The 20 year fallacy

Trenton M. Clark, P.E., Virginia Asphalt Association

2021 marks the 28th year since I graduated from Virginia Tech. Except for a week or two in Dr. Martin’s Transportation Engineering class, I never took a pavement class while in college. After graduating from Tech, I landed with a pavement specialty firm—PCS/LAW, located in Maryland. Over the next three decades, I worked for PCS/LAW, VDOT, and now Virginia Asphalt Association (VAA). During that time, I have seen a lot of asphalt and learned to question some common assumptions. I performed pavement designs, analyzed existing pavements, and conducted “failure” investigations. I designed new interstate pavements, and I designed subdivision pavements. I have analyzed airport pavements in the Middle East and shipping ports in the Bahamas. Within the Commonwealth of Virginia, I have had the pleasure of conducting pavement investigations on nearly every type of facility maintained by VDOT. I do not share my background to boast but lend credence to what I’m about to relay in this article.

My first job after college was as a staff pavement engineer. I was asked to crunch numbers, conduct visual condition surveys and occasionally design a pavement. With no formal education in pavement design, I decided to read the 1993 AASHTO Guide for the Design of Pavement Structures (1993 AASHTO) from cover to cover. While not classic literature, it gave me a background on pavement structures. Also, I was blessed to have numerous leaders in the pavements field in our office, including Dr. Matt Witczak, Dr. Chuck Schwartz, Dr. Gonzalo Rada, Pete Stephanos, and John Miller. Each came with different strengths in the area of pavements. I recall Dr. Witczak saying one day, “Pavements are the only civil engineering structure we design to fail.” While it may only be a pothole, a pavement at some point will fail. Repeated loads, aging, environmental conditions, and other factors will lead to failure. The key is balancing the road’s needs and the funds available to provide a pavement structure for a specified length of time.

Figure 1: Obvious Bottom-Up Fatigue Cracking
Figure 2: Top-Down Cracking and Middle-Up Cracking

As my career progressed, I moved from office work with limited field visits to spending more time on projects. I got to see several of my designs constructed and investigated pavements in apparent need of reconstruction or major rehabilitation. Early on in my career, I was under the impression that pavements lasted a specified number of years (or ESALs) and that asphalt pavements fail from bottom-up fatigue cracking and rutting. While other distresses may be present, these two indicated the end of the pavement’s structural design life.

The premise of a finite pavement life goes back decades. In VHTRC Report 77-R24 titled “A Review of the Pavement Performance on Virginia’s Interstate System,” Research Engineer Ken McGhee noted that flexible pavements were designed for 20 years (Pg. 3). This was based on the AASHTO Road Test results in the late 1950’s/early 1960s that were implemented in Virginia by Dr. Vaswani. By 1995, VDOT increased the design life to reconstruction from 20 to 30 years (Flexible Pavement Design Guide for Primary and Interstate Roads in Virginia—Revised January 1995). Five years later, when VDOT migrated from Vaswani Method to the 1993 AASHTO, the design life ranges stayed between 20 and 30 years. Again, as the pavement approached the design ESALs while in service, expected load-related distresses would manifest, and eventually, the pavement would need reconstruction or major rehabilitation. But there was one problem, the vast majority of the projects I investigated had performed well beyond their initial design life and handled more ESALs than ever anticipated. So it led me to an “aha” moment—the 20- and even 30-year design life of asphalt pavement is a fallacy!

Of course, I wasn’t the first to have this “aha” moment. In the Fall/Winter 2020 edition of Virginia Asphalt magazine, Dr. David Timm of Auburn University wrote about perpetual pavements. He pointed out the theory that supported the concept of perpetual pavements, and at a certain asphalt thickness, no more asphalt thickness is needed. While this concept is undoubtedly applicable to pavements on interstates and primary routes like US highways, what about other roadways? Do those pavements get reconstructed?

Figure 3: Typical Pavement Design Life
Figure 4: Reconstruction and Major Rehabilitation

Unscientific Survey

In preparation for this article, I performed a completely unscientific survey of pavement practitioners and contractors across the United States. The survey was simple and only asked a few questions. How long do you typically design a pavement to last? What percentage of the pavements have been reconstructed at the end of their design life? What percentage has needed significant rehabilitation at the end of their design life? These questions were asked of experienced engineers and contractors who have the benefit of actually seeing the life cycle of pavements.

A total of 67 people responded to the survey from across the US and internationally. The survey confirmed my experience and identified a looming concern. How long are most pavements designed to “last” given a set of design parameters and expected truck loadings? As shown in Figure 3, most are not designed for super long lives, but only 15 to 20 years. This led to follow-up questions for both the pavement designer and contractor. Both were asked what percentage of pavements were reconstructed at the end of their design life (Figure 4). Both groups reported a small percentage of pavements designed for 15 or 20 years were reconstructed (Figure 4). This may be due to the conservative nature of 1993 AASHTO and other pavement design methods. It may also support the premise of a perpetual pavement described by Dr. Timm in the Fall 2020 issue. Either way, the vast majority of pavements are not reconstructed.

The next question focused on the significant rehabilitation of asphalt pavements at the end of their design life. In the replies, both the pavement designer and the contractor noted the need to perform a major rehabilitation of the pavements—but still less than 30 percent of the time (Figure 4). In the survey, a major rehabilitation was defined as removing and replacing a portion of the existing asphalt layers or adding at least two inches of additional asphalt to strengthen the structure. Again, at the end of the design life, most asphalt pavements do not require major rehabilitation.

Finally, I will mention two looming concerns from my survey. Of the 67 respondents, 13 had less than 15 years of experience, and two had less than five years. It could be that those with less than 15 years in the industry were not comfortable or experienced enough to complete the survey. It also seems to point out the limited number of younger individuals in our industry and that a possible technical workforce shortage is on the horizon—if not already here!

Closing Thoughts

Now, I am not an asphalt pavement apologist who refuses to admit failures do occur. I recognize asphalt pavements have failed for various reasons. Again, I have investigated and identified the source(s) of the failure. Some pavements failed for lack of asphalt structure. A thin farm-to-market type of pavement was not designed to handle an extensive logging operation. The pavement will likely fail, and no one should be shocked. Pavements have failed due to a lack of sub-surface drainage. The subgrade soils and aggregate become saturated, lose strength, and repeated loadings by trucks lead to fatigue failure. These issues can be resolved with proper design or retrofitting drainage structures. Some aggregates are not suitable for asphalt mixtures, and some mixtures are not very good. Advances such as SUPERPAVE™ and Balanced Mix Designs have weeded out many of those problem aggregates and mixes. And simply tacking between lifts of asphalt during paving will minimize the occurrence of delamination-
related cracking and potholes.

For over 20 years, the asphalt industry has promoted the theory of perpetual pavements. While not wanting to appear biased, the pavement design community promotes the term “long-life pavements.” Even the other secondary paving product in some parts of the US has embraced the long-life terminology. Still, only asphalt has a long-running record of performance to support the claim—not projected hopes. So while I have a deep respect for Dr. Witczak, we do not have to design pavements to fail—at least not from a bottom-up fatigue standpoint. We have the knowledge and tools to design, produce, and construct asphalt pavements that fail from the top down where maintenance is easy and economical. They do not need to last for only 20 or 30 years; they can last perpetually!