Investigation on the Mechanical and Pore Structure Evolution of Cyan Sandstone Under Freeze–Thaw and Different Cyclic Loading Paths

ABSTRACT Periodic disturbances induced by traffic loads in cold regions induce cumulative damage and microstructural changes in the crushed rock within crushed‐rock embankments, significantly altering their physicomechanical properties. This study selects commonly used cyan sandstone in crushed‐rock embankments in cold regions as the research material and conducts cyclic loading tests under freeze–thaw conditions using three different loading paths: constant amplitude loading (Path 1), graded loading (Path 2), and constant lower bound incremental loading (Path 3). The experiment aims to reveal the degradation characteristics of mechanical properties and, in combination with nuclear magnetic resonance (NMR) T 2 spectroscopy, to investigate the evolution of pore structure in freeze–thaw rocks. Experimental results indicate that all three cyclic loading paths increase the strength of cyan sandstone under freeze–thaw conditions and reduce porosity by decreasing mesopores and macropores. The largest changes occur after 30 freeze–thaw cycles, with strength rising by 13.20%–23.05% and porosity decreasing by 3.41%–5.06%, a finding that confirms cyclic loading substantially alters the rock's pore structure. Total damage of cyclically loaded rocks, determined by the relative change rate of the total area in NMR T 2 spectra, shows a significant linear correlation with freeze–thaw damage , providing an important reference for understanding the damage evolution mechanism of rocks subjected to freeze–thaw and cyclic loading.