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Highway Quality Compendium
Innovations in Hot-Mix Asphalt
by David Newcomb
The era of innovation started with recycling, moved to Superpave and SMA, and might just get us to roll-your-own roads.
Competition demands product improvement through innovation to achieve an advantage in terms of quality, cost, and convenience. The hot-mix asphalt industry has answered the call of competition with innovations that better match materials and mixtures to their applications, and with manufacturing techniques and technology that produce these products in the most environmentally friendly manner possible.
And these innovations have been cost effective. The cost of the product must be the lowest it can be while ensuring that performance expectations remain at their highest. Likewise, the long-term costs of maintenance and rehabilitation need to be minimized in order to justify the selection of the product.
At one time, contractors were essentially told what to do and how to do it. The expertise in dealing with hot-mix asphalt resided primarily in the various agencies that specified asphalt, and the contractor provided the equipment and personnel to build the project.
But, beginning in the 1970s, the industry began to change its way of doing business by recycling asphalt. The oil crisis of that period made recycling very advantageous. Not only was it saving asphalt cement, it was saving aggregate, energy, landfill space, and just as importantly, it was saving money. Agencies wrote specifications allowing the contractor to retain ownership of the material and decide how much recycled material, within reason, to put into the new mix. This innovation has become a hallmark of the industry and asphalt is America's most recycled material as a result.
As the 1970s started to give way to the 1980s, agency concern over asphalt pavement performance led to intensive efforts to improve specifications and mix design processes, and to tailor mixes to specific circumstances. The introduction of quality control/quality assurance and end-result specifications in this period began the shift of responsibility to the contractor. As a result, contractors increased their knowledge of the product and saw the genuine economic benefits of reduced reliance on method specifications.
New Mix Designs
Late in the 1980s, the Strategic Highway Research Program started as an initiative by the state highway agencies through the American Association of State Highway and Transportation Officials. By 1993, a new asphalt binder specification and a new mix design procedure had been put in place by means of the Superpave system. In the 10 years since, many refinements have been made and continue to be made to simplify the procedure, to remove redundant requirements, and to improve performance. These refinements were the direct result of the asphalt industry working with AASHTO and individual agencies through the Binder and Mixture Expert Task Groups. The major missing piece of the Superpave system, a suite of performance tests, is about to be completed through National Cooperative Highway Research Program Project 9-19.
About the same time that Superpave was being developed, a joint industry-agency scan tour of Europe was made under the auspices of the Federal Highway Administration to investigate technology that might be of benefit to the U.S. That trip introduced the industry to a new type of hot-mix asphalt-Stone Matrix Asphalt. As a surface mixture, SMA had a proven record of rutting and cracking resistance under heavy traffic. The combination of a strong aggregate structure with a binder-rich matrix proved to be a winning recipe for high volume roadways. States such as Georgia, Maryland, Wisconsin, Illinois, Louisiana, Texas, and others now specify SMA as their premium surface mixture. Illinois plans to further this innovation by developing an SMA for low-volume roads.
Experimentation with open-graded friction courses began in the 1960s as a way of improving the skid resistance of pavements. Implementation took place in areas where they were successful, and further refinements to the mix design process were made. In the last couple of years, the new generation of open-graded friction-course materials and mix design were brought over from Europe and standardized at the National Center for Asphalt Technology. These mixes are more porous and more durable than their predecessors. These improvements are taking place just as greater benefits and more uses are being identified for the new generation OGFC.
Another benefit of OGFC-surfaced pavements is they vastly improve visibility in rainstorms by reducing the amount of splash and spray generated by traffic. Furthermore, OGFC surfaces greatly reduce the amount of pavement-tire traffic noise, an increasingly important benefit to landowners adjacent to highways.
