Frost damage in unsaturated porous media

Frost damage in porous materials is a weathering mechanism that can cause dangerous rockfalls or damage to built cultural heritage. The volume expansion of 9% when water freezes can be one of the cause of frost damage. This does not, however, explain why partially saturated porous stones also show damage despite the fact that ice should have room to grow. By performing experiments both at the scale of a single pore and in a real stone, we investigate the mechanism of frost damage at low water saturations at the pore scale and how it relates to macroscopic damage. We observe that the meniscus at an air-water interface confines the water in the pores. Because of this confinement, ice that forms will exert a pressure on the pore walls rather than growing into the pore. The amplitude of stress is found to be larger in small pores and when the meniscus has a larger contact angle with the walls. The contact angle is also observed to increase in the case of multiple freeze-thaw cycles, which increases the likelihood of damage. We find that cracks start first in the ice (being weaker than the confining material), followed by damage in the material itself. Remarkably, when multiple air-water interfaces are induced within limestone samples through a hydrophobic surface treatment, the stones are much more susceptible to frost damage than are uncoated stones, with cracks appearing preferentially at the hydrophilic-hydrophobic interface. This shows that indeed the meniscus confining the water during freezing and consequently the wetting properties are the relevant factors for frost damage in partially saturated porous stones Reference: R. Le Dizès Castell, R. Sinaasappel, C. Fontaine, S. H. Smith, P. Kolpakov, D. Bonn, and N. Shahidzadeh, “Frost Damage in UnsaturatedPorous Media,” Physical Review Applied, vol. 20, p. 034025, Sept. 2023.