A Thick Dry Stake

Asphalt Article January 21, 2002
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At state transportation departments across the nation, acceptance is growing rapidly for the concept of perpetual pavements bui


At state transportation departments across the nation, acceptance is growing rapidly for the concept of perpetual pavements built with thick hot-mix asphalt (HMA).


More and more states are beginning demonstration programs for perpetual pavements.


Under the concept, total reconstruction—the remove-and-replace option—is rendered virtually obsolete. Only surface replacement would be needed at about 20-year intervals. As a result, state DOTs are attracted to the cost effectiveness and reduced traffic delays that perpetual pavements would bring about. In fact, states including Illinois, California, Virginia, Michigan and Ohio recently have launched demonstration projects or taken action regarding long-life HMA pavements and principles.


The concept of the perpetual pavement is not a new one; full-depth and deep-strength asphalt pavement structures have been built since the 1960s. Today, however, recent efforts in materials selection, mixture design, performance testing and pavement design make it possible for performances to exceed 50 years, while periodically replacing the pavement surface and recycling the old pavement material.


Perpetual pavements have three distinct features: a rut-resistant and wear-resistant surface layer; a rut-resistant and durable intermediate layer; and a base layer with the combination of sufficient asphalt thickness and flexibility to resist deep fatigue cracking.


"I think there’s a lot of promise to perpetual pavements," said Kevin Herritt, the chief of the office of state pavement design for the California Department of Transportation (Caltrans). Caltrans is building a long-life HMA pavement demonstration project on I-710, the Long Beach Freeway. "That’s why we’re looking at that philosophy here. The biggest concern I have is that there are a lot of variables in production and placement. If we focus on reducing the variability in those variables, we should be successful in pulling this off."


A lot of "good"


The longevity of HMA pavement structures, with only periodic resurfacings, has been demonstrated in Washington state roadways and on other highways including the New Jersey Turnpike. A report recently released by the Washington State Department of Transportation (WSDOT) on I-90 documents asphalt pavements there have performed well for up to 35 years—thus showing the classic characteristics of a perpetual pavement.


The report, done for WSDOT by University of Washington engineering professor Joe Mahoney, covers 300 miles of I-90. Mahoney’s research shows that HMA structural sections have remained intact over 23 to 35 years since their original construction.


Average daily traffic (ADT) counts vary from the Seattle area to Spokane. In the Seattle area, traffic counts range from 18,000 to 70,000 with about 5% trucks, while in the Spokane area they range from 20,000 to 50,000 and higher with 14 to 15% heavy trucks. ADTs in the center of the state average around 10,000 to 12,000 with 21% heavy trucks.


The original thickness of I-90’s asphalt base and wearing surface courses was 14.5 in. in the Seattle area and 9.5 in. in Spokane. All of I-90’s HMA was placed on granular base material.


"The thinnest structural sections were about 6 in. and the thickest were about 14 in.," said Mahoney. "I can say that there were no pavement failures when there was sufficient asphalt thickness."


Mahoney said some top-down cracking did occur on wearing courses, but that the cracks stopped at the bottom of the wearing surface layer. The cracks had no effect on the structural integrity of the pavement.


In the Seattle area, most of the pavements have received one overlay; in Spokane two overlays have been put down. Most overlays were about 2 in. thick, Mahoney said. The average duration of the original asphalt pavement without overlay in Seattle is between 15 and 16 years, and in Spokane it’s more than 12 years. In Seattle, climate and temperatures are more moderate, but the Spokane area has freeze-thaw cycles not found in the west.


Philosophically, yes


"We’re not calling it a perpetual pavement, but it has that philosophy built into it," said Caltrans’ Herritt of the state’s project on I-710 in southern California. Now under construction, the project comprises 3 miles of six-lane divided highway. Most of the HMA will go down over cracked and seated concrete pavement, but where the pavement passes under bridges, the concrete will be removed and replaced with full-depth HMA.


In the cracked-and-seated portions, the total thickness of the new HMA will be 9 in., said Herritt. An 8-in. structural section is to be topped by a 1-in. open-graded friction course (OGFC).


