High-performance concrete in fire-exposed reinforced concrete sections

The ultimate behavior of reinforced high-performance concrete (HPC) structures at high temperatures is still not fully understood, but the increasing number of important HPC structures at risk of a fire requires further studies, either at high temperatures or after cooling. Herein, the ultimate behavior of three different reinforced concrete square sections subjected to an eccentric axial force is investigated with reference to a typical HPC and to several fire durations. In addition to the M-N envelopes of the undamaged sections, those at high temperatures (that is, during afire) and after cooling are worked out. Comparisons are made with normal-strength concrete (NSC) sections having the same size or the same ultimate capacity, as well as with HPC sections characterized by different bar arrangements or protected by a cementitious lining, and a few performance indicators are introduced to describe the marked reduction of the failure envelopes. The main findings are: the sections can still be analyzed by limiting their effective area to a critical isothermal line (lower than in NSC); thin insulating layers are sufficient to minimize the greater HPC sensitivity to high temperatures; and different bar arrangements have marginal effects on section performance, except in the case of pure bending. In summary, HPC sections are more temperature-sensitive than NSC sections, but not as much as one might expect.