Numerical evaluation of temporal moisture variations in unbound pavements with thin seals

Abstract

The moisture condition of pavements is a crucial factor determining both their long-term and short-term performance in terms of the strength and stiffness of pavement materials. Unbound pavements with thin seals, the most common type of pavement in Australia and New Zealand, are vulnerable to moisture-related performance loss. Thin seals can be either bituminous or sprayed, the latter type being most common in Australia. This type of pavement can easily exchange moisture between the surrounding environment and the pavement structure, since the seal is relatively permeable. Therefore, changes in climatic factors such as precipitation, atmospheric temperature, evaporation, relative humidity and water table cause temporal moisture variations in pavement layers. These temporal moisture variations also affect performance losses. Hence, a reasonably accurate model is required to predict possible temporal moisture variation during service life in order to advance current Australian pavement design. This paper reports the development of a model to predict moisture conditions in terms of the degree of saturation (Sr), a critical parameter which determines rutting in the pavement layers of unbound pavements with sprayed seals. The model simulates the coupled moisture, vapour and heat flow through pavement layers in the middle section of the pavement, where the edge effects are not significant, employing 1-D numerical modelling. After validation of the model against some field data, the model was employed to analyse both the long-term and short-term hydraulic behaviour of pavements. Environmental factors substantially affect moisture conditions in the base and sub-base layers. Sensitivity analyses were performed to investigate the effects of the hydraulic properties of unbound granular materials (UGMs), the initial condition and the water table depth on long-term Sr and suction variation in unbound pavements with thin seals. It was found that the equilibrium moisture condition does not depend on the initial conditions, and the influence of the water table on subgrade equilibrium is significant only when the water table is within 6 m from the surface.