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Highway Quality Compendium

Asphalt's Generation of Change

by Tom Kuennen

Perhaps nothing in the road industry has changed more in the past two decades than asphalt pavement technology.

Now well into the 21st century, the asphalt paving industry is light years from what it was just a generation ago.

But if a generation is defined as the interval between the birth of parents and the birth of their children, those light years actually are only about 20 years.

In those two decades the industry has been transformed from being a producer of a dependable, but plain-vanilla product for overlays and low-volume roads, to a producer of an environmentally friendly, high-tech paving medium adaptable to different climates, traffic loads, end-use applications, and suitable for recycling and reclamation.

And it's all happened on our watch.

Consider these near-tectonic shifts in hot-mix asphalt that have occurred in just two decades:

  • The industry has shifted from conventional Marshall mix designs, based on a binder's viscosity or resistance to penetration, to performance-related, more durable Superpave binder mix designs.
  • That shift has spawned a new generation of lab and field mix-testing equipment and new full-scale accelerated pavement testing facilities, and it has supported a new class of technically trained lab technicians. One result of this is a tremendous, growing body of data that is being used every day to make mix design decisions and improve long-term asphalt performance.
  • Complementing Superpave, new, extremely rugged mix designs like stone matrix asphalt (SMA) have been imported from Europe and elsewhere to benefit highway agencies and their motorist patrons.
  • The chemistry of liquid asphalt has been enhanced by a new generation of asphalt modifiers, boosting the performance of Superpave mixes (Superpave Plus), open-graded friction courses, and thin-lift overlays.
  • Abetted by innovative new equipment and research, recycling of reclaimed asphalt pavement and other industrial or waste materials into pavements now has spread through the asphalt paving establishment, benefiting the environment and reducing costs.
  • The industry has adapted to most of its work being done on existing rights-of-way and accommodating ever-growing traffic loads by embracing night work as a standard way of doing business.
  • Major changes in state department of transportation staffing and philosophy have put more responsibility with the contractor for quality assurance and quality control. These changes have also compelled contractors to offer long-term warranties for their work, and have forced them to develop new equipment and new ways of providing the super-smooth pavements that taxpaying motorists want.
  • While infrared and electronic technology improve quality and placement of mix, new asphalt paver designs permit faster, safer, and more versatile paving.

"In my time, hot-mix asphalt has changed dramatically," said National Asphalt Pavement Association 2003 chairman Peter A. Wilson, senior vice president of Barriere Construction LLC, in his inauguration address. "We have had our own evolution and revolutions. Today, we know so much more about our product and its characteristics than we did 25 years ago."

In 2003, the industry knows how to design a pavement that will not rut, and one that holds up to the heaviest traffic, the Louisiana contractor said, adding today's asphalt pavements are smoother, quieter, more cost-effective to maintain, and longer-lasting than ever before.

"We have all these things going for our industry because we have a new and improved product, a product much different and much better than we have ever had," Wilson says.

Durable Mixes

The number-one change over the last 20 years has been the rise of more durable asphalt mixes and the advent of performance-related binder specifications under the Superpave system of asphalt mix design.

No other development in HMA in the past generation has had so much impact in so many areas of the asphalt industry as the advent of Superpave.

Superpave-a registered trademark of the Transportation Research Board-was a product of the Strategic Highway Research Program, authorized by Congress in 1987.

And while Superpave launched the modern era of asphalt paving, Superpave itself was launched with the 1984 Transportation Research Board Special Report 202, America's Highways: Accelerating the Search for Innovation, also known as the Strategic Transportation Research Study or the STRS ("Stars") report. This report laid the foundation of the Strategic Highway Research Program, which gave birth to Superpave.

Under the guidance of an expert steering committee, STRS settled on six areas of study in which focused, accelerated, results-oriented research promised significant benefits.

Foremost was asphalt, with an objective of improving "pavement performance through a research program that will provide increased understanding of the chemical and physical properties of asphalt cements and asphalt concretes," STRS said.

The other research topics were long-term pavement performance, the cost-effectiveness of maintenance, protection of concrete bridge components [from chlorides], cement and concrete in highway pavements and structures, and chemical control of snow and ice on highways.

