Engineering properties and sustainability assessment of UHPFRC incorporating recycled tyre steel and polymer fibers as substitutes for manufactured fibers

This study aims at achieving eco-efficient ultra-high performance fiber reinforced concrete (UHPFRC) by substituting manufactured copper-plated steel fiber (CPSF) with recycled tyre steel fiber (RTSF), and polypropylene fiber (PPF) with recycled tyre polymer fiber (RTPF). A total of sixteen UHPFRC mixes with varying substitution rates from 0 % to 75 % were examined regarding their workability, compressive strength, flexural performance, and dry shrinkage, so as to explore their development characteristics and reinforcement mechanisms. The incorporation of RTSF and RTPF led to a decrease in workability and compressive strength, which can be attributed to the shape, surface characteristics, size, length, and residual rubber of RTSF and RTPF. When the substitution rates of RTSF and RTPF exceeded 50 %, the compressive strength exhibited a significant reduction falling below 120 MPa. The experimental investigation of flexural performance demonstrated that the bridging ability of RTSF delayed the bending fracture process and peak load, as well as improved toughening, while RTPF weakened the effects to a certain extent. Besides, a substitution rate of less than 75 % effectively mitigated dry shrinkage, while the substitution of RTPF exacerbated this phenomenon. Finally, the carbon emission performance index, cost performance index, and desirability function were introduced to assess the environmental and economic benefits, and optimal fiber substitution ratios were identified as 50 %RTSF for single-type fiber substitution and a combination of 50 % RTSF and 25 % RTPF for dual-type fiber substitution.