Experimental investigation on the local splitting behavior under TBM hydraulic jacks

Tunnel segments are generally reinforced with conventional rebars in order to resist the tensile stresses both at Serviceability (SLS) and Ultimate Limit States (ULS). In the two last decades, Fiber Reinforced Concrete (FRC) was also used in several precast tunnel segments in combination or not with conventional rebars. For structural purposes, Steel Fiber Reinforced Concrete (SFRC) is generally utilized, even though some types of structural polypropylene fibers have been recently introduced in the market. Moreover, polypropylene fibers are now able to impart significant toughness and ductility to concrete. The proposed paper concerns an experimental study aimed to evaluate the opportunities offered by Polypropylene Fiber Reinforced Concrete (PFRC) in precast segments for use in tunnel linings. 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. For these reasons, an experimental campaign aimed to investigate the local splitting behavior which occurs in the segment regions under the TBM hydraulic jacks was arranged at University of Brescia. Tests on concrete prisms (with or without fibrous reinforcement) were carried out in order to evaluate the beneficial effects of polypropylene fibers in controlling typical splitting cracks occurring in presence of high-concentrated loads. Due to the rather limited use of PFRC in structural applications, the experimental campaign presented herein is of paramount interest to the scientific community, since the results show the feasibility of using polypropylene fibers in tunnel segments.