U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Date: May/June 1998|
Issue No: Vol. 61 No. 6
Date: May/June 1998
"We cannot expect the technology of the World War II era to meet the demands of our nation today," said Federal Highway Administrator Kenneth Wykle. "In the past 10 years alone, traffic volume in this country has doubled, and pavement loadings have quadrupled."
Wykle made these remarks in a speech at the "Superpave: Today and Tomorrow" conference April 21-23 in St. Louis. At the conference, sponsored jointly by the Asphalt Institute and the Federal Highway Administration (FHWA), the latest news on trends and technology related to Superpavetm - the high-performance asphalt pavement mix design system - were reported.
"Working together, we can have better pavements," Wykle continued. "That means using the best technology and providing training to understand the need for proper mixing; proper temperature when applying [pavement] to avoid tender zones; proper compaction; sensitivity to weather conditions; and prevention of bleeding, rutting, and cracking. It also means knowing the specifications and staying within tolerances. Quality is 'Job One' with Superpave."
The number of Superpave projects has skyrocketed to 1,339 for this construction season. This represents 30 percent of awarded asphalt tonnage and about 16 percent of state asphalt paving programs, compared to just 93 projects representing 2 percent of the market in 1996. Three-quarters of the states plan to implement the Superpave mix design system by 2000, and all but four states will have implemented the Superpave binder specification by then.
Some of the most valuable and timely information that was exchanged at the conference was related to field construction. Superpave mixes can require a greater level of compactive effort.
"The key words are attention to detail," said Dale Decker, vice president of the National Asphalt Pavement Association (NAPA). Decker said 15 percent of respondents to a recent NAPA survey said that they had problems achieving specified compaction densities in Superpave projects. Both compaction expert Charles Deahl and New York state's Ron Sines warned against using excessive force, which may cause materials to fracture. On vibratory rollers, higher frequencies (vibrations per minute) and lower amplitudes work better, Deahl said. He recommended using a vibratory read tachometer. Sines, who is supervisor of New York's Field Engineering Unit, recommended running "team" breakdown rollers, using two vibratory compactors on the mix while it is still very hot to get as many passes as possible while monitoring the mat temperature.
"The trend toward bigger, heavier rollers with the Superpave system can damage pavement structure on smaller roads," Sines said. "A large part of our system in New York state is rural roads, and we want to design some finer Superpave mixes for lower volume roads."
The mysterious "tender zone" - a temperature zone (generally reported between 93 to 116 degrees Celsius) in which some mixes become unstable - was discussed a great deal. "It's not in all the mixes. Sometimes it's not there in the morning and shows up in the afternoon. We don't know what causes it," Sines said.
The tender zone phenomenon seems to be related to ambient temperatures as well, and in some areas of the country, it has not been a problem at all. About two-thirds of those reponding to the NAPA survey reported some tenderness in the mix. Recommended procedures are to either: (1) avoid compaction if tenderness is observed when the mat temperature reaches those temperatures; then resume compaction after the mat cools; or (2) use a rubber-tired roller, with proper confinement of the mix, for rolling in the tender zone.
"Each roller operator needs to understand what temperature zones he should be rolling in," Sines said.
Use of RAP (recycled asphalt pavement) in Superpave is becoming more commonplace. Ongoing research is aimed at delivering more knowledge of how recycled materials may affect binder properties.
"Contractors who aren't successful with RAP just dump it in. If you do that, you can't use very much of it," said Richard Schreck of the Virginia Asphalt Pavement Association. "If you test it, characterize its properties, and treat it like any other material, you can use a lot more of it." RAP is used in about one-half of Virginia's Superpave projects.
Current guidelines on the use of RAP in Superpave were issued by the FHWA's Mixture Expert Task Group in March 1997. The guidelines call for a three-tiered approach. Aged materials can be stiff and cause cracking. At low levels of RAP use, there is no recommended change to the binder grade. At medium levels, the binder grade is reduced by one level. At high levels, further analysis is required based on "blending charts."
These guidelines are based on empirical experience. The North Central Superpave Center is conducting research to produce, by sometime next year, improved guidelines based on improved analytical understanding.
Permeability problems have emerged on some Superpave projects as well, most notably in northwest Florida, where the available aggregates make mixes more susceptible to this problem. Recommended solutions include high in-place densities with strict density-monitoring specifications, well-sealed longitudinal joints, and thicker lifts. Florida now uses a 4-to-1 ratio between lift thickness and maximum aggregate size. Standard Marshall recommendations call for a 3-to-1 ratio.
Superpave's impact on the aggregate industry was discussed by several speakers, including industry spokespersons who pointed out that new equipment and procedures are required, placing a capital investment burden on many suppliers. Don Green of United Metro Materials in Arizona described how his firm successfully geared up for Superpave, changing equipment and processes to accommodate the demand for higher quality, more uniformly graded aggregates.
Lawrence Warren, a Mississippi contractor and the current NAPA president, said, "We love the mix. It's the prettiest mix we've ever run. There are no more Marshall hammers in the state that invented them. Mississippi is 100-percent Superpave." He called for a "renewed commitment to training to enable the contracting industry to deal with new specifications, materials, and equipment."
Uniformity of the application of the Superpave specifications remains a much-debated issue. Some states and contractors call for modification of the system to allow greater use of local materials. Paul Mack, leader of the Lead States Superpave Team for the American Association of State Highway and Transportation Officials (AASHTO), encouraged states to comply with the AASHTO Superpave specifications and to work through the AASHTO standards process to achieve consensus on any future changes in specifications.
"Superpave is a system just like your car is a system. If you go out to your auto and start taking parts out of it, pretty soon it's not a system any more, but a pile of parts," said Mack, who is from New York. "Superpave can be improved, and research is underway that will improve it. But in the meantime, to the extent your state follows specifications, you will be in a position to immediately implement future improvements."
"We are committed to assisting the states with new technology, but the states must decide when and how to use the technology," FHWA Administrator Wykle said. "FHWA is committed to work with the industry and the states to provide the best pavement technologies available in the world."
For more information on Superpave construction issues, readers may wish to consult a new report, Superpave Construction Guidelines, published by the National Asphalt Pavement Association (NAPA) under a cooperative agreement with FHWA. The report, available from NAPA, points out the differences between Superpave mixes and conventional mixes, and it provides tips on how to prevent these differences from disrupting mix production and pavement construction.
Karen Haas Smith is a communications consultant in Rockville, Md. She provides ongoing support to FHWA's Superpave Technology Delivery Team.