STRS recommended a radical increase in research funds-to the tune of $150 million over five years-funded by 0.25% of federal-aid highway funds. The American Association of State Highway & Transportation Officials, representing all state departments of transportation, agreed to a 0.25% "take-down" from the federal-aid highway funds, and the Strategic Highway Research Program was born.

Superpave is Performance-Based

Superpave is a performance-based system of specifications for designing asphalt pavements to hold up to the traffic loading and weathering stresses of the new century.

The three major elements in the Superpave system are an asphalt binder specification geared to pavement loading and local climate, a volumetric mix design and analysis system, and mix analysis tests and a performance prediction system that include computer software, weather database, and environmental and performance models.

Superpave's volumetric properties include the percentage of air voids, voids in the mineral aggregate, and voids filled with asphalt. Superpave allows civil engineers to fine-tune asphalt mixes to specific traffic loads and climates, thus producing pavements that are more durable and less likely to rut in extremely hot weather or to crack in extremely cold weather.

Switch to Superpave

The Marshall system of mix designs served well from World War II, but under the crushing loads of modern traffic, had to be reconsidered.

"If there was a single problem with asphalt in the 1980s, it was rutting," said Gerry Huber, P.E., research engineer, Heritage Research Group, at the Superpave 2003 conference, held March 17-19 in Nashville. "Rutting became a national epidemic in the 1980s. If you look in the trade press, in the technical journals you will see there was a huge amount of emphasis placed on rutting of asphalt pavements."

The rutting of the 1980s was different than much of the rutting of the past, he said. "This rutting was occurring within the mixture itself, not structural rutting," Huber said. "Rutting had tended to be a structural process, in which the subgrade or granular layers underneath the pavement were giving away, with a gentle settlement in the pavement causing the rut in the wheel path. Here, a shallow rutting was taking place, in which the upper 3 or 4 inches of HMA were rutting, with sideways displacement of the mix in the asphalt layer itself."

To fix required a new approach toward rutting. The existing penetration grading of asphalt binders dated to the 1890s, and derived from the force required for a physical push of the thumb, and later, No. 10 sewing machine needle, to make a displacement at the top of a barrel of asphalt.

"In the 1970s we decided we should get a lot more sophisticated than the penetration test, and we began to look at viscosity penetration," Huber said. "We began to develop devices to measure the viscosity of asphalt instead of penetration."

But both specification methods had a common drawback: They only measured how stiff the asphalt was at a certain temperature. "They didn't tell anything about the properties of the asphalt at other temperatures," Huber said. "Asphalt's stiffness properties are affected by temperature and time of loading. SHRP was tasked with coming up with performance-based specifications for asphalt binder, mix design, and aggregates. SHRP addressed rutting, fatigue cracking, and low temperature cracking."

At the start of 2003, 47 states included Superpave specifications as a standard specification, if not the only spec allowed, for state DOT paving. The University of Texas at El Paso found that for the 2002 construction season, the most recent reliable data available, 4,726 scheduled projects were designed using Superpave procedures.

That's triple the number of Superpave projects built in 1998 and about 60% of the state asphalt paving projects scheduled for letting in 2002. Superpave has become the national standard.

Asphalt Modifiers Boost Performance

While additives to asphalt have been promoted since the days of the asphalt "patent mixes" of the 19th century (for this purpose, 1871-1918), the specialized asphalt modifiers of the last two decades have improved asphalt performance, and made possible designs such as open-graded friction courses and thin-lift overlays.

And today modifiers make it possible to improve lower-performing performance-graded asphalt binders to the point where they can meet stringent PG specifications for Superpave mixes. The performance properties of asphalt modifiers for Superpave work now are being studied and publicized by the Federal Highway Administration under its Superpave Plus program.

"Modified asphalt binders are typically used in high stress applications," says FHWA's John D'Angelo, P.E. "They have been used in intersections with stop-and-go traffic, high-volume truck routes, and high-volume interstates. Modifiers have also been used in extreme climate conditions to reduce aging in desert climates and to help produce binders for extreme low-temperature applications."

