Page 34 - MODELING OF ASPHALT CONCRETE

12 Cha pte r T w o

asphalts used in the United States are modified asphalts. Modification of asphalts is,
therefore, an integral part of this field and understanding its variables is necessary.
Modification of asphalt binders is usually performed to improve one or more of the basic
asphalt properties that are related to one or more of the pavement distress modes (Bahia
1995; Terrel and Epps 1989). The basic properties that have been targeted include

Rigidity: Total resistance to deformation which can be measured by complex
∗
moduli like G under dynamic loading or by creep stiffness, S(t), under quasi-
static loading. Higher rigidity is favorable at high temperatures or low loading
rates to resist rutting while lower rigidity is favorable at intermediate and low
temperatures to resist fatigue and thermal cracking, respectively.
Elasticity: Recovery of deformation using stored energy applied. It can be
measured either by the phase angle (d) or by the logarithmic creep rate (m). To
resist rutting and fatigue damage more elasticity is favorable. To resist thermal
cracking, less elasticity and more ability of relaxing stress by flow is favorable.
Brittleness: Failure at low strains is the best definition of brittleness. To improve
resistance for fatigue and thermal cracking, brittleness should be reduced by
enhancing strain tolerance or ductility.
Storage stability and durability: Oxidative aging, physical hardening, and
volatilization are key durability properties. Resistance to all of these changes is
favorable.
Resistance to accumulated damage: Rutting and fatigue damage are recognized
as the two most important load induced types of distress. They represent
progressive failure mode which is not necessarily measured using small stress or
small strain testing.

Modeling Critical Properties of Asphalt Binders
Critical properties of asphalt binders can be classified into two main groups:
mechanical properties and durability properties. Testing of mechanical properties can
be further sorted, based on progress in testing technologies into three groups: traditional
rheological, linear viscoelastic, and damage resistance characterization. Durability
properties include aging during production and construction and for modified binders,
compatibility or storage stability are important properties. The following sections describe
these different properties.

Traditional Rheological Properties
Traditional or index rheological properties include many standardized tests that were
used mostly before the completion of the Strategic Highway Research Program (SHRP)
in the early 1990s. Although in North America and some European countries a new set
of rheological test methods are rapidly being introduced, the traditional tests continue
to be used widely in other parts of the world. Table 2-1 includes a list of the main
traditional tests (Isacsson and Lu 1995).
The most common independent traditional measures used for asphalts include
penetration, ductility, softening point, and viscosity. The problem with the first three
measures, in addition to being one point measurements, is that the empirical nature
complicates the derivation of any meaningful engineering property. The problem with