Mechanical properties of concrete reinforced with reused tyre steel fibres

This study presents a design-oriented experimental investigation on the use of reused tyre steel fibres (RTSF) as structural reinforcement in fibre-reinforced concrete, addressing both mechanical performance and environmental considerations. Two RTSF types with nominal lengths of 20 mm (FX-20) and 25 mm (FX-25) were evaluated at dosages of 15, 30 and 45 kg/m³ in three concrete mixes representative of common structural applications: precast segment lining, concrete pavements and sprayed concrete. The experimental programme included fresh-state characterisation (workability), hardened-state properties (compressive strength and residual flexural strength), and non-destructive assessment of fibre content and orientation using an inductive method. Residual flexural strengths obtained from notched beam and Barcelona tests were subsequently translated into constitutive stress-crack width and stress-strain relationships compatible with the fib Model Code 2020 and the forthcoming Eurocode 2 Annex L. The results show that FX-20 fibres provide a more robust contribution to compressive strength and cracking resistance, while FX-25 fibres enhance post-cracking behaviour and crack control due to their greater anchorage capacity. Inductive measurements revealed that effective fibre content, dispersion and orientation jointly govern the variability of residual strengths, supporting their use as complementary parameters for interpreting mechanical performance. Design-oriented verification examples demonstrated the direct applicability of the calibrated constitutive laws to typical structural elements. Finally, the environmental performance of RTSF was quantified using Environmental Product Declarations (EPDs) and benchmarked against industrial steel fibre EPDs, showing a substantially lower cradle-to-gate global warming potential for the recycled fibres. Overall, the study demonstrates that RTSF can be effectively used as structural reinforcement within current design frameworks, offering a technically sound and environmentally favourable alternative to conventional steel fibres. • Two RTSF types and dosages were assessed in three structural concretes: segment lining, pavement and sprayed concrete. • RTSF improved residual flexural strength, ductility and toughness in all concretes tested. • RTSF are a viable substitute for industrial steel fibres in structural fibre-reinforced concrete. • RTSF use supports circular economy strategies and lowers the concrete sector carbon footprint.