Saturation of Granular Base Material Due to Water Vapor Flow during Freezing: Laboratory Experimentation and Numerical Modeling

Water vapor flow in response to temperature gradients within a pavement structure can be an important mechanism of moisture ingress in aggregate base layers during winter in cold regions. The objective of this research was to investigate the occurrence of vapor flow in a non-frost-susceptible granular material typical of base layers within the jurisdiction of the Swedish National Road Administration. A cylindrical specimen 145 mm in height and 123 mm in diameter was positioned 100 mm above a water table inside an insulated plastic tube, which was instrumented with thermistors to facilitate monitoring of the specimen temperature profile. A computer-controlled cooling element was utilized to freeze the specimen surface to a constant temperature of –1°C for 41 days, and the bath water was maintained at a temperature of 5°C. The specimen weight was measured weekly to quantify the vapor flow rate, and the specimen was then physically separated into four layers and oven-dried to determine the moisture profile at the end of the testing. Additional characterizations were also performed to facilitate material classifications, and numerical modeling was conducted to simulate the observed laboratory results. Analyses of the data indicate that the average vapor flow rate was 12.8 g/week and that the majority of the incoming water accumulated just above the freezing front. The water content near the freezing front was 10.8 percent, which corresponds to a degree of saturation of 115 percent, while the water content was only 3.9 percent near the base of the specimen. Although redistribution of moisture residing within the specimen prior to the start of testing could be partially responsible for the higher water content that developed in the vicinity of the freezing front, the overall increase in specimen weight is attributable only to the movement of water vapor from the bath into the specimen; the condensation and freezing of water vapor within the pores of the specimen caused localized super-saturation of the material. The data clearly show that water vapor can be an important source of increased water content within pavement base layers subjected to sustained freezing temperatures.