VAA Virginia Asphalt Spring/Summer 2021

VAASPHALT.ORG 23 THE 20 YEAR FALLACY 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 AASHO 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, Figure 1: Obvious Bottom-Up Fatigue Cracking Figure 2: Top-Down Cracking and Middle-Up Cracking continues on page 24 △ 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!