A Study on the Bending Stiffness of Reinforced Concrete Tunnel Segments with Added Steel Fibers

In recent years, steel-fiber-reinforced concrete (SFRC) has been increasingly applied in shield tunnel engineering. However, most research on SFRC segments focuses on the load-bearing capacity, while the tunnel deformation is an equally critical indicator that decides if the tunnel can operate safely during service conditions. Therefore, it is essential to also study the stiffness variations in SFRC segments, which is closely connected to the serviceability limit state (SLS). To investigate the influence of SFRC on segment stiffness, full-scale four-point bending tests and analytical calculations are carried out on both traditional reinforced concrete (RC) segments and SFRC segments with rebars. A C55 plain concrete is used in the RC segment, and for SFRC, 30 kg/m³ steel fibers are added. The segment stiffnesses are calculated and analyzed, and compared between test and analytical results. This study shows that the addition of steel fibers to traditional reinforced concrete segments can enhance the bending stiffness. This effect becomes apparent only after the segments crack. Initially, the effect is strong but then becomes weaker, with increasing load. The added 30 kg/m³ steel fibers generate a maximum of 33% in stiffness increment for a segment with 2.1% reinforcement. Further analysis indicates that the transfer of stresses in the cracked SFRC results in a stiffness improvement, but after cracking, the contribution of the reinforcement to the flexural resistance increases while the contribution of the SFRC gradually decreases. Thus, the effect is weak at high load levels. This paper contributes to a better understanding of the effect of SFRC on the stiffness of segments, as relevant for SLS requirements.