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September/October 2002
Superpave Comes of Age
by Cathy FryeSUPERPAVE® named for a system of components that work together to provide a SUperior PERforming asphalt PAVEment is the hot ticket these days across the United States. The use of Superpave has grown steadily. In 1999, no fewer than 2,515 projects used 73 million metric tons of Superpave designed hot-mix asphalt (HMA). This figure rose to 3,166 projects by 2000, and the upward trend continued in 2001 with 3,846 projects slated for construction.
Market share climbed to 47 percent of total projects and 53 percent of total tonnage in 2000, an appreciable increase from 32 and 45 percent respectively in 1999.
In its 2000-2001 National Implementation Report, the Transportation Research Board's (TRB) Superpave Committee reported that "most States" are implementing the new system, adding that "Superpave has become the hot-mix asphalt design method of choice."
Photo by: Tom Harman, FHWAPaver laying down new test section of polymer-modified Superpave mix for the FHWA Accelerated Load Facility (ALF).
What Is Superpave?
Superpave gives highway engineers and contractors the tool they need to design and construct asphalt pavements that perform better and last longer under extremes of temperature and a wide range of traffic loads. The researchers seek to improve the performance of flexible pavements through a greater understanding of the fundamental chemical and physical properties of asphalt binders and mixes.
Superpave research has resulted in performance-based tests, equipment, and procedures for both binders and mixes. The procedures and evaluation methods encompassed under Superpave assess binder and mix performances in terms of the most common causes of pavement failure: pavement deformation (rutting), fatigue (structural) cracking, and low-temperature (thermal) cracking. Superpave also considers the detrimental effects of age hardening and moisture damage on these distresses. By applying Superpave principles, engineers and designers can customize the binder and mixture to make the most appropriate mix for a particular pavement's environmental and traffic conditions.
Many variables determine the appropriate pavement for a particular location. One size certainly does not fit all. A pavement that wears well and lasts beyond its expected life on a rarely traveled highway in the Nevada desert could crack and rut within months of use in much colder Minnesota on a busy metropolitan road that carries a heavy daily volume of stop-and-go traffic. Even within the same geographic area under similar climatic conditions, the type of traffic, including trucks, can make a major difference.
Superpave enables the hot-mix asphalt designer to customize the pavement components to create a pavement that provides optimal durability and longevity. This customization, in turn, cuts costs by reducing repairs and the need for early replacement. "The idea is to get in, get out, stay out," says Rick Dunn, research and technology engineer in the Federal Highway Administration's (FHWA) New York Division, quoting the FHWA motto. Extended service-life and less maintenance provide a host of other benefits, including fewer road closures and traffic delays.
Superpave: A Complex System of Choices
Pavements sound like simple products when looking at their key ingredients: a binder mixed with stones and a bit of air. Nowhere nearly as simple as it sounds, combining pavement components to create a reliable product is a complex science in which even slight variations can result in pavements that perform quite differently from each other.
When using the Superpave system, for example, binders are graded according to their laboratory performance. To select an appropriate binder grade, the designer first must determine the pavement temperature range for a specific project and the corresponding design pavement temperatures. The designer also must consider the traffic volume and typical traffic speeds at the project site. The type of aggregate used plays an important role as well. Aggregates make up roughly 95 percent of the mixture by weight, and hot-mix asphalt performance is tied to the quality and characteristics of the aggregates used. Properties that are considered most important in pavement performance are coarse aggregate angularity, fine aggregate angularity, flat and elongated particles, and clay content.
Angular coarse and fine aggregates are recognized as having the greatest shear resistance and, therefore, provide a high resistance to rutting. Superpave has minimum requirements for angularity that depend on traffic level and whether the mix will be in the top or bottom portion of the pavement. However, high angularities produce hot-mix asphalt mixtures with the highest voids in the aggregate. Voids—or air pockets that are filled by binder when designing a mixture—are regulated under the Superpave system so that excessively high binder content is not used. Current Superpave standards allow no more than 10 percent of coarse aggregate particles to be flat and elongated because excessive numbers of flat, slivered particles have a tendency to create slippage planes and reduce the ability of the aggregate to interlock in the mix. Clay content also is regulated under Superpave standards because it can make the mixture moisture-sensitive.
All of these variables—and more—go into designing an appropriate pavement.
Photo by: Tom Harman, FHWASuperpave mixture is loaded into a paving machine in preparation for placement and roller compaction.
"Superpave means construction and material savings, as well as economic benefits to highway users," Dunn says. "Just think of the trucking industry and traveling public who otherwise may be delayed, sitting in traffic backed up due to pavement construction operations."
Minimizing construction and maintenance delays saves travelers time and aggravation, makes transportation more profitable for commercial haulers for whom time is money, and also decreases the potential pollution from idling engines. Other beneficiaries of Superpave's continued success are manufacturers of supporting equipment and products.
