A study of temperature gradient effects on mechanical properties of concrete at elevated temperatures

Abstract

The outbreak of fire in a concrete structure can have serious consequences, including structural damage, loss of property, business interruption, and possibly loss of life. To ensure adequate fire performance of concrete structures, detailed knowledge of fundamental mechanical properties of concrete at elevated temperatures is crucial. In addition, realistic performance-based structural fire design, which has been developed in many countries (e.g. Australia, UK, New Zealand), requires detailed knowledge of fundamental properties of concrete at elevated temperatures to be re-examined in a holistic and scientific manner. Despite extensive research over the past decades, current knowledge of the fundamental properties of concrete at elevated temperatures remains primarily based on data collected from conventional tests in which inconsistent thermal boundary conditions are highlighted as limitations of the current testing method. In addition, the effect of temperature gradients has been intentionally minimised when determining the mechanical properties of concrete at elevated temperatures. Consequently, the influence of processes linked with temperature gradients, including thermal stress, moisture transport, and pore pressures, has not been directly addressed. This gap in knowledge is potentially important considering the steep temperature gradients formed within concrete structural members subjected to real fires. This thesis reports the outcomes of a research programme aimed at re-examining the performance of concrete in fire accounting for temperature gradients using a newly-developed testing method. This research has taken an integrated approach combining analytical, experimental, and numerical studies to investigate the effect of temperature gradients on mechanical properties of high strength concrete at the material level, focusing on a standard cylindrical concrete specimen (Ф100mm × 200mm). The outcomes of the thesis can be summarised as follow: - Analytical solution for evolution over time of in-depth temperature in a cylinder specimen subjected to a constant uniform heat flux boundary condition was obtained by solving the fundamental heat transfer equation. The analytical solution was demonstrated to agree well with numerical solutions using finite difference and finite element methods. This agreement gives confidence to the validity and reliability of the three approaches. Such validity of the models’ predictions is then confirmed through their good agreement with the corresponding measured temperatures during testing of concrete cylinder specimens. - By heating using radiant panels, well-defined and consistently controlled heat flux boundary conditions on concrete cylinders (Ф100mm×200mm) were achieved. The repeatability, consistency, and uniformity of thermal boundary conditions were demonstrated using measurements of heat flux, temperature profile, and compressive strength. The newly-developed test setup enabled a more reliable study of the mechanical properties of concrete at elevated temperatures under various stress-temperature paths, forming the basis for their re-examination with the effects of temperature gradients appropriately taken into account. - Analysis of the initially obtained data shows that the incident heat fluxes, and thus the associated temperature gradients, have potentially significant effects on concrete properties at elevated temperatures. Recommendations for further research to quantify such effects and to develop models for their inclusion into effective performance-based fire design and analysis of concrete structures are also proposed.