Heat in concrete: structural behavior and failure modes

The studies on the materials and structural behavior in fire-related situations have recently received new impetus as a consequence of tragic fires, that occurred in the mid and late nineties in a few road and railway tunnels. However, beside these events there is a general consensus on the need to know more about the interaction among fires, high temperatures, materials and structures, even more since the number of major structures at risk of fire is increasing (industrial buildings, entertainment halls, off-shore platforms, tall buildings and containment shells). Since concrete - either reinforced or prestressed is the most extensively used material in civil engineering, the vast majority of the recent studies has been devoted to cementitious composites, as required also by the frenzy of innovation that cement-based materials have exhibited in the last fifteen years. (Each composite has its own thermal properties and mechanical decay under fire and after cooling to room temperature). In this paper, several aspects of the many-faceted mechanical response of R/C structures and members under fire and at high temperature are recalled and investigated, the emphasis being mostly on R/C sections and single members (deep beams and circular slabs) in the first part, and on continuous beams and plane frames in the second part. Specific issues treated in the first pan are those concerning high-performance concrete (HPC), as well as light-weight concrete (LWC and HPLWC), while nonlinear modeling of R/C members is dealt with in the second part, where a 'fiber model for R/C sections is implemented into an existing FE code, to investigate the complex interaction among the restrained thermal elongation, the ensuing axial forces and the bending/shear behavior of continuous beams and frames. Whenever possible, reference is made to the most recent code proposals on fire design, that is once more recalled in the concluding remarks, where a set of indications resulting from the tests and from the parametric analyses is presented.