Enhanced tensile performance of ultra-high-performance alkali-activated concrete using surface roughened steel fibers

To improve the mechanical performance of ultra-high-performance alkali-activated concrete (UHP-AAC), the surface of steel fiber was modified using an electrolyte solution containing ethylenediaminetetraacetic acid (EDTA). The longer the steel fiber was exposed to the EDTA-electrolyte solution, the more the longitudinal peeling of the steel fiber surface was induced and the surface roughness increased. By exposing the specimens to the solution for up to 6 h, the highest fiber bond strength (11.96 MPa) and a maximum tensile strength of UHPAAC (14.7 MPa) were obtained. The mechanical properties of UHP-AAC were also investigated using conventional long straight and twisted steel fibers. The increase in bond strength due to the triangular cross-sectional shape and untwisting torque of the twisted fiber had an overall positive effect on the mechanical properties of UHP-AAC. However, the best tensile performance of UHP-AAC, in terms of tensile strength and energy absorption capacity, was obtained when the straight steel fibers surface-refined by the EDTA-electrolyte solution for 6 h were adopted. Based on the Pearson correlation coefficient, the Weibull distribution was applied as a crack width prediction model. The results showed that the median microcrack widths formed in UHP-AAC were marginally influenced by the surface treatment using EDTA-electrolyte solution. However, the longer straight and twisted steel fibers produced wider microcracks than their counterparts. The greatly increased tensile strain capacity of UHP-AAC by using the surface-refined steel fibers was thus caused by the formation of more microcracks rather than the increase in the microcrack width.