Influence of steel fibers corroded through multiple microcracks on the tensile behavior of ultra-high-performance concrete

Abstract This study aims to investigate the effects of pre-formed microcrack properties and immersion duration in a 3.5% NaCl solution on the corrosion degree of steel fibers in ultra-high-performance concrete (UHPC). Two types of steel fiber, i.e., straight and twisted, two pre-strain levels causing different microcrack properties, i.e., 0.45% and 0.6%, and four immersion durations of 0, 4, 10, and 20 weeks were investigated. Energy-dispersive X-ray spectroscopy (EDX) analysis was conducted using scanning electron microscope images for a quantitative evaluation of the degree of surface corrosion. Test results indicated that steel fibers embedded in the multi-cracked UHPC are oxidized due to permeation of the NaCl solution and longer immersion durations lead to a higher corrosion degree in general. Better tensile performance was achieved if both the straight and twisted steel fibers were moderately corroded. The twisted-fiber-reinforced UHPC was more susceptible to corrosion than the straight-fiber-reinforced UHPC, indicated by the earlier deterioration in tensile performance and lower tensile parameter ratios. Given identical immersion durations, the composites with fewer and smaller microcracks provided better tensile performance due to the moderately corroded steel fibers. Steel fiber corrosion influences the energy absorption capacity more significantly than the tensile strength, irrespective of the fiber type and microcrack property.