Page 41 - Mechanics of Asphalt Microstructure and Micromechanics

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34 Ch a p t e r w o

2.1.1 Modeling of Asphalt Binder
Asphalt binder at relatively high temperatures is usually considered a viscous fluid.
Viscosity models in either linear, shear thinning, or shear hardening can be used to
model their behavior. At low temperatures, it behaves like an elastic medium. In the
temperature range encountered in pavement, it is a viscoelastic/viscoplastic material.
Several models that are often used in AC include the Kelvin Model and the General
Maxwell model for this range of temperatures. For viscoplastic behavior, one may adapt
the viscoplastic models presented in Chapter 7.
Asphalt binder is a polymeric material. It consists of several constituents including
asphaltenes, resins, aromatics, and saturates. Resins, aromatics, and saturates are col-
lectively named as maltenes. These constituents have different properties (Whiteoak,
1990). Depending on the volume fractions of these constituents, asphalt binder could
have quite different physical, chemical, and mechanical properties. The determination
of the chemical composition of polymers can be achieved by polarity analysis (Petersen,
1984), differential thermal analysis, and differential scanning calorimetry. A detailed
literature review on binder composition analysis and the binder composition mechani-
cal properties relationship can be found in (Cheung and Cebon, 1997a; Anderson et al.,
1994). For simplification, only a brief description is presented here so that readers can
have some working knowledge about composition and properties, which link the entire
book, from micro-structure to macro-behavior.

2.1.2 Glass Transition of Asphalt Binder
Glass transition is an important concept in understanding the properties of asphalt
binder. It is a temperature at which a material transitions from a liquid to solid or vice
versa. The glass transition temperature of asphalt binder ranges from –40°C to 0°C. It
increases with the volume fraction of asphaltenes for the binder from the same source
(Wada and Hirose, 1960; Hirose et al., 1963; Schmidt and Barral, 1965). The empirical
concept of high or low temperatures is actually related to this temperature.

2.1.3 Linear Viscous Model
At temperatures well above the glass transition temperature, asphalt binder behaves
like a viscous fluid. Its stress is related to the strain rate in the following equation:
σ = ηε (2-1)
Where h is the viscosity.

Generally, this relationship is not followed. The viscosity usually varies with strain
rates. If the viscosity decreases with the strain rate, it is shear thinning. The opposite
is named as shear thickening. The power law model better describes the behavior of
binder.

2.1.4 The Power Law Model (PLM)
At a particular temperature, the power law equation can be written as:

(2-2)

·
Where n is the creep exponent, and e r is the strain rate at a reference stress s r .