Debonding behaviors and micro-mechanism of the interface transition zone in sandstone-concrete interface in response to freeze-thaw conditions

Comprehensive understanding the micro-debonding behavior and micro-mechanism at the interface transition zone (ITZ) of rock-concrete interfaces is essential for protecting shotcrete in tunnels. The ITZ is a weak area of the rock-concrete interface and is vulnerable to freeze-thaw (F-T) deterioration. Such deterioration can cause tunnel support failure. We used nanoindentation technology, stereo microscopy, and scanning electron microscopy (SEM) and considered several F-T actions to obtain four primary findings: (1) The elastic modulus and hardness values of the ITZ decrease continuously as the number of F-T cycles increases, and the trends of these changes are highly consistent. However, the decline of hardness value exceeds that of the elastic modulus as F-T cycling continues. (2) The thickness of the ITZ has a positive correlation with the number of F-T cycles. For example, the ITZ thickness increases from 60 μm (0 F-T cycles) to 80 μm (20 F-T cycles). (3) The volume fraction of micropores at the interface has a positive linear correlation with the number of F-T cycles; however, the volume fractions of low-density (LD) calcium-silicate-hydrate (C-S-H) gel, high-density (HD) C-S-H gel, and calcium hydrate (CH) at the interface exhibit negative correlations with the number of F-T cycles. These relationships indicate that micropores exert a degradation effect in response to F-T conditions. (4) The ITZ has high porosity and low sensitivity to debonding near the sandstone side. ITZ debonding is induced by F-T cycles because frost expansion and thaw contraction are significantly different in sandstone and cement paste. This study provides a reference for understanding the behavior and mechanism of debonding in rock-concrete interfaces subjected to F-T conditions.