Tunnel linings made by precast concrete segments

Tunnel segments are generally reinforced with conventional rebars that are placed in r.c. elements to resist the tensile stresses both at Serviceability (SLS) and Ultimate (ULS) Limit States. As far as the service conditions are concerned, in recent years durability issues have became of paramount importance. Durability design gen-erally requires rebars protection against corrosion that can be achieved by reducing concrete porosity and crack width. The former can be obtained by using a matrix with a low water/cement ratio while the latter can be achieved by using a diffused reinforcement; to this aim, fiber reinforcement may represent an optimal solu-tion since it is diffused in the concrete matrix. As far as the ultimate conditions are concerned, localized stresses (due to bending actions) are better contrasted by localized reinforcement (rebars) while diffused stresses are better resisted by diffused reinforcement as fibers. For the reasons mentioned above, since both localized and diffused stresses are present, an optimized reinforcement for tunnel linings can be obtained by using a combination of conventional (rebars) and fiber reinforcement. In this Section, the results of a research program on concrete tunnel segments are presented. After the experimental determination of the material properties, the structural behaviour of the lining was simulated with FE nonlinear analyses based on fracture mechanics. Some of the main issues concerning the design process of segmental tunnel linings are pointed out and a the attention is devoted to the beneficial effects of fiber reinforcement. The load cases mainly concern the force exerted by TBM pressure on the ring during the excavation process (the so-called thrust jack phase).