The National Center for Asphalt Technology at Auburn University has been an important part of the change in the way asphalt mixtures are tested. They developed the NCAT oven, which allowed for the determination of mix asphalt content while eliminating the need to dispose of solvents. NCAT has played a central role in the refinements of the Superpave system, and provided the industry and agencies with guidance on mix design procedures for SMA and OGFC. The test track has answered questions regarding the rutting performance of surface mixtures and it will be key in the evaluation of new structural pavement design procedures.
Construction equipment and practices have also changed in an effort to improve the quality of the product, speed of construction, and production rates.
The invention of the milling machine ranks high in this category because it allows resurfacing to be done accurately and quickly. Additionally, it serves to reduce the number of steps in recycling the material by essentially sizing it before it goes to the plant.
Remixers and material transfer devices improve the smoothness of the roadway and minimize mix segregation.
Improvements in paver screeds, automatic screed controls, and rollers have also raised the bar on the quality of the HMA placed in the field, and engineering controls have been placed on paving machines to reduce fumes in the work environment.
In the future, contractors will have access to rollers that show the quality of compaction as they proceed down the pavement, and this will be tracked using global positioning technology.
HMA plants have also advanced, especially in terms of environmental friendliness. New plants incorporate innovations that make them run quietly, efficiently, and with fewer emissions.
The notched wedge joint was developed in Michigan over a concern for safety. This innovation allows contractors to place an overlay in one lane and not have to pull the other lane even before opening the road to traffic. This reduces construction time, costs, and improves smoothness.
Other innovations in longitudinal joints include joint tape and sealers which are applied to unconfined joints to improve joint performance.
Not all innovation is the result of broad sweeping changes in mix design, pavement design, or construction. Some of it is just people doing their jobs to meet the demands of the situation or the demands of specific requirements.
For instance, sometimes a road must get built when it's rainy or cold, and contractors have shown they can deal with it. On the New Mexico Highway 44 project, for example, contractors FNF and E.L. Yeager both used pavement heaters in the fall in order to keep the operation going. The construction management group of Koch Performance Roads noted that the heaters were an effective means of maintaining good paving conditions during cool weather and were crucial to the early delivery of the project.
Lakeside Paving in Seattle uses a trailer-mounted jet engine with the exhaust directed at the pavement to dry wet surfaces before paving. This improves the bond between the old and new layers and keeps the new mix hot, which gives them more opportunity to obtain density.
When confronted with a functional requirement of meeting a light reflectivity specification in a tunnel in France, Colas used waste from a mirror production plant in the surface of the roadway to provide the needed result.
There are further innovations in HMA technology occurring now and on the horizon which will respond to the needs of the future transportation system.
For instance, Perpetual Pavements, which are already gaining acceptance, will provide agencies with the means to reduce the life-cycle cost of owning a pavement and minimize user delay by having a long-lasting structure that only requires periodic resurfacing.
Porous asphalt pavements will present developers and land owners an alternative to current stormwater management practices by reducing runoff and providing groundwater recharge underneath parking lots meeting the proper criteria. Facilities using porous asphalt have been in use for up to 25 years of service, and they're still performing. NAPA has just released IS-131, Design, Construction and Maintenance Guide for Porous Asphalt Pavements to help designers and contractors who are interested in building porous pavements.
New methods of producing HMA will actually reduce the heat required by using warm-mix technology. Processes are being developed in Europe to reduce the mixing and compaction temperatures of HMA in order to reduce fumes and the energy required to make the mix. This technology will be showcased at the 2004 World of Asphalt Show and Conference, March 16 -18, in Nashville.
What's next? Asphalt you roll out like a carpet? Don't laugh, it's been done in the Netherlands as a part of their "Roads to the Future" project!
We are in a transition where responsibility for the final product is shifting from agencies to contractors. In the pursuit of providing a quality product that is economical and environmentally friendly, the industry is coming forward with numerous ideas and approaches to coping with the realities of construction and delivering the best possible performance.
David Newcomb is vice-president, Research and Technology, at the National Asphalt Pavement Association.
Reprinted from Better Roads, November 2003. Better Roads can be visited online at www.BetterRoads.com.