The Superpave System -- Better Roads, Big Savings

Nearly two-thirds of States have adopted_or are in the process of adopting_the Superpave binder specification, which is the first step in implementing the Superpave system. Many States are already planning to take the next step_namely, implementation of the Superpave volumetric mix design procedures by 2000, in line with the target date set by the Asphalt Technical Working Group. These States are making the right choice, according to the findings of the Strategic Highway Research Program (SHRP) assessment project. By producing longer-lasting pavements, the Superpave system can stretch highway dollars and save motorists time and money.

The assessment of the Superpave system focused exclusively on the role of the asphalt binder in mix performance. The properties of the binder have a significant effect on the performance of an asphalt mix, leading TTI to estimate that switching to the Superpave binder specification could increase the service life of an asphalt overlay by 25 percent, to 10 years from 8 years. Based on States' experiences, the Texas Transportation Institute (TTI) estimated that one-fourth of the asphalt paving projects in the United States use binders that do not meet the Superpave binder specification.

Those conservative assumptions yield tremendous potential savings over a 20-year period. If, for example, all highway agencies adopt the Superpave binder specification within 5 years, TTI calculates that the savings to highway agencies nationwide could reach $637 million each year. Even if it takes highway agencies 10 years to implement the specification, the annual savings could still reach $484 million.

The potential savings to motorists are even more impressive. Because longer-lasting overlays will mean fewer maintenance-related delays and less wear and tear on vehicles, TTI calculated that motorists could save as much as $1.7 billion per year over a 20-year period if highway agencies were to adopt the Superpave binder specification within 5 years. If it takes 10 years to adopt the technology, the savings to motorists could still reach $1.3 billion per year.

Superpave research was the single biggest item in the SHRP budget, costing $53 million. All told, research, development, and implementation of the Superpave binder specification will cost an estimated $230 million over 20 years. But even if highway agencies take 10 years to fully implement the Superpave binder specification, they'll still save more than twice that amount every year for the next 20 years. Motorists will save more than five times that much every year.

States Report Success

States that have already built Superpave pavements agree that the new system should produce longer-lasting roads.

• In 1995, the Alabama Department of Transportation (DOT) resurfaced 8 km (5 mi) of Route 165, which was badly rutted, with a Superpave mix. Despite heavy truck traffic and extremely hot weather, the pavement shows virtually no signs of rutting, and Alabama DOT expects the pavement to last longer than its conventional mix.

• Early Superpave test sections constructed on Interstate 43 in Waukesha County and on Interstate 94 in Monroe County, Wisconsin, are holding up better, after 4 years of cold winter weather and heavy traffic, than adjacent sections constructed using Wisconsin's conventional mix.

• Delaware DOT used a Superpave mix to widen US Route 113, which carries high volumes of beach-bound traffic and lots of heavily loaded trucks. Jim Pappas of Delaware DOT says, "With our normal mix, we expect a 10- to 12-year life for the pavement. With this new mix design, we hope to add a couple of years to the life."

• Texas DOT has built several projects using asphalt binders that meet the Superpave binder specification. Darren Hazlett of Texas DOT says, "Using the conservative assumptions that 25 percent of the asphalt Texas now uses doesn't meet the specification and that pavement life will gain about 2 years where we use binders that meet the Superpave specifications, I came up with an extremely rough estimate of $2.2 billion in savings over 30 years."

TTI's analysis included consideration of the following factors:

• Highway agencies in the United States spend $10 billion on hot-mix asphalt annually.

• Traffic loads and volumes are expected to increase by 2.1 percent annually.

• Studies by Texas DOT found that one-fourth of its overlay projects do not use the right grade of binder for the conditions at the project site. By converting to the Superpave binder specification, the DOT expects those overlays to last 50 percent longer. In its analysis, TTI used a more conservative estimate of a 25 percent increase in pavement life.

• Based on early projects, Superpave binders are, on average, more expensive than other grades of binders, increasing the cost of overlays by about 7 percent.

• As States implement the Superpave system, they will have higher upfront costs for purchasing and maintaining equipment and for employee training. These costs were subtracted from the projected benefits.

• The cost savings to motorists are based on reductions in delays and in vehicle operating costs, which result from improved pavement condition.

• The cost savings to highway agencies are based on longer pavement life, which reduces the frequency of overlays. For example, over a 40-year period, a roadway would require four overlays built with a Superpave-graded binder, as opposed to five overlays built with a conventional binder.

By producing longer-lasting pavements, the Superpave system can stretch highway dollars and save motorists time and money.

What Makes the Superpave Binder Better than Conventional Grading Systems?

Traditionally, engineers have tested binders for characteristics such as viscosity or penetration. These tests do not measure a binder's low-temperature properties. As a result, highway agencies do not always choose the best binder for a job. The Superpave binder specification, in contrast, classifies binders according to the pavement temperatures under which they will serve.

The Superpave binder specification sets forth the physical properties that all binders must have; what varies, however, is the temperature at which those properties are reached. The properties indicate a binder's ability to resist rutting and low-temperature cracking.

Pavement designers determine the pavement temperatures expected at the project site and then select the appropriate Superpave binder grade. For example, a Superpave binder classified PG58-22 will meet the required physical properties at pavement temperatures ranging as low as -28 ºC (-18 ºF) and as high as 58 ºC (136 ºF).

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