Approximation of concrete thermophysical properties to solve the nonlinear problem of heat conduction of reinforced concrete structures under fire conditions

Introduction. This paper is dedicated to the development of methods for approximating the thermophysical properties of concrete when solving a nonlinear heat conduction problem. The research is relevant in the context of assessing the fire resistance of building structures and enhancing their safety during fires. The goal of the work is to create universal models that describe the dependence of the thermal conductivity coefficient and specific heat capacity on temperature, which will improve the accuracy of temperature field calculations. Materials and methods. Experimental data from numerous sources, as well as known standards (e.g., Eurocode 2), were used for analysis. The study employs the method of least squares (MLS) for constructing regressions and approximations. Various types of concrete — from lightweight to heavyweight — and their behaviour when heated to high temperatures were considered. Special attention was given to the influence of porosity, aggregate composition, and moisture content on the material’s thermophysical characteristics. Results. New approximation formulas for the thermal conductivity coefficient of concrete as a function of density and temperature have been obtained. It has been shown that this dependence can be successfully described by a universal exponential model. A linear approximation for specific heat capacity was proposed, demonstrating good agreement with experimental data. It was established that effective heat capacity increases within certain temperature ranges due to endothermic processes such as cement stone dehydration and limestone decarbonization. The results were confirmed by comparison with existing models and regulations. Conclusions. The approximation models developed in this work allow for more accurate predictions of concrete behaviour under high-temperature exposure. This has significant practical importance for designing fire-resistant structures, especially in nuclear energy and other sectors where safety plays a crucial role. The findings can be used to improve existing engineering approaches and develop new standards. Future research is planned to consider additional factors, such as mass transfer and changes in material structure during heating.