Partial Safety Factor for the Residual Flexural Strength of FRC Precast Concrete Segments

The use of fibre reinforced concrete (FRC) to produce segmental linings of TBM-constructed tunnels is an increasing tendency. So far, more than 50 tunnels have been constructed with this structural material, in some of these even using solely fibres as reinforcement. Moreover, several design guidelines (e.g., fib Model Code 2010) already include the FRC as structural material. There also exist specific guidelines for the design of FRC precast segment linings (e.g., ITAtech Report/7-15 and ACI 544.7R-16). These guidelines deal with the design of FRC considering the traditional limit state safety format. Therefore, partial safety factors for both the loads (gamma(L)) and material strengths (gamma(M)) must be considered. In particular, the magnitude of gamma(M) considered for compressive and tensile FRC strengths are assumed to be the same. Nonetheless, this assumption can be unrealistic, particularly in terms of flexural residual strength (f(R)) since this property present higher scatter than the compressive strength (f(c)). This is particularly true for elements with a reduced cracking surface (e.g., beams) due to the higher impact that uncertainties like fibre orientation and distribution have on the variability of f(R). Therefore, this assumption can lead to lower reliability indexes (beta) than those established for traditional reinforced concrete structures. However, this variability tends to decrease with the increase of the width of the cracked sections (e.g., slabs). The results of a structural reliability analysis carried out to calibrate partial safety factors for f(R) is presented. Full-scale bending tests on precast segments with different dimensions, amounts and type of fibers were considered. This partial safety factors could be used in the design of future precast FRC tunnel linings.