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.
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