Protocol for Evaluation
of
Superpave Gyratory Compactors
By John R. Bukowski, Chair
Superpave Mixture Expert Task Group
Draft - April, 1997
The Superpave Gyratory Compactor (SGC) creates a cylindrical specimen from loose, hot mix asphalt through a gyratory (kneading) effort. The components inducing the gyratory effort are: angle of gyration, consolidation pressure, and speed of gyration. The SGC compaction procedure is described in AASHTO TP 4, "Method for Preparing and Determining the Density of Hot Mix Asphalt (HMA) Specimens by Means of the SHRP Gyratory Compactor."
Two models of SGCs were purchased and evaluated by the Federal Highway Administration (FHWA) in a "pooled" fund purchase for the State highway agencies: Pine Instruments Company, 101 Industrial Drive, Grove City, Pennsylvania 16127, model number AFGC125X; and Troxler Electronic Laboratories, Inc., 3008 Cornwallis Road, P.O. Box 12057, Research Triangle Park, North Carolina 27709, model number 4140.
The evaluated units conformed to the equipment requirements in AASHTO TP 4. In addition, these units provide comparable results to the SHRP prototype SGC. The following protocol is suggested for all manufacturers of new SGCs. This protocol is intended to provide a uniform process to verify that the candidate SGC meets the operational requirements outlined in AASHTO TP 4 and that the candidate SGC can provide comparable compaction results to the two models of SGC evaluated in the FHWA "pooled fund" purchase.
1 Protocol for Evaluation:
A gyratory compactor, herein referred to as the candidate gyratory, shall be fabricated in compliance with the operating characteristics defined in AASHTO TP 4. In addition, the candidate gyratory should be evaluated through a volumetric comparison to a previously evaluated SGC. The volumetric comparison should be performed by one of the laboratories listed in § 1.01.
1.1 Laboratories: The following is the list of recommended
laboratories for the volumetric comparison, herein referred to
as the evaluation laboratory:
1.1.1 Superpave Regional Centers (SRC):
- a. Northeast SRC: Penn State University, State College, Pennsylvania
- b. Southeast SRC: Auburn University, Auburn, Alabama
- c. North-Central SRC: Purdue University, West Lafayette, Indiana
- d. South-Central SRC: University of Texas, Austin, Texas
- e. West Coast SRC: University of Nevada at Reno, Reno, Nevada
SHRP Research Originating Laboratory: Asphalt Institute, Lexington, Kentucky
1.2 Volumetric Comparison: Volumetric comparison should be conducted employing a minimum of four (4) evaluation mixtures as described in § 1.03. The evaluation mixtures should satisfy the Superpave volumetric requirements.
1.2.1 A single operator should be used throughout the evaluation, herein referred to as the operator.1.2.2 The candidate compactor is to be compared to one of the SGCs previously evaluated. If more than one previously evaluated SGC is available at the evaluation laboratory, it is recommended they be employed in the evaluation.
1.2.3 Prior to compaction, the operator shall perform, on all compactors included in the evaluation, the manufacturer's calibration procedures to ensure the angle of gyration, consolidation pressure, speed of gyration, and height measurement conform to AASHTO TP 4 and the manufacturer's machine tolerances. A summary of the calibration results, including date and time, should be provided in the final report, as described in § 1.05.
1.2.4 Adequate amounts of each evaluation mixture shall be prepared to yield six (6) gyratory specimens per compactor and two (2) maximum specific gravity specimens (Rice, Gmm ).
1.2.5 Split samples of each of the evaluation mixtures will be compacted in the candidate compactor and previously evaluated SGC(s), according to AASHTO TP 4. Six (6) gyratory specimens per evaluation mixture shall be compacted for each compactor.
Specimens are compacted to the maximum number of gyrations (Nmax) as described in § 1.03. Specimen height is recorded at each gyration and should be provided in the final report, as described in § 1.05. The specimens are extruded. The bulk specific gravity (Gmb ) of each specimen is determined according to AASHTO T 166. Bulk specific gravity at Ndes and Nini are calculated for each specimen and data reported on Table 1.
