Evaluation of hydraulic permeability of open-graded asphalt mixes using a full numerical simulation

Permeability of open-graded asphalt mixes is defined as the ability to transmit fluids through the voids. The infiltration of water affects the durability and safety of pavement systems. Traditionally, an empirical approach, based on laboratory tests or on field measurements, is used to evaluate hydraulic permeability. In general, such an approach allows the ex post calculation of the drainage capability of the open-graded pavement. In order to evaluate the hydraulic permeability during the phase of mix design, in this paper a novel method is proposed, based on simulation of unsteady flow of water through open-graded mixture. In particular, a real sample of asphalt mix is simulated in the shape of a Marshall test sample. It is composed of a set of particles, selected according to the real grading, positioned in the space domain following the random sequence adsorption approach (RSA) and covered by a film of bitumen. The flow of water through the synthetical sample is simulated through the lattice Boltzmann method. In this method, the particles of water propagate and collide while moving on a lattice, obtained by discretizing the physical space. This lattice is formed by equidistant nodes, where each node corresponds to a void or to a solid. The velocity of the particles depends on a set of microscopic velocity vectors. Simulation results allow prediction of the permeability of open-graded pavement through a theoretical calculation based on the Kozeny-Carman equation. The model is validated using experimental tests