OGFCs were abandoned in the 1970s and 1980s because the liquid asphalt was not stiff enough, creating drain-down of asphalt into dense "fat" spots, while encouraging raveling of the top layer of aggregate. Today's polymer-modified asphalt mixes are mixed at higher temperatures, thus more efficiently drying the aggregate in the drum and improving adhesion.

Modifiers developed over the last 20 years include:

  • Styrene butadiene rubber, "latex." SBR stiffens the binder and can improve adhesion and cracking resistance. It is usually added at a minimum rate of 3% by weight of binder.
  • Styrene-butadiene-styrene. SBS polymer increases stiffness, crack resistance, and adhesion of the binder, and is added at a rate of 3 to 5% by weight of binder.
  • Ethyl vinyl acetate. EVA polymer boosts stiffness and cracking resistance at temperatures above freezing, but doesn't provide good low-temperature cracking resistance.
  • Crumb rubber modifiers. CRM, added at less than 20% by weight of binder, are used with great success. The crumb rubber is blended with asphalt cements at elevated temperatures ("wet" process) and improves resistance to cracking. A "dry process" adds the crumb rubber as an aggregate in the drum.

RAP Becomes Part of Industry

The energy crisis of the late 1970s led to another tremendous change in the way asphalt is manufactured and placed-the adoption of reclaimed asphalt pavement specifications.

When the Arab Oil Embargo and subsequent energy crisis triggered skyrocketing oil prices and petroleum conservation programs, aged asphalt pavement changed from a waste material destined for landfills to a valued product to be stockpiled and reused in many ways.

But it would not have been possible without the refinement of the cold milling machine.

"By the mid- to late-1970s, high-horsepower cold milling machines took over and became an integral part of the rehabilitation process," said NAPA president Mike Acott in late 2001. "The operation was seamless, and best of all it could be done under traffic. It restored the [road] profile and traffic could ride on the milled surface."

In 2003, RAP is commonplace; it is reused as inexpensive road base, added to virgin hot-mix asphalt as a tested material, used for driveways, bike paths, recreational trails, and much more.

Asphalt pavement is unquestionably the nation's most widely recycled product. A 1993 study by the FHWA and EPA says about 73 million of the 91 million tons of asphalt pavement that are removed each year during resurfacing and widening projects are reused as part of new roads, roadbeds, shoulders, and embankments. That's a recycling rate of 80%.

The 73 million-ton volume of recycled asphalt pavement is about one-third higher than the total volume of 60.7 million tons of post-consumer recycling. And it's double the volume of paper, glass, plastic, and aluminum combined, the FHWA/EPA reports.

Use of RAP also saves valuable aggregate resources. While there are plenty of construction aggregates in place in the ground, there are fewer and fewer aggregate sites that are permitted for extraction.

Existing quarries or gravel pits once outside of a city now are being surrounded by new suburbs-and neighbors who don't like living near quarries and will fight any kind of expansion.

But RAP contains aggregates that have already been acquired, permitted, shot, loaded, crushed, screened, stockpiled, reloaded, and hauled, saving time, money and resources.

And reclaimed asphalt pavement isn't the only product recycled in asphalt pavements or below them. Others include reclaimed demolition portland cement concrete as base material; crumb rubber from old tires, added to asphalt pavement or reused as bases for temporary traffic signs, traffic cones, or in rubber railroad crossing pads; crushed, rounded broken glass as a mineral aggregate in asphalt; waste sand from metal-casting foundries; reclaimed asphalt roofing shingles; and in California, crushed toilets in road base.

Full-Scale Testing

The performance of Superpave, SMA, RAP, and different types of aggregates are being tested year after year under real-world conditions at a variety of full-scale, accelerated testing facilities.

These facilities have included MnRoad in Minnesota, WesTrack in Nevada, and most recently, the Pavement Test Track of the National Center for Asphalt Technology.

MnROAD. The Minnesota Road Research Project is the world's largest and most comprehensive outdoor pavement laboratory, distinctive for its electronic sensor network embedded within 6 miles of test pavements. Located 40 miles northwest of Minneapolis/St. Paul, its design incorporates 4,572 electronic sensors. The sensor network and extensive data-collection system provide opportunities to study how heavy, commercial truck traffic and the annual freeze/thaw cycle affect pavement materials and designs. Unlike WesTrack and NCAT, MnROAD is not exclusively devoted to HMA research.

