Superpave - What's the Latest Scoop in Research
December 1999
Thomas Harman (202 493-3072 or Tom.Harman@fhwa.dot.gov)
FHWA Asphalt Pavement Team Leader
Turner-Fairbank Highway Research Center (TFHRC)

In 1992 the asphalt industry had its first glimpse of the new Superpave system. At that time, Superpave provided us with a framework asphalt binder specification, a volumetric mix design procedure using gyratory compaction, first generation performance models, and DOS-based software. Most of the equipment needed was either prototype or research grade. And no AASHTO specifications were available. In 1992 only a few dozen engineers understood the new system and less than a handful of contractors had any experience with placing the new mixes.

Since 1992 our industry has been working hard through expert task groups, user-producer groups, national and local associations, and lead states. Today we have standard equipment and test specifications. Every State highway agency has the Superpave equipment. Over three thousand engineers and technicians have been trained. And over 5000 projects have been placed. So. . .

"Not quite." We still have to address several user needs by continuing efforts in several research areas. State highway agencies and the asphalt industry (the Superpave users) are looking for Superpave to include a simple performance test in the near future - a laboratory test or tests to assure that a volumetric mix design will perform in the field. Users are also looking for improved binder characterization, improved guidance for RAP usage, a better moisture sensitivity test, resolution of the construction tenderness issue, updated Superpave software, and refined aggregate standards. On the research end, we are looking for continued development of performance models and the development of performance-related specifications (PRS) for construction.

The good news is efforts are underway to address this list of the most significant user needs currently on the Superpave horizon. What I would like to do is to share with you the status of some of key activities and who you may want to contact for more information.

Before I get started, I need to make you aware of what happened to the FHWA asphalt technical working group (TWG) and the various expert task groups (ETG's). The current highway bill entitled, "Transportation Equity Act for the 21st Century, TEA-21," greatly reduced the flexibility of resources available for highway research and technology transfer. Subsequently, AASHTO, through the National Cooperative Highway Research Program (NCHRP), has stepped to the plate to help FHWA fund and manage critical Superpave activities. In addition, the Transportation Research Board (TRB) will be managing the ETG's and has created the TRB Superpave Committee to replace the TWG. So it's not business as usual, but we are getting the job done.

A simple performance test: As you know, Superpave mix design is volumetrically based. It does not include a "strength test" such as Marshall stability and flow or Hveem stabilometer. This is being addressed under a contract, originally started by FHWA, now under NCHRP. By the Spring of 2000, NCHRP 9-19, "Superpave Support and Performance Models Program" will identify/develop a simple performance test to assess Superpave mixes' resistance to rutting and if possible, fatigue cracking. To find out about this, or any NCHRP project, check out the National Academy of Sciences' web page at http://www4.nationalacademies.org/trb/crp.nsf/. The project manager at NCHRP for this project is Dr. Ed Harrigan (202 334-3232 or eharriga@nas.edu) and the principal investigator for the study is Dr. Matt Witczak (408 965-2759 or Witczak@asu.edu).

Improved binder characterization: The Superpave binder specification has been refined under the watchful eyes of the binder expert task group (ETG) chaired until recently by FHWA's John D'Angelo (202 366-0121 or John.DAngelo@fhwa.dot.gov). The binder ETG, now under TRB's direction, during its October 1999 meeting has proposed a new binder specification, AASHTO MP-1(A), to include a refined version of the direct tension test (DTT) - for low-temperature characterization. The DTT will be used in combination with the bending beam rheometer to determine the low temperature grade of the asphalt binder. The combined tests will apply to both neat and modified binders. Specifying agencies will have the choice of using either MP-1 (without the DTT) or MP-1(A). FHWA has just awarded a contract to Instron to supply the State highway agencies with the new DTT.

In addition, the binder ETG is looking closely at the findings of NCHRP 9-10, "Superpave Protocols for Modified Asphalt Binders," being conducted by the University of Wisconsin's Dr. Husain Bahia (608 265-4481). Specifically, the FHWA will be evaluating the LAST and the PAT tests. The LAST test stands for Laboratory Asphalt Stability Test and measures a modified binders storage stability at elevated temperature with time. The PAT test stands for Particle Additive Test and measures the particle content of a binder; which effect the validity of the dynamic shear rheometer. This work is being conducted by FHWA's Turner Fairbank Highway Research Center, (see Jack Youtcheff (202 493-3090 or Jack.Youtcheff@fhwa.dot.gov)).

Improved Guidelines for RAP usage: Guidelines for the use of RAP in Superpave were developed by the mixture ETG chaired until recently by FHWA's John Bukowski (202 366-1287 or John.Bukowski@fwha.dot.gov) and myself. Building on this effort is NCHRP 9-12, "Incorporation of Reclaimed Asphalt Pavement in the Superpave System," being conducted by the North Central Superpave Center's (Purdue University) Rebecca McDaniel (phone or E-mail). Please see http://www4.nationalacademies.org/trb/crp.nsf/ for the complete status of this project.

A better moisture sensitivity test: During the SHRP research program an effort was made to develop a better moisture sensitivity test. The result was the environmental conditioning system (ECS). However, subsequent research showed the ECS did not work as well as AASHTO T-283 (modified Lottman procedure). But, the modified Lottman procedure calls for 4 inch Marshall specimens, not 150 mm gyratory specimens. Enter NCHRP 9-13, "Evaluation of Water Sensitivity Tests," being conducted by the University of Nevada-Reno's Jon Epps (phone or E-mail). The goal of this study was to refine the modified Lottman procedure to effectively use gyratory specimens. Jon and his team have met the goal of the study and a draft final report has been completed. For complete status, check the web site.

