A generalised model for direct prediction of stresses in concrete at high temperatures

A novel theoretical model capable of quantifying the stress causing transient thermal strain (TTS) in heated concrete is presented. It is applicable to numerical models and as a design equation for assessment of stress caused by combined actions of loading, heating fluxes and end restraints during heating. Three categories of concrete members are classified: heated then loaded; loaded then heated; heated with full restraint with or without load. The TTS equation is incorporated in a novel thermo-mechanical finite element model developed in ABAQUS. Material models capable of capturing degradation of concrete properties under elevated temperatures have been selected and adapted to include TTS and temperature dependence. The finite element model and stress equation have been successfully validated against experimental results under various loading configurations and boundary conditions. A number of observations were made: (i) stress developed under restraint is lower than under applied preload, but stress relaxation, in part owing to TTS, is higher under preload; (ii) the limit of elastic behaviour is reduced under free thermal expansion compared with restrained samples; (iii) free thermal expansion is critical to the overall stress–strain response and appears to vary with applied stress while significantly increasing at high temperatures.