Perpetual Pavement an Inch at a Time
Better Roads Staff
NAPA SPECIAL FEATURE
Perpetual pavement is a concept that was developed and has traditionally been marketed for high-volume applications like freeways and interstates. There is no reason, however, that this life-extending design approach can’t be applied to lower-volume roads, or to rehabilitations of thinner pavements. Even pavements that were not originally designed to be perpetual can become perpetual pavements — one inch of asphalt at a time.
In a nutshell, perpetual pavements are designed to develop distresses from the top down rather than from the bottom up. This preserves the integrity of the pavement structure and confines damage to the top layer, where it can be easily managed without requiring full-depth repairs or major rehabilitation. Asphalt pavements can be designed to never develop bottom-up fatigue-related distress, regardless of how many loads are applied to the pavement, and no matter how heavy those loads may be.
Organizations managing lower-traffic-volume applications may not be aware that a perpetual pavement is an option for them, because of a common false impression that they must build interstate thicknesses in order to have a perpetual pavement. Not so, according to Jim Huddleston, executive director of the Asphalt Pavement Association of Oregon.
“Even if an existing county road or city street was not originally designed to be perpetual, it can become perpetual by adding as little as one inch of asphalt to the structure,” he says.
Huddleston says that adding one inch of asphalt to the pavement can double the fatigue life of the structure. If a three-inch pavement is expected to last 20 years, for example, adding one inch of asphalt will increase the fatigue life to 40 years.
“It is the cheapest inch of asphalt you’ll ever buy.”
- Jim Huddleston, executive director, Asphalt Pavement Association of Oregon
Adding another inch will double the fatigue life again, clearly qualifying the pavement as perpetual without constructing interstate thicknesses. A road need only have an asphalt thickness of five to six inches in order to have perpetual properties (such as cracking and rutting developing from the top down rather than from the bottom up), says Huddleston.
For a new construction project, adding one more inch of asphalt typically amounts to no more than the cost of the material itself and delivery to the site. “It is the cheapest inch of asphalt you’ll ever buy,” Huddleston adds.
A Traditional Approach
Perpetual pavements have traditionally been thought of as an option for new construction only. In such cases, each layer of the pavement structure is designed to contribute to the perpetual nature of the pavement.
Up to three layers of pavement are typically involved. Two factors – material properties and thickness – are evaluated as they apply to the structural demands of each layer. For example, the thickness, binder content, and binder properties of the bottom layer are selected for optimum fatigue response and resistance to bottom-up fatigue cracking.
The middle layer relies on the stone-on-stone contact and a high-modulus asphalt cement to provide resistance to bending and rutting, in turn reducing the magnitude of the tensile strain at the bottom of the pavement. The surface layer typically employs a long-life mix such as stone-matrix asphalt (SMA), Superpave, or open-graded friction course (OGFC) to resist rutting, weathering, thermal cracking and wear.
In considering the options for each layer, the variables of material and thickness must also be evaluated in terms of how they will respond to the environmental conditions and traffic load of the particular location. The foundation beneath the layers is critical as well, and must meet minimum requirements for support throughout construction as well as for the life of the pavement.
The Fatigue Factor
Many agencies have traditionally approached their pavement designs by putting a thin asphalt layer on top of a thick rock base, believing this would produce the most cost-effective design.
This approach can be an outcome of traditional empirical design methodologies that are unable to model or predict fatigue development in the asphalt layers.
Huddleston recommends using only enough aggregate or improved sub-base material to support construction equipment and properly grade the site. The remaining structural requirements should be focused on the asphalt layers.
Experience and studies show that the fatigue life of the asphalt pavement is not highly influenced by the thickness of the aggregate base course, but is very sensitive to the thickness and properties of the asphalt layers themselves. Beyond optimized performance, there are economic advantages to trading base rock thickness for asphalt thickness.
A New Approach
Research confirms that properly designed asphalt pavements, with thicknesses of five to six inches or more, tend to produce top-down cracking rather than bottom-up cracking. Rutting in thicker asphalt pavements is confined to the surface layers, and does not develop in base or subgrade layers. These are some of the primary characteristics qualifying a pavement as perpetual.
This evidence indicates that thinner county roads and city streets that were designed for low-volume traffic (and are still in good condition) can be made perpetual by adding asphalt thickness to the surface. It is not necessary to build these structures to high-volume highway or interstate standards, nor is it necessary for the structure to have been originally designed and built as a perpetual pavement in order to reap the longevity and cost-saving benefits that perpetual pavements provide. Rather, in the case of existing pavements, Huddleston promotes the concept of achieving perpetual pavement “one inch at a time.”
This gradual approach to achieving perpetual pavement includes a few key factors:
• Determine the ultimate thickness needed to become perpetual for your specific site and loading conditions. Generally this would require the addition of one to two inches of asphalt to an existing 20-year-design pavement that has not yet failed from the bottom up.
• Schedule overlays to achieve optimum thickness (typically a minimum of five to six inches of asphalt) before full-depth structural distress occurs. Applying a surface treatment (like a chip or slurry seal) prior to achieving optimum thickness is a risky decision, and by not addressing the structural integrity of the road, can lead to full-depth failures.
• Manage the surface – i.e., mill and fill as necessary – to ensure confinement of distress to the top layer of the pavement.
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.
It also provides the highest level of service to the driving public with the least amount of interruption.
Thin overlays and mill-and-fill treatments, unlike surface treatments, can greatly improve smoothness, can be done at night when traffic is minimal, and can be opened to traffic immediately since no curing is required. And, the new pavement is environmentally friendly, utilizing high quantities of recycled asphalt pavement (RAP) produced during the milling process.
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