Now refining the Lottman procedure to address bigger specimens does not necessarily give us a better moisture sensitivity test. The Lottman procedure has always worked okay, but we still need something better. On July 28, 1999, the "Moisture Sensitivity Focus Group" convened to discuss moisture sensitivity and develop a problem statement for a new NCHRP project. In the interim, some agencies and researchers are augmenting their analysis to include testing of water saturated beams in torture tests, such as the Asphalt Pavement Analyzer (a.k.a. Georgia loaded wheel tester) and the Hamburg wheel tracker, and binder testing using the pull-off test (see Jack Youtcheff (202 493-3090 or Jack.Youtcheff@fhwa.dot.gov)).

Resolution of the construction tenderness issue: During the placement of some Superpave mixes, contractors experienced tenderness during compaction. General guidelines for handling tender mixes are available through the National Asphalt Pavement Association (NAPA) (301 731-4748) in special report 180, "Superpave Construction Guidelines." In addition, FHWA (John D'Angelo) has contracted with the National Center for Asphalt Technologies (NCAT) to develop more specific guidelines on addressing the tenderness issue. Dr. E. Ray Brown (NCAT) has provided FHWA with a draft report entitled, "Hot Mix Asphalt Tender Zone." It is succinct and comprehensive. The report will be available Spring 2000.

Updated Superpave software: During SHRP, the researchers set out to develop a computer-based system to allow everyone to do Superpave mix designs. That meant it had to work on all computers. At the time of SHRP, Bill Gates was only a millionaire and very few people had even heard of Windows. So the software was written in a DOS-based language. By the time the software was completed it was obsolete. Since then we have been writing spread sheets, using gyratory manufacturers' software, or using the FHWA demonstration project 90's database (see John D'Angelo).

Today something new is on the horizon. Developed as part of the FHWA models contract, a new, windows-based software package has been developed. The principle in this effort is the University of Maryland's Dr. Charles Schwartz (301 405-1965). Superpave version 2.1 is a Microsoft Access-based program with separate modules for testing, mix design, binder classification, field quality control, and quality acceptance. In a workshop hosted by Maryland State Highway's Larry Michael on July 8, 1999, contractors were given a chance to review the final product. Overall, this is a very comprehensive package. It takes time to learn, but it provides an excellent platform to Superpave. The software was showcased at the annual AASHTO Subcommittee on Materials (SCOM) meeting in August. This software will be sold as AASHTO-Ware.

Refined aggregate standards: The Superpave consensus property standards are being examined currently in studies being conducting by the International Center for Aggregate Research (ICAR at Texas A&M), University of Illinois, and Purdue University. The mixture ETG will review the research findings and propose any future changes to the Superpave specification.

Continued development of performance models: One of the ultimate goals of asphalt research is to have a unified material and structural analysis system. To achieve this, there are many steps along the way. Currently NCHRP 9-19, the models contract, is developing material characterization procedures and a framework models system. A future NCHRP project will generate environmental effect models to be combined with the framework models system and this will be followed by another NCHRP project to calibrate and validate the refined models.

In parallel with this effort, the AASHTO structural design guide will be updated to incorporate the new mixture modeling concepts. It is anticipated all of this work will be completed over the next 10 years.

Development of performance-related specification (PRS): FHWA's first major step in the development of PRS was the construction and loading of a test track in Nevada, known as WesTrack. FHWA's Dr. Terry Mitchell (202 493-3147 or Terry.Mitchell@fhwa.dot.gov) was the contract manager. As many of you are aware, some of the mixes at WesTrack rutted prematurely. FHWA pulled together an expert team to look into the failures at the track. The team included, Ray Brown (NCAT), Larry Michael (MD SHA), Ron Sines (NY DOT), Gerry Huber (Heritage Research), Erv Dukatz (Mathy Construction) and Jim Scherocman (Consultant). The forensic team's report has been published and is available through the FHWA's Report Center at (301) 577-0818, Report Number FHWA-RD-99-134 or at the TFHRC web page (www.tfhrc.gov). The WesTrack project is being completed as NCHRP 9-20, "Performance-Related Specifications for Hot-Mix Asphalt Construction (WesTrack)."

The prototype PRS and software will be validated and field tested on at least five projects around the United States beginning in 2000. This effort will be conducted under NCHRP 9-22, "Beta Testing and Validation of Hot-Mix Asphalt Performance-Related Specifications."

Lastly, I would like to make you aware of a few other activities going on here at the FHWA Turner-Fairbank Highway Research Center (TFHRC). First we are completing an extensive study of the Superpave binder specification using the full-scale, accelerated load facility (ALF) machines at TFHRC. This project is being headed up by Kevin Stuart (202 493-3073 or Kevin.Stuart@fhwa.dot.gov). The results of this study are helping us to better understand the complexity of modified binders and to assist other contracting efforts like the Models contract. Right now we are developing the new experiment entitled, "Predicting mix performance with new complex binders." Kevin is again heading up the effort. This time our focus will be to look more into understanding and addressing the issue of fatigue cracking. This will be achieved through both laboratory and ALF testing.

Well, I hope you found this article informative. If you have any questions or comments, please let me know.