Perpetual Pavement

Perpetual Pavement

The Perpetual Pavement concept was first articulated in 2000 and the concept has rapidly gained acceptance. The APA’s newest technical document on the subject is Perpetual Asphalt Pavements: A Synthesis. This comprehensive publication captures the activities that have taken place over the last decade, synthesizes the information in way that is useful to providing guidance for Perpetual Pavement design and construction, and provides a vision for further research and development to refine Perpetual Pavements.

Since the inception of the Perpetual Pavement Awards program, the Washington State Dept. of Transportation has been awarded four Perpetual Pavement Awards.

An Article on the Basics of Pavement Design and Perpetual Pavements

Assuming an entirely new road where there has been no pavement before, the process begins with an examination of the right of way and the projected mix of traffic and the expected loading. Then, the soil is prepped, in some cases amended, compacted, and graded (F in the figure). During this stage, drains, sewers, and other features are placed. Next comes the pavement foundation, usually gravel that is placed and compacted (E).

Once the foundation is in place, actual paving begins. Asphalt is usually placed in layers, which can range from several inches thick to a fraction of an inch thick, depending upon the mix, position in the structure, and the purpose (A, C, and D in the figure).

Asphalt mixtures are like recipes, there are a lot of options — the size of the aggregates, the Performance Grade of the binder, recycled materials, additives, etc. — and the goal is to build a strong, long-lasting pavement. Ideally, the mixes should be designed and placed in a way that distributes the strains of traffic loading throughout the pavement in a way that localizes any distresses to the surface. If an asphalt pavement cracks at the surface (A in the figure), it is a fairly quick and easy process to mill off the surface and to replace it with a new surface, allowing the structure to remain in service indefinitely. If cracks happen at the bottom (regardless of pavement type), the pavement full depth of the pavement has to be replaced/repaired — a much more time intensive and costly proposition.

Asphalt mixes are produced at an asphalt production facility according to the mix design formula (or recipe) and loaded into a haul truck for delivery to the job site. On the site, the mix is transferred to a paver that places the mix at the desired width and thickness. The paver is followed by a compactor that tamps down the asphalt to the desired level of compaction. Once compaction is achieved and the pavement has sufficiently cooled, the next layer can be placed (or the road opened to traffic). Between each layer of a pavement, a tack coat (usually an emulsion of asphalt cement) is used to glue the layers together, improving the bond between the layers and strengthening the structure (B in the figure).

If an existing road is being repaired or replaced, the work can be as simple as milling off a few inches and replacing it with a few inches of new asphalt mix. (It’s worth noting that since asphalt is 100% recyclable, when a pavement is milled the millings are almost always saved and reused, usually to replace a percentage of the virgin asphalt cement and aggregates in a new pavement.) Or it may be a more extensive project that involves an entirely new structure, such as when an old concrete pavement is rubblized to create a foundation for a new road.

Author: T. Carter Ross, Communications Director for the National Asphalt Pavement Association.
Article originally appearing on Quora. com, June 9. 2014.