08-11-2012, 11:17 AM
Characterization of Modified Asphalt Binders in Superpave Mix Design
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INTRODUCTION AND RESEARCH APPROACH
This section of the report presents the statement of the
problem, objectives, and descriptions of the phases and tasks
of the research project.
STATEMENT OF PROBLEM
NCHRP Project 9-10, “Superpave Protocols for Modified
Asphalt Binders,” was initiated to confirm whether the binder
and mixture test methods of Superpave®, an asphalt–aggregate
mixture design and analysis system developed under the Strategic
Highway Research Program (SHRP), are generally suitable
for use with modified asphalt binders. If they are not, the
Superpave methods may need to be altered to better characterize
modified binders and hot-mix asphalt (HMA) containing
modified binders. By the same token, the specification
limits, criteria, and models developed through the SHRP
asphalt research program may require revision.
OBJECTIVE
The objectives of this research project were to
1. Recommend modifications to Superpave asphalt binder
tests for modified asphalt binders, and
2. Identify problems with the Superpave mixture performance
tests in relation to mixtures made using modified
binders.
The project did not include a significant field validation of
the results. Additional research work, using field performance
data from other projects, is needed to evaluate and refine the
findings of this project. This additional research, if conducted,
should be focused on establishing appropriate binder specification
limits for modified asphalt binders, as well as criteria
and models for mixtures containing modified asphalt binders.
FINDINGS
TYPES OF ASPHALT MODIFIERS
Asphalt modifiers can be classified in several ways: on the
mechanism by which the modifier alters the asphalt properties;
on the composition and physical nature of the modifier;
or on the target asphalt property that needs improvement
or enhancement. Based on the review of published literature
(1–12), and the survey information collected in Phase I of the
project (13), a list of the types of modifiers used currently in
the asphalt industry is given in Table 2.1. The modifiers are
classified based on the nature of the modifier and the generic
types of asphalt modifiers. The target distress shown in the
table corresponds to the main distress the additive is expected,
or claimed, to reduce. The information is based on an interpretation
of the published information for brands of modifiers
that belong to the modifier classes shown. In many cases the
reported effects are based on limited data and should not be
generalized to all asphalt sources.
CHARACTERISTICS OF MODIFIED BINDERS
In Phase II of the project, extensive rheological and failure
testing was conducted on a large number of modified binders
with the Superpave binder protocols. To address the deficiencies
in the current testing protocols, methods for determining
the type and amount of additive (the PAT) and for
determining the storage stability (the LAST) were developed.
In addition, a modification of the current rolling thin film
oven test (AASHTO T240) was developed. The results were
summarized in the second interim report of the project (30);
the main findings are discussed in the following subsection.
PAT
One of the alternatives to using microscopy to determine
the nature of the asphalt additives is separation of the additive
from asphalt. With separation, the general type of the
additive and its characteristics can be determined. In a PAT,
a diluted solution of the asphalt binder is passed through a
sieve to separate particulate additives from the base asphalt.
Particulate additives can result in potential separation or in
interference with test sample geometry. In the current standard
Superpave binder test methods, the particulate size is
limited to 250 μm, selected arbitrarily as one-fourth (1⁄4) of
the minimum testing sample dimension. The PAT separates
material larger than 75 μm using a No. 200 (0.075-mm)
mesh. This size was selected because larger-size particulates
are commonly considered part of the mineral aggregates in
the asphalt mixture.