Phase-field modeling of thermo-mechanical fracture in heterogeneous concrete

Concrete structures often face severe deterioration when subjected to elevated temperatures, which can significantly impair load-bearing capacity and long-term durability. However, the precise mechanisms governing crack formation and propagation at elevated temperatures remain insufficiently clarified and understood, particularly at the mesoscopic level, for improving safety assessments of concrete structures. To address this gap, we develop and validate a two-dimensional phase-field fracture model that considers the coupled effects of temperature on heterogeneous concrete mesostructures. The proposed mesoscale model integrates graded aggregates generated via Voronoi tessellation, mortar matrix, an explicitly defined interfacial transition zone, and random pores synthesized by thresholding a Gaussian-smoothed random field. The model captures temperature-dependent damage, crack propagation, and microstructural interactions by coupling thermo and mechanical fields in Abaqus through user-defined subroutines. We calibrate our methodology against established experimental data, revealing that increasing aggregate volume content enhances load-bearing capacity but increases localized stress concentrations. At the same time, higher porosity accelerates crack initiation and promotes distributed failure. Thermo softening substantially reduces peak compressive strength at temperatures up to 800 °C and shifts crack propagation from localized to more diffuse patterns. These results highlight the critical influence of mesoscale phases, such as pores and aggregate content, on the thermo-mechanical behavior of concrete. Consequently, our model provides a robust framework for predicting crack propagation patterns in numerical simulations, validating its effectiveness in assessing the mechanical behavior of heterogeneous materials.