Performance Evaluation on Rubberized HMA – Gap Graded with Asphalt Mixture Performance Tester (AMPT)

Abstract: This paper presents research conducted by California Department of Transportation (Caltrans) - Materials Engineering and Testing Services (METS), to explore the feasibility and effectiveness of using the Asphalt Mixture Performance Tester (AMPT) to evaluate the performance characteristics of Rubberized Hot Mix Asphalt-Gap Graded (RHMA-G).

A ½-inch RHMA-G mix was selected for this project which included 7.5% asphalt binder by total weight of mixture. The base binder grade used was PG 64-16. The GPS coordinates for locations where the HMA was placed were tracked for analysis in the AMPT test result analysis software- FlexMATTM. There were six sampling events conducted on three dates (two events per date) from late September 2022 through mid-October 2022 at the HMA plant. The samples collected for METS were labelled as Event #2, Event #4, and Event #6. AMPT specimen preparation involved both large specimen (AASHTO R 83) and small specimen fabrication (AASHTO PP 99) using the Superpave Gyratory Compactor (SGC). The mixing temperature was set as 149 °C (300 °F). The SGC compaction pressure was maintained at 700 kPa. An additional 30 minutes of holding time was applied to the RHMA-G mix after the completion of compaction according to the job mix formula.

The stiffness (AASHTO TP 132), rutting resistance (AASHTO TP 134), and fatigue damage resistance (AASHTO TP 133) of the RHMA-G mix were evaluated using the AMPT.

The Rutting Strain Index (RSI) demonstrates that this RHMA-G mix provides sufficient rutting resistance as a surface layer. RSI results indicate this mix can support a “Heavy” traffic tier (10 to 30 million ESALs) at the project paved location based on the threshold values recommended by FHWA (TechBrief: FHWA-HRT-21-044).

The apparent damage capacity (Sapp) represents the amount of fatigue damage this mix can tolerate. Based on the recommendation by FHWA (TechBrief: FHWA-HIF-19-091): the mixtures sampled at all Events can support “standard” traffic (traffic less than 10 million ESALs). It should be noted that the fatigue damage curves (stress*N versus number of cycles) of the RHMA-G did not show a distinct curve peak which typically represents fatigue failure, as observed in the ones of dense-grade HMA. This phenomenon could result from the rubber particles in the RHMA-G that still hold the mix together after microcracking has been induced. Since FlexMATTM defines fatigue life (Nf) based on the peak of Stress*N vs cycles, the absence of peaks in the RHMA-G curves raises questions about the validity of calculations from FlexMATTM for RHMA-G used in California.
The analysis of the data generated from this AMPT evaluation of RHMA-G mix supports the following findings:
• The evaluated RHMA-G mix is expected to have sufficient fatigue and rutting performance at the current project location under standard to heavy traffic respectively.
• As distinct peaks may not be observed from the fatigue damage curves of RHMA-G, it will be difficult to capture the accurate fatigue life from FlexMATTM analysis software.