Flexural performance and microstructural characterization of seawater sea sand high performance concrete reinforced with sisal, polypropylene, and hybrid fibers

This study examines the flexural behavior of seawater sea sand high-performance concrete (SWSSHPC) reinforced with polypropylene fibers (PPF), sisal fibers (SSF), and their hybrid combination. Fiber type, volume fraction (0.5%, 1.0%, 1.5%), and length (5, 10, 15 mm) were evaluated in a unified experimental program. Compressive strength, three-point bending, flowability, and microstructure were assessed using FTIR, XRD, and SEM. Fiber addition increased flexural strength by about 16–79% compared with the plain mix. The highest gain, 79.3%, was achieved with SSF at 0.5% and 5 mm, while the best PPF mix reached 59.9% at 1.0% and 5 mm. Hybrid fibers produced more stable flexural performance and the greatest deformation capacity. Compressive strength showed a weaker response, with gains up to 23.6%. Microstructural analysis showed stronger SSF matrix interaction than PPF matrix. A regression model predicted flexural strength accurately (R2 = 0.915).