Ruggedness Evaluation of AASHTO TP4

The Superpave Gyratory Compactor
The Federal Highway Administration, Office of Technology Applications recently sponsored a research project aimed at evaluating the ruggedness of AASHTO Method TP4, Preparing and Determining the Density of Hot Mix Asphalt (HMA) Specimens by the SHRP Gyratory Compactor. The purpose of a ruggedness evaluation is to determine whether tolerable variations in testing parameters and operator technique cause significant changes in test results. Five laboratories participated in the experiment:

  1. The Asphalt Institute,
  2. Illinois DOT (conducted at The University of Illinois),
  3. Texas DOT (conducted at The South Central Superpave Center at The University of Texas at Austin),
  4. Washington State DOT, and
  5. Heritage Research, Inc.
The ruggedness evaluation was conducted in accordance with ASTM Method C1067, Conducting a Ruggedness or Screening Program for Test Methods for Construction Materials. Extremes of seven testing parameters in AASHTO TP4 were evaluated:

  1. Angle of Gyration,
  2. Mold Loading Procedure,
  3. Compaction Pressure,
  4. Precompaction,
  5. Compaction Temperature,
  6. Final Specimen Height, and
  7. Aging Period @ 135 °C.
As an example, TP4 specifies a compaction angle of 1.25 degrees +/- 0.02 degrees. Thus, the ruggedness experiment evaluated whether a change in compaction angle from a low of 1.23 degrees to a high of 1.27 degrees, caused a significant change in the bulk specific gravity of a compacted specimen. The experimental data was analyzed by engineers at the Asphalt Institute and The Superpave Center at Austin. They arrived at the following conclusions.

  1. the tolerance on compaction angle (+/- 0.02 °) is reasonable and necessary,
  2. a transfer bowl is preferable but not necessary for mold loading,
  3. the tolerance on compaction pressure (+/- 18 kPa) is too high,
  4. precompaction using a blunt nosed rod is ineffectual,
  5. the tolerance on equiviscous compaction temperature (+/- 0.030 Pa-s) is reasonable,
  6. the tolerance on specimen height (+/- 1 mm) is too narrow, and
  7. for asphalt binders similar to the one used in this experiment, the 30-minute compaction temperature equilibration period can be included in the four-hour aging period as long as mixture volumetrics are the main response variable.
The tolerance on compaction pressure in AASHTO TP4, which is +/-18 kPa, should be changed to +/- 10 kPa. This pressure tolerance is more of a compactor design/capability issue than operator tolerance issue.

AASHTO TP4 should be amended to require a nominal specimen height of 115 mm with a tolerance of +/- 5 mm. This tolerance is considerably more practical than the current tolerance of +/-1 mm.

Verbiage in AASHTO TP4 regarding mold loading should be changed from, "Place the mixture into the mold in one lift" to "Quickly place the mixture into the mold using a transfer bowl or other suitable device".

A report was prepared that presents a thorough analysis of the ruggedness evaluation of AASHTO TP4. Its recommendations are currently under review by AASHTO for possible inclusion in a future release of AASHTO TP4. The FHWA Office of Technology Applications will distribute the final report. For more information concerning this evaluation, contact Bob McGennis at The South Central Superpave Center.

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