Synergistic effects of hybrid macro basalt fibers and micro fibers on the mechanical properties of UHPC

This study aims to optimize the mechanical properties of ultra-high performance concrete (UHPC) reinforced with macro basalt fibers (MBFs). Six types of fibers, containing micro fibers and macro fibers, were employed to reinforce UHPC by mono or hybrid method. The micro fibers contained micro basalt fiber, polypropylene fiber, and polyvinyl alcohol fiber corresponding to the modulus of 90, 3.5, and 41 GPa, respectively. The macro fibers contained MBF, macro polypropylene fiber, and polyvinyl alcohol fiber corresponding to the modulus of 43, 2.9, and 23 GPa, respectively. The flowability, and compressive and flexural behavior of UHPC were tested and analyzed. The micro fibers with high modulus effectively restricted microcracks owing to the dense fiber distribution and strong restraint, thereby significantly enhanced the mechanical properties of UHPC before cracking; thus, UHPC with 0.3% micro basalt fiber showed the highest compressive and flexural strength of 132.6 and 26.10 MPa. The macro fibers showed pullout failure and consumed energy during fiber pullout process, leading to a ductile failure and enhancement in mechanical properties of UHPC after cracking. UHPC with 3% MBFs had the highest compressive, flexural first-cracking and post-cracking strength of 151.8, 24.97, and 26.32 MPa, owing to the great energy consumption, low damage to fiber–matrix interface and strong macrocrack resistance supported by MBFs. For UHPC with hybrid fibers, UHPC with 3% MBFs and 0.3% micro fibers had the best comprehensive performance, corresponding to the flexural first-cracking and post-cracking strength of 27.95 and 28.01 MPa. It was because that MBF and micro basalt fibers with proper content, which had the highest modulus, synergistically limited microcracks and macrocracks before and after UHPC cracked. The principle, choosing the fiber combination with high modulus and proper content, applies to the improvement of mechanical properties of UHPC with different mixture in practice.