Tensile behavior of UHPFRC with recycled steel fibers: Insights into fracture evolution and substitution feasibility

Recycled steel fibers (RSFs) recovered from end-of-life tires offer a promising route for reducing the environmental and economic burdens associated with ultra-high-performance fiber-reinforced cementitious composites (UHPFRC). However, their irregular geometry and surface characteristics raise concerns regarding tensile performance. Five UHPFRC mixtures were investigated, with manufactured steel fibers (MSFs) replaced by RSFs at 0%, 25%, 50%, 75%, and 100% on a steel-fiber mass basis. Direct tensile tests and digital image correlation (DIC) characterized the tensile response and crack evolution. Increasing the RSF replacement level progressively reduced peak tensile strength and tensile strain capacity, with pronounced deterioration beyond 50% replacement. Tensile responses and DIC observations indicated less extensive multiple cracking and earlier crack localization at higher RSF replacement levels. Crack-width distributions at peak tensile stress were skewed and well described by a log-normal model, with mean and maximum crack widths of 0.019–0.035 mm and 0.102–0.132 mm, respectively. A micromechanics-based model provided preliminary estimates of peak tensile strength under prescribed assumptions regarding fiber orientation and spatial homogeneity. Full RSF replacement reduced material cost and embodied carbon by 33.98% and 41.22%, respectively, relative to the MSF-only reference mixture. Multi-criteria analysis identified 50% RSF replacement as providing the best overall balance among the investigated mixtures. It reduced embodied carbon and material cost by 20.61% and 16.99%, respectively, while retaining 70.80% of the reference peak tensile strength and 89.96% of its post-peak fracture energy. These findings demonstrate the feasibility of partial RSF substitution in UHPFRC while clarifying its mechanical limitations and sustainability trade-offs.