Mechanical properties of multi-scale mono/hybrid non-metallic fiber-reinforced ultra-high performance seawater sea-sand concrete

Ultra-high performance seawater sea-sand concrete (UHP-SSC) is a promising construction material with excellent mechanical and durability properties, addressing the deficiency of materials like fresh water and river sand. Nevertheless, the steel fibers that reinforce UHPC are unsuitable for UHP-SSC, resulting from the corrosion problem. In this regard, this study investigated the impacts of multi-scale (micro and macro) mono and hybrid non-metallic fiber reinforcements on the mechanical properties of UHP-SSC. The experiments, including flowability, compression, and third-point bending, were carried out. The surface morphologies and microstructures of UHP-SSC were characterized by scanning electron microscopy and low-field nuclear magnetic resonance. The mono-fiber comprised three micro-fiber types: polyvinyl alcohol fiber (PVAF), carbon fiber (cF), and basalt fiber (bF); along with two macro-fiber types: polypropylene fiber (PPF) and basalt fiber reinforced polymer fiber (BF), while the hybrid fiber reinforcement was composed of different hybridization of foregoing fibers. The results indicated that the compressive and flexural properties of plain UHP-SSC were enhanced by adding micro- or macro-fiber. The enhancement is positively related to the elastic modulus of fibers. Notably, utilizing macro-fiber can effectively improve the flexural toughness, particularly for BF, which induces strong strain-hardening behavior. Microscopic and microstructure observations confirmed the strength and toughness evolution and illustrated the relevant mechanism. In the case of fiber hybridization, the combination of micro- and macro-fiber has a positive synergy effect, and the mixture with bF and BF exhibits the utmost synergistic influence when the volume of bF is controlled within 0.4 vol%.