Fire resistance of RC columns with regard to spalling of concrete

At date a common agreement seems to be established among researchers that for a proper prediction of fire-induced concrete spalling on structural fire resistance a fully-coupled thermo-hygro-mechanical model should be used in general. Such sophisticated models may engage complex systems of equations. For everyday practical applications, however, the accompanying computational procedures may be found to time-consuming. Proper input material data such as permeability tensor and diffusion coefficients may be found difficult to define as well, especially for modern special-type concretes. Thus, simplified procedures for rough assessments of spalling are still desirable. For reinforced concrete (RC) columns, one of such is proposed here. The model combines two essential analyses, i.e. a hydro-thermal and a mechanical analysis, and runs them separately but simultaneously. For rough estimation of spalling different semi-empirical criteria are used. They consider concrete pore pressure, velocity of spalled-off pieces and buckling length of delaminated concrete. The proposed model is validated against experimental data and shows promising results. In the final part of the paper, the model is implemented in an investigation of the effects of different spalling scenarios on a fire-engulfed column. This shows that spalling may reduce its ultimate fire resistance as much as by 70%.