The Durable Performance of Perpetual Pavements
By Audrey Copeland, Ph.D., P.E.
When the Iowa Department of Transportation (IDOT) opened a new stretch of U.S. Highway 151 in Linn County back in 1966, they wanted a road that would last a long time, but they never imagined the traffic loads and heavy-weight trucks that drive on U.S. highways today. Despite this, U.S. 151 is still in place and in use 47 years later, handling more than 10 million equivalent single-axle loads (ESALs) since its initial construction.
During that time, U.S. 151 has required two overlays, but has never had to be reconstructed. The original pavement structure remains, and with each round of surface maintenance it seemed like a brand-new road to drivers. For this reason, IDOT won a 2012 Perpetual Pavement Award for this 3.5-mile stretch of northbound U.S. 151 in Linn County.
This is the durable performance offered by an asphalt pavement especially designed for long-life, commonly called a Perpetual Pavement.
Perpetual Pavement — or, as some states term it, Long-Life Pavement — isn’t an asphalt mix; it’s an advanced approach toward pavement design and construction that began around 2000 and places a high value on constructing a pavement structure where all the layers of the pavement — from the subbase to the surface — work together to limit strain and achieve long-lasting performance and durability.
The Perpetual Pavement design starts with a flexible fatigue-resistant base, topped with a thicker, high-modulus rut-resistant material, and, finally, a high-quality surface course is applied. The flexible fatigue- resistant base in combination with the rut-resistant material prevents bottom-up and structural distresses. Thus, the impact of traffic and climate over the years is confined to the surface layer, which is easily maintained and/or replaced as necessary.
These maintenance costs are predictable over the long life of a Perpetual Pavement. They also pale in comparison to the enormous cost of reconstructing the pavement structure at the end of its design life.
Without maintenance, every pavement grows rougher and noisier and gradually deteriorates. Durability is not the same thing as “maintenance free.” Your teeth are some of the strongest bones in your body, but they require brushing and flossing (i.e., maintenance) to remain in top condition. The same is true of pavements. Like your teeth, with a Perpetual Pavement that is properly maintained you will not lose your pavement structure. Any deterioration occurs only at the surface.
When the time for maintenance arrives, it is a simple matter to mill off a small layer of asphalt and to place a new surface layer, restoring the original ride quality and smoothness. This also allows new innovations, such as open-graded friction courses that reduce road spray, to be added as they are developed. And, since it is asphalt, any material removed during maintenance can be recycled in new asphalt pavements.
Recently, the New York State Department of Transportation (NYSDOT) completed a research project that included monitoring of a new Perpetual Pavement on Interstate 86. The pavement was constructed and instrumented in 2008 and data was collected from 2008 through 2011. The pavement has been exposed to heavy truck loading and extreme weather variations and after three years has experienced no distress. For the NYSDOT, the Perpetual Pavement design concept has been validated; it is now recommended that Perpetual Pavements be required for specific projects.
But what about long-term performance? Since 2001, the Asphalt Pavement Alliance (APA) has awarded Perpetual Pavement Awards to 85 pavements in 30 states and Canada, including U.S. 151 in Linn County, Iowa. All the award winners have been in service for at least 35 years without structural rehabilitation. Further, maintenance is performed no less than 13 years between maintenance cycles.
The Perpetual Pavement method is highly recommended for consideration for all new construction, but can also be considered for existing pavement structures. Asphalt pavements in good condition can be retrofitted to meet the Perpetual Pavement requirements by adding layers to increase expected design life.
For drivers and road owners alike, Perpetual Pavements are a wise investment that offer durable performance that lasts decades.
About the author: Audrey Copeland, Ph.D., P.E., is Vice President for Engineering, Research, and Technology for the National Asphalt Pavement Association (NAPA) and is recognized as a technical expert on using recycled materials in asphalt pavements.
Editor’s note: Be sure to check out more coverage of Perpetual Pavements in the October 2013 print edition of Better Roads.