News Perpetual Pavement: Extending the Life of Roads

The term “perpetual pavement” may conjure images of an endless road spooling out over a distant horizon, or perhaps the idea of an endlessly self-renewing road that never needs maintenance. The reality of perpetual pavement is a little less whimsical, but no less amazing: it is a pavement management strategy for increasing the life expectancy of a road from an average of 17 years to up to 30 years. It stretches government funds to enable more roadway treatments and an improved level of service.

Sound too good to be true? Read on!

Anatomy of a road

Before we can dive into perpetual pavement, we first need to understand the layers that make up the typical roadway:

1. Surface Course – Also called the riding surface, this is the asphalt or concrete layer that vehicles travel on
2. Intermediate and Base Courses – This is a strong, stable support layer of asphalt pavement, typically made from larger aggregate material; it resists compaction from heavy loads on the surface course
3. Sub-Base – This is the main load-bearing layer of pavement, typically made from large crushed aggregate material; it also provides drainage and frost protection
4. Sub-Grade – Also called the formation layer, this is the ground the road is built on, the place where excavation stops and construction starts

Why roads fail, and what to do about it

As the surface course is exposed to vehicle loads and environmental stresses from sunshine, wind, rain, ice, temperature changes, and pollutants, the asphalt or concrete slowly breaks down. Once the integrity of the surface course is compromised, water and other elements can begin to penetrate to the base course and the sub-base, leading to deeper damage and eventual failure.

Some failures in the surface course, like potholes and cracks, can be repaired with patches or crack sealing. Eventually, a deteriorated surface course needs to be removed and replaced. For asphalt pavement, this is typically accomplished by grinding away one to two inches of old asphalt and replacing it with fresh asphalt, a process called mill-and-overlay or mill-and-fill.

But when the base course or sub-base fails, patching and filling aren’t an option. Instead, the surface, base, and sometimes sub-base need to be removed and replaced, an expensive and highly disruptive process referred to as full reconstruction.

What is perpetual pavement?

Perpetual pavement is a pavement management strategy that focuses on preserving the integrity of the surface course to protect base and sub-base from damage so that only the surface course needs to be periodically repaired and replaced.

With perpetual pavement, the asphalt surface course can undergo mill-and-overlay multiple times with only minor repairs to the base course as needed. The asphalt surface is not permanent, but the base course and sub-base will, ideally, remain in place perpetually.

Perpetual pavement does not mean that a road lasts indefinitely without intervention—in fact, perpetual pavement requires pavement owners to be proactive about applying preservation treatments and making repairs.

How does the perpetual pavement process work?

The perpetual pavement process takes place at all stages of a road’s life. An example maintenance schedule for perpetual pavement might look like this:

• Year 0: Construct a new road with an adequate stone base (ABC), typically 6–8 inches deep, with a 2–3-inch asphalt surface course
• Year 1–3: First application of rejuvenators to replenish the maltenes (oils) that keep the asphalt from becoming brittle and prevent water from penetrating to the base and subbase
• Year 5–8: Second application of rejuvenators; crack sealing and localized patching as needed
• Year 10–13: First application of a thin-lift overlay such as a micro surface, slurry seal, or cape seal
• Year 17–20: Second application of a thin-lift overlay
• Year 24–27: No more treatment options; road surface must be replaced via mill-and-overlay when end of life is reached

The goal of all these actions is to prevent surface water from getting into the ABC and sub-base. Whenever surface water migrates through the ABC, it leaves behind voids that create soft spots. The more water that seeps into the stone, the more likely there are to be failures in the base and sub-base. When more than 50% of the base is compromised, we consider reconstructing the entire road structure either by total replacement or by full-depth reclamation (FDR)—activities that have both the highest cost of maintenance and cost of ownership.

Understanding cost of maintenance vs. cost of ownership

Cost of maintenance is the total cost of applying one or more repair or replacement strategies to a roadway segment or network. It is the bill the community pays for the treatment. When comparing treatments, pavement owners often focus on the cost of maintenance, because this is the amount that must be approved in the budget.

Cost of ownership, on the other hand, is the cost of maintenance divided by the number of years the treatment lasts or provides a benefit. It provides a more holistic view of pavement costs over the life of the pavement.

Consider the following example:

Anytown has completed a pavement condition survey and noted that Main Street shows significant cracking. It would cost $14 per square yard to resurface Main Street with asphalt overlay and $20 per square yard to mill-and-fill the road. Because the cost of maintenance for the asphalt overlay is cheaper, it might seem like obvious choice.

But experience shows that asphalt overlays typically degrade to their original condition within 7 years. The cost of ownership for asphalt overlay is therefore $2.00 per square yard per year. Mill-and-fill, on the other hand, is likely to last 17 years. That means the cost of ownership for mill-and-fill is $1.17 per square yard per year. It is ultimately cheaper in the long run for Anytown to mill-and-fill Main Street than to apply an asphalt overlay.

Perpetual pavement and level of service

Level of service (LOS) is typically represented by the average pavement condition index (PCI) score for the roadway network. A higher level of service is good; a lower level of service is bad. Desired level of service can vary from community to community and over time.

It is cheaper to maintain a road network at a higher level of service than it is to maintain it at the lowest level of service. At the lowest level of service, roads are on or over the brink of failure, and the only way to fix them is to perform a total replacement or full-depth reclamation (FDR)—which, as mentioned above, have the highest cost of maintenance and the highest cost of ownership. Unless the community has unlimited funding, there is no way to catch up.

Perpetual pavement relies on regular preservation treatments to slow degradation and avoid the most costly forms of replacement while maintaining a high level of service for as long as possible.

The bottom line

While perpetual pavement requires more frequent intervention and may have a higher cost of maintenance, it ultimately leads to a lower cost of ownership and better level of service over the life of the roadway network.