A Meso-scale study to predict high-temperature behavior of concrete structures in a hygral-thermal-chemical-mechanical framework

Abstract

Fire has become one of the common hazards that a concrete structure faces in today’s world. Considering the heterogeneity of concrete and the contrasting nature of the constituents at high temperatures (cement paste dehydrates and shrinks while aggregate expands in volume), the use of a homogeneous macroscopic model to predict concrete's thermo-mechanical performance is questionable. Therefore, a more realistic description of concrete (in terms of explicit consideration of coarse aggregate and cement mortar rather than homogenized concrete medium) is needed to characterize the behavior of a concrete structure under such severe loading conditions (e.g., fire). In fact, the performance of a macro-scale (meter size or higher) concrete structure is the coupled effect of the various physico-chemical processes that occur within its constituents at meso-scale, a scale (mm-cm scale size) where coarse aggregates are randomly distributed in a mortar paste medium. Therefore, it is required to develop a model that explicitly considers the constituents' contrasting nature and the different thermo-hygral-mechanical processes at the scale of occurrence of these processes. Hence, in this contribution, a meso-scale based model is developed in a coupled hygro-thermo-chemo-mechanical (HTCM) framework to investigate the effect of high temperature on the thermo-mechanical performance numerically (e.g., in terms of spalling, deformation, residual capacity, etc.) of fire-exposed plain and reinforced concrete structural elements (e.g., slab, beam, etc.). Simulated results and validation with experimental data (taken from the literature) highlight several crucial aspects related to obtaining a more precise residual capacity of a concrete structure, which is impossible to reproduce with a homogenized macroscopic model. The study highlights that the failing of random concrete parts at different times during high-temperature exposure can not be simulated with a homogenized assumption. Further, a mesoscopic model does not require transient creep strain to be specified explicitly. The primary influencing factors behind this transient creep strain, such as the different thermo-chemical and thermo-mechanical damage of the concrete constituents (cement paste, aggregate), thermal-incompatibility of the components and associated damages, etc., are implicitly taken into account in a meso-scale model.