Combining fiber-reinforced concrete with traditional reinforcement in tunnel linings

New procedures to design cast-in-situ steel fiber reinforced concrete (SFRC) tunnel linings are briefly presented in this paper. The ductile failure of such cement-based structures is ensured by adding a suitable amount of steel fibers to ordinary steel bars. The capability of SFRC to carry tensile stresses, also in the presence of cracks, allows designers to reduce the minimum area of ordinary steel reinforcement, generally computed in compliance with American or European code requirements. In the serviceability stage, to evaluate crack widths more accurately, a suitable block model is introduced. This model is able to take into account the bridging effect of fibers, as well as the bond slip between steel bars and concrete in tension. The proposed approaches have been successfully applied to the design of tunnel linings in Italy.