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Modeling of Asphalt Binder Rheology and Its Application to Modified Binders 41

the response under loading can be highly dependent on sample geometry. Additives
such as fibrous materials with high aspect ratio, and other particulate additives of
relatively large random shapes are examples that can result in anisotropic behavior of
the modified binders. It is known that as the ratio of particle size to sample size increases,
and/or as the volume concentration of the additive increases, the geometry of the
sample can interfere with the measurement. The size and the volume concentration of
particulate additives are not fully controlled in the current specification. The only
existing limitation is particulates should not exceed 250 μm, which was selected
arbitrarily (AASHTO TP5). Size and volume concentrations are both required in limiting
the amount of additive that can result in interference with measurements. It is, therefore,
necessary that the presence of solid additives be detected and the nature of the additive
be checked. A new test, called the PAT is described in this article to address this need.
Stability of binders in the current Superpave binder specification has also been
oversimplified by considering oxidation as the only mechanism by which asphalt binder
properties can change in service. Modified asphalt binders, however, can undergo changes
due to factors other than oxidation. It is recognized that modified binders are multiphase
systems in which the modifiers are dispersed into the asphalt cement phase. This dispersion
is generally accompanied by a degree of incompatibility that is affected by various physical,
thermal, and chemical factors. Excessive incompatibilities can negatively affect the
performance of these binders as gross separation occurs during storage and handling of
the binder, production and transportation of asphalt mixtures, and during the construction
of pavement layers. There are four separation mechanisms (physical, thermal, chemical,
and oxidation) that need to be considered. In the current Superpave system, there are
protocols for characterizing the oxidative stability [Pressure Aging Vessel PAV and RTFO].
There are also proposed protocols for measuring physical separation (cigar tube test).
There are, however, no provisions to separate these effects and to take into account the
different physical, thermal, and chemical treatments expected in the field.

New Classification of Asphalt Binders
Two main conclusions can be drawn from the review of the Superpave binder test
protocols and specification and the existing knowledge of modified binders:

1. The existing protocols cannot be used to fully characterize all asphalt binders
modified with different additives. The main reason is that they are based on
simplifying assumptions that cannot be reliably extended to modified binders.
2. Some additives can result in binders that are too complex to be evaluated by
any binder-only protocols. Such additives will result in anisotropy or
interference with testing geometry such that only actual replication of films
that will exist in mixtures will allow reliable estimation of their role in pavement
performance.

To apply the current Superpave binder protocols for modified binders these two
conditions have to be satisfied. In other words, the modified binders have to be “simple”
rheological systems. Based on this concept, asphalt binders should be classified into
simple binders and complex binders, as follows.
Simple Binders: Asphalt binders with simple behavior that do not violate the
assumptions, which the PG-grading system is based upon; these assumptions
include