Photo by: Tom Harman, FHWASuperpave is placed during an asphalt paving operation.
Building on National Research
Superpave was one of seven core technology areas originally identified for implementation under the Strategic Highway Research Program (SHRP), a 5-year research effort established by Congress in 1987. Through a provision in the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA), Congress provided more than $100 million through 1998 to support the development of SHRP products, with approximately $40 million going to Superpave.
Superpave at the National Level
Located at the FHWA Offices of Pavement Technology in Washington, DC, and Infrastructure Research and Development at the Turner-Fairbank Highway Research Center in McLean, VA, the national experts on Superpave provide assistance to the States in a variety of ways. "We've worked with States to put together implementation strategies," says Tom Harman, asphalt pavement team leader, "to help them establish training programs, and with research and development."
As asphalt pavement technology continues to evolve, new solutions are emerging. "We're refining and redeveloping equipment and specifications," says Harman, citing a handful of past and current activities, including the refinements to the Pressure Aging Vessel (used to age asphalt binders in the laboratory) and the Direct Tension Test (used to assess low-temperature properties of the asphalt binder).
Another innovation, the Dynamic Angle Validation (DAV) Kit, is designed to ensure uniformity between gyratory compactors. Measurable differences in the bulk specific gravities of field specimens have been traced to differences between individual compactors, and this can mean the difference between passing and failing test results. DAV measures the angle of gyration inside any individual compactor and the data provided is then used for more accurate calibration. The AASHTO Subcommittee on Materials is considering adoption of DAV as part of the standards associated with Superpave.
Photo by: Tom Harman, FHWAThese machines are Superpave gyratory compactors.
Also coming soon are 12 new pavement test lanes at Turner-Fairbank. The test lanes will include five polymer-modified binders, one fiber-modified binder, one air-blown asphalt, and one "neat" or unmodified binder. Four of the eight mixtures will be placed at two thicknesses. The research will be used to validate proposed changes to the Superpave binder specification and will provide additional validation data for assessment of the proposed simple performance test and 2002 Pavement Design Guide models. "The main product of this research will be an enhanced binder specification that fully addresses the benefits of polymer-modified binders," says Harman. (See Along the Road for more information.)
Photo by: Tom Harman, FHWAThis scene shows the National Center for Asphalt Technology (NCAT) Test Track.
In 1998, when funding was endangered, representatives of FHWA, the American Association of State Highway and Transportation Officials (AASHTO), the National Research Council, and several State departments of transportation worked to obtain additional money from sources such as TRB's National Cooperative Highway Research Program (NCHRP). According to NCHRP's Web site, the program's involvement continues today with tasks such as the "development and validation of an advanced material characterization model, creation of the associated calibration and testing procedures for hot-mix asphalt, and the pursuit of a simple Superpave performance test."
As a result of these and other efforts of the "Saving Superpave Team," about $2.5 million in additional funds were obtained for critical Superpave projects. Most of the laboratory equipment was purchased using pooled fund money so that one procurement could be used to purchase each piece of equipment. This eliminated the need for each State to procure the equipment on its own and ensured that the equipment for all States met the most up-to-date specifications.
Photo by: Tom Harman, FHWAKevin Stuart of FHWA and Dr. Walaa Mogawer of the University of Massachusetts Dartmouth during reconstruction of FHWA's Accelerated Loading Facility (ALF).
Under the 1991 Surface Transportation Act, Congress charged FHWA with leading the implementation. The development of Superpave has been a dynamic process in which laboratory science, the test environment at WesTrack (a pavement testing facility in Nevada), and practical on-road experience all provided invaluable information. The development of better equipment and specifications has continued through national expert working groups (ETGs) and by training given by FHWA and other organizations.
Superpave has benefited from the strong leadership of participating managers and the technical expertise of ETGs. They coordinated the work of the large and varied group of stakeholders across the Nation to minimize duplication of effort.
As with most ambitious projects, Superpave has collected a number of lessons learned. Perhaps the most valuable has been to include the end-user in all phases of research, development, and implementation. Throughout the history of Superpave, feedback from implementers in the field to researchers in the lab has resulted in reiterative improvements. In an annual survey conducted by the New York State Department of Transportation and FHWA's New York Division, end-users have the opportunity to note deficiencies and difficulties, and their comments have been used to fine-tune the Superpave system and track implementation.
Photo by: Tom Harman, FHWAThis Virginia Paving Co. hot-mix drum plant produces asphalt for test lanes at Turner-Fairbank Highway Research Center.
The Lead State ProgramAs with any other innovative technology, a market for Superpave had to be developed. Prospective users needed to be educated about the new technology and convinced that it would benefit them and their customers.
