The Quest for Long-Life Asphalt Pavement

Perpetual pavement is a marketing campaign, 
but it’s an engineering concept, too.
 Here’s what the buzz is about.

by Bob Bushmeyer

Asphalt pavers have seen the future, and it includes perpetual pavement.

For years, the conventional wisdom has been that asphalt roads have a lower initial cost, a shorter effective life, and a higher life-cycle cost than concrete roads. For that reason, even though asphalt is specified for most roads and for a huge portion of road surfaces in North America, portland cement concrete is often specified for new road construction or complete reconstruction involving long-term lane closures.

To combat the notion that asphalt roads are short-lived — and expand the market for hot-mix asphalt technology — the asphalt industry has created long-life asphalt pavement designs and is mounting a campaign to make road engineers and managers aware of them.

From Virginia to Michigan to California, and across the oceans, the particulars of perpetual pavement design are being recognized and applied.

What is a perpetual pavement?

Long-lived asphalt pavements aren’t new, proponents say. What is new is the perpetual pavement design, which takes an already successful design to another level.

Today’s perpetual pavement design is a three-layer hot-mix asphalt pavement that is intended to provide pavement life spans of 50 years or more, with occasional asphalt overlays to maintain optimum rideability. The layers are constructed of different asphalt designs. They are topped with a sacrificial friction course intended to be cold-milled and overlaid with asphalt at 15-20 year intervals to restore drivability. In practice, the actual composition and depth of the sections will vary according to anticipated conditions and traffic loads, including the percent of truck traffic.

As envisioned, a perpetual pavement starts with a lower layer specifically designed to resist bottom-up fatigue cracking. The middle layer uses an asphalt mix designed to support anticipated traffic loads.

This design of massive, flexible bottom layers prevents pavement distress which develops in the bottom layers. Distress is now confined to the surface course, where it is more easily repaired. This surface layer may be of any current HMA design, be it Superpave, stone matrix asphalt, or modified asphalt open-graded friction course. The combined thickness of all the layers prevents rutting in the base or subgrade.

“The design starts with a strong HMA base layer, flexible enough to prevent bottom-up, structural fatigue cracks,” says Gerald Waelti, executive director, Wisconsin Asphalt Pavement Association.

“As a long-lasting and smoother paving material, perpetual pavements using HMA could spell dramatic savings for state and federal transportation agencies striving to balance tight road budgets,” Waelti says. “And because asphalt pavements are 100% recyclable, perpetual pavements offer further cost advantages as well as environmental benefits.”

Underlying engineering principles

There is little doubt among promoters that perpetual pavements will perform with reliability because they are based on sound engineering principles.

These principles are articulated in a concept paper titled Perpetual Pavements, by Jim Huddleston, P.E., Asphalt Pavement Association of Oregon; Mark Buncher, Ph.D., P.E., The Asphalt Institute; and David Newcomb, Ph.D., P.E., National Asphalt Pavement Association.

With Perpetual Pavements, typically there are three asphalt layers:

• A durable, fatigue-resistant base layer.
• A rut-resistant and durable intermediate layer.
• A rut-resistant, impermeable, and wear-resistant surface layer.

The material in each asphalt layer is specifically designed to resist pavement distresses. The outcome is a thinner overall section than those using a conventional long-life design.

With this design, the potential for fatigue cracking may be reduced, and pavement distress may be confined to the upper layer or surface of the structure, according to Huddleston.

“Most pavement design procedures do not consider the characteristics of each pavement layer relating to fatigue, rutting, and temperature cracking,” the engineers write. “Since each pavement layer has its own part to play in performance, a new structural design method is needed to analyze each pavement layer. The mechanistic-empirical approach meets this need.”

With mechanistic design, they write, “knowing the critical points in the pavement structure, one can design against certain types of failure or distress by choosing the right materials and layer thicknesses. In the case of a perpetual pavement, this consists of providing enough stiffness in the upper pavement layers to prevent rutting and enough total pavement thickness and flexibility in the lowest layer to avoid fatigue cracking from the bottom of the pavement structure.”

