The evolution of flexible pavement thickness design began with experience-based approaches before the 1950s through empirical methodologies first created in the 1960s based on full-scale road tests.
These empirical methods, most notably the AASHTO series of empirical design guides, have reigned supreme in the U.S. over the past 50 years with the vast majority of paved mileage designed, constructed, and rehabilitated according to their design equations. More recently, pavement design has evolved into mechanistic-empirical (M-E) approaches with AASHTO now using the Mechanistic-Empirical Pavement Design Guide (MEPDG) and accompanying software, AASHTOWareTM Pavement ME Design, as their standard approach. Many states are transitioning into using M-E design. Still, the process can sometimes advance slowly due to the need to evaluate, calibrate, and validate the new M-E approach before taking the next evolutionary step.
Regardless of the approach, pavement design has always attempted to predict distress development as a function of time and traffic application. Using equations 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 limited 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 continuous 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 pavements 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 thicknesses 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 forces near the top are critical to a perpetual 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, but more than 150 pavement sections around the U.S. have demonstrated that asphalt pavements can be designed and built to prevent deep structural distresses. Many of these sections resulted from over design in earlier design systems. More modern approaches, such as that implemented in PerRoad and PAVEXpress, allow for an optimized perpetual pavement cross-section typically ranging from 8 to 15 inches, depending on the design conditions which are practical and economically feasible.
For More Information
The National Asphalt Pavement Association has just published a new document (Quality Improvement Publication 130), “Perpetual Pavements: A Manual of Practice” for those interested in much more detail on the design, materials selection, and construction of perpetual pavements.
Castro, A., N. Tran, F. Gu, D.H. Timm and C. Wagner, “Limiting Strain Distribution Criteria for Perpetual Pavement Design Using AASHTOWare Pavement ME Design,” Journal of the Association of Asphalt Paving Technologists, Volume 87, 2018, pp. 275–304.
Newcomb, D.E., M. Buncher, and I.J. Huddleston, “Concepts of Perpetual Pavements,” Transportation Research Circular No. 503. Perpetual Bituminous Pavements. Transportation Research Board. Washington, D.C., 2001, pp. 4–11.
Tran. N., M.M. Robbins, D.H. Timm, J.R. Willis and C. Rodezno, “Refined Limiting Strain Criteria and Approximate Ranges of Maximum Thicknesses for Designing Long-Life Asphalt Pavements,” Report No. 15-05, National Center for Asphalt Technology, Auburn University, 2015.