A thick, dry stake

News January 07, 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, 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"

In fact, 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. "There are sections out there that have performed for 35 years intact and they continue to perform," said Mahoney. "I could find no pavements that have been reconstructed on that particular highway."

Average daily traffic (ADT) counts vary from western Washington (the Seattle area) to eastern Washington (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 western Washington and 9.5 in. in eastern Washington. 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 western Washington most of the pavements have received one overlay; in eastern Washington 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 the west is between 15 and 16 years, and in the east it's more than 12 years. In the west, climate and temperatures are more moderate, but eastern Washington has freeze-thaw cycles not found in the west.

Among other conclusions in the report:

* WSDOT's use of a 40-year design period in its Pavement Guide is valid;

* Asphalt sections on I-90 generally fall into the "good" category as defined in the Federal Highway Administration's Long Term Pavement Performance program;

* The International Roughness Index (IRI) measurements for the I-90 asphalt pavements fall into the "good" category; and

* HMA resurfacing may be necessary after 12 to 16 years.

For more on the story, read the January issue of ROADS&BRIDGES.

Overlay Init