WesTrack. As Superpave unfolded, the Federal Highway Administration became involved in developing performance-related specifications for HMA. Its first major step was the construction and loading of a test track in Nevada, WesTrack, near Reno. The WesTrack Road Test was conducted from 1996 to 1999. Its purpose was to evaluate the direct effects of deviations of materials and construction properties on pavement performance. WesTrack was a 1.8-mile oval track divided into 34 test sections. The track was loaded over a two-year period using driverless vehicle technology. Test results provided useful information in areas such as quality control/quality assurance, construction methods, pavement rehabilitation, and materials specifications. But WesTrack had its critics. Some experts felt the driverless vehicles did not provide a real-world effect of wear on the pavement surface. The computer-controlled trucks continuously traveled in the same wheel path causing extreme wear in that path.

NCAT. Today, NCAT's Pavement Test Track is attempting to answer some of the questions raised at WesTrack.

In 1986, members of NAPA endowed the National Center for Asphalt Technology at Auburn University, providing a centralized, systematic approach to asphalt research. In 2000 NCAT opened a new research center and 1.7-mile test track and is now the world's leading institution for asphalt pavement research.

The first phase of testing at NCAT's track incorporated 46 sections, averaging 200-feet long, each of a different permutation of asphalt mix or lift design, as specified by one of the co-sponsoring state DOTs, or by the Federal Highway Administration.

In mid-December 2002 the NCAT Test Track ended a two-year cycle in which 10 million equivalent single axle loads-equal to 1.6-million miles-were logged on the track using professional drivers.

Among the findings after two years: Negligible rutting of the performance-based binder sections took place in test sections, and that occurred mainly during the first summer. Rutting decreased in the second summer, and stopped after the seven-day average high air temperature was below 82 degrees F. The small amount of rutting observed probably was related to "densification," or long-term compaction of mix under traffic.

In October 2003, following reconstruction work, the NCAT track was to resume its experimentation. Reconstruction in summer 2003 included milling and inlaying 14 sections with new rutting study mixes, and deep removal of eight sections to facilitate a small, instrumented structural experiment. Truck traffic will continue on the remaining sections to extend the original 2000 experiment over a second application of design traffic (for example, another 10 million ESALs).

Demands Won't Stop

As asphalt has gotten bigger, better, and more complex, and the work environment more complicated, the state DOTs and taxpaying motorists have benefited. From the point of view of the contractor and asphalt supplier, though, the challenges have gotten harder and more costly. Unfortunately, that's a cycle that's not going to stop. Fortunately, America's asphalt contractors have pledged themselves to setting the pace and not stopping the improvement.

"The asphalt pavement industry has demonstrated an ongoing commitment to quality improvement and product innovation," says NAPA's Acott. "It's the versatile pavement that meets every customer's specific needs."

Reprinted from Better Roads, November 2003. Better Roads can be visited online at www.BetterRoads.com.

Perpetual Pavement Launches a New Chapter in Asphalt Design

Perpetual Pavements are designed with thick layers of asphalt of different mix types, with a sacrificial driving course on top. This driving or friction course is intended to be periodically cold-milled and overlaid to restore the ride quality and surface friction.

Perpetual Pavements are designed and built from the bottom up, with a lower layer designed to resist bottom-up fatigue cracking. The middle layer uses an asphalt mix designed to support anticipated traffic loads, and the top layer may be customized to local conditions and requirements, from SMA for high-volume urban interstates, to OGFC for noise reduction and safety, to Superpave for other applications.

"We now have the capability of designing the most cost-effective mix using local materials, selecting the appropriate mix type for the traffic conditions and environment," says NAPA's president Mike Acott. "Through an understanding of the interrelationships between material characterization, pavement thickness, fatigue, and rut resistance, we are able to develop Perpetual Pavement structural designs that result in the lowest life cycle cost."

Promotion of Perpetual Pavement designs continues, and each year the Asphalt Pavement Alliance recognizes long-performing, existing pavements which meet Perpetual Pavement design criteria with the Perpetual Pavement Award.

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Updated: 06/27/2017
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