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Expert Task Group on Asphalt Mixtures & Construction; Asphalt Binders; and Models Technology April 2001 - Chairman's Report
Mr. Joseph A. Mickes
Dear Mr. Mickes:
The TRB Superpave Mixture and Aggregates Expert Task Group (ETG) met on April 3 and 4, 2001 in Tempe, Arizona. A summary of the issues discussed, which resulted in an action by the ETG is given below:
Superpave Gyratory Compactor (SGC) Comparisons and Calibration
A number of states and suppliers in the asphalt industry have become more concerned about the correlation between various Superpave Gyratory Compactor (SGC) models. While many of the SGC models currently available on the market have all been evaluated in accordance with AASHTO PP35 (Standard Practice for Evaluation of Superpave Gyratory Compactors), there are still a number of reports that indicate that various compactors may give significantly different results for certain mixes.
One area that has surfaced as a potential source of the variability is the angle of gyration. AASHTO T312 (Method for Preparing and Determining the Density of Hot Mix Asphalt Specimens by Means of the Superpave Gyratory Compactor) requires the angle to be 1.25 ± 0.02°, however, this angle is measured externally, in a static, unloaded condition. Compounding this issue is that each of the SGC manufacturers have developed their own unique method of measuring this external angle. In response to these concerns, the FHWA developed a device, which has the potential to independently measure the internal angle on the specimen under load and in a dynamic condition. This device, designated the Angle Validation Kit (AVK). The AVK fits inside the mold during compaction and provides a real-time measurement of the angle of gyration. The AVK was first developed over a year ago, and is now commercially available for approximately $9000. The device is NIST traceable for angle and alignment.
At the September 2000 ETG meeting, a Task Team was formed to work with the FHWA on the development of an experimental plan to fully evaluate this device. The Team consisted of Gerry Huber (Team Lead), Mike Anderson, Erv Dukatz, Julie Nodes, and Larry Michael. In addition to working on the long-term evaluation plan, the team also proposed some potential short-term solutions related to the problem of correlating SGC's:
Consequently, the ETG decided to issue short-term guidelines to be used when situations occur where there are differences in Gmb values that appear to be SGC related. It will direct users to first establish that the proper equipment and procedures are being followed and only then to determine if additional action is needed. Proper procedures for comparison of mixture produced by multiple SGCs are presented in the attached procedure proposed for AASHTO consideration and adoption. The Task team will continue to work with FHWA on the development of a long-term evaluation plan.
Update on Subcommittee on Materials (SOM)
Based on the timeline necessary for ETG recommendations to be incorporated into the SOM balloting process, the ETG has decided to hold their winter/spring meeting in February in order that any recommended changes to AASHTO methods are available by March/April. The second meeting of each year would be held in August and would be used to advance new problem statements as deemed necessary.
AASHTO Standards Options
At the September 2000 ETG meeting, a Task Team was established to review current AASHTO test procedures associated with Superpave. The Task Team, under the direction of Dr. Kevin Hall, initially focused on AASHTO T209 (Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures). The team summarized a number of differences between T209 and ASTM D2041 (which has recently been revised). It was noted that one problem that frequently occurs is that test methods typically allow a number of optional steps or sub-methods when running the test. Although this allows users greater flexibility in determining which variation better fits their needs, it does create problems, particularly in areas of test method standardization and uniformity. Consequently, the next step the Task Team will focus on will be to identify which variations within T209 could be eliminated. This will be based on input from State DOT's. This information, once compiled, will be forwarded to Staff at NCHRP for their inclusion in the NCHRP 9-26 Project (Precision Statement for AASHTO T-312).
Future work for this Task team includes the review of AASHTO T-166 (Bulk Specific Gravity of Compacted Mixtures Using Saturated Surface-Dry Specimens) and T-283 (Resistance of Compacted Bituminous Mixture to Moisture Induced Damage).
Aggregate Gradation Optimization
AASHTO PP-28 (Standard Practice for Superpave Volumetric Design for Hot Mix Asphalt) requires the development and evaluation of three aggregate blends through a trial and error process during the design procedure. Although this is considered a good construction practice, PP-28 does not give any additional requirements for the blends, nor does it include a rational basis for selecting the trial or alternate gradations. At previous meetings, the ETG discussed the "Bailey Method" as a possible approach to improve how mix gradations are determined. Some ETG members believe a Bailey Method type of packing analysis is needed to improve how target gradations are established, especially as we move away from using the restricted zone and other similar limitations.
