Optimized reinforcement for precast tunnel segments with macro-synthetic fibers

The use of fiber reinforced concrete (FRC) in tunnel linings, with or without conventional rebars, has increased in the two last decades, especially in segmental linings. Nowadays there is a growing interest in the scientific community on macro-synthetic fibers for use in underground structures. Within this framework, the present study investigates the possibility of using macro-synthetic fiber reinforcement in precast tunnel segments in a possible combination with conventional rebars. An experimental program based on 3 Point Bending Tests was carried out on FRCs characterized by different fiber contents in order to assess their post-cracking nominal residual stresses. The corresponding stress vs. crack opening laws, representative of the FRCs investigated, were calculated through inverse analysis procedure. Then, a typical tunnel lining having small diameter was adopted as reference to optimize the reinforcement solution (macro-synthetic fibers and conventional rebars, i.e. hybrid solution). Particular attention was devoted to the TBM thrust phase, in which high-concentrated forces are introduced in the linings.