A multiaxial constitutive model for concrete in the fire situation

Performance-based design in fire engineering leads to increasing demand for advanced temperature-dependent material models for the load bearing materials used in building structures. These models must be valid in natural fire situations including cooling down phase and must be sufficiently robust for complex numerical calculations such as, for example, the analysis of tensile membrane action in composite slabs. Although structural concrete is widely used in civil engineering, proper modelling of its thermo-mechanical behaviour remains a challenging issue for engineers mainly because of the complexity of the phenomena that result from the microcracking process in this material and because of the necessity to ensure the numerical robustness of the models.
This paper presents a new multiaxial concrete model based on a plastic-damage formulation and developed to meet the specific requirements of structural fire engineers and researchers. The model, which incorporates an explicit term for transient creep strain and encompasses a limited number of material parameters, has been implemented in a finite element software dedicated to the nonlinear analysis of structures in fire. The paper presents a series of numerical simulations conducted to highlight the model ability to capture the main phenomena that develop in concrete under fire (permanent strains, degradation of the elastic properties, unilateral effect) as well as its ability to be used for the fire analysis of large-scale structural elements. As an example, the new concrete model is used in the numerical analysis of a full scale fire test on a composite steel-concrete slab and it is shown that the computed and measured results agree.