Perpetual Pavement an Inch at a Time
Better Roads Staff
Huddleston notes that for this strategy to be successful, it is critical to have an accurate assessment of the thickness and condition of the lower layers of the existing pavement structure. Also, if cracks are present at the surface, don’t assume they are bottom-up cracks, especially if the pavement is greater than five inches thick. Take a core sample to confirm the direction and extent of the cracks.
Surface Rehabilitation as a Step to Perpetual Pavement
Once a pavement structure begins to show distress on the surface, there are two ways to address it – either with an asphalt overlay or surface treatments like slurry or chip seals.
Surface treatments can seem attractive for their initial low cost. These treatments do not, however, add any structural value to the pavement, nor do anything to increase the pavemnt life expectancy.
If you apply a surface treatment to a ten-year-old pavement that is designed for 20 years of traffic, 50 percent of the pavement’s fatigue life is already consumed and the surface treatment will do nothing to extend the life of the pavement. The surface treatment also does nothing to prevent a bottom-up fatigue crack from developing or expanding during the next ten years of the road’s design life.
Agency pavement management systems will often program additional surface treatments for such roads at 16 or 17 years of age, possibly further masking a pending structural failure. Full-depth distress will develop, reflecting through any future treatments and eventually manifesting as potholes. At that point, complete structural rehabilitation or entire reconstruction are required.
In the same example, if you applied a one-inch asphalt overlay to a 10-year-old pavement that is designed for 20 years of traffic, you will have effectively increased the fatigue life of the pavement to 40 year.
Now, at ten years of age, the pavement has consumed only 25 percent of its fatigue life. When the pavement is 20 years old, another one-inch overlay could be applied, doubling the pavement’s fatigue life again, extending it well beyond 50 years.
A Credit Policy where
A Credit Policy is Due
Many agencies, by policy, have awarded no structural credit for thin preservation pavements like the ones addressed here. While a grade of “no credit” is appropriate for seal coats and chip seals, it is clearly not appropriate for thin asphalt layers.
An asphalt overlay does more than just disguise problems. It adds structural value to the pavement, providing increased tensile strength and protection against bottom-up failure. This can delay or entirely avert a costly, full-depth rehabilitation.
In accordance with the principles of perpetual pavement design – whether the structure is perpetual from the start or becomes perpetual over time – the resulting asphalt pavement can be maintained indefinitely without adding more thickness to the pavement (just mill-and-fill as the need arises).
This simplifies the job of the pavement manager and saves significant financial resources over time by eliminating the cost of future major rehabilitation or reconstruction projects.
This maintenance method provides many benefits beyond removal of surface distress. It allows the road surface to maintain its grade, which is important for fitting existing curbs, manhole covers and drainage features, as well as maintaining bridge clearances.
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