Page 64 - MODELING OF ASPHALT CONCRETE

42 Cha pte r T w o

1. Wide linear range (independence of strain)
2. Nonthixotropy (independence of mechanical working effects)
3. Isotropy and independence of sample geometry (no additives that result in
geometric effects)
Complex Binders: Asphalt binders that cannot be classified as simple binders because
their behavior violates one or more of the PG-grading system assumptions.

This new classification is based on the hypothesis that the role of simple binders in
mixture and pavement performance can be estimated using the existing (or slightly
revised) Superpave binder protocols, regardless of their constituents or the method of
production. The role of complex binders in mixture and pavement performance, on the
other hand, cannot be estimated using binder testing. Mixture testing will have to be used.
An asphalt binder can be classified as a complex binder because of the physical
characteristics of the modifier or because of the nature of the effect of the modifier. Binders
modified with particulate matter can be complex because of their dependency on sample
geometry. Other binders can be complex because they are thixotropic or strain dependent.
To qualify asphalt as a simple binder, two initial tests were recommended: (a) strain
sweep, and (b) time sweep. Each should be done at high and intermediate temperatures. Both
these tests are easily performed using the current version of the Dynamic Shear Rheometer
(DSR). A change by more than 10 percent for either test will indicate a complex behavior.
If the binder passes both tests, an evaluation of the nature of additive was
recommended. A new test called the PAT is proposed to separate the additive and
evaluate its nature. If the binder does not contain more than 2 percent by volume of a
particulate additive, and passes the strain and time sweep tests, it can be graded as a
simple binder and can be graded according to the PG system.
The current Superpave-testing protocols do not cover stability under nonoxidative
conditions. The stability under nonoxidative conditions requires a new test method.
The LAST was proposed to measure potential for phase separation and thermal
degradation of asphalts. The LAST can be used to simulate the conditions of hot storage
with and without agitation under minimal exposure to oxidation. It is proposed that
potential rate factors for degradation (K ) and for separation (K ) be calculated from the
d s
results of this test used to evaluate the stability of asphalts. In addition to these new
tests, the RTFO test needs modification to make it more effective in handling the highly
viscous polymer modified asphalts. The following sections give a brief overview of the
PAT and the LAST. A detailed description of the background and the initial data
collected is available in another publication by the author (Bahia 1999).
The finding of the study on modified asphalts in the NCHRP 9-10 project (Bahia et al.
2001) resulted in considering a different approach to testing of asphalts in general. The
approach is to evaluate damage resistance as apposed to linear viscoelastic properties. It
is becoming clear that the critical role of modification is not reflected in the linear
viscoelastic range but is more profoundly shown in the damage resistance range. The
following section describes this approach.

Damage Resistance Characterization
In the previous section the limitations of applying the Superpave binder specification to
modified binders was described. It was shown that modified asphalts are generally
nonlinear viscoelastic materials that exhibit complex rheology. They also exhibit unique