Thermal Damage Evolution of Meso-Heterogeneous Concrete: Insights from Phase-Field Simulations

Concrete is a typical heterogeneous material with a complex internal structure. The damage evolution and heat transfer behavior of concrete in high-temperature environments are closely related to its heterogeneity. This study develops a coupled thermomechanical phase-field model that integrates the heterogeneity of the mesoscale components to investigate the influence of meso-heterogeneity on the thermal damage behavior of concrete. A random aggregate model of concrete is established and integrated with the Weibull distribution to characterize the heterogeneity of aggregates, mortar, and interfacial transition zone (ITZ) in concrete. Subsequently, based on the theory of variational fracture phase-field, the coupled thermomechanical phase-field model considering the deterioration of concrete performance is developed to predict the failure behavior in concrete. The accuracy of the proposed numerical model is verified through simulations of two random aggregate samples. Further, taking the concrete model of a thick-walled cylinder with a typical thermal stress field as an example, a detailed study is conducted on the influence of heterogeneity on the thermal damage process under two representative heating rates (fast and slow heating). The results indicate that heterogeneity can lead to changes in crack paths and also cause local microdamage in concrete. Under the rapid heating regime, microdamage is more concentrated adjacent to the heating boundary. The propagation of main cracks in concrete induces stress release, thereby retarding the progression of microdamage. Under the slow heating regime, microdamage is more evenly distributed throughout the entire geometry of the concrete. Under both the rapid and slow heating rate regimes, the contribution of heterogeneity exhibits a consistent trend, namely, the stronger the heterogeneity, the larger the area of microdamage, and the more severe its degree. Furthermore, the microdamage promoted by heterogeneity affects heat transfer in concrete, resulting in a significant reduction in the residual thermal conductivity of concrete. Under the slow heating rate regime, the final microdamage ratio of the mesoscale model is 18.45%; whereas for the meso-heterogeneous models with homogeneity degrees (shape parameters in Weibull distribution) of 6, 3, and 1.5, the final microdamage ratios increase to 21.20%, 36.27%, and 88.10%, respectively. For the mesoscale model, the final thermal conductivity decreases by 10%; in contrast, for the meso-heterogeneous models with homogeneity degrees of 6, 3, and 1.5, the final reductions of thermal conductivity increase to 11%, 15%, and 45%, respectively. Such significant contrasts are also observed between the meso-heterogeneous models and the mesoscale model under the rapid heating rate regime. This study reveals a sequential promotion relationship between heterogeneity, microdamage, and thermal degradation, providing new mechanistic insights and an evaluation perspective for concrete deterioration in fire or high-temperature service scenarios.