Performance of hybrid fiber reinforced concrete (HyFRC) in tunnel segment: Full-scale tests under thrust and flexural loads

Macro fibers have been demonstrated to be superior as a partial replacement for steel rebar by effectively bridging structural cracks. The advantages of macro fibers make them highly suitable for substituting steel rebar in controlling crack widths in tunnel segments. In this study, the performance of a full-scale reinforced concrete (RC) segment with a steel reinforcement ratio of 0.45 %, incorporating hybrid macro fibers (RC-HyFRC), is evaluated and compared to that of a conventional reinforced concrete segment with a steel ratio of 0.64 %, under both flexural and thrust loading conditions. A reduction of up to 34.72 % in the amount of steel rebar is achieved in the RC-HyFRC segment by incorporating 30 kg/m³ of macro steel fibers (SF) and 4 kg/m³ of macro polypropylene fibers (PPF). In addition, the digital image correlation (DIC) technique is employed to analyze crack patterns and widening during the flexural test. The analysis of the test results indicates that after replacing the rebars with macro hybrid fibers, the RC-HyFRC segments demonstrate comparable structural performance to RC segments in terms of load bearing capacity. Furthermore, the crack control performance of RC-HyFRC segments during the service stage is enhanced compared to RC segments. Thrust load test shows that after substituting steel rebar with macro hybrid fibers, the macro hybrid fibers in the RC-HyFRC segment effectively limit the opening of bursting cracks compared to the RC segment. The results above quantitatively confirm that, for a RC segment with a steel ratio of 0.64 %, a partial substitution of steel rebar with macro hybrid fibers can be realized in RC-HyFRC segments with a much low steel ratio of 0.45 %. These findings also demonstrate that macro hybrid fibers significantly enhance flexural crack resistance, even in segments with reinforcement ratios exceeding those previously investigated.