Full implementation of Multiple Stress Creep Recovery (MSCR)

A look at three Northeast U.S. states

By Greg Harder, P.E.

Back in the early 90s the advent of Superpave gave us a performance-graded (PG) binder specification known as AASHTO M320. The new PG specification considered the climate including both high and low pavement temperatures, traffic loading rates including both speed and volume plus incorporated aging of the binder within the testing framework to consider the binders performance with respect to permanent deformation and cracking.

Developed around neat asphalt binders, it was soon recognized that M320 did not properly characterize modified binders – especially those modified with elastomeric polymers. The PG specification was refined with the introduction of the Multiple Stress Creep Recovery (MSCR) test (AASHTO T-350) along with its corresponding specification AASHTO M332. While this change has been a significant improvement in the characterization of asphalt binders, its implementation nationally has been quite slow.

The northeast states have been quite aggressive in their efforts to implement M332 – either fully, partially or by allowing substitution. Three of the first states in the U.S. to fully implement MSCR for all binder grades were New York (NYSDOT), Connecticut (CTDOT) and Maryland (MDSHA). I spoke with personnel from each of the three agencies and asked the following questions regarding their implementation of MSCR. I have summarized their responses below each question.

1. Why did you decide to fully implement MSCR (M332) for all binder grades?

NYSDOT: We implemented the MSCR specification based on recommendations from the Asphalt Institute and FHWA. The MSCR spec provided a sound and reasonable approach that better characterized modified binders. It eliminated the PG plus (elastic recovery) test which was lengthy. The new spec also eliminated grade bumping which was done under the old PG specification. We’ve always had concerns with testing binders at temperatures higher than our pavements would ever see. Testing at one environmental temperature simplified lab operations. We chose to implement for all grades since with any spec change comes a great deal of effort, so why go through it again in a few years for the unmodified binders.

CTDOT: We recognized the improvement to modified binder characterization that the MSCR specification brought and reduced the amount of time required for testing in the lab. The transition to MSCR was smooth as we only use two grades of binder (PG 64-22 and PG 76-22 (by AASHTO M320) or PG 64S-22 and PG 64E-22 (by AASHTO M332). We had several results that confirmed that the binder met both AASHTO specifications. Additionally, the timing was right as we were in the process of reviewing our bituminous concrete specifications.

MDSHA: The MSCR specification provides us with a better picture of our binders with regards to modification. We also liked the concept of testing all binders at the environmental temperature without grade bumping. Our division chief always liked keeping specs as uniform as possible so he really encouraged full implementation for all grades. This made sense as it takes a long time to go through the process of specification changes so we decided to do it all at once rather than going through the effort twice.

2. How did you implement (what steps or actions did you take)?

NYSDOT: We worked closely with our state associations and NEAUPG in an effort to implement at the regional level. State personnel and suppliers were all involved so there were no surprises. We tested all of our binders under both specifications to identify and discuss any potential changes with suppliers. An engineering bulletin was developed and distributed which provided guidance on a phase-in period where MSCR graded binders were allowed (and encouraged but not required) and included an effective date for full implementation of MSCR.

CTDOT: Prior to the implementation, we tested all of our supplier’s split samples per AASHTO M320 and M332 and confirmed that at the time the binder met the requirements for both. As such, we transitioned to M332 in the first two years by allowing both M320 and M332 grading.

MDSHA: We worked closely with our binder suppliers to discuss our planned direction and address any potential issues. We (and others) gave many presentations at association meetings and conferences regarding MSCR. We also held multiple roundtable meetings across the state with MDSHA staff, industry and local agencies to discuss these and other specification changes. MDSHA first began specifying MSCR graded binders in September 2014 and the first 12 months were used as a phase-in period allowing either PG (M320) or MSCR (M332) binders during that time.

3. What grades did you specify prior to MSCR implementation and what grades are you now specifying? Were there any changes?

NYSDOT: Prior to MSCR we specified PG 64-22 and PG 64-22P upstate and PG 70-22 and PG 76-22 downstate. We also used a little PG 58-34 up north. We now specify PG 64V-22 upstate, PG 64E-22 downstate, and PG 58E-34 up north (still not used very often) as NYSDOT is moving towards 100 percent polymer usage statewide. For truing and leveling courses and/or temporary pavements we will still use neat PG 64S-22 upstate and PG 64H-22 downstate. The only change that took place transitioning to MSCR was that the PG 64-22P required additional polymer to meet the new PG 64V-22 specification.

CTDOT: Prior to MSCR we used two grades of binder – PG 64-22 and PG 76-22. We now use PG 64S-22 and PG 64E-22. Very preliminary analysis shows there may be a slight decrease in PAV DSR stiffness (G*sinδ), the BBR stiffness and a slight increase in BBR m-value.

MDSHA: Before implementing MSCR, MDSHA specified PG 76-22, PG 70-22, PG 64-22, and PG 58-28. We now specify PG 64E-22, PG 64S-22, and PG 58S-28. Projects that would have required PG 70-22 in the past are now specifying either PG 64E-22 or PG 64S-22. Going to a PG 64S-22 has been done on local projects with typically lower traffic volumes and with mixtures containing RAP. To help eliminate any confusion early on, our binder suppliers dual labeled their binder shipments with both M332 and M320 designations i.e. PG 64E-22 (PG 76-22). This helped to ensure that the right binders were delivered to the HMA plants.

4. Did you experience any issues during or after implementation?

NYSDOT: We experienced no unusual issues – maybe a few failures at start of the first season when MSCR was required but not really any more than usual for the start of a season.

CTDOT: No issues at all.

MDSHA: The only real issues were with suppliers’ concerns with changes needed to their reporting systems for certificate of analysis and/or bill of lading. We did experience one problem which was on a special project that was specified PG 76E-22 but the contractor bid the project as a PG 76-22. Fortunately for the contractor it was a small project. Aside from that it has been smooth sailing. We have had no field performance issues.

5. How are you handling “recycle” grades i.e. softer binders required with higher RAP mixes?

NYSDOT: We do not change binder grade in our recycle mixes – we use less than 20 percent RAP in surface courses without a grade change.

CTDOT: For RAP contents greater than 15 percent and up to a maximum of 20 percent, Connecticut requires a blending chart and supporting test results in accordance with AASHTO M 323 Appendix X1, or by testing that shows the combined binder (recovered binder from the RAP, virgin binder at the mix design proportions, warm mix asphalt additive and any other modifier if used) meets the requirements of the specified binder grade.

MDSHA: For higher RAP quantities the final blend of RAP and virgin binder from the finished mix must meet the specification requirements. The RAP binder is true graded and based on the use of blending charts the virgin binder grade is determined. The virgin binder alone would then be tested at the test temperature for that binder grade regardless of environmental temperature such as a PG 58S-28 being tested at 58C instead of the environmental temperature for MD of 64C.

6. Have you done any comparison testing since implementation i.e. M332 vs. M320? If so, what have you found?

NYSDOT: Yes, and the results are pretty comparable to what we were getting except PG 64V-22 typically has a little more polymer than the PG 64-22P we were using.

CTDOT: We still continue to run both grades in random samples. So far, all samples tested meet both grades.

MDSHA: We are currently doing comparison testing and found that some of our PG 64E-22 will marginally meet PG 76-22 according to M320. The transition to the MSCR specification has been virtually seamless for all three states and none of them have any regrets in their decision.

Harder is an Asphalt Institute Senior Regional Engineer based in New York.