Superpave Protocols for Modified Asphalt Binders
Project NCHRP 9-10
Superpave Protocols for Modified Asphalt Binders
Summary of Research Hypothesis and Concept for the
Experimental Testing Plan of Modified Binders
Dr. Hussain U. Bahia
Principal Investigator
University of Wisconsin - Madison
Overview
NCHRP 9-10 is a research project aimed at evaluating the veracity
of the Superpave protocols in testing modified asphalt binders.
This summary describes the revised working plan for Phase II
of the project. It includes the experimental program developed
to fully characterize a representative set of modified asphalt
binders (Task 4), the plans for modification of Superpave binder
protocols to allow their use for modified asphalt binders (Task
5), and the plans to prepare an experimental program to evaluate
the effect of modified binders on Superpave mixture performance
tests.
The results of the literature search and surveys conducted under
Task 1 of the project were used to revise the working plan originally
proposed for Task 4. A new classification system of modified
binders was established to separate modified binder into simple
binders and complex binders. A total of 17 modifiers were selected
for the project. Two asphalt binders with different chemistry
were selected for the study. These asphalts are from the source
as the SHRP asphalts AAK and AAZ. The main criteria used for
selecting these asphalts were asphaltene content and being one
of the sources used in the SHRP program.
The revised experimental program includes three main tasks. Task
4A is the largest task that will focus on testing simple modified
binders. Task 4B will include testing complex binders modified
with selected additives that are known to result in complex binder
systems. Task 4SS includes four special studies that focus on
special characteristics of modified binders and evaluating specific
testing protocols that are not part of the current Superpave testing protocols.
Classification of Modified Binder
The Superpave binder test protocols are based on two main assumptions:
- The binder behavior is independent of the film thickness and
sample geometry.
- That the binder is evaluated based on the properties within
the linear visco-elastic range within which the behavior is independent
of the strain or stress level.
The essences of these two assumptions is that the asphalt binder
is a simple system that can be characterized using linear visco-elasticity
and simple geometries within which stress and strain fields are
simple to calculate. 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, modified binders
should be classified into simple binders and complex binders described
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
- Wide linear range (independence of strain),
- Non-thixotropy (independence of mechanical working effects)
- 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.
Simple binders can be evaluated using the existing Superpave binder
protocols and can be classified as PG-graded binders. Complex
binders should be evaluated using Superpave mixture protocols
or specialized testing protocols. Although binder protocols can
still be used to characterize certain aspects of their behavior,
they cannot be considered as PG-graded binders.
Overall Research Hypothesis
The role of simple binders in mixture and pavement performance
can be estimated using the existing (or 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.
Testing Program
The testing program is divided into three main tasks
- TASK 4A : Characterization of Selected Simple Modified Binders
- TASK 4B: Characterization of Selected Complex Modified Binders
- TASK 4SS: Special Studies to Evaluate Properties of Modified
Binders
- Subtask 4SS1: Effect of size and concentration of particulate
additives
- Subtask 4SS2: Protocol for measuring potential of thermal
degradation of additives.
- Subtask 4SS3: Protocol for separation of effects of oxidative
aging in the PAV procedure.
- Subtask 4SS4: Protocol for measuring potential for phase separation
of additives.
For more information on project NCHRP 9-10, please contact Dr. Hussain U. Bahia.
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