Experimental study on the application of a hybrid form combining straight and deformed steel fibers in ultra-high-performance concrete under uniaxial and biaxial flexural stress conditions

This study investigates the uniaxial and biaxial flexural performance of ultra-high-performance fiber-reinforced concrete (UHPFRC) using four types of steel fibers at a fixed 2% fiber content. The primary objective is to evaluate the impact of deformed steel fibers in hybrid reinforcement systems compared to straight steel fibers. The results reveal that hybrid-reinforced UHPFRC generally exhibits higher compressive strengths, with a minimal variation of approximately 10% across specimens. Increasing beam thickness enhances flexural strength but reduces deflection capacity in uniaxial UHPFRC beams. Higher incorporation rates of hooked-end (H) steel fibers decrease the flexural strength and toughness of UHPFRC, particularly at larger deflections. In contrast, twisted steel fibers (T3 and T6) maintain toughness up to incorporation rates of 1% and 1.5%, respectively, despite moderate reductions in flexural strength. Under biaxial stress conditions, hybrid-reinforced UHPFRC with T3 and T6 fibers demonstrates greater flexural strength and toughness compared to specimens reinforced with straight fibers or H fibers, highlighting their effectiveness in hybrid configurations. Lastly, UHPFRC panels outperform beams in normalized deflection capacity, microcrack development, and flexural strength. • Compressive strength of hybrid UHPFRC is less sensitive to fiber type than its flexural performance. • More hooked fibers decrease flexural strength and toughness of UHPFRC beams at large deflections. • Twisted fibers maintain toughness better than hooked fibers, especially in hybrid biaxial conditions. • UHPFRC panels excel in deflection capacity, microcrack resistance, and flexural strength over beams.