VAA Virginia Asphalt Fall/Winter 2020

VAASPHALT.ORG 29 PERPETUAL PAVEMENTS THE NEXT EVOLUTIONARY STEP built into the various methods, designers typically determine the pavement cross- section that will deteriorate to a failed state in a prescribed amount of time or load repetitions. Structural failure in asphalt pavements is often defined by a critical level of bottom-up fatigue cracking (e.g., 20% of lane area cracked) or terminal rutting (e.g., 0.5-inch rutting). However, during this evolutionary process, many asphalt pavement sections were built that far outlived their expected performance periods without experiencing any deep structural distresses. In 2001, the Asphalt Pavement Alliance began identifying these pavement sections in the U.S. through a perpetual pavements award program. Since that time, over 150 sections have been awarded under a wide range of conditions. According to the Asphalt Pavement Alliance (APA), to receive a perpetual pavement award, a pavement section must meet the following criteria: • At least 35 years of continuous service; • Minimal structural improvements lim- ited to less than 4-inches of additional thickness added during its service life; • Infrequent resurfacing, requiring at least 15 years between resurfacing; • No deep structural distresses such as bottom-up fatigue cracking and deep structural rutting. Pavements meeting these criteria can be nominated to APA, which relies on staff at the National Center for Asphalt Technology (NCAT) to carefully review the pavement’s historical record to determine if it is perpetual. Figure 1 shows the number of awards in each state through 2019, and, notably, all climate zones are represented. Though many sections are on interstates, a significant number represents lower volume state, county, and municipal routes. Many more pavement sections likely meet the criteria that simply don’t have the continu- ous historical records to prove their perpetual status. Interested readers may peruse the winners, organized by year, at: http://www.asphaltroads.org/perpetual- pavement/award-winners/ . Since all of the awarded perpetual pave- ments were designed primarily using pre M-E approaches, they were likely achieved through the older empirical design guides’ inherent over-conservative nature. Another common factor is they were all very well built, with well-designed materials, to withstand the test of time and traffic as layer thickness alone will not overcome poor construction or inferior materials. Figure 2 illustrates a sample of perpetual pavement award winners taken from interstate sections in seven different states. Total AC thicknesses in this sample ranged from 7.35 inches (Montana) to a whopping 21.5 inches (Iowa). Obviously, the thinner section is more practical and sustainable from a cost and materials perspective. More modern approaches to perpetual pavement design have aimed to optimize the pavement section. It is just thick enough to prevent deep structural distresses without wasting materials, which could be spread further rather than thicker. Figure 3 provides guidance regarding layer thick- nesses and material types in a modern and typical perpetual pavement section. This figure highlights the need to build on a strong foundation, and should also consider drainage, as needed. Above that, selecting the appropriate materials and thicknesses to withstand tensile forces generated at the bottom of the asphalt concrete and high shearing and compressive For those unfamiliar with structural pavement design, determining a perpetual pavement cross-section may seem like a difficult task. Fortunately, there are some free, highly-evolved design tools available. continues on page 30 △