Pavement Unbound Materials Stiffness-Moisture Relationship during Spring Thaw

In the study presented here, the mechanical behaviour of a flexible pavement structure and its correlation to the pavement profile moisture condition was investigated during the spring thaw and the recovery periods. The study was carried out based on continuous pavement environmental data measurements as well as frequent Falling Weight Deflectometer (FWD) tests during 2010. Measurements were taken on a relatively thin flexible pavement (≤ 100 mm Hot Mix Asphalt, HMA) in southern Sweden where frost penetration of 1.2 m was observed during the winter. Considerable decrease in the pavement bearing capacity was observed as the moisture content in unbound layers increased during spring thaw. Both pavement deflection basins and backcalculated layer stiffness indicated clear correlation with thaw penetration and measured moisture content. Pavement structure was weakest as the thaw penetrated the subgrade. During the same period, highest annual moisture content at the upper part of the subgrade was also registered. Furthermore, using the field moisture measurements and backcalculated layer stiffness, a moisture-stiffness model was developed for granular layers and subgrade. The backcalculated layer stiffness and the pavement moistures measurements fell on a unique curve and showed promising agreement with the M_R-Moisture laboratory-based predictive model used in ME-PDG.