"The lower 3 in. is a mix rich with asphalt cement to minimize the fatigue cracking that might come up from the bottom," said Herritt.


The rich bottom mix will have about 0.5% more asphalt cement than Caltrans’ normal mix for that use. And the lower 3 in. will have a low design air voids content of 3%.


"The remaining 5 in. is designed to minimize the potential for rutting," said Herritt.


The top 3 in. of the structural section will be modified with polymers to improve the resilience of the pavement.


"We wanted to get it to perform for 30 or 35 years. We designed it for 200 million ESALs (equivalent single axle loads) total and we backed into the 30- to 35-year time frame by looking at the current traffic load. The difficulty is to project traffic loads over that time."


The project’s pavement thickness design came from a combination of mechanistic design and local empirical factors. The University of California at Berkeley performed the mechanistic-empirical analysis. (Mechanistic pavement design simply means that a mathematical model is used to calculate the reaction of the pavement to traffic loads.)


"We’ve tempered those (mechanistic) models with climate, material and loading information that relates to California," said Herritt. "That’s how we came up with the whole 8-in. thickness. And the 1 in. was added on because the district wanted a friction course to reduce splash, spray and noise."


Caltrans has chosen a rubberized asphalt for the OGFC, "because that’s our longest-lasting friction material." Periodically, Caltrans plans to mill off the OGFC and replace it.


With a construction cost of $18 million, the I-710 project is slated for completion in early 2003.


5 to 30


Illinois DOT officials have begun work on a program of five projects to demonstrate the feasibility of what they’re calling "extended-life HMA." The first project, on I-70 near the Illinois-Indiana line, "will use some of the concepts of perpetual pavement," said Eric Harm, bureau chief of materials and physical research, Illinois DOT. For the second through the fifth projects, the state plans to move toward a more pure perpetual pavement design complete with three layers of HMA: a rich bottom layer; a rut-resistant intermediate layer; and a durable, rut-resistant, renewable wearing layer. All five projects are slated to be built by 2003.


Design of the 11-plus-mile-long I-70 project had not been totally completed at press time.


"We’re taking a look at using a stone matrix asphalt-type surface, and for added durability we probably will use hydrated lime throughout the mix," said Harm. Hydrated lime is an anti-stripping additive.


Much of the I-70 job will be built over a rubblized 8-in.-thick concrete pavement with continuous reinforcement. Harm said IDOT plans to place a 17.5-in. thickness of HMA over the rubblized concrete. And the state may reduce the design compaction level slightly in the hot mix below the top 7.5 in. of pavement section.


Mechanistic-empirical


The Michigan DOT will start construction this summer on a 1.1-mile demonstration project that uses long-life design principles. Located near Dearborn, the project will occupy four northbound lanes of U.S. Highway 24.


"Our project was designed for 20 years, so the design life really doesn’t come up to that of a perpetual pavement, which is generally 40 years or more," said Harold Von Quintus, senior research engineer with Fugro-BRE Inc., the Austin, Texas, firm that provided design input to the DOT for the project. "The project uses the principles of a long-life pavement design. We used mechanistic-empirical design principles."


In terms of the mechanistic design, Von Quintus said he designed for a maximum of 10% fatigue cracks on the surface at the end of the design life, and 1Ú2 in. of surface distortion (rutting).


According to the cross section that Fugro-BRE recommended to the DOT, the pavement will have three layers with a total thickness of 10 in. The surface wearing course will be 2.5 in. thick and use PG 70-28 asphalt cement in either a dense-graded or a gap-graded Superpave mix. The 3-in. intermediate layer will use PG 70-22 in a Superpave binder mix, and the 4.5-in. base mix will be designed at 3% air voids—1% less than the other two layers.


PG 70-22 asphalt cement (AC) will be used in higher-than-normal amounts in the base mix. The AC in all three layers will be polymer-modified.


"Anytime you use higher AC content and lower air voids, you increase the fatigue resistance of that pavement," said Von Quintus. "Our project was designed for 3.7 million ESALs over a 20-year design life. If it were designed as a perpetual pavement, the asphalt layers would be a little thicker and it would have a design life of at least 40 years."


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