Perpetual pavement research at the NCAT test track

Also known as long-life or extended life, these pavement structures are intended to meet traffic needs in excess of 50-plus years with only minimal surface treatments to maintain a good wearing surface. While long-life pavements are certainly nothing new — the Asphalt Pavement Alliance has been granting Perpetual Pavement Awards to recognize existing long-life pavements — only recently has attention been focused on intentionally designing the structure for extended life.

As more attention from the industry has been placed on perpetual pavements, research has been directed toward examining the many factors that contribute to perpetual pavements. One such study, at the National Center for Asphalt Technology Test Track is examining the effects of using stiffer upper asphalt layers combined with an elevated asphalt content in the bottom lift of the flexible pavement. It is theorized that the stiffer upper layers will help control surface rutting, while using additional asphalt in the bottom lift (optimum +0.5% asphalt content) will increase the fatigue resistance of the pavement. 

The perpetual pavement test section at the NCAT Test Track is part of a larger pavement structural study, sponsored by the Alabama Department of Transportation, the Indiana Department of Transportation, and the Federal Highway Administration, that is examining dynamic pavement responses under live truck loading to establish mechanistic links between pavement response (i.e., stress or strain) with performance (i.e., rutting and cracking).

The eight test sections that comprise the study are pictured in an accompanying figure, where sections seven and eight are designed to explore perpetual pavement concepts. Sections seven and eight are comprised of a Stone Matrix Asphalt surface course for rut resistance on top of unmodified hot-mix asphalt. Section eight contains the rich bottom with the increased asphalt content.

Central to the study are strain and pressure gauges in each section to monitor pavement responses under live traffic loads. Another figure illustrates the construction process with the gauges under cover material just prior to paving. Additional gauges, to monitor pavement temperatures and subgrade moisture, were also installed. The strain gauges provide data that quantify the pavement response under live loading, and each axle group is clearly visible in the strain trace. These responses can then be compared between test sections, monitored over time, and linked to pavement performance. The perpetual pavement section has typically lower strain than the other 7-inch sections. In fact, the response is similar to the thicker 9-inch sections. This is important since lower strain typically leads to longer pavement life.

The project is currently 30% complete and traffic is scheduled to be complete in December of 2005. Until then, traffic will continue to be applied, dynamic data will be collected, and performance will be monitored on a weekly basis.  Ultimately, in the context of perpetual pavement design, this research will help quantify the effects of using stiffer upper layers and rich bottoms in long-life flexible pavement design. More details about the project can be found at www.pavetrack.com.

GRAPHS MENTIONED IN ARTICLE:

David H. Timm, P.E., Ph.D. is Assistant Professor of Civil Engineering at Auburn University.

Reprinted from Better Roads Magazine
November 2004

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