Optimized layer stiffness

As the asphalt pavement is tailored to resist specific distresses in each layer, the materials selection, mix design, and performance testing need to be specialized for each layer, according to Huddleston. “The stiffness of each layer must be optimized to resist rutting or fatigue cracking, depending upon which layer is being considered,” they write. “Durability is a primary concern for all layers.”

Base Lift, or Layer. The base layer must resist the tendency to crack from bending under traffic loads, they write. One mix characteristic that can guard against fatigue cracking is a higher asphalt content. Combined with an appropriate total asphalt thickness, this helps prevent fatigue cracking from the bottom layer.

Engineers can also use pavement thickness, instead of special mixes, to ensure long-term resistance to fatigue cracking. If the total pavement is thick enough, its structural stiffness can reduce tensile strain at the bottom of the asphalt layers to insignificant levels. This allows for the use of a single mix design in the base and intermediate layers, and precludes the need to change mix types in the lower pavement structure.

Intermediate Lift, or Layer. The intermediate or binder layer must combine the qualities of stability and durability, write Huddleston, Buncher, and Newcomb. “Stability in this layer can be obtained by achieving stone-on-stone contact in the coarse aggregate and using a binder with an appropriate high-temperature grading,” they write. The mix design should be a standard Superpave mix, and the design asphalt content should be the optimum.

Wearing Surface or Friction Course. The design of the wearing surface depends on local requirements and economics. “In some cases the need for rutting resistance, durability, impermeability, and wear resistance may dictate the use of SMA [stone matrix asphalt, a low-fines, stone-on-stone mix],” says Huddleston. “This may be especially true in urban areas with a high percentage of truck traffic.”

In instances where the overall traffic is not as high, or in cases where the truck traffic is lower, the use of a well-designed, dense-graded Superpave mix may be more appropriate, they say.

First project in California

The first major perpetual pavement placement in the United States began last spring in Southern California.

Initial work began in March 2001 on this $16.7-million project, which will rehabilitate a 2.5-mile stretch of I-710 (Long Beach Freeway) between the Pacific Coast Highway and the San Diego Freeway in Long Beach. Initial preparation and structural work continued through 2001, with the actual paving to begin this year.

“The project marks the first large-scale use of asphalt concrete, long-life pavement on a major California freeway,” says a spokesman for the California Department of Transportation. “The goal is to develop and demonstrate new techniques that can replace aging pavements throughout the state with minimum traffic delay and less inconvenience to motorists.”

The new pavement design is the result of partnering at every level of project interest. A coalition of refineries, aggregate and emulsion suppliers, pavers, and the California Asphalt Pavement Association worked with the University of California-Berkeley’s Pavement Research Center and Caltrans as part of its Longer Life Pavement Rehabilitation Team to develop the performance specification.

“The designs developed for the project are based on accommodating 200-million equivalent single-axle loads for a design period of 30 years, significantly more than the typical pavement design period,” say researchers Fenella Long and C.L. Monismith.

Full-depth under overpasses

To maintain vertical clearance under overpasses, workers will remove existing pavement and construct a full-depth section of asphalt pavement, which uses perpetual pavement principles.

This full-depth section — 500 feet on either side of an overpass — will consist of a 3-inch rich bottom layer, with 6 inches of a standard asphalt mix, and a 3-inch modified asphalt pavement surface course, topped with a 1-inch open-graded friction course for the wearing surface.

Long and Monismith report that the full-depth asphalt concrete was designed using multi-layer elastic analysis. “The procedure requires determination of the principal tensile strain on the underside of the asphalt concrete pavement in order to mitigate bottom-up fatigue cracking,” they write.

“Determination of the vertical compressive strain at the sub-grade surface is also required to minimize the contribution of the layers below the asphalt concrete to surface rutting,” they say. “Fatigue resistance of mixes was determined using the SHRP-developed flexural fatigue test, which permits determination of the relationship between the applied tensile strain and the load repetitions to cracking.”

