Constitutive Models of Concrete at Elevated Temperatures: Studying the Effect of Temperature Gradients

The outbreak of fire can have serious consequences in the structural performance of a load-bearing concrete structure. To assure adequate fire performance, detailed knowledge of fundamental mechanical properties of concrete at elevated temperatures is crucial. This paper first highlights limitations of existing knowledge regarding the mechanical response of concrete at elevated temperatures, including the inconsistent thermal boundary conditions and intentionally-minimised temperature gradients in "standardized" conventional concrete material testing. Accordingly, it is argued that the effect of temperature gradients within concrete on its fire performance has not been extensively or directly addressed. On this basis, the paper outlines key features of an ongoing research programme at The University of Queensland aimed at studying the performance of concrete in fire using a novel medium-scale testing method. By heating using radiant panels, well-defined and consistently-controlled heat flux boundary conditions on concrete cylinders (phi 100mm x 200mm) have been achieved. The repeatability, consistency, and uniformity of thermal boundary conditions are demonstrated using measurements of heat flux, temperature profile, and compressive strength. Analysis of initial obtained data shows that the incident heat fluxes, and thus the associated temperature gradients, have potentially significant effects on concrete properties at elevated temperatures. Further research is thus ongoing to quantify such effects and also to develop models for their inclusion into effective performance-based fire design and analysis of concrete structures.