavior of hybrid fiber UHPC under dynamic loading (impact, blast) and high temperature conditions. Specifically targeted studies are needed to observe crack propagation and failure mechanisms at high strain rates and elevated temperatures. High-speed imaging, detailed fracture analysis (e.g., via scanning electron microscopy or X-ray CT), and controlled fire tests can shed light on how different fiber types contribute to toughness in extreme conditions.