Perpetual pavement: extending the life of roads
Nothing lasts forever. But when it comes to your transportation network, the concept of perpetual pavementaims to stretch the life of roads, more so than standard asphalt resurfacing.
By extending the life of a road and keeping it pliable for as long as possible, a municipality stretches those hard-to-come-by tax dollars and government funds.
What exactly is perpetual pavement? WithersRavenel Director of Pavement Management Steve Lander and Construction Manager Rob Holland have together worked on thousands of miles of North Carolina roads. They have valuable insights into the perpetual pavement process, and how it aligns with the concept of effective early road maintenance.
The term perpetual pavement might lead some to think of it as build a road and forget about it.But it is anything but that.
According to Lander, when thinking about pavements it is not just the asphalt riding surface. You also must consider the stone base and subbase. So, in a perpetual pavement process, you are trying to protect and maintain the riding surface to avoid base and subbase failure which is a major expense to fix.
And at 30 years, if you have stretched it out that long and are fortunate enough to have a really good base, then you are performing a full-depth mill and fixing any soft spots in the base. And then you are starting over, so it is perpetual, but the asphalt itself is not perpetual,Lander said.
Holland said that once the underlying material is compromised, it becomes extremely expensive to repair a road. So that’s why we refer to it as reconstruction vs. rehabilitation. You could have a perpetual foundation, but not the asphalt, which has a finite life. One can argue about what that span of life is, but you cannot change the fact that it is finite. The number of years in that finite life varies, and that number is dependent on multiple factors.”
The perpetual pavement process takes place at all stages of a road’s life. It starts with new construction. Providing adequate stone base (ABC), typically 6 to 8 inches, and 2 to 3 inches of asphalt as a riding course are typical for municipal roads. Between 1 and 4 years after the resurfacing, rejuvenators are usually applied to the new asphalt surface to replenish the oils (maltenes) depleted during the construction process and environmental conditions. This treatment is again repeated for a second application 5 years after the initial application to keep the asphalt pliable,Lander said.
During this time, you are likely to see some cracking that needs to be repaired with crack sealing and localized patching. Usually, 5 years after the second rejuvenator application, a thin-lift overlay (micro surface, slurry seal or a cape seal) can be applied. Typically, 5-7 years after that, another thin-lift overlay is applied. Once you get to the end of the life of the second thin-lift overlay (5-7 years), you are looking to perform a rehabilitation (mill and overlay) of the existing asphalt.”
Lander says the key is to prevent surface water from getting to the stone base (ABC) and the subbase.
Whenever you get incompressible material and voids in the stone base, caused by the migration of surface water through the base, it creates soft spots. The more water that seeps into the stone, the more likely you are to see possible failures in your base and subbase. If we can limit that, we are ahead of the game,he said.
Basically, we are trying to protect the base at all costs. When over 50% of the base is compromised, we consider reconstructing the entire road structure either by total replacement or by full-depth reclamation (FDR).This is the highest cost maintenance activity with the highest cost of ownership.
Holland said that one way to explain the process to clients is that we are S-T-R-E-T-C-H-I-N-G the life of the asphalt, and that’s what perpetual pavement is all about. We have got to do it to minimize the cost-of-ownership per square yard per year. It’s easy to say, I will just resurface it after 10 years.That is expensive. It works but it is not cost-effective. We are looking for methods and technologies to stretch the life of that pavement.”
With the following car analogy, Lander explains why we need perpetual pavement processes in place. It’s like buying a vehicle and driving it off the lot, and presuming you are good for 10 years without regular oil changes or routine maintenance. With this approach, it won’t be long before the engine will need to be replaced. Most people don’t do this with their cars, but it’s a typical approach to road maintenance. Like regular oil changes and routine maintenance, the perpetual pavement process ensures we are incorporating the right maintenance activity at the right time which minimizes the cost of ownership and extends the life of the pavement.”
For many clients, resurfacing might seem like the go-to and quicker solution, but that does not take into account the cost of ownership.
Let’s say we invest $14 per square yard to overlay a street with a significant cracking pattern. Our experience suggests that within 7 years we are back to the same condition where we started, The cost of that resurfacing is $2.00 per square yard per year,Lander said.
Instead, if that road is milled and overlayed at $20 per square yard, it is likely to last 17 years, and the cost of ownership is $1.17 per square yard per year,he said.
It all comes down to the dollars per square yard year,Lander said. Minimizing this cost is our goal.”
It is cheaper to manage your road network at the higher level of service provided by the perpetual pavement process than to manage it at the lowest level of service, Lander said. Many municipalities get into the rut of managing at the lowest level of service, so all they are doing is rehabilitation (mill and overlay) and reconstruction. You do a mill and overlay this year, what happens to the road next year? There are treatments we can apply at every stage along a road’s useful life. But if you end up just going after streets in bad condition, you never catch up, and the whole roadway network deteriorates to the most expensive maintenance activity.”
And unless you have unlimited funding, you will never catch up,said Holland.
Pavement preservation and the perpetual pavement process are the answers to the lowest pavement ownership cost! WithersRavenel can help you get there.
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