Asphalt is Forever
In a throw-away world in which everything has a shelf life, it seems impossible for something to last perpetually. But asphalt continues to prove, year in and year out, that is not only sustainable, economical, and useful, but when constructed properly, can provide a road system that lasts indefinitely. That means asphalt roads designed sixty years ago are still in use and asphalt roads designed today may still be in use 100 years from now. Such an enduring road system would not be possible without pavement designed to be Perpetual Pavements.
- adding just one inch of asphalt to an existing asphalt road can double the fatigue life of the pavement.
- adding one inch of asphalt can cut the aggregate base requirement by four inches.
What is Perpetual Pavement?
Perpetual Pavements have unintentionally been designed, constructed, and maintained for decades. Asphalt roads that were built decades ago have inadvertently become Perpetual as the lower lifts are still in place serving as the base while the surface has either been milled and replaced or had additional lifts added to it. The Perpetual Pavement concept was formally recognized 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). At that time, it was recognized that many well-built, thick asphalt pavements that were categorized as either full-depth or deep-strength pavements had been in service for decades with only minor periodic surface rehabilitation to remove defects and improve ride quality. In an effort to identify the pavement systems in service for the longest, APA defined criteria by which Perpetual Pavements from the past could be categorized and pavements of the future could be built to meet.
The Perpetual Pavement concept advances the notion that deep, structural pavement distresses that destroy pavement life could be altogether avoided if distresses were kept below the design threshold for strains, stresses, and deflections. In short, avoiding damage at the bottom or base of pavements and keeping wear at the surface level creates a system in which the initial road design remains in use indefinitely with only the top lift requiring maintenance. Building a base capable of such performance requires designing for the heaviest vehicle. This ensures the road is strong enough to resist stress and remain in sound structural condition. This design system not only creates quality roadways but is the most economical and financially responsible approach to infrastructure.
The approach to designing Perpetual Pavements is different than empirical methods which state the heavier volume of heavy loads, the thicker the pavement. Although Perpetual Pavements require a sound base, it does not simply mean designing thicker roadways on an abundance of stone. It means designing to reasonably withstand the heaviest load without over designing. Notably, while current design standards discourage overdesign, for many years, ‘the thicker the better’ was the mindset. These thick pavements unintentionally became the foundation of our sixty year old perpetual pavements. We now know over design is not necessary; smart design is the ultimate goal.
Missouri Perpetual Pavements
Since 2001, the Asphalt Pavement Alliance has recognized 69 roadways for meeting Perpetual Pavement requirements. In order to receive the award, pavements must be in place for over 35 years and have its original base structure in place. Missouri is proud to have received four awards for Perpetual Pavements. These include:
- 2002: Interstate 44, Jasper & Lawrence Counties
- 2003: US 63, Texas County
- 2007: US 54, Camden County
- 2009: Route 47, Franklin County
perpetual pavement benefits
Designing pavements to be perpetual is not difficult or unfeasible. Unfortunately many are under the misconception that a pavement designed to last indefinitely must cost more. However, that is not the case. Research has shown that Perpetual Pavements have the following benefits (Timm and Newcomb, 2006):
- they provide a more efficient design which eliminates overdesign and unnecessary costs
- they eliminate reconstruction costs because they don’t exceed a pavement’s structural capacity.
- they lower rehabilitation-induced user delays
- they reduce the use of non-renewable resources like asphalt and aggregates
- they diminish energy costs while the pavement is in service
- they reduce the life cycle cost of the pavement because the lower layers will remain and only the surface will require rehabilitation
This means Perpetual Pavements often times cost less than traditional pavement designs. Furthermore, Perpetual Pavements are more economical over the long term than conventionally designed pavements.
Just as Perpetual Pavements don’t cost more to build, they are not more difficult to design. One component of Perpetual Pavements is full depth pavement. Full depth pavements are constructed by placing asphalt layers on modified or unmodified subgrade material. This base allows engineers to use a thinner total pavement section than if a thick granular base were used. This design helps to keep cracking at the surface level in turn allowing pavements to far exceed their design life.