Determine the average of the bulk specific gravities at Nmax of each set of six (6) specimens. No individual specimens bulk specific gravity at Nmax should deviate from its average of all six specimens in the group by more than 0.020. If the bulk specific gravity of any one specimen is not within 0.020 of its group average, the specimens should be discarded. A notation of this should be made in the final report. Checks to assure compactor operation and specimen preparation is occurring properly should be taken. A new batch of mixture to produce six specimens for each of the subjected compactors should be prepared and the above procedure should be repeated. If the deviation of any specimen of this new set again exceeds the group average for either compactor another set will not be prepared. At the discretion of the evaluation laboratory and the manufacturer, a decision will be made whether to continue testing on that mixture. A note to this effect will be made in the final report.
1.2.6 Two (2) split samples of each of the evaluation mixtures shall be used to determine the maximum specific gravity of the mixtures (Rice, Gmm ), according to AASHTO T 209. The supplemental dry-back procedure of AASHTO T 209 is recommended.
1.2.7 The evaluation mixtures need to have a minimum of one percent (1.0 %) air voids at the maximum number of gyrations to be used in the evaluation.
1.2.8 Tables 1, 2, and 3 should be used to compile the data.
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Table 2: Summary of Bulk Specific Gravities (Gmb
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1.2.9 The standard deviations of Gmb for the candidate
compactor and the SGC(s) at Nini, Ndes,
and Nmax are to be reported on Table 2 and should
be comparable. If candidate compactor results are repeatedly
greater this should be commented on in the final report.
1.2.10 The difference in Gmb from the candidate compactor,
average of all 6 specimens, to the previously "evaluated"
SGC(s), average of all 6 specimens, at Nini, Ndes,
and Nmax (as reported in Table 3) should be
less than 0.01 to be considered comparable.
1.3 Evaluation Mixtures: Evaluation mixtures should be designed according to AASHTO PP 28 and MP 2. Two of the evaluation mixtures shall have 19.0 millimeter (mm) nominal maximum size blends, referred to as 19 mm mixtures. One of the 19 mm mixtures should be on the coarse side of the restricted zone, (coarse 19 mm mixture). The coarse 19 mm mixture should be designed for a traffic level greater than 10 million equivalent single axle loadings (ESALs) at an Ndes of 109 . The other 19 mm mixture should be on the fine side of the restricted zone, (fine 19 mm mixture). The fine 19 mm mixture should be designed for a traffic level greater than 1 million ESALs at an Ndes of 86.
1.3.1 The additional evaluation mixtures should be designed according to AASHTO PP 28 and MP 2. It is recommended a 12.5 mm and a 25.0 mm nominal maximum size blend mixtures be employed. Each of these mixes should be designed for properties commensurate with a traffic loading of greater than 10 million ESALs at an Ndes of 109.
1.4 Review: An evaluation of the above data should be reviewed by a minimum of one of the other evaluation laboratories, as given in § 1.01. All reviewer comments should be included in the final evaluation report, as given in § 1.05.
1.5 Reporting: A final report should include the
following:
- a. An executive summary of evaluation findings,
- b. Tables 1, 2, and 3 from § 1.02.08,
- c. A summary of the evaluation mixtures' designs, and
- d. A summary of the calibrations performed on each unit.
1.6 Distribution: In addition to providing the manufacturer a copy of the final report, a copy of the final report will be supplied to all evaluation laboratories, § 1.01, and the FHWA Office of Technology Applications. This will help to ensure proper retesting, should any be required. The copies of the report are to be held in confidence by all recipients and results not to be divulged without the manufacturers permission.
Prior to commencement of testing under this protocol, the evaluation lab will ensure that the manufacturer is made aware of the above intended final report distribution, and his agreement obtained.
1.6.1 Future Evaluations: If a candidate compactor is found not to compare to the previously "evaluated" SGC(s), future evaluations should not be performed by any of the evaluation laboratories unless the manufacturer modifies the unit.
1.6.2 The evaluation is only valid for the model tested. If the manufacturer elects to modify the unit, a new evaluation is necessary. If these tests are performed on a "proto-type" candidate compactor, they should be repeated on an actual production model prior to utilization by agencies of that model.
1.6.3 Evaluation Cost: All costs associated with the evaluation are the responsibility of the manufacturer of the candidate compactor. Costs for the evaluation are at the discretion of the evaluation laboratory. Costs should include compensation for any reviewing laboratories, § 1.04.