Mechanical performance of concrete with steel–carbon fibre hybridisation

Hybrid-fibre reinforcement is an effective approach for enhancing the mechanical performance of concrete by combining the complementary characteristics of different fibre types. While previous studies have mainly focused on high-strength or high-performance concrete, limited research has examined the hybridisation of steel fibres (SF) and carbon fibres (CF) in medium-strength concrete. This study experimentally investigates the effects of hybrid hooked-end SF (0–1.0% by volume) and CF (0–0.3% by volume) on the workability, ultrasonic pulse velocity (UPV) and mechanical properties of medium-strength concrete. Ten concrete mixtures were prepared and tested for slump, UPV, compressive strength split tensile strength and flexural strength after 28 days of curing. The results showed that hybrid-fibre-reinforced concrete significantly improved mechanical performance, although workability decreased with increasing fibre content. The maximum compressive strength improvement of 43.8% was achieved with 1.0% SF and 0.1% CF, while split tensile and flexural strengths increased by up to 53.2% and 70.4%, respectively, compared with control concrete. UPV results indicated good internal concrete quality. A regression model for flexural strength, validated using leave-one-out cross-validation, demonstrated satisfactory predictive capability. The findings highlight the potential of SF–CF hybridisation for improving medium-strength concrete performance and guiding the development of crack-resistant cementitious composites.