Mechanical and Post-Cracking Performance of Recycled and Commercial Steel Fiber-Reinforced Concrete

Recycled steel fibers (RFs) recovered from end-of-life tires (ELTs) are a potential alternative to manufactured steel fibers in concrete, but their irregular geometry may limit their effectiveness. This study compared RF and commercial hooked-end steel fibers (CFs) in ordinary Portland cement concrete produced at water-to-cement ratios (w/c) of 0.40, 0.50, and 0.60 with fiber dosages of 15, 30, and 45 kg/m3. Workability, compressive strength, flexural strength, residual flexural strengths (fR1 and fR3), and toughness were determined, and a multi-response desirability approach combining compressive strength, fR3, and toughness was applied. Both fiber types improved the post-cracking response; however, CF provided higher residual strengths and toughness, whereas RF reduced workability more and showed a greater loss of residual capacity between fR1 and fR3. Compressive strength was governed mainly by w/c rather than fiber dosage. The highest overall desirability was obtained with 45 kg/m3 of CF and a w/c of 0.40, with a geometric mean (D) of 0.653, while for RF the maximum was obtained with 30 kg/m3 and w/c = 0.40 (D = 0.285). RF effectiveness depends on matrix quality and dosage, and RF should be selected according to the required post-cracking performance rather than as a direct replacement for CF.