Experimental study on the effect of air pores on the uniaxial compression properties of S2-type columnar freshwater ice

The mechanical behavior of S2 columnar sea ice is central to ice-load evaluation in polar and cold-region engineering. In natural sea ice, this behavior is affected by both brine pockets and gas pores, whose individual contributions are difficult to separate. In this study, S2 columnar freshwater ice was used as a brine-free material system to isolate the effect of gas-pore characteristics on uniaxial compressive behavior, thereby providing a decoupled baseline for understanding pore effects in columnar ice. A laboratory preparation method was developed to fabricate specimens with controlled air porosity and pore diameter while preserving the S2 columnar texture. A total of 139 uniaxial compression tests were conducted at -15 °C, with loading applied perpendicular to the columnar growth direction at nominal strain rates ranging from 1 × 10 −4 to 1 × 10 −2 s −1 . The observed responses were classified into ductile, ductile-to-brittle transition, and brittle regimes based on stress-strain behavior, post-peak load drop, and failure observations. In the ductile regime, gas pores did not change the overall failure mode but strongly affected crack propagation. The crack-propagation area, quantified using a semi-quantitative image-based damage indicator termed the critical crack density, decreased with increasing porosity and pore diameter. In the brittle regime, pore-free and low-porosity specimens mainly failed by longitudinal splitting, whereas high-porosity specimens with heterogeneous pore morphology exhibited large-scale local spalling or explosive fragmentation. The characteristic strain rate corresponding to the peak compressive strength decreased as porosity and pore diameter increased. Within the tested specimen set, pore diameter appeared to influence the ductile-to-brittle transition more strongly than pore spacing, although this conclusion should be interpreted within the limited pore-parameter range examined here. At ductile strain rates, increasing porosity reduced the area-corrected compressive strength and weakened its strain-rate dependence. At brittle strain rates, the compressive strength showed no clear correlation with pore characteristics during longitudinal splitting, but remained relatively low when local spalling occurred. These findings provide a gas-pore-specific baseline for ice-load parameterization, ice-structure interaction modeling, model ice preparation, and future investigations into the distinct roles of brine and gas pores in sea ice.