MODELING MOISTURE TRANSPORT IN ASPHALT PAVEMENTS
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Many of the pavement distresses such as pot holes and surface cracks are caused by moisture damage, which is due to the destruction of adhesive bond between aggregate and the binder in the presence of moisture. These distresses can cause excessive pavement roughness that might necessitate replacement of the entire pavement layer. Hydraulic conductivity has traditionally been used to characterize the moisture transport in asphalt pavements. However, laboratory or field measured unidirectional hydraulic conductivity only provides information about the flow in one direction and does not represent flow in other directions. Numerical modeling of fluid flow within the pores of asphalt pavements is a viable alternative to characterize the directional hydraulic conductivity as well as pore pressures and viscous shear stresses.
Three-dimensional lattice Boltzmann (LB) fluid flow models were developed and validated using analytical solutions and laboratory experiments. An excellent agreement was observed with second order accuracy. Three-dimensional real pore structures of the specimens were generated using X-ray CT technique and used as an input in the LB models. Numerous steady and unsteady fluid flow simulations were conducted on different asphalt specimens to study the moisture transport characteristics. Analysis of hydraulic conductivity tensor indicated that the asphalt specimens are isotropic comparing two horizontal directions and anisotropic comparing horizontal and vertical directions. Therefore, it is recommended that a new field testing standard be developed to account for this anisotropy. Analysis of hydraulic conductivity at different depths revealed a rapid decrease in the hydraulic conductivity as the analysis depth was increased. The decrease was more pronounced when compaction effort was increased; therefore, the field compaction effort could be adjusted to control the depth of water penetration. Local pressure gradients and shear stresses at constrictions during steady fluid flow were up to one order of magnitude higher than their average values. Unsteady flow simulations revealed that dynamic hydraulic conductivities of asphalt specimens were relatively close to their steady values. The pressure gradients and viscous shear stresses due to dynamic flow were much higher at the surface as compared to their steady values and the dynamic effect decreased with increasing depth.