A Coupled Micromechanical Model of Moisture-Induced Damage in Asphalt Mixtures: Formulation and Applications

Abstract

The deleterious effect of moisture on the structural integrity of asphalt mixtures has been recognized as one of the main causes of early deterioration of asphalt pavements. This phenomenon, usually referred to as moisture damage, is defined as the progressive loss of structural integrity of the mixture that is primarily caused by the presence of moisture in liquid or vapor state. Moisture damage is associated with the development of different physical, mechanical, and chemical processes occurring within the microstructure of the mixture at different intensities and rates. Although there have been important advancements in identifying and characterizing this phenomenon, there is still a lack of understanding of the damage mechanisms occurring at the microscopic level. This situation has motivated the research work reported in this dissertation. The main objective of this dissertation is to formulate and apply a numerical micromechanical model of moisture-induced damage in asphalt mixtures. The model focuses on coupling the effects of moisture diffusion—one of the three main modes of moisture transport within asphalt mixtures—with the mechanical performance of the microstructure. Specifically, the model aims to account for the effect of moisture diffusion on the degradation of the viscoelastic bulk matrix of the mixture (i.e., cohesive degradation) and on the gradual deterioration of the adhesive bonds between the aggregates and the asphalt matrix (i.e., adhesive degradation). The micromechanical model was applied to study the role of some physical and mechanical properties of the constitutive phases of the mixtures on the susceptibility of the mixture to moisture damage. The results from this analysis suggest that the diffusion coefficients of the asphalt matrix and aggregates, as well as the bond strength of the aggregate-matrix interface, have the most influence on the moisture susceptibility of the mixtures. The micromechanical model was further used to investigate the influence of the void phase of asphalt mixtures on the generation of moisture-related deterioration processes. Two different probabilistic-based approaches were used to accomplish this objective. In the first approach, a volumetric distribution of air voids sizes measured using X-Ray Computed Tomography in a dense-graded asphalt mixture was used to generate probable void structures in a microstructure of an asphalt mixture. In the second approach, a stochastic modeling technique based on random field theory was used to generate probable air voids distributions of the mixture. In this second approach, the influence of the air voids was accounted for by making the physical and mechanical properties of the asphalt matrix dependent on probable voids distributions. Although both approaches take into consideration the characteristics of the air void phase on the mechanical response of the mixtures subjected to moist environments, the former explicitly introduces the air phase within the microstructure while the latter indirectly includes its effects by modifying the material properties of the bulk matrix. The results from these simulations demonstrated that the amount, variability and location of air voids are decisive in determining the moisture-dependent performance of asphalt mixtures. The results from this dissertation provide new information on the kinetics of moisture damage mechanisms in asphalt mixtures. In particular, the results obtained from applying the micromechanical model permitted identification of the relative influence of the characteristics of the constitutive phases of a mixture on its moisture-related mechanical performance. This information can be used as part of design methodologies of asphalt mixtures, and/or as an input in life-cycle analysis models and maintenance programs of road infrastructure