Page 28 - MODELING OF ASPHALT CONCRETE

6 Cha pte r O n e

Part 2—Stiffness Characterization
Part 2 of this book focuses on asphalt concrete stiffness. Stiffness is critically important
for mechanistic modeling of both the pavement response and the pavement performance.
Chapter 3 discusses explicitly the importance of this factor for such analysis and also
details the major factors affecting the material stiffness. Particular attention is paid in
Chaps. 4 and 5 to the stiffness characterization of asphalt concrete via the complex
modulus. Two different test methods are demonstrated. The first is part of the proposed
simple performance test protocol and involves testing cylindrical asphalt concrete
specimens in the axial direction. The second method strives to overcome shortcomings
associated with using the geometry of the first method to evaluate the stiffness of field
cores using the indirect tension test. There are numerous advantages to assessing
material stiffness via the dynamic modulus in the frequency domain; however, many of
the mechanistic models presented in this book require stiffnesses in the time domain.
Linear viscoelastic theory and mathematical manipulation are used in Chap. 6 to
demonstrate different methods of converting the dynamic modulus into time domain
functions such as creep compliance and the relaxation modulus.

Part 3—Constitutive Models
Part 3 of this book focuses on the constitutive modeling of asphalt concrete. Three
approaches are presented in detail in this part. These approaches utilize different
principles to describe the deformation behavior and performance of asphalt concrete,
but are similar in that they attempt to form a unified model encompassing different
performance characteristics by accounting for various constitutive factors.
Chapter 7 in this part incorporates the theory of viscoelasticity, continuum damage
mechanics, and the theory of viscoplasticity to arrive at a so-called viscoelastoplastic
continuum damage (VEPCD) model as a constitutive relationship for the behavior of
asphalt concrete. Implementation of the VEPCD model into the finite element program is
discussed. Chapter 8 presents a constitutive model based on the hierarchical disturbed
state concept (DSC). The chapter describes the capabilities of the DSC for various pavement
distresses such as permanent deformation and different types of cracking. Analysis of both
two- and three-dimensional pavement problems is given using the DSC model, and a
unified methodology with DSC for design, maintenance, and rehabilitation of pavement
structures is proposed. Chapter 9 uses the DBN (Di Benedetto and Neifar) law to describe
the behavior of asphalt concrete under a broad range of conditions. It explains how the
different types of behavior can be modeled using the same formulation.
Part 4—Models for Rutting
In this part, the mechanisms of permanent deformation are described and modeled in
two chapters. Information documented in Chap. 10 is the result of the SHRP A-003 study
and illustrates that shear deformation contributes a significantly greater portion of total
permanent deformation (rutting) in asphalt concrete than volume change. Based on these
findings, the shear test was proposed to measure the propensity of a mix for rutting. The
issue of sample size is discussed in the light of RVEs. The data presented illustrate the
efficacy of the simple shear test, performed in the repeated load, constant height mode,
for mix design and performance evaluation. Chapter 11 summarizes the findings from
the more recent NCHRP 9-19 project. This chapter is composed of three main sections: (a)
a review of mechanistic-empirical modeling approaches, and in particular the permanent-
to-resilient strain ratio model adopted for the NCHRP 1-37A MEPDG; (b) the VEPCD