Evaluation of de-icing chemical and moisture mass transfer in freezing soils

Abstract

Highway subsoils in cold countries are subject to increased thermal conductivity, disruption of natural moisture circulation as well as dynamic loading and application of de-icing chemicals in the winter months. In this work, the moisture mass transfer in a state of vapour flow and the de-icing chemical migration were considered during unidirectional freezing. The moisture mass transfer in a gaseous state was previously widely neglected in the exploration of frost heave. To conduct freeze-thaw cycles with increased lengths of soil samples and a modified slow freezing technique, an environmental chamber of nine samples capacity was designed. Supplying the non-saline samples with either 11 or 22 g/L sodium chloride solution signified chemical mass transport over the sample length and a significant change in temperature-moisture distribution when compared to deionised water supplied test results. The presented conceptual model with vapour mass transfer was based on the thermodynamic equilibrium of vapour density with temperature change and the phase transition to ice during thermal energy withdrawal. Compared to the widely used coupled heat-mass models, the vapour flow based model clearly explained the driving forces and presented a much easier algorithm for calculation. The de-icing chemical displacement was explained as the migration of the dissolved ions together with hygroscopic water transport, which in turn, was driven by cryosuction forces. The reduction of hydraulic conductivity during the secondary salinisation with sodium chloride was caused by chemical osmosis, which tended to equalise the solute concentration in pore water over the sample length. The research outcomes indicate a significant contribution to the future perspectives on frost heave modelling and prognosis. Further research could extend this work by inclusion of the vapour mass transfer in quantitative analysis for soil freezing. The effect of secondary salinisation should be also foreseen in the long term prognosis for highway subsoils exploitation.