Frost salt scaling of concrete is related to cyclic freezing and melting of a few millimeter thick deicer solution on the surface of the concrete. It is almost absent when pure water is freezing and reaches a maximum at a so-called pessimum concentration that for NaCl is around 3%. Different mechanisms have been suggested to explain this pessimum and frost salt scaling in general, ranging from the transport of moisture and growth of ice within the pore space of concrete (“cryogenic suction”) to crack formation in the saline ice layer followed by spalling off the surface (“glue-spall”). Though in these theories the saline ice layer, that forms in concrete frost salt scaling experiments, plays a major role, so far little is known about its properties. We present a characterisation and an analysis of the microstructure of this saline ice layer by means of 3D X-ray microtomography. We found that the morphology of the saline ice is very similar to young, columnar sea ice, with lamellae of ice and brine oriented in the direction of freezing. On the basis of the microscopic 3D image data, we formulated percolation-based models of macroscopic properties (e.g., strength, thermal expansion coefficient, porosity metrics) relevant for different proposed frost salt scaling mechanisms. Model results and observations suggest that the ice growth velocity, direction and confinement, have a major impact on the pore structure of saline ice, thereby governing both mechanical and transport properties. These properties in turn are expected to affect proposed frost salt scaling mechanisms of concrete. The microstructure length scales in the ice-brine composite (lamellar spacing, pore width) are comparable to those for concrete (air void spacing and size), suggesting complex poro-mechanical interaction at the interface of concrete and saline ice. The results highlight the importance of studying saline ice properties to improve predictions of frost salt scaling processes.

We simulate the glue-spall stress due to mechanical interactions between a frozen saline solution (brine-ice composite) and a non-air entrained concrete surface including the impact of the micro-structure of the frozen solution. The presence of brine channels at the ice/concrete interface was found to be a prerequisite to induce stress during freezing and hence for scaling to occur. Pure ice does not result in scaling as it does not have brine channels. Furthermore, the size of the brine channels and their distribution was found determinant for the magnitude of the glue-spall stress in the concrete and the experimentally observed pessimum effect of a medium salt concentration was explained based on the change of the microstructure of the brine-ice composite at different salt concentrations and temperatures. The predicted results are in good agreement with the experimental observations and the few numerical demonstrations related to frost salt scaling in the literature.