This full-depth structural section includes the use of a rich-bottom design for the lower portion of the full-depth HMA. Binder content for this section is 0.5% higher than the design binder content, according to Long and Monismith. “Increasing the binder content facilitates greater compaction,” they say, “which improves the fatigue resistance of the mix; and, because this layer is at the bottom of the asphalt concrete, the rutting resistance of the pavement is not compromised.”

This section under the overpasses will consist of an AR-8000 mix (for its higher stiffness), and a PBA-6a mix (for its greater rut resistance). Use of both mixes gave the thinnest pavement section while ensuring the fatigue and rutting performance, in keeping with perpetual pavement principles.

For the I-710 driving or wearing (friction) course, an open-graded friction course with an asphalt rubber binder will be used. Because of its porous structure, open-graded friction course will reduce tire splash, the potential for hydroplaning, and tire noise.

Crack-and-seat for open pavement

For the rest of the I-710 pavement in Long Beach, the existing concrete pavement will be cracked and seated to create a sound base, then topped with asphalt overlays meeting perpetual pavement criteria.

A 1-inch leveling course over the cracked-and-seated PCC base will be topped with a geotextile fabric. Another 4 inches of asphalt will be placed over the geotextile fabric, followed by a 3-inch modified asphalt pavement surface course, and a 1-inch open-graded friction course for the wearing surface.

A finite element analysis was performed to select the total thickness for the overlay section above the cracked-and-seated concrete, Long and Monismith say. The same materials as those used in the full-depth replacement sections under overpasses will be used there as well.

Ohio Turnpike work continues

In Ohio, the Ohio Turnpike Commission is widening 160 miles of the 241-mile-long Ohio Turnpike — the biggest and most complex asphalt rehabilitation project in Ohio’s history — using perpetual pavement principles, the Asphalt Institute reports.

The multi-year project adds a third traffic lane in each direction to the most heavily traveled portion of the highway from Toledo to Youngstown. Work began in 1995 and continues into the 21st century.

A life-cycle cost analysis performed early in the design phase by consulting firm Resource International showed that hot-mix asphalt was the most economical pavement type for the huge widening project, reports The Asphalt Institute.

Since the original concrete pavement in the 160-mile section had been overlaid with asphalt, turnpike officials wanted the same surface on the third lane. But the question was whether to use asphalt or concrete in the 10 inches of base underneath. The commission ultimately decided to use a hot-mix asphalt base for the entire roadway except for 15 miles where PCC was used due to very low subgrade strength.

The structure of the new third lane begins with 6 inches of 2-inch (maximum size) crushed aggregate base placed on compacted subgrade. Placed on top of that is 10 inches of large stone (2-inch maximum size) asphalt base course, followed by a nominal 3.75-inch (variable thickness) intermediate course. Completing the pavement structure is a 1.25-inch surface course with crushed slag aggregate added for skid resistance.

On top Down Under

Asphalt pavements incorporating some perpetual pavement principles are outperforming expectations in Sydney, Australia, according to the Asphalt Pavement Association of Australia. One example is Southern Cross Drive, on Sydney’s Orbital Route, which provides the main access to Sydney Airport and its southern suburbs.

This full-depth asphalt pavement was constructed in 1969. “It’s a pavement which was virtually maintenance-free for 25 years, before deterioration became apparent and rehabilitation was required,” the AAPA says.

One profile is of full-depth asphalt on a sandstone/sand subgrade, and the other profile is of a deep-strength pavement type. The asphalt throughout the depth of each pavement profile is of a very stiff mix to reduce strain in the subgrade.

“Australia and the USA are experiencing the same sort of advantages from well designed and constructed deep strength and full depth pavements on major roads,” the AAPA says.

“The indications are that these sorts of pavements can perform beyond widely held expectations concerning their life and maintenance needs,” the AAPA says. “The adoption of appropriate design concepts for full depth AC heavily trafficked pavements help immeasurably, as do the other key ingredients for success: good design and construction implementation.”

