THERMODYNAMIC CONSISTENT GRADIENT POROPLASTIC THEORY FOR COUPLED THERMO-CHEMICALMECHANICAL PROCESSES OF CONCRETE UNDER HIGH TEMPERATURE

G. ETSE; M. RIPANI; J. MROGINSKI

Barcelona, España

Conferencia; International Conference on Computational Modeling of Fracture and Failure of Materials Structures (CFRAC 2011); 2011

International Center for Numerical Methods in Engineering, CIMNE

Concrete materials are strongly affected by long term exposure to high temperature. Most of their relevant mechanical features such as cohesion,friction, strength and stiffness suffer severe and complex degradation processes that are strongly dependent on the temperature, temperature gradient, and on the particular governing stress stage. These degradations in its mechanical properties lead to relevant changes in the failure mechanism and response behavior of concrete when subjected to long term high temperature field. Regarding concrete failure mechanism, the question arises on how strong is the variation from ductile to brittle or localized failure mode due to the temperature effect.At the macroscopic level of observation, concrete temperature exposure causes the well-known spalling failure due to the porous overpressure. From the structural engineering stand point, accurate constitutive theories are required to capture the complex changes of concrete mechanical properties when it is subjected to long term exposure of high temperature. They should be capable to realistically predict strength, stiffness and ductility variations with the applied temperature. In the framework of finite element analysis, these model formulations should be able to objectively reproduce the dramatic variation from ductile to brittle failure modes of concrete component when subjected to high temperature regimes. This requires, on one hand, the consideration of the porous features of concrete to take into account the humidity conditions and its variation with temperature. On the other hand, nonlocal model strategies need to be considered based on temperature dependent characteristic lengths to reproduce the variation from ductile to brittle failure modes. In this work, a thermodynamically consistent gradient poroplastic model for concrete subjected to high temperature is proposed. The constitutive theory follows the concept of open reactive porous media according to Ulm et al (1995), Ulm et al (1999) and Coussy (2004) that is particularized here for the partially saturated case. Regarding the gradient formulation a restricted form is considered in this work, following Vrech and Etse (2009) and Mroginski et al (2010), whereby the state variables are the only one of non-local characters. After describing the main features of the coupled thermo-chemo-mechanical model formulation the presentation focuses on the analysis of the failure indicators and of its variations with the considered temperature. The main objective is the evaluation of the temperature dependence of the transition point between ductile and brittle failure mode of concrete.