Enhancement of tensile performance and cracking behavior of ultra-high-performance alkali-activated concrete using engineered steel fibers

This study investigates the influence of surface-treated steel fibers on the tensile performance of ultra-high-performance alkali-activated concrete (UHPAAC) at fiber volume fractions of 2.0 % and 1.5 %. Three distinct surface treatments were applied to the steel fibers: EDTA-electrolyte treatment, CaCO 3 coating, and nano-SiO 2 coating. Fiber morphology and tensile characteristics of UHPAAC were evaluated using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), direct tensile tests, and digital image correlation (DIC) analysis. At a 2.0 vol% fiber content, the surface treatments significantly enhanced UHPAAC tensile strength (by 7.1 %–12.6 %) and strain energy density (by 7.1 %–25.7 %) compared to pristine steel fibers. Even at a reduced fiber content (1.5 vol%), UHPAAC containing surface-treated fibers showed improved tensile strength (23.3 %–38.6 %), strain capacity (190.6 %–246.1 %), and strain energy density (247.1 %–373.7 %) relative to conventional reinforcement with 2.0 vol% short straight steel fibers. Additionally, UHPAAC with 1.5 vol% surface-treated fibers exhibited up to a 31.1 % increase in strain energy density compared to UHPAAC reinforced with 2.0 vol% pristine long steel fibers even at reduced fiber content. DIC analysis revealed that UHPAAC with nano-SiO 2 fibers provided the most effective crack width control, reducing maximum crack width by 67 % at 2.0 vol% fiber content compared to conventional fiber reinforcement. Notably, even at a reduced fiber content of 1.5 vol%, surface-treated fibers maintained or enhanced crack control performance, enabling a fiber content reduction of 0.5 % while simultaneously improving durability. These results confirm the potential of surface-treated steel fibers as effective alternatives to conventional reinforcement, enhancing both tensile performance and crack control in UHPAAC. • Surface-treated steel fibers significantly improved the tensile performance of UHPAAC with 2 % SS fibers. • UHPAAC with 1.5 % surface-treated fibers shows 31.1 % higher strain energy density than 2 % pristine fibers. • Nano-SiO₂-coated fibers (2 %) reduce maximum crack width by 67 % at 0.2 % tensile strain. • 1.5 % CaCO₃-coated fibers increase critical tensile strain for 50 µm cracks by 252 % versus 2 % pristine. • 2 % nano-SiO₂-coated fibers improve crack counts by 223 % and reduce average crack width by 82 %.