5570 - ON THE DESIGN OF STEEL FIBER REINFORCED CONCRETE TUNNEL LINING SEGMENTS

The structural applications of Steel Fiber Reinforced Concrete (SFRC) have recently been increasing due to the improvement of material properties, such as in the material toughness under tension and durability. However, because the behavior of such structures is fairly different from conventional Reinforced Concrete (RC) structures the classic design method should be critically reviewed considering the post-cracking resistant mechanism. This work focuses on the application of SFRC in tunnel lining segments, as an alternative to conventional RC segments. Based on an accurate experimental investigation on full scale specimens, a smeared crack model, which implements the Hilleborg’s criteria was used. In order to assess the SFRC reliability, a wide population of tensile tests on cylinders drilled out from a reference full scale specimen was carried out. The tensile constitutive relation, which is the fundamental property for SFRC materials, was chosen on a probabilistic fashion accounting for the actual dispersions of fiber in the tunnel segment due to the casting procedure. According to the finite element analysis, the structural response of such structures was found to be very sensitive to the fiber dispersion. Finally, the AFREM recommendation for SFRC materials and the simplified ‘struts and ties’ model were evaluated by means of a parametric analysis. 1. INTRODUCTON The steel fiber reinforcement not only improves the toughness material, the impact and the fatigue resistance of concrete, but it also increases the material resistance to cracking and, hence to water and chloride ingress with significant improvement in durability of concrete structures. Therefore, the use of SFRC in tunnel structures represents an attractive technical solution with respect to the conventional steel reinforcement, because it reduces both the labor costs (e.g. due to the placement of the conventional steel bars) and the construction costs (e.g. forming and storage of classical reinforcement frames, risks of spalling during transportation and laying). For these reasons several European pilot projects have been undertaken to assess the reliability of SFRC in tunnel structures, as examples the 2 Heinenoord tunnel (Netherlands) [1] and the two SFRC tunnel linings in Essen and Ruhr-Region (Germany) [2]. These preliminary SFRC examples exhibited reduced crack development and a lower risk of leakage and the falling off of concrete flakes, which often represents a concrete issue for tunnel road. Furthermore, the steel fiber reinforced details, such as the shear tooth of ring joints, was found to exhibit a higher ductility under localized force. Due to the current lack of design rules for Steel Fibers Reinforced Concrete (SFRC) structures, engineers have usually designed SFRC tunnel lining segments by adopting the same rules that are valid for concrete with conventional reinforcement. However, the post-cracking behavior of SFRC structure is dramatically different from conventional RC structures. After the cracking onset, SFRC structures exhibit a markedly non-linear behavior to the extend that strain softening behavior may occur with the fiber volume fraction frequently used in practice (Vf = 0.3 0.6%; Figure 1a). On the other hand, the conventional reinforcement keeps an almost linear behavior (with a lower stiffness with respect to the uncracked stage) until the reinforcement yields (Figure 1b). It is possible to reproduce the actual behavior of SFRC structures from the