Freeze-thaw-induced microstructural damage in polyester fiber-reinforced cementitious composites revealed by X-ray microtomography

Fiber-reinforced cementitious composites are increasingly used in cold-region infrastructure because of their robustness, rapid installation, and sustainability. However, their long-term behavior under freeze-thaw cycles (FTCs) is still poorly understood at the microstructural scale. This study investigates frost-induced damage in a polyester fiber-reinforced cementitious composite using a fully saturated, closed-system protocol over 100 laboratory FTCs. High-resolution X-ray micro-computed tomography (11 µm voxel) performed at nine time points on a single specimen, combined with deep-learning segmentation, yields fiber-resolved pore and crack evolution and a mechanism-informed interpretation. The combined pore-plus-crack volume fraction rose from 10.0 % to 21.0 %, with local maxima up to 24.8 % in fiber-sparse zones. Thermomechanical modeling of a 185 µm cylindrical pore shows that differential thermal expansion between ice and matrix generates hoop stresses far exceeding those from crystallization pressure, identifying thermal-dilation mismatch as the dominant cracking mechanism under full saturation. Although polyester fibers usually toughen the composite under mechanical loading, freeze–thaw induces internal pore expansion that produces tensile hoop stresses in the matrix and concentrates them at fiber-matrix interfaces. These results provide new microstructural insight into durability limits and underscore the need to optimize pore structure and fiber-matrix interfaces to enhance freeze–thaw resistance. • Polyester reinforcement fibers were found to locally amplify freeze–thaw cracking by concentrating stresses around fiber–matrix interfaces in the cementitious composite. • The geometry and connectivity of the pore network were shown to control the initiation and growth of freeze–thaw damage, with local porosity rising from 10 % to 24.8 % after 100 cycles. • A deep-learning segmentation approach enabled reliable phase separation in three-dimensional tomographic images of the heterogeneous composite despite overlapping grey levels. • Thermo-mechanical analysis revealed that thermal expansion mismatch between ice and the cement matrix generates hoop stresses exceeding 120 MPa, identifying it as the dominant cracking mechanism under full saturation.