Microstructural analysis of freeze–thaw degradation in rubber-modified cement-stabilized crushed stone using X-ray computed tomography

This study investigates the microstructural mechanisms governing the freeze–thaw (F–T) degradation of cement-stabilized crushed stone modified with recycled rubber powder. Specimens with rubber contents of 0, 10, 20, 30, and 45% by volume of fine aggregate were evaluated. Frost resistance exhibited a non-monotonic relationship: It initially decreased at low rubber contents before increasing significantly at higher contents. To explain this, we employed high-resolution X-ray computed tomography to analyze the internal pore structure evolution. We introduce a novel metric, the pore orderliness expansion index (POEI), derived from the spatial correlation of damage between progressive F–T stages. A strong, statistically significant positive correlation was found between the POEI and frost resistance. High POEI values, indicating an orderly expansion of existing pores, corresponded to superior durability, whereas low POEI values signified disordered microcrack propagation and poor performance. The findings clarify that high rubber concentrations enhance frost resistance not merely by adding ductility but by promoting a more benign, orderly pore expansion mechanism, thereby inhibiting the formation of new, damage-inducing microcracks.