Uniaxial model for concrete under variable temperature and stress

A plasticity model using a strain-rate formulation is presented to describe the uniaxial response of concrete when subjected to combined thermal and mechanical actions. The total strain rate is resolved into three individual components: mechanical strain, thermal strain, and transient creep strain. Each component is formulated individually. The mechanical strain rate is assumed to consist of an elastic strain rate and a plastic strain rate, which itself is taken as temperature-dependent. In describing the plastic strain rate, the nonlinear part of the uniaxial stress-strain curves is assumed to be a quarter of ellipse, defined for a given temperature in the range 20°C-800°C. The model was first used to analyze uniaxial (experimental) data under variable load but isothermal conditions, and then to investigate the effect of a sustained load on the deformational response of a concrete specimen under heating. Thirdly, a set of experimental relaxation tests was studied, in which temperature and stress vary continuously under a zero total strain condition. Here, the importance of a variable temperature and stress, and the significance of the history, is demonstrated.