VAA Virginia Asphalt Spring/Summer 2021

24 SPRING/SUMMER 2021 THE 20 YEAR FALLACY 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? 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 rehabilita- tion 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 Pavement Design Life 25 Years or Longer 6% 15 Years 33% 20 Years 61% Figure 3: Typical Pavement Design Life Pavement Activity at End of Design Life 30% 25% 20% 15% 10% 5% 0% Designer Contractor Reconstructed Major Rehabilitation Figure 4: Reconstruction and Major Rehabilitation △ continued from page 23