Chapter 1. Introduction
1.1. Problem Statement
Asphalt concrete pavement, one of the largest infrastructure components in the United States, is a complex system that involves multiple layers of different materials, various combinations of irregular traffic loading, and various environmental conditions. Therefore, a realistic prediction of the long-term service life of asphalt pavements is one of the most challenging tasks for pavement engineers. The performance of asphalt concrete pavements is closely related to the performance of asphalt concrete. In order to predict the performance of asphalt concrete with reasonable accuracy, a better understanding of its deformation behavior under realistic conditions is urgently needed.
Asphalt concrete is a viscoelastic particulate composite that consists of aggregate particles and an asphalt binder matrix. When the asphalt-aggregate composite is subjected to repeated traffic loading at low temperatures, distributed microstructural damage occurs primarily in the forms of microcrack nucleation and growth due to the embrittled binder and high-stress concentrations along the aggregate-binder interfaces. Therefore, the role of the binder and the variables that influence the properties of the binder (e.g., aging, adhesion, etc.) become important to the study of this type of damage. At high temperatures, the asphalt binder becomes too soft to carry the load, and thus, the principal type of damage is permanent deformation due to volume change (i.e., densification) and rearrangement of aggregate particles. Therefore, a reliable performance prediction model should account for the effects of various constitutive factors that affect the aggregate-binder and aggregate-aggregate interactions.
With the goal of accurate pavement performance evaluation, researchers at North Carolina State University (NCSU) have been developing advanced models for asphalt concrete under complex loading conditions. Over the past decade, they have successfully developed material models that can accurately capture various critical phenomena such as microcrack-induced damage, which is critical in fatigue modeling, strain-rate temperature interdependence, and viscoplastic flow, which is critical for high-temperature modeling. The resulting model is termed the viscoelastic continuum damage (VEPCD) model. While the initial development of the VEPCD model focused on uniaxial tension behavior, the accurate performance prediction of an asphalt mixture in a pavement structure requires a multidimensional model.
To predict the performance of real pavement structures, it is also important to incorporate the material model in a pavement model that considers the vehicle and climatic loads as well as the boundary conditions. The finite element method is best suited for this purpose due to the nonlinear material behavior. The group has an in-house finite element code (FEP++) which can analyze general nonlinear dynamical systems. FEP++ is a research code and requires several modifications in order to be used for routine pavement modeling.
1.2. Objectives
The long-term goal of the asphalt pavement modeling research at NCSU is to develop a mechanistic asphalt pavement performance prediction methodology that can be used by State highway agencies. This research focuses on the following objectives to accomplish this goal:
- To develop a multiaxial viscoelastoplastic continuum damage (MVEPCD) model for asphalt concrete in both compression and tension.
- To enhance FEP++ so that nonlinear analysis of pavements can be easily conducted.
1.3. Research Scope
This research includes four of the mixtures used in the Federal Highway Administration's (FHWA) Accelerated Load Facility (ALF) current study that is funded through the pooled-fund study, TPF-5(019). Three of the mixtures contain asphalt binders modified with polymeric additives; the remaining mixture contains an unmodified asphalt binder. Material behavior under both tension and compression is addressed in this research. For the current research in multiaxial model development, only the unmodified mixture is being used.
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