Page 45 - MODELING OF ASPHALT CONCRETE

Modeling of Asphalt Binder Rheology and Its Application to Modified Binders 23

of loading of the pavement under traffic needs to be simulated in measurement to
obtain a reliable estimate of binder contribution to pavement performance.
The third temperature zone is the low-temperature zone at which thermal cracking
(Chaps. 14 and 15) is the prevailing failure mode. During thermal cooling, asphalt
stiffness increases continuously and thus results in higher stresses for a given shrinkage
strain. Simultaneously, thermal stresses relax due to viscoelastic flow of the binder. To
reliably predict binder contribution to cracking, both the stiffness of a binder and its
rate of relaxation need to be evaluated. The stiffness of the binder is directly proportional
∗
to G and the rate of relaxation is directly related to d. A lower stiffness and higher rate
of relaxation are favorable for resistance to thermal cracking. As with other temperature
zones, a single measure of the stiffness or viscosity of the binder is not sufficient to
select better binders that will resist cracking at the lowest pavement temperatures.
The above discussion of the relation between asphalt binder properties and pavement
performance is further complicated by the aging phenomenon. Asphalts are hydrocarbon
materials that oxidize in the presence of oxygen from the environment. This oxidation
process changes the rheological and failure properties of the asphalt. As shown in Fig. 2-2,
∗
the rheological mastercurve slope decreases with aging, which indicates higher G and
smaller d values for the unaged binder at all temperatures. These changes translate into
∗
less sensitivity of G and d to temperatures or loading frequency and into more elastic
component (lower d). Significant oxidation effects usually appear after considerable
∗
service life. Increased G values and lower d values are favorable changes with respect to
rutting performance, but they are unfavorable for thermal cracking performance. For
∗
fatigue cracking, the increase in G is not favorable while the decreased d is generally
favorable, depending on the type of pavement and mode of fatigue damage.

Modeling of the Viscoelastic Properties of Asphalts
Many attempts have been made to use simple mechanical analog, such as the generalized
Burgers model and the Prony series, and phenomenological models, defined by curve
fitting of experimental data, to describe the viscoelastic properties. The latter approach
has seen more acceptance particularly with the advancement of computers and the
flexibility of these models. Some of the most notable models that followed the pioneering
work by Van der Poel in the early 1950s include work by Jongepier and Kuilman (1969)
who proposed that asphalts can be considered as simple liquids whose rheological behavior
can be approximated by log Gaussian distribution of relaxation times. These authors used
a width parameter to represent the dependency of rheological behavior on loading time
and an equiviscous temperature to represent the temperature dependence. Dickinson and
Witt (1974) reported a study related to the work done by Dobson (1969). The authors
proposed a new mathematical function for representing the loading-time dependency of
the rheological parameters and adopted the same mathematical functions developed by
Dobson for the temperature dependency (Dobson 1969). These early models were evaluated
in several following works, and the accuracy of the models have been tested using many
types of unaged as well as aged asphalts (Pink et al. 1980; de Bats and Gooswilligen 1995;
Maccarrone 1987). These researchers, although sometimes not consistent, in their
observation all agree that asphalts can indeed be represented as linear viscoelastic materials
that are thermorheologically simple. They also agree that to characterize such materials
two behaviors need to be defined: the dependency of rheology on loading time, and the
dependency of rheology on temperature.