In 1996, FHWA funded a Lead State Program to reduce the new technology's learning curve by sharing technical expertise and experience. The six lead States—Florida, Indiana, Maryland, New York, Texas, and Utah were proactive in implementing Superpave and trying new concepts and practices. The program also reduced unnecessary and costly duplication of effort. Because changes in the choice of pavement technology for public roads involve both public and private sector organizations, effective communication was key.
"It took the efforts of lots of individuals to make this happen, and the initial players have stayed with Superpave," Dunn notes. "If the FHWA field perspective of Superpave implementation had to be summed up in a word, that word would be dedication."
Paul Mack, retired deputy chief engineer for the New York State Department of Transportation (NYSDOT), adds, "We learned a great deal about what it takes to make fundamental changes in how an industry delivers its products."
With road surfaces, the stakes are high. The States collectively spend more than $10 billion annually to pave with hot-mix asphalt. Significant improvements in performance quickly translate into savings.
"The public sector—precisely because it is spending the public's money—is, by its nature, a risk-averse institution," Mack says. "In one sense, this is a good attribute but, at times, it can impede improvement. If public decisionmakers are to take calculated risks in the implementation of new technologies, they must first work in an atmosphere that rewards rather than punishes risk-takers."
As Mack sees it, research and its implementation begin with awareness of the opportunities and picking those that have a good chance for success.
He adds, "Contrary to common belief, government does not have an infinite supply of resources, and one must choose wisely where to invest so as not to waste precious resources and miss alternative opportunities."
Pushing the Margins of Performance Adds Up to Big Savings
Just what kind of savings can be expected from Superpave? As a case in point, New York State is responsible for about 72,420 lane-kilometers (45,000 lane-miles) of highway. When considering an infrastructure of that magnitude, small increments in performance can pay large benefits over time. Superpave is a technology that works at the margins of performance. It is not a "silver bullet" that will make pavements last indefinitely, but it is expected to add service life.How much life will it add? At this time, that is an unknown. However, according to Paul Mack, "New York knows that if we are able to extend the service life of all of our pavements by just 1 year, we will save $1 billion over 30 years. New York, and many other like-minded States, aggressively adopted Superpave because we understood that even marginal increases in performance would return value to our customers and taxpayers."
After identifying the research that should be pursued, the next step is selling it to others. "Of particular importance is to sell as high on the organizational chart as you can," says Mack. "This is really the development of champions who have the authority to commit resources and the ability and commitment to stay the course when things go badly, which they will."
Next is developing a plan that identifies the steps needed to implement the research successfully. These steps can include training and equipping staff, conducting pilot programs, refining the plan, and developing a rollout schedule. "Then you must work the plan and work it consistently," says Mack. "One good rule is to always maintain progress, never step backward, and maintain momentum until you have fully implemented the research product.
Finally, never assume it has ended."
During the early days of Superpave, the barriers and hurdles seemed endless. As with most new ventures, Superpave was met with both excitement and hesitation. Naysayers abounded while others continued to work to make Superpave the best method for designing mixtures and specifying materials.
"Implementation is a tough business," says Rick Dunn. "When things are going well, you will find yourself in a crowd. If things should falter, it's the lonely hearts' club."
Photo by: Tom Harman, FHWAWorker performs a compacted density check to ensure proper compaction of Superpave.
Looking at the Future of Superpave
Strong staff support by AASHTO and training offered by FHWA's headquarters offices enabled the lead-State concept to succeed. By 2000 the lead States concluded their mission because it was apparent that Superpave had moved beyond the initial implementation stage. By then, Superpave had entered a new era in which it was no longer the struggling upstart of hot-mix asphalts, but the primary pavement system consistently chosen by States across the Nation.
TRB's Superpave Committee has picked up where the lead-State team left off. The committee has developed a long-range plan for the delivery of needed improvements to the current Superpave system and to performance-prediction models. The Superpave committee has been successful in obtaining approval for the annual research elements of the plan.
One major goal on the horizon is the development of a simple performance test to tie mix design to structural design. Another is to continue good communication practices by creating one definitive source for information. To fulfill Superpave's mission to be the universal system of choice for all U.S. hot-mix asphalt, implementation must move beyond the State level to the municipal and commercial levels as well.
The vision is that Superpave will become the standard system of hot-mix asphalt specification, design, and construction in the United States. That's a heady goal for yesterday's young upstart, and it's validation that Superpave is an investment that has already paid off handsomely.
Cathy Frye is a contract writer for the Federal Highway Administration.
Other Articles in this issue:
The Bridges That Good Planning and Execution Rebuilt
Bridge Rebuilt on the Fast Track
Stop. You're Going the Wrong Way!
Toledo's New Signature Structure
Superpave Comes of Age
