Designing a pavement system with a strong, durable foundation and a replaceable surface makes it possible to create a pavement that performs well for decades.
By: Troy Kaiser
Senior Consultant
Pavement is one of the most critical infrastructure components of any facility, yet its longevity and performance are often challenged by the elements, traffic loads, and time. While it's impossible to create an asphalt surface that lasts forever, the concept of perpetual asphalt pavement offers a compelling solution to extend the lifespan of pavements, reduce long-term costs, and minimize environmental impact. Designing a pavement system with a strong, durable (crack-resistant) foundation and a replaceable surface makes it possible to create a pavement that performs well for decades, with only periodic surface renewal.
What is Perpetual Pavement?
The concept of perpetual pavements was introduced in 2000 by the Asphalt Pavement Alliance (APA). They defined a perpetual pavement as “an asphalt pavement designed and built to last longer than 50 years without requiring major structural rehabilitation or reconstruction and needing only periodic surface renewal in response to distresses confined to the top of the pavement” (APA, 2002).
Perpetual Pavement Design Philosophy.
At Benchmark, we embrace the philosophy of "the right method at the right time," tailoring our approach to pavement design and maintenance to ensure each pavement serves its intended purpose for as long as possible. In this article, we'll explore the principles behind perpetual asphalt pavements, from the importance of a robust foundation to the cost-saving benefits of surface replacement. Whether you're planning a new parking lot or looking for ways to extend the life of existing pavement, understanding the concept of perpetual pavement can help you make smarter, more sustainable decisions for your pavement management needs.
As pavement designers, our goal is to build pavements that last as long as possible at a reasonable cost. Of course, like any infrastructure asset, pavement must be maintained, or it will not perform as expected. The general timeline for asphalt pavements includes:
- sealing cracks in the initial years of a pavement's life and repairing isolated failures as the cracking progresses to structural deficiencies,
- performing a structural overlay at the age and condition in which maintenance becomes cost-prohibitive for the return on life extension, and
- eventually reconstructing the pavement in the late stages of the pavement's life when an overlay is no longer effective.
You might be asking, what does all of this have to do with building a perpetual asphalt pavement? Due to environmental aging factors such as sun, wind, and rain compounded by the subgrade soil composition and traffic use, we cannot truly build a perpetual asphalt surface because, eventually, the surface will have to be replaced. However, we can build a perpetual asphalt pavement system consisting of stone base, multiple lower lifts of asphalt, and an asphalt surface that is above and beyond the structural necessity for the thickness design. The asphalt surface is then replaced every 15 years or so as part of the preventive maintenance program. Essentially, it is building a permanent subsurface system to support a replaceable surface.
This system may also include enhancers below the surface such as subgrade fabric or geogrid layers in addition to a drainage system to keep the subgrade materials dry and structurally sound. Lower asphalt layers should also be designed to be more flexible and resistant to cracking while upper asphalt layers are designed to be more resistant to rutting. The benefit of this is a long-lasting asphalt pavement, reduced future reconstruction costs, and minimized downtime for the construction work to occur.
Pavement Foundation
The key to a perpetual asphalt pavement system, or any pavement system for that matter, is its foundation. Without the right base, the upper layers of a perpetual pavement system are bound to fail.
A formula for pavement thickness design is the strength of the combined parts of the pavement system must be greater than deficiencies in the subgrade plus the planned traffic loading.
This process starts with geotechnical investigation to determine the subgrade soil's consistency and strength, and the moisture content. Armed with this information, we can now determine what will be required for subgrade improvements and the stone base depth needed to support the construction equipment and work in combination with the asphalt layers to structurally support the traffic loadings over time. A properly designed foundation consists of a variety of components, such as chemically or mechanically modified or unmodified soils along with crushed aggregate and recycled products like crushed concrete or milled asphalt.
The Pavement Structure
The lower lifts of asphalt must be designed robust enough to withstand the traffic loads without breaking down and should be designed to resist fatigue cracking. Often, you will see an intermediate course (middle layer) of asphalt and even a second intermediate course for heavy duty and high traffic volume situations. This layer, or layers, should be designed to carry most to all of the traffic load. The asphalt surface course (top layer) should be thick enough to remain serviceable until replacement is needed. This layer should be designed to resist surface-initiated distresses such as top-down cracking and rutting.
Before perpetual pavements, we would see asphalt pavements built no more than 3" to 5" thick in trucking areas compared to concrete pavements which are typically 8" in depth. The stone base the asphalt was placed on would vary from 8", typically on the high end, to no stone base. This type of design can result in major failures of the asphalt and lead to full-depth reconstruction at an earlier stage in the pavement's life, which is costly.
Multiple Asphalt Lift Core Sample.
By creating a solid foundation along with designing and building asphalt pavements in multiple layers with greater overall thickness, a perpetual asphalt pavement system is created that will perform as intended for a longer period of time. The surface course can be more easily milled off and replaced repeatedly without the need for costly and time-consuming reconstruction in the future.
Perpetual Cost Savings
With perpetual pavement, you will only need to restore your surface two or three times over the 50-year lifespan. That compares to a standard asphalt pavement system which would require a greater level of structural overlaying and eventual replacement in order to achieve that same 50-year life. Concrete pavements, at a higher initial cost, in theory would also not reach a 50-year life without replacement. The cost of a pavement overlay can range from 65-80% less than pavement reconstruction; however, overlays cannot compensate for structural deficiencies in lower pavement layers and the base course that accelerates with age and traffic loadings.
If the budget costs for the initial investment are too high, one option is to design the first rehabilitation overlay earlier in the pavement life, such as at 10 years instead of 15-20 years.
Summary
Not only will perpetual pavement extend the service life of your asphalt, but you also significantly reduce the amount of raw resources required to maintain your truck drives, yards and parking lots as well as decrease the volume of demolition materials that can end up in the landfill since the renewable surface is 100% recyclable.
Perpetual asphalt pavements are not new and similar long-term systems such as deep-strength pavements have been designed and constructed for years. As industry knowledge surrounding structural pavement design and materials has increased, there have been major advances in strategies for long-term performance and cost-effectiveness of perpetual pavement. Before you start your next pavement project, evaluate the fit of perpetual pavement. When properly designed, installed, and maintained, it can provide a cost-effective and sustainable option for long-term performance.