Characterization of frost damage in fractured red sandstone during unidirectional freezing and thawing process

The common freeze-thaw environments in engineering construction in cold regions are unidirectional and all-directional, while most of the rock freeze-thaw tests conducted in the laboratory are performed only in the all-directional direction. In this paper, the freeze-thaw damage characteristics of fractured red sandstone under different freeze-thaw loading modes are investigated. By designing unidirectional and all-directional freezing and thawing tests, the frost swelling force, temperature change and strain during freezing and thawing were monitored, and the microporous change mechanism was analyzed by scanning electron microscopy. It was found that the peak frost gravity of unidirectional freezing was lower, with 3.6 MPa for unidirectional freezing and 4.5 MPa for all-directional freezing. during unidirectional freezing, the peak frost swelling force appeared with a time lag, and there were no frost cracks in the samples. The temperature monitoring results showed that the temperature at the bottom of the unidirectionally frozen samples decreased slower and was higher than that at the top of the samples, and the temperature gradient effect was obvious. Scanning electron microscopy observation showed that after 20 freeze-thaw cycles, the inter-granular cementing material of sandstone decreased significantly and the smoothness of the granules increased, indicating that the freeze-thaw cycle damaged the microstructure of sandstone, but the degree of damage of the unidirectional freezing sample material was weak.