VAA Virginia Asphalt Fall/Winter 2020

30 FALL/WINTER 2020 forces near the top are critical to a perpet- ual pavement. Crack-resistant materials should be considered in the bottom layers, and high-quality rut resistant material in the upper layers. The total thickness through a perpetual pavement design process will depend on the prevailing design conditions (e.g., traffic, materials, environment) but will typically range between 8 to 15 inches for higher volume roadways. Similarly, Table 1 shows ranges of perpetual pavement thicknesses determined in a sensitivity analysis conducted by NCAT (Tran et al., 2015). The designs assumed a 6-inch aggregate base between the soil and asphalt concrete, and each row represents a different set of foundation conditions from relatively low to high strength. The calculated thicknesses were determined for a range of climate conditions, with the last column capturing the range across the climate conditions. Clearly, the perpetual pavement thickness depends on all these factors and should be determined on a case-by-case basis through a rigorous design process. But the final result is in the same range as shown in Figure 3 of about 8- to 15-inches. 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. PerRoad and PAVEXpress (Figure 4) are two modern tools that employ the same fundamental design concepts and differ primarily in appearance and user interface. PerRoad is a traditional Windows-based program, while PaveXpress is available online and runs in a web browser. Both will produce the same cross-section given the same design inputs. They also have online interactive help available to facilitate the design. They are available at: • PerRoad: http://www.asphaltroads.org/ perpetual-pavement/about-perpetual- pavements/ • PAVEXpress: https://pavexpress.com/ Flexible pavement thickness design has evolved over the last 70 years from experience- based approaches through empirical, mechanistic-empirical, and now perpetual. Before the latest evolutionary branch, pavements were designed to reach failure, △ continued from page 29 Iowa 7.5 7.35 18 5 4.25 3.25 13.75 8 11.5 6 6 13 12.5 9.3 18 1 1.5 8 4 11.4 13.2 14 6 Montana Virginia Kentucky Tennessee Mississippi Oklahoma 2 Oklahoma Depth from Pavement Surface, in. Surface AC 0 5 10 15 20 25 30 35 Surface AC Aggregate Base Aggregate Subbase Treated Subgrade Subgrade FIGURE 2: Sample Perpetual Pavement Award-Winning Cross Sections (Castro, et al., 2018) Typical Depths 1.5–3” 4–7” 3–4” Materials High Quality AC High Modulus, Rut Resistant AC Fatigue Resistant AC Strong Pavement Foundation (consider drainage) FIGURE 3: Perpetual Pavement Cross Section Guidance (Newcomb et al., 2001) FIGURE 4: Modern Perpetual Pavement Design Tools: PerRoad and PAVEXpress.