Asphalt pavement, where do we begin? We begin from the bottom up. Without a good base, we cannot have a good wearing surface. Anyone driving a car or truck north of the Mason-Dixon knows about bumping and thumping into pot holes and over frost heaves in areas with a freeze-thaw cycle. The national average per vehicle for extra wear and tear due to poor pavement preservation is over $400 per year. With a good subbase, base course and proper drainage, pot holes, frost heaves and alligator cracking can be eliminated.
When constructing a roadway, a new material for a subbase is not always required. It depends upon the existing material, the expected volume of traffic, the expected wheel load, and the climate. The subbase material may require treatment such as liquid asphalt or lime for stabilization. Ideally, a good subbase consists of a granular material properly graded and compacted, with drainage that prevents the intrusion and settlement of fines and water.
Onward, upward to the base course. The depth and materials used for our base course depend upon the subbase and, of course, the climate. It must contain a high percentage of large granular material and is usually treated with a binding material such as asphalt emulsions, fly ash, lime, or cement.
Next up is a binder course that acts just as the name implies: It binds the base course with the wearing course. The binder course has less depth and contains smaller aggregate than the base course with a higher percentage of asphalt.
We're finally ready for the wearing surface. This is where the latest advancements in technology have occurred. Let's start with Superpave. One highly successful development in Superpave research is permeable pavement. I can remember having my driveway paved with a wearing course with such fine aggregate that, on a hot day, women with high heels would lose 2 inches in height. There certainly was no chance of any water seeping through that pavement. Now it's the base or binder course that's impervious; the friction or wearing course of permeable pavement, as the name implies, is pervious. Water drains right through the open-graded friction course to the impervious base or binder course and on to the shoulder.
What are the features of this permeable pavement? Basically it's an open-graded aggregate mix with no fines. The larger sized aggregate with more voids allows the water to drain right through to the base or binder course. The asphalt wearing surface, without the usual fines in the mix, needs modification to bind the aggregates effectively. The Superpave asphalt used for these permeable pavements can be modified with polymers and stabilized with fibers. The usual thickness of this course is between 0.75-inch and 2 inches. Because of the thinness of the course, the base course or existing pavement should be well graded and without cracks before the permeable pavement is laid. The end result is a high-performance pavement but at a slightly higher cost.
There's another new wearing surface rehab system being applied and tested with origins in France: the Nova Chip System. As always, we need a sound base course. A special paver places a thin (0.375-inch to 0.75-inch) coarse aggregate hot mix over a special polymer-modified asphalt membrane in one pass. Because of the thin hot mix course over the emulsion membrane, the new wearing surface can be open to traffic within minutes.
We have talked about the success and research of permeable pavements. Another success story of asphalt pavements is perpetual pavement. This system gives new life to old pavements every 15 to 20 years, so that they can live forever. The key ingredients, as in any pavement, are:
- A well-graded, well-drained subgrade.
- A firm durable asphalt base course.
- A fatigue-resistant asphalt binder course.
- An impervious friction wearing course.
As we all know, no pavement can hold up without a good base. The principle of perpetual pavement is that, with a good base, the skin or wearing surface can be economically replaced when necessary. The key here is a good structural base to resist rutting and fatigue cracking. When deemed necessary, the asphalt wearing surface on a sound structural base can be milled down and replaced with a new wearing surface economically and with minimum disruption of traffic. It's sort of like the annual sanding and staining the surface of my wooden deck that's as sound as the day it was built some 30 years ago.
One of the best examples of the success of perpetual pavement is the New Jersey Turnpike. This is one of the busiest highways in the United States and was built over 50 years ago, before the advent of the Interstate Highway System.
"Even though more than 50 years of heavy use have punished that pavement, motorists on the New Jersey Turnpike are still traveling on the original pavement structure," says Mike Kolos, Asphalt Pavement Alliance (APA) chairman. "Only surface treatments have been used to maintain the pavement."
What are the main benefits of all these asphalt pavement innovations?
- Less traffic delay.
- Better traction and skid resistance.
- Reduced back spray creating greater visibility.
- Reduced long-term cost to the taxpayers.
We haven't mentioned the many improvements in the paving and compaction equipment and the GPS systems that guide them, and also the permanent and portable asphalt plants that deliver the material to the jobsite. These have an equal importance to the improvements to our asphalt highways today.