Investigation on the flexural behaviour and crack propagation of hybrid steel fibre reinforced concrete with a low fibre content for tunnel structures

Fibre-reinforced concrete (FRC) has been suggested as a feasible substitute for steel reinforcement in tunnel lining segments. To enhance the flexural strength and toughness of tunnel linings while retaining costeffectiveness, two scales of steel fibres were mixed with the reinforcement. Four-point bending tests were performed and the cracks were captured using the digital image correlation (DIC) method. The results showed that performance and toughness of the concrete in the pre-peak stage improved with the hybrid steel fibres. Moreover, the DIC measurements showed that for the same crack width at higher replacement rate of the short straight steel fibre (SSSF) was associated with at greater length of the primary cracks. However, SSSF can prevent the concrete cracks from spalling. Furthermore, a quantitative calculation method for the synergistic effect in hybrid steel fiber reinforced concrete (HSFRC) with a certain proportion is proposed. Finally, based on a comprehensive fibre reinforcement index, analytical equations were established to predict the flexural performance and toughness of the HSFRC by considering the fibre aspect ratio, content, and tensile strength. This study can enhance the understanding of the mechanism of low-content hybrid fibre reinforcement and assist in more accurate evaluation and optimization of the HSFRC performance during engineering design.