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Unified Disturbed State Constitutive Modeling of Asphalt Concr ete 213

of separate viscoelasticity with plasticity, damage, and fracture mechanics models may
not lead to unified and economical models for pavement materials.

Unified Model
Hence, although continuing improvements have occurred for pavement distress analysis,
no unified mechanistic models have yet been developed and validated for design,
maintenance, and rehabilitation. A unified model should be able to characterize all
significant material responses in a single framework. This chapter presents an integrated
methodology based on the unified constitutive model called the disturbed state concept
(DSC) for modeling of pavement materials, interfaces, and joints. It is believed that the
DSC with two- and three-dimensional computer FE procedures provides a fully
mechanistic approach considered to be desirable in pavement engineering. It can provide
a unified model that is considered to be superior to other available models including the
ad hoc combinations described before.
Factors in Mechanistic Unified Model
The basic issue is the prediction of the performance of a pavement under repetitive
mechanical and environmental (thermal, fluid, etc.) loadings. The mechanical loading
is due mainly to the repeated application of traffic-wheel load. The thermal-loading
arises form the variation of temperature with time—daily and seasonal. The fluid in
pavement materials can be due to the ingress of water, which may lead to full or partial
saturation of the materials.
In some conventional procedures, the materials in the pavement are assumed to be
linearly elastic and isotropic; then, the models such as elastic layered theory are used to
predict displacements, stresses, and strains (Huang 1993). However, both the bound
and unbound materials in the pavement exhibit nonlinear behavior, which is affected
by factors such as the state of stress and strain; initial or in situ conditions like stress,
pore water pressure, and inhomogeneities; irreversible (plastic) deformations; viscous
or creep response; stress path; volume change; anisotropy; temperature; fluid and type
of loading. Hence, although the assumption of elastic behavior may yield satisfactory
results, their validity is highly limited. For a full mechanistic characterization, it is
necessary to use constitutive or material models that allow for the foregoing factors.

Disturbed State Models
The descriptions and statements of following topics are included in mechanistic DSC
approach: (1) brief description of the unified and hierarchical DSC constitutive model,
(2) capabilities of the DSC for various pavement distresses such as (a) permanent
deformations; (b) microcracking, fracture, and reflection cracking; and (c) thermal
cracking, as affected by plastic and creep strains under mechanical and environmental
loading, (3) identification of parameters in the DSC model and their determination
from laboratory tests, (4) validation of laboratory test data using the DSC model,
(5) implementation of DSC in two- and three-dimensional finite element procedures,
(6) statement of validation for a number of laboratory simulated and field problems in
geotechnical and pavement engineering, and analysis of both 2-D and 3-D pavement
problems, and (7) unified methodology with DSC for design, maintenance, and
rehabilitation of pavement structures.