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
University of Wisconsin - Madison
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
- 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.
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
- 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.