Precast segments under TBM hydraulic jacks: Experimental investigation on the local splitting behavior

The design process of segmental concrete linings in ground conditions generally refers to standard load cases of de-molding, storage, embedded ground condition and grouting process. Nevertheless, the application of the Tunnel Boring Machine (TBM) thrust jack forces is a crucial temporary loading condition during construction, which may govern the design procedure as well as the other stages. Tunnel segments were traditionally reinforced with conventional rebars in order to resist the tensile stresses at both Serviceability (SLS) and Ultimate Limit States (ULS). In the two last decades, Fiber Reinforced Concrete (FRC) has been also used in several precast tunnel segments with or without conventional rebars. For structural purposes, steel fibers are generally used, even though some types of structural macro-synthetic fibers, which are able to impart significant toughness and ductility to concrete, have been recently introduced in the market. For these reasons, an experimental program aimed to investigate the local splitting behavior in the segment regions under the TBM hydraulic jacks was carried out at the University of Brescia. Tests on concrete prisms (with or without fibrous reinforcement) under Line Load (LL) or Point Load (PL) configurations were carried out in order to evaluate the beneficial effects of polypropylene (PP) fibers in controlling typical splitting cracks occurring under the jack loads, which represent one of the most severe loading conditions for tunnel segments. Experimental results show the feasibility of using polypropylene fibers in tunnel segments, since they significantly enhance both the splitting bearing capacity and the ductility. Fibers lead to a stable development of the splitting crack, which allows a redistribution of stresses after cracking. However, the effectiveness of fibers is influenced by the casting direction, which leads to a different fiber orientation. When the latter is favorable (fibers expected to be mainly transversal to the splitting crack) the bearing capacity is higher than in case of an unfavorable casting direction (fibers expected to be mainly parallel to the splitting crack)