Page 60 - MODELING OF ASPHALT CONCRETE

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Complexity of Modified Binders
As discussed previously, one of the main objectives of the SHRP was to develop test
methods for characterization of asphalts that are equally applicable to unmodified or
modified asphalt cements, collectively called asphalt binders (Anderson et al. 1994).
There were, however, two problems that raised concerns about applicability of PG
specification to all asphalt binders. The first was that the majority of the testing during
SHRP was done on unmodified asphalts of certain PG grades that did not cover the
extreme grades required by the new specifications. A review of the asphalts included in
the SHRP Materials Reference Library (MRL) indicates that they range between a PG
64-28 and a PG 46-34 with one PG 70-22. This range of grades does not cover extreme
grades that are being specified for high-volume traffic in warm regions and grades
being considered in many cold regions. The second problem was the fact that these
extreme grades, such as 76-22, 82-22, 64-34, and 58-40 did not exist at the time SHRP
research was active.
The concerns about the Superpave binder specification applicability to all asphalt
binders resulted in the initiation of the NCHRP 9-10 project, “Superpave Protocols for
Modified Asphalt Binders” (Bahia et al. 2001). The first phase of the project included a
survey of users and producers of modified binders to identify the types of asphalt
additives most commonly used in practice, to summarize concerns about the use of
Superpave protocols for modified asphalts, and to define the current and future needs
for modified asphalts. It also included a comprehensive literature review to evaluate
the research done to evaluate modified binders using the Superpave protocols.
The first phase resulted in recommendation for classifying asphalt binders into
simple and complex binders. Based on this classification, it is recommended that the
Superpave binder specification be used for asphalts that exhibit simple rheological
behavior. The first phase has also resulted in the recommendation for the addition of
new or revised testing procedures to characterize specific properties that are important
for asphalts modified with additives. These procedures include modification of the
rolling thin film oven test (RTFOT) procedure, development of the particulate additive
test (PAT) and the laboratory asphalt stability test (LAST) (Bahia et al. 1998). The
following sections cover the details of the deficiencies in the existing SHRP PG grading
protocols and the recommended modifications.

The Assumptions in the Superpave Binder System
The Superpave binder specification contains criteria based on assumptions that were
made to simplify the testing required and evaluate characteristics that are most critical
to pavement performance. These assumptions although were validated for neat
asphalts, may not be valid for asphalts modified with different additives. Based on
detailed review of the SHRP Project A-002A report (Anderson and Kennedy 1993;
Anderson et al. 1994) and other recent published literature (Bahia et al. 1998; Bahia et al.
1999), the following assumptions are found to be the most important that are related to
the behavior of modified binders.
1. No strain/stress dependency of rheological response (wide linear viscoelastic
range).
2. No shear rate dependency of viscosity (wide Newtonian range).
3. Testing at one loading rate is sufficient (similar loading rate dependency).