Page 42 - MODELING OF ASPHALT CONCRETE

20 Cha pte r T w o

through the SHRP project where major emphasis was placed on the development of
testing equipment and/or test protocols to standardize the use of asphalt tests that were
relatively blind to the binder composition. The test protocols, although developed with
the intention of being suitable to unmodified and modified binders, focused on linear
viscoelasticity as a compromise between the difficulty of rheological testing and the need
to produce practical and realistic test protocols for an industry that has historically used
mainly simplistic index testing to qualify asphalts (Anderson et al. 1991).
In 1992–1993 a new set of testing techniques and a new grading system was
introduced. The testing and grading systems are based on fundamental properties that
are related in a more rational way to pavement performance. The viscoelastic properties
of asphalt binders are measured at application temperatures and grading criteria are
based on sound understanding of pavement failure mechanisms.

The Viscoelastic Nature of Asphalt Binders
At any combination of time and temperature, viscoelastic behavior, within the linear range,
must be characterized by at least two properties: the total resistance to deformation and the
relative distribution of that resistance between an elastic part and a viscous part. Although
there are many methods of characterizing viscoelastic properties, dynamic (oscillatory)
testing is one of the best techniques to represent the behavior of this class of materials.
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In the shear mode, the dynamic modulus (|G |, for simplicity, denoted as G hereinafter)
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and phase angle (d) are measured. G represents the total resistance to deformation under
load, while d represents the relative distribution of this total response between an in-phase
component and an out-of-phase component. The in-phase component is an elastic
component and can be related to energy stored in a sample for every loading cycle, while
the out-of-phase component represents the viscous component and can be related to energy
lost per cycle in permanent flow. The relative distribution of these components is a function
of the composition of the material, loading time, and temperature.
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Rheological properties can be represented either by the variation of G as a function
of frequency at a reference temperature (commonly referred to as a mastercurve) or by
∗
the variation of G and d with temperature at a selected frequency or loading time,
commonly called isochronal curve. Although time and temperature dependency can be
related using a temperature-frequency shift function (Ferry 1980), for practical purposes
it is much easier to present data with respect to one of the variables. Figure 2-1 depicts
typical rheological properties of an AC-40 and an AC-5 asphalt binder at a wide range
of temperatures and frequencies. Figure 2-1a is a mastercurve at 25°C and Fig. 2-1b is an
isochronal curve at 10 rad/s. Some common unique characteristics of the rheological
behavior of asphalt can be seen in the typical plots of Fig. 2-1:

• At low temperatures or high frequencies, both asphalts tend to approach a
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limiting G value of approximately 1.0 GPa and a limiting d value of 0°. The 1.0
GPa reflects the rigidity of the carbon hydrogen bonds as the asphalts reach
their minimum thermodynamic equilibrium volume. The 0° value d represents
the completely elastic nature of the asphalts at these temperatures.
∗
• As the temperature increases or as the frequency decreases, G decreases
continuously while d increases continuously. The first reflects a decrease in
resistance to deformation (softening) while the second reflects a decrease in
elasticity or ability to store energy. The rate of change is, however, dependent