In order to maximize the benefits of Perpetual Pavements, engineers must know the heaviest anticipated traffic loads without grossly overdesigning. In short, understand what traffic loads the road will receive and design accordingly. An acceptable approach to designing Perpetual Pavements in the mechanistic-empirical method which uses rational engineering to evaluate a pavement in terms of stresses, strains, and displacements during the pavements expected life.
Engineers should approach designing Perpetual Pavements using the following philosophy (Walubita et al., 2008; Merrill et al., 2006):
- Perpetual Pavements must have enough structural integrity and thickness to preclude distresses such as fatigue cracking, permanent deformation, and structural rutting.
- Perpetual Pavements must be durable enough to resist damage from traffic, abrasion, and the environment.
Research has shown limiting horizontal strains at the bottom of the asphalt base can help control fatigue cracking (Shook et al., 1982; Al, 1982). When the tensile strain at the bottom of the asphalt layer is reduced, tensile strains are relocated and experienced at the surface level instead of at the base (Mahoney, 2001; Rolt, 2001). This means pavements need to be designed from the bottom up. This in turn ensures the stresses are confined to the surface level and do not result in deep structure cracking. Ultimately, the base lifts remain sound and can be left in place while the surface, when necessary, can be milled and replaced rejuvenating the entire roadway at a fraction of the cost.
This has been proven through ample research. Research has indicated that fatigue cracking does not occur so long as the tensile strain at the base of the pavement does not exceed 200µ€ (Nishizawa et al., 1996). Many pavement bases have a subgrade limiting strain of 200µ€ while the design value only requires 150 µ€. Because many of the existing asphalt pavements were deisnged with a value to withstand 200µ€, they inherently meet the requirements for Perpetual Pavemnt classification.
subgrade/base: a good foundation
Because Perpetual Pavements require designing from the base up, engineering for a quality base is pertinent. A pavement subgrade or foundation may be comprised of compacted subgrade, chemically stabilized subgrade, or stabilized granular material, as well as unstabilized granular material like crushed stone or gravel. No matter which foundation material is utilized, consideration must be taken to ensure it meets a minimum requirement for stiffness throughout construction and expected life span. This means as long as the criteria are met, Perpetual Pavements can be constructed on a variety of bases.
mix design for appropriate lifts
Designing a Perpetual Pavement does not mean simply increasing pavement thickness. It is important to not look solely at the number of inches, but also at the design. Designing multiple lifts with each respective lift able to withstand the wear and tear experienced is more important than the number of inches. Considering the function of each lift is equally important as designing a sound base.
The base lift is designed to be incredibly durable and resist fatigue cracking and should have lower voids and a higher binder content than the other lifts. The intermediate lift provides durability and rutting resistance and must combine the qualities of stability and durability. Stability in the intermediate lift can be obtained by stone on stone contact and a binder with a high temperature grading. The surface layer must be designed to withstand traffic and direct exposure to the environment. It should be designed with consideration of traffic conditions, environment, local experience, and economics. The surface course should be resistant to rutting and surface cracking, have good friction, and reduce pavement noise. Integrating RAP or RAS in Perpetual Pavement design is often beneficial as it helps to create a stiff mix that is more resistant to rutting.
A common misconception about Perpetual Pavements is that they are more difficult to construct. However, construction of Perpetual Pavements does not differ from conventional pavements. It just requires attention to detail and a commitment to build from the bottom up ensuring perpetual characteristics.
When building Perpetual Pavements, normal best practices used in the construction process should be observed. There are five main considerations to remember when building your Perpetual Pavement:
- A strong uniform foundation must be present
- Optimum density in the asphalt mixture must be achieved.
- The asphalt mix design, production, and placement must be uniform.
- Bonding between all pavement layers is essential.
- Normal quality control procedures should be followed throughout the construction process.