Analysis of open graded pavement performance through microstructure and hydraulic simulation

Abstract

The infiltration of water causes lots of adverse effects on flexible pavement and reduces its performance. Moisture related damage in asphalt pavements, that consists in the loss of strength and durability caused by the presence of water, is global concern. This kind of damage is induced by the loss of bond between aggregate and binder surface and loss of bond within the binder itself. These two modes of failure occur in asphalt pavements as stripping, ravelling, cracking and even permanent deformation. To prevent these problems and to control the conditions of the mix, one of the primary requirements for a flexible pavements, since the advent of asphalt paving technology, is that hot mix asphalt are impermeable. By minimizing moisture infiltration, adequate support from the underlying unbound materials is obtained. In 1993, Superior Performing Asphalt Pavement was introduced as a part of Strategic Highway Research Program. With the adopting of Superpave mix design system, open-graded pavements have been produced with coarser gradation. This application of novel material limited distresses, however other issues related to higher permeability values have to be considered. The traditional approach used to evaluate the expected drainage capability of open-graded pavement is controversial. It is based on laboratory tests using hydraulic permeameters or measures on the field, and it is unable to estimate correctly the permeability due to many approximations in measurement and to the wide variety of variables that influence the hydraulic behavior of asphalt pavement. It often happens that the mixture is designed adopting an overestimated value for hydraulic conductivity. It increases the susceptibility to moisture induced damage in the asphalt pavement, promoting the oxidation of asphalt, and it produces many disadvantages and problems from an economic and environmental point of view. Mix design has consequently to be approached as an optimization problem. In order to solve this problem in the last decades several researchers have attempted to calculate permeability of porous media through analytical model, by defining empirical formulas derived from 4 experimental experiences. These formulas are not able to define exactly and univocally the infiltration process, causing uncertainty and approximation of prediction. Recently, in order to investigate fluid flow in open-graded mixes, some studies, based on numerical hydrodynamic simulations at the micro-structural levels, have been carried out. These models require the knowledge of the internal microstructure, that produce expensive and onerous experimental procedures. In this thesis a novel simulation approach is proposed, based on different numerical algorithms, in order to predict the properties of an open-graded asphalt pavement during its service life. This novel approach reduce computational complexity of the numerical models and increase the reliability of empirical methods. The simulation model allow to investigate three main characteristics of open-graded pavement, namely: i) physical characteristics, through generation of the physical microstructure of the mixture from the design inputs using RSA algorithm; ii) hydraulic properties, by the simulation of flow of water within the numerical sample, based on the Lattice– Boltzmann method; iii) acoustic performance, by the creation of a new experimentation and by the definition of additional variables. The results of the new procedures, validated through experimental data and analytical prediction, are encouraging in order to support and to optimize the design of the open-graded asphalt mixes.