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SWUTC Research Project Description

Improving Fracture Resistance in Asphalt Binder with Verification on Asphalt Mixtures Cracking Performance

University: Texas A&M University

Principal Investigator:
Fujie Zhou
Texas Transportation Institute
(979) 458-3965

Funding Source: SPR Program

Total Project Cost: $221,937

Project Number: 0-6674

Date Started: 9/1/12

Estimated Completion Date: 8/31/13

Project Summary

Project Abstract:
The current performance grading (PG) specification for asphalt binders was developed during the Strategic Highway Research Program (SHRP), it is based primarily on the study of unmodified asphalt binders.  Over the years experience has proven that the PG grading system, while good for ensuring overall quality, fails in some cases to guarantee good rutting and cracking performance, particularly as it applies to modified binders.  Specifically, recent studies on mixes with highly modified binders from out of state (Minnesota) found that the MnRoad mixes have substantially improved cold weather cracking properties than mixes currently used in Texas, while still passing TxDOT’s Hamburg rutting requirements.

To eliminate this type of apparent discrepancy AASHTO has adopted a new PG grading system based on the Multiple Stress Creep and Recovery (MSCR) test which is claimed to encourage the adoption of binders which are softer but still retain the required rut resistant.  The benefits of the new grading system have not been verified for Texas asphalt binders and mixes.

Project Objectives:

  1. Determine if the new AASHTO MSCR-based binder grading system is superior to the current TxDOT binder grading system;
  2. Identify/develop a simple test method or methods to characterize fracture and adhesive properties of asphalt binders and associated tentative specification limits;
  3. Determine if the asphalt binders not currently used in Texas would potentially improve overlay performance, if so conduct an associated cost-benefit analysis;
  4. Identify optimal asphalt binder/aggregate combinations for different environmental zones in Texas;
  5. Develop and initially populate a catalogue of all these measured (binder, binder/fine aggregate mastic, and asphalt mix) properties with relevant information that can be used to track the field performance of pavements constructed using these asphalt binders.

Task Descriptions:
Task 1: Literature Search and Brainstorm Workshop

Task 2: Evaluate and Identify/Develop Test Methods to Characterize Asphalt Binder Properties: Rutting, Fracture/Fatigue, and Adhesion/Moisture Damage

Task 3: Evaluate the DMA Test for Characterizing Asphalt Mastic Fatigue and Moisture Damage Resistance

Task 4: Conduct Laboratory Mixture Tests to Validate Binder/Mastic Tests for Rutting, Fatigue/Fracture, and Moisture Damage

Task 5: Construct and Monitor Field Test Sections to Validate Results from Tasks 2 to 4

Task 6: Predict Pavement Performance of Test Sections and Validate Existing Models

Task 7: Develop a Prototype Statewide Catalogue of Recommended Binder Types and Mix Designs for Asphalt Overlays (and/or New Pavements)

Task 8: Recommend Specification Change

Task 9: Develop and Teach Workshop for New Binder Tests and Specification Change

Task 10: Evaluate the Potential of Using the New Binder Tests to Characterize Reflective Paints and Coatings

Task 11: Reports

Implementation of Research Outcomes:
The current performance grading (PG) specification for asphalt binders was developed based on the Strategic Highway Research Program (SHRP) and is based primarily on the study of unmodified asphalt binders. Over the years, experience has proven that the PG grading system, while good for ensuring overall quality, fails in some cases to predict rutting and cracking performance, particularly as it applies to softer but highly modified binders. Therefore, it is necessary to improve current asphalt binder performance parameters and associated specification limits, especially for modified binders.

This report documents the laboratory evaluation of several new tests for rutting, fatigue, and adhesion property of asphalt binders, including the multiple stress creep and recovery (MSCR) test, linear amplitude sweep (LAS) test, double edged notched tension (DENT) test, elastic recovery test, pull-off test, pneumatic adhesion tensile testing instrument (PATTI), dynamic mechanical analyzer (DMA) test, and surface energy test.

Impacts/Benefits of Implementation:

This research impacting the USDOT goal of state of good repair concluded the following:

  • Asphalt mix rutting test results showed that that the MSCR test and associated specification works better than the current G*/sin  based PG specification, especially for those highly modified asphalt binders (such as PG64-34). MSCR Round Robin results among five laboratories clearly indicated that both Jnr0.1 and Jnr3.2 results are very repeatable and reproducible. The R3.2 results are acceptable in terms of repeatability and reproducibility, but both Jnrdiff and R0.1 have pretty high variability. Since Jnrdiff is one of the parameters for grading asphalt binder, TxDOT should exercise caution when grading the slightly modified asphalt binders (such as PG64-28) using the MSCR specification.
  • This study further confirms the poor relationship between the parameter G*sinδ and the binder fatigue resistance. Neither the MSCR nor the elastic recovery test shows good correlation with the asphalt mix OT cracking test. Both the LAST and the DENT tests provide similar ranking as that of asphalt mix OT cracking test. Considering the test equipment requirements of both the LAS and DENT tests, the DSR-based LAS test is recommended for asphalt binder fracture test, since the DSR has been widely used in last 20 years and laboratory technicians and researchers are very familiar with it.
  • Four types of laboratory tests for measuring asphalt binder adhesive property were evaluated. It was found that the PATTI test is the only promising test for evaluating adhesive properties of asphalt binders. All other three tests, the pull-off test, DMA, and surface energy test, were not successful in this study for evaluating asphalt binder adhesion property.

Obviously, these findings are based on laboratory test results only, and further field validation is definitely needed. Additionally, one needs always to keep in mind that the binder alone does not determine rutting, fatigue cracking, and moisture damage of asphalt pavements. Mix characteristics as well as the pavement structure itself, traffic, and the environment within which it is located have a significant role in determining pavement performance.

Web Links:
Final Technical Report