What if you could live forever, without suffering injuries or the inevitable decline that comes with age? Imagine if this were possible, and the key to doing it involved protecting yourself by developing an extra strong, tough layer of skin, but periodically removing that skin and growing a new layer. Well, in a simplified form, this is how perpetual asphalt pavements are supposed to work. In this edition of the RoadReady newsletter, we’ll examine the essentials of what makes a perpetual pavement and the benefits these pavements can provide.
Definition of Perpetual Pavement
The standard definition of a “perpetual pavement” is an asphalt pavement that is designed for a service life of 50 years or more without requiring major structural rehabilitation or reconstruction. Other terms used for perpetual pavements include “long-life asphalt pavements,” “deep-strength pavements,” and “extended-life pavements.”
Using the word “perpetual” to describe the pavement is not meant to suggest that maintenance is unnecessary, but it does mean that a large portion of the pavement structure can remain in place indefinitely. Pavement distresses should be confined primarily to the surface of the pavement, which can be replaced or overlaid with a new layer relatively easily. This lets the pavement achieve an extended service life by regularly renewing the surface layer so that the overall pavement structure can remain in place perpetually.
Perpetual Pavement Design
Perpetual pavement design is focused on two goals:
- Ensuring that the pavement has the strength necessary to resist structural distresses (those that affect the entire pavement structure, such as deformation, full-depth rutting, or bottom-up fatigue cracking), and
- Designing a pavement surface that can withstand anticipated traffic and environmental effects without significant damage due to abrasion or moisture.
In order to achieve these goals, the pavement structure is typically designed with an emphasis on stiffness in the upper layers to prevent rutting. For the lowest layer of asphalt, the focus is more on total thickness and flexibility, to help prevent bottom-up cracking.
As with any pavement, some wear and surface distress will become apparent over time, but in a perpetual pavement, this should be addressed by scheduling a repair or overlay of the pavement surface before the distress extends into other layers. In order to achieve the design goals, the replacement materials would need to have the same characteristics as the original surface in terms of durability, rut resistance, and permeability.
The design of a perpetual pavement is based on using multiple layers of asphalt mix. This does not mean simply building a thicker pavement; for each layer, a mix type should be selected that contributes the desired characteristics to the overall pavement structure. An example of a perpetual pavement design concept is shown in the figure below.
Design concept for a perpetual pavement.For example, the surface course could consist of a stone matrix asphalt (SMA) selected for improved rut resistance and to provide a wear-resistant top layer that is impermeable to moisture. Another approach might be to use an open-graded friction course (OGFC) that will allow water to drain from the surface, as long as the layer immediately below is impermeable to maintain the integrity of the structure. Below the surface course, the intermediate layer or layers would be fairly thick, acting as the main structural layers to absorb load stresses and limit strain responses in the pavement. Finally, the asphalt layer placed directly on the original subgrade should be a rich (high asphalt content) bottom layer designed for fatigue resistance.
Layer Design and Thickness
The structural design can be evaluated using mechanistic-empirical design to predict the pavement’s response to stresses and strains. Important criteria include minimizing the vertical compressive strain (normally measured at the top of the subgrade) caused by wheel loads on the pavement surface, as well as the horizontal tensile strain at the bottom of the lowest asphalt layer. Perpetual pavements constructed using the Mechanistic-Empirical Pavement Design Guide (MEPDG) should have the structural integrity necessary to achieve an extended service life. However, perpetual pavement concepts can also be applied in any approach to pavement design. Many high-quality existing asphalt pavements could effectively qualify as perpetual pavements if they are given appropriate treatment and maintenance.
By incorporating pavement response into the design, perpetual pavement concepts help ensure that each layer has the appropriate thickness. Keep in mind that the thicknesses of the layers will vary based on the expected traffic loads the pavement needs to deal with. If properly designed, a perpetual pavement may seem fairly thick, particularly if it must deal with high traffic volumes, but in many cases a perpetual pavement may actually end up being thinner than a pavement designed using conventional principles.
Benefits of Perpetual Pavements
The potential benefits of perpetual pavements go beyond simply the fact that they have an extended service life. Because the pavement is designed so that maintenance and rehabilitation needs are limited to the surface layer, future costs to preserve the pavement in good condition should be lower. A perpetual pavement design may lead to higher initial construction costs than a conventional pavement, often around 10% or so. However, much like building a house with quality workmanship and good materials, the perpetual pavement can save that amount and more in the long run. As you can verify using a life cycle cost analysis, the reduced maintenance costs along with the longer overall pavement life will often make perpetual pavement the superior option financially.
Perpetual pavements are capable of being designed for high traffic loads, including roadways that see a heavy volume of truck traffic. Not only is this structural capacity a benefit of perpetual pavement design, but a related benefit appears when it comes to maintenance on high-traffic roadways. By spacing out maintenance and rehabilitation needs and limiting treatments to the surface layer, perpetual pavements can reduce road user costs associated with lane closures and more intensive rehabilitation activities.
Pavements with Staying Power
Many drivers imagine that once a road is built, that’s all there is to it, and the road can stay the same forever. Those of us who understand the need for pavement maintenance and rehabilitation know that isn’t really the case. A perpetual pavement gets pretty close, however, and does the job about as well as we know how to do. If designed, built, and maintained correctly, a road constructed as a perpetual pavement really can last for a very long time.
Transportation Research Circular 503, Perpetual Bituminous Pavements: http://onlinepubs.trb.org/onlinepubs/circulars/circular_503.pdf