Previously, ETG members volunteered to work with the Bailey Method in an effort to gain experience and provide insight into its potential application. Gerry Huber volunteered to assist those members working with the Bailey Method through on-site training and technical support.
Gerry Huber and Bill Pine gave an update on their work since the last ETG meeting. They have evaluated 25 mixtures in various states. Organizations involved with the evaluation included APAC, Arizona DOT, FHWA, Lafarge and PJ Keating. In general, the method was successful in predicting/optimizing volumetric properties, although it was noted that the method is somewhat complicated. In at least one evaluation, the method has predicted opposite results.
The next step would be to draft in AASHTO standard format a guideline/procedure/ specification/standard practice that could be used by designers as a tool that could be used to help evaluate and select aggregate gradations for use in Superpave.
At the next ETG meeting, an effort will be made to bring closure to this issue. A decision will be made to either recommend the Bailey Method as an official AASHTO procedure or just as a recommended tool.
As a related subject, the ETG recommends that a recommendation be forwarded to NCHRP to conduct a synthesis on various procedures to develop HMA gradation blends.
4.75 mm Superpave Mixture
At the previous meeting, the ETG identified the need for a 4.75 mm Superpave mix design specification, for use as leveling or a thin surfacing mix. The ETG agreed that an AASHTO specification needs to be developed for inclusion of this mix into MP-2 and PP-28, in order to prevent the development of a wide variety of 4.75 mm specifications. Larry Michael distributed two 4.75 mm specifications currently in use (MD, GA). However, it was noted that the Southeast Superpave Center at NCAT was currently in the process of developing specifications for this mix, and a decision was made by the ETG to defer to NCAT's work rather than to have two parallel efforts. The NCAT 4.75 mm specification will be reviewed and further discussed at the next meeting.
At the previous ETG meeting, a need was identified for a standardized field mix design verification procedure. John D'Angelo prepared for ETG discussion, a standard practice in AASHTO format for field mix verification. The ETG was asked to review the procedure, and at the next meeting a decision will be made on whether the procedure should be further developed as an AASHTO procedure or if it should be referred to NCHRP 9-7.
NCHRP 9-19 Superpave Support and Performance Models Management
Dr. Matthew Witczak, of Arizona State University and principal investigator of NCHRP Project 9-19 briefed both the Binder and Mix ETG's on the status of activities to identify a simple performance test, which is one of the remaining "gaps" in the Superpave System. Dr. Witczak discussed the experimental plan used for identifying potential candidate tests in the areas of stiffness/permanent deformation (rutting) and fracture.
The 9-19 work evaluated a number of candidate test methods, involving a number of mixture response parameters, and related them to field performance at the FHWA ALF, MnRoad, and WesTrack facilities. The tests that have correlated the best with respect to permanent deformation, albeit with limited samples , are the triaxial dynamic (complex) modulus (E*), static triaxial creep (Flow Time), and triaxial repeated load (Flow Number).
Binder and Mix ETG members then toured the ASU asphalt laboratories and reviewed the equipment being used to perform the test protocol that has been tentatively identified as having the greatest potential for being selected as the Simple Performance Test.
Impact of Raised Dust-to-Binder Ratio
As a carry-over item to the September 2000 meeting, the ETG recommends that the following note be added to MP-2 (Superpave Volumetric Mix Design):
"The effect of the filler on stiffening of the binder-dust mastic may be greatly oversimplified by the mass ratio of the dust and effective binder. Research through the past several decades clearly shows that characteristics of the dust (mineral filler) such as its particle size distribution, shape, and surface properties influences the dust's stiffening effect on the binder and performance of the mixture. Direct measurement of the stiffness of the binder-dust mastic should be considered for dust-to-binder ratios of 1.2 or greater."
Additional information on this suggested change can be found in the October 21, 2000 Chairman's Report.
If you have any questions regarding this information please contact me at 352-337-3150. The next Superpave Mixture and Aggregate ETG meeting will be held in Washington, D.C., on August 28 and 29, 2001.