Modelling the transient strain behaviour of concrete exposed to elevated temperatures

Abstract

The available transient strain models for concrete are investigated and, where appropriate, modified through a multi-level analysis procedure in which the models are explored in general, in terms of qualitative behaviour in FE simulations of generic experimental conditions, and in terms of FE simulations of specific experimental conditions. The previously ignored issue of the nature of the transient strain response to states of tension is explored through simulation of the resultant axial strain behaviour of three mathematical models for tensile transient strain, defined a priori. An original analytical tool and a novel calibration technique are developed to allow this work to be performed. The method of ‘Gauss point mean variables’ plotting allows the values of any model parameters to be explicitly examined throughout a simulation, while the ‘iterative feedback technique’ allows for rapid parametric calibration while maintaining the accuracy of the FE method. Using these techniques, it is found that the Nielsen model and the Terro model allow the reproduction of experimental results, with the remaining models, including the commonly applied Anderberg model, shown to be a relatively poor approximation. Application of these models to three realistic examples of concrete technology shows that transient strain is extremely important in conditions of heating, consistently increasing structural resilience. In particular, simulation of concrete beams subjected to restrained expansion shows convincingly that the Anderberg model is fundamentally unable to reproduce experimental behaviour. Results suggest that no transient strain develops due to conditions of tension. However, due to the relatively small specimen size of the experimental conditions, this conclusion may be considered to be tentative. A continuation of this research using experimental data obtained using larger specimens is recommended. The response of transient strain to states of tension is shown to have a profound effect on the development of damage in columns subjected to fire conditions. Further, a brief investigation into the load-dependence of transient strain implied by the experimental data reveals that a small deviation from linearity may be suitable. A further exploration of this deviation, including into the effects on overall results in experimental conditions, may be useful for transient strain modelling.