Acoustic Emission Characteristics and Damage Evolution of Initially Damaged Limestone Under Freeze–Thaw Action

To investigate the effects of freeze–thaw (F-T) action on the mechanical properties, pore structure, internal progressive damage evolution laws, and failure characteristics of initially damaged limestone, intact limestone specimens were subjected to different initial damage states and numbers of F-T cycles. Subsequently, the porosity and T2 spectrum distribution of the specimens were tested using nuclear magnetic resonance (NMR). Finally, uniaxial compression tests were performed while monitoring the acoustic emission system. The test results showed that, as the number of F-T cycles increased or the initial degree of damage intensified, the peak strength of the limestone decreased, and the porosity increased. The higher the number of F-T cycles of limestone, the wider the distribution range of ringing counts in the middle and later stages of loading. As the number of F-T cycles increased, the proportion of tensile cracks in the limestone interior gradually increased and became dominant. The b-value evolution curves generally showed a sudden drop in the later loading stage. The damage variable of limestone did not show regular changes with an increase in the F-T cycles. This results from the superposition of the initial damage and the F-T cycles.