A thermo-mechanical model for SFRC beams or slabs at elevated temperatures

The bearing capacity of steel fibre reinforced concrete (SFRC) at elevated temperatures is the subject of significant ongoing research, as the effect of steel fibres on concrete performance at higher temperatures is poorly understood. On one hand, steel fibres increase the average thermal conductivity of the concrete cross section and lead to greater heating within a concrete structural member during fire exposure, and on the other, fibres reduce crack widths and prevent excessive spalling. The former effect negatively impacts SFRC performance at high temperature, whereas the latter effect protects the inner structure from direct fire exposure. Additionally, the decreasing strength of steel at higher temperatures can result in the sudden failure of fibre or traditionally reinforced concrete. Within this contribution, a coupled thermo-mechanical model is developed in order to investigate the influence of steel fibres on the thermal loading of concrete. The effect of fibres on heat transmission within concrete, the length of time for which concrete can sustain thermal loads, and on the bending stiffness of reinforced concrete beams or slabs is investigated. The heat transfer process is modelled using Fourier's partial differential equation of transient heat conduction. A modified plastic hinge model and moment-curvature relations are used to describe stress-dependent deformations. Thus, two alternative approaches are used to adequately track the localisation of damage for single cracks and for distributed and multiple cracking. Thermo-mechanical coupling is achieved by means of temperature-dependent stress-strain relations. These are derived for SFRC based on experimental data from the literature. Experiments are performed in which concrete slabs reinforced with variable amounts of fibres and rebar are exposed to combined thermal and mechanical loadings. The results of these experiments are used to validate the proposed model. The measured and predicted results agree well and indicate that steel fibres have a positive effect on the fire resistance of structures, assuming additional rebar is provided to prevent crack localisation. Additionally, it is shown that temperature fields within concrete remain almost unaffected by variations in fibre content.