Page 228 - MODELING OF ASPHALT CONCRETE

206 Cha pte r Ei g h t

versatile approach for constitutive modeling of materials in the pavement structures,
and with the nonlinear FE codes can provide a novel approach for analysis and design
in pavement engineering. Further research could involve detailed (laboratory) testing of
asphalt concrete, concrete, and interfaces in the pavements, together with measurements
and validations of simulated (in the laboratory) and field problems.

Introduction
The need for improved mechanistic procedures for design, maintenance, and
rehabilitation of highway and airport pavements has been recognized for many years
now. Mechanistic procedures are based on the principles of mechanics in contrast to ad
hoc and empirical procedures that are often used.
Accurate predictions for the response of pavements under mechanical and
environmental loads require consideration of important factors such as multi-
dimensional geometry, realistic loading, and appropriate constitutive models. A vital
ingredient that influences the response is the nonlinear behavior of materials in
pavements. Elastic, plastic, and creep strains, microcracking, softening, and healing
under repetitive mechanical and environmental loading, and initial or in situ stress
conditions, are among the important characteristics that need to be considered in
modeling and testing for the nonlinear behavior.
In order to incorporate the nonlinear material response and multidimensional
effects in the solution procedures for design, maintenance, and rehabilitation, it becomes
necessary to invoke the basic principles of mechanics so as to develop unified and
mechanistic procedures.

Scope
The scope of this chapter includes (1) brief review of some existing models for materials,
(2) discussion of the limitations of existing procedures based mainly on empirical and/
or empirical-mechanistic approaches, (3) description of the unified modeling approach
called the disturbed state concept (DSC) that provides a mechanistic model for the
significant factors that influence behavior of pavement materials, (4) brief description
of two- and three-dimensional nonlinear finite element computer procedure in the
implementation of the DSC models, and (5) description of the capabilities of the DSC
procedures to handle major distresses: permanent deformation (rutting), microcracking
and fracture, and reflection cracking under mechanical and thermal loading, and typical
application and validations. This chapter is based on various previous publications, for
example, on Desai (2001), Desai and Ma (1992), and Desai et al. (1986) for development
of constitutive and computer models, and on Desai (2002, 2007) for their application for
pavement analysis. The major emphasis in this chapter is given to asphalt concrete.

Approaches for Pavement Analysis and Design
Figure 8-1 shows a schematic of various approaches for design, maintenance, and
rehabilitation of pavements. The empirical (E) approach is based on experience and/or
knowledge of certain index properties such as California bearing ratio (CBR), limiting
shear failure and limiting deflections (Huang 1993). The index and empirical models do
not include effects of multidimensional geometry, loading, material behavior and