Assessment of the post-fire residual bearing capacity of FRC and hybrid RC-FRC tunnel sections considering thermal spalling

A design-oriented numerical model for the analysis of RC, FRC, and RC-FRC tunnel sections exposed to fire with different spalling parameters is presented. The numerical model is conceived in two steps: the first is the determination of the temperature field in the cross-section exposed to fire according to the spalling parameters; and the second is the determination of the bearing capacity of sections based on the thermal field in the section. At last, a parametric study was conducted to evaluate the effect of the fire curve, the spalling parameters, the reinforcement type, and the rebar’s concrete cover on the bearing capacity of the sections. The results showed that the use of FRC as total or partial substitution to RC mitigates the fire-related reduction in the bearing capacity of the sections. Moreover, increasing the RC concrete cover is beneficial only if thermal spalling is avoided. When thermal spalling occurs, the FRC and RC-FRC solutions yielded the lowest reductions in the bearing capacity among the reinforcement solutions tested.