Virginia evaluates specs

Last year, the Virginia Transportation Research Council was evaluating hot-mix asphalt density specs in its project, Evaluation of Techniques to Measure Asphalt Pavement Density and Permeability.

The project evaluated lab and field permeability devices for their potential to improve VDOT’s current density specification or to be used in a replacement specification. It also evaluated the state’s hot-mix asphalt pavement density specification, and sought to develop a statistically sound quality assurance program for density testing that required a minimum amount of VDOT staff time.

“Based on preliminary testing that indicates severely inadequate density levels, the service life of Virginia’s pavements could be improved by 50% or more,” VTRC reports, regarding the benefits of denser pavements. “Developing specifications that provide impermeable asphalt pavement surfaces will help protect base asphalt from moisture damage and act as a step toward perpetual pavements.”

And in the Badger State, the Wisconsin Asphalt Pavement Association reports that a perpetual pavement project was built in 2000 on Wisconsin S.R. 50 near Lake Geneva, and a project was designed in 2001 for a weigh station off ramp on I-94 in Kenosha County. The latter project involved an 11-inch asphalt structure designed from the bottom up to include layers that are fatigue- and rut-resistant, and topped with a skin layer of high-performance mix that can more easily and quickly be rehabilitated when needed.

Vigorous new marketing

Perpetual pavement designs are being popularized in the United States by the new promotion arm of the asphalt paving industry, the Asphalt Paving Alliance.

Launched in 2000, APA is an industry coalition composed of the Asphalt Institute, National Asphalt Pavement Association, and the State Asphalt Pavement Associations, an umbrella group representing local associations in 36 states. APA’s activities include publications, outreach through industry meetings and conferences, and targeted communications to public officials and the general public.

(illustrations from article and sidebar notes:)

Turnpike honored with first perpetual pavement award

The New Jersey Turnpike Authority was honored with the first-ever Perpetual Pavement Award last September. The turnpike’s asphalt pavement was honored by the Asphalt Pavement Alliance for a half-century of service.

“Even though 50 years of heavy use have punished that pavement, motorists on the New Jersey Turnpike are still traveling on the original pavement structure,” says Mike Kolos, APA chairman, “only surface treatments have been used to maintain the pavement.”

Construction of the turnpike began in January 1950 and was completed in 23 months. Fifty years later, and 148 miles long, the New Jersey Turnpike is one of the most heavily traveled roadways in the nation.

To celebrate this anniversary, the New Jersey Historical Society opened a new exhibition on the turnpike entitled What Exit?

“The New Jersey Asphalt Pavement Association congratulates the New Jersey Turnpike Authority for 50 years of outstanding service as New Jersey’s premier superhighway,” says John Post, president of the association.

“It is entirely fitting that the APA’s first ever Perpetual Pavement Award recognizes the fact that not one mile of this durable road has ever had to be reconstructed. Our members helped build and maintain the Turnpike. We are a part of its history and its future.”

More information on perpetual pavements

Even though the concept of long-life asphalt pavements is in the early stages, many resources exist for more information.

You will find an abundance of material through the main marketing arm for perpetual pavements, the Asphalt Pavement Alliance. Visit the APA’s Web site at http://www.asphaltalliance.com/, where you’ll find the Buncher, Newcomb, and Huddleston concept paper noted in this article, and other technical documents.

New in January is APA’s Perpetual Pavements: A Synthesis, a scholarly document that summarizes all the existing literature on perpetual pavements in the U.S. and Europe. It can be downloaded free from the APA web site or purchased for $1 per printed copy.

Long and Monismith’s technical paper describing the mix selection process for Caltrans’ I-710 project in Long Beach can be found on the Web site of the University of California-Berkeley, Institute of Transportation Studies, Technology Transfer Program at www.its.berkeley.edu/techtransfer/resources/newsletter/01summer/I-710-pics.html.

And in November, APA released What if Roads Could Last a Lifetime, an interactive CD-ROM espousing permanent pavement principles. The CD is available from APA for $2.25 each.

Reprinted from Better Roads Magazine
February 2002

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