The asphalt pavement industry is committed to and focused on innovation. Mixing aggregates with asphalt binder and applying heat and pressure has been a familiar, cost-effective method for creating long-lasting pavements since the 1800s. And since the evolution of the automobile in the early 1900s the asphalt pavement industry continues to invest in research and engineering to make asphalt pavements stronger, more economical and more sustainable. Some efforts focus on specialty mixes, such as porous asphalt or stone-matrix asphalt, for specific applications, others focus more broadly on best practices, enhanced durability, and increased use of recycled materials.
This research is performed at universities and public research facilities across the country, as well as at the National Center for Asphalt Technology, a state-of-the-art asphalt pavement research facility and pavement test track at Auburn University.
State departments of transportation (DOTs), public works agencies, municipalities, toll authorities, and other pavement customers across the county have access to the most advanced technologies and tools to build roads thanks to the work being done at NCAT and elsewhere. These innovations make roadways safer, quieter, and smoother for drivers and their families, as well as more sustainable and economical.
With today’s limited budgets and interstate highways and bridges reaching the end of their design life, it’s important for road owners to utilize advanced design methods and tools for developing long-lasting pavement designs that can be maintained in an economical manner without having to be rebuilt.
According to the American Association of State Highway and Transportation Officials (AASHTO) Guide for the Design of Pavement Structures, the basis for most current DOT pavement design guidelines, primary factors to be considered during pavement selection include traffic, soil characteristics, weather, construction considerations, end-of-life considerations, and cost comparisons. Secondary factors include similar pavements in the area, adjacent existing pavements, conservation of materials and energy, and availability of local materials.
Many agencies are implementing or considering implementing mechanistic-empirical pavement design guide (MEPDG) tools, primarily AASHTOWare Pavement ME Design, to develop and validate their designs. As with other pavement design methods, close attention must be paid to the local calibration of variables within MEPDG software to ensure that roads are not over- or underdesigned for local conditions or needs. Several mechanistic-empirical tools are available to aid in the development of long-life asphalt pavements (also known as Perpetual Pavements), including PerRoad and PerRoadXpress. A series of five webinars, conducted in 2013 by leading asphalt paving technologists, is available to help officials with calibration.
PaveXpress Design is a new, free, web-based tool using the AASHTO 93/98 method for designing flexible and rigid pavements. The tool is designed for use by local agencies, engineers, architects, consultants, and engineering students who need a reliable way to quickly determine the necessary pavement thickness for a given section of roadway or project. The software only asks users for inputs required to create technically sound pavement designs, and it suggests industry-accepted defaults where appropriate. PaveXpress also includes support for designing asphalt overlays and porous asphalt pavements, as well as for analyzing pavement structures, estimating material costs, and performing a life-cycle cost analysis. The PaveInstruct online tutorial program can help in learning how to take full advantage of PaveXpress and to access all of its modules and functions.
To help model the expected lifetime costs of an asphalt pavement, the industry has developed free life-cycle cost analysis (LCCA) software that uses the principles recommended by the Federal Highway Administration (FHWA) to compare the economics of alternative designs for a given road project over time, taking into account construction, use and removal, replacement, or remaining value at the end of a study period.
Asphalt pavements can be divided into three broad categories based on the sort of voids in the mix. Each type can be designed for different uses, locations, and traffic volumes, but they also have distinct purposes and characteristics.
Most asphalt pavements are dense-graded and are used effectively in all pavement layers and for all traffic conditions. Dense graded asphalt pavements are commonly designed using the Superpave design method, including performance-grading of the asphalt binder, but can also be designed using the Marshall or Hveem method. Like all asphalt pavements, dense-graded mixes can provide a smooth, high performance surface that is easy to maintain.
While most pavements are impervious, open-graded asphalt pavements are designed specifically to allow water to drain through the pavement. These can be constructed as full-depth porous pavements, where water drains through the pavement to the soil; or they can be constructed as an open-graded friction course (OGFC), which helps move water to the side of a pavement, improving friction while reducing both road spray and noise.1 Full-depth porous pavements are an EPA best practice for stormwater management and they can help reduce pollutant concentrations.2 In fact, even OGFCs have been demonstrated to help filter possible pollutants from highway runoff.3
Stone-matrix asphalt (SMA) is a gap-graded asphalt pavement designed to improve rut resistance and durability through the use of a stable stone-on-stone skeleton held together by a rich mixture of asphalt cement, along with stabilizing agents such as fibers and/or asphalt modifiers. SMA is primarily used to pave high-volume U.S. interstates and highways, achieving high levels of rutting resistance and durability.4 In addition to improved durability and rutting resistance, SMAs have very good friction characteristics.5 They have been shown to be effective in reducing road spray6 and traffic noise.7 SMAs have also been successfully used on high-volume urban roadways with heavy bus and truck traffic.8
The basic components of an asphalt pavement are aggregates and an asphalt binder. These are combined according to certain ratios and sometimes additives are used to ensure desired production or performance characteristics. By varying the mix design used, different asphalt mixes can be created to suit different climates, traffic volumes, or other local needs.
Aggregates are hard, inert materials that typically make up about 95 percent of an asphalt pavement. Aggregates can be rocks or gravel of various, controlled sizes, as well as sand and dust. A mix of different size and types of aggregates is used to achieve desired pavement characteristics. Some pavements use reclaimed asphalt pavement (RAP) or other recycled materials or byproducts, such as slag, fly ash, and even glass, for a portion of the aggregate. Most often aggregates are sourced locally, sometimes at a quarry that is co-located with the asphalt mix prodution facility, but in some cases desired chemical or physical properties are not inherent in local aggregates and they have to be sourced from further away.
The asphalt binder makes up about 5 percent of the typical asphalt pavement. Also referred to as “liquid asphalt,” “asphalt cement,” or “bitumen,” asphalt binder is the glue that holds aggregates together in a pavement. Liquid asphalt forms naturally, such as at Pitch Lake in Trinidad,9 but most of the asphalt binder used is derived during the refining process that converts crude oil to fuel.10 In some cases, asphalt binders are modified through the addition of polymers, ground tire rubber, or other materials to adjust physical and chemical properties of the asphalt binder and altering how it acts within a pavement.
In the U.S., performance grading is used to characterize asphalt binders under the Superpave design method. They are usually identified by the high and low surface temperatures, in Celsius, the pavement will experience.11 For example a PG 64−22 binder would be expected to yield good performance at surface temperatures from −22°C to 64°C (−8°F to 147°F).
Additives and Improved Mix Processes
A variety of additives can be incorporated into asphalt mixtures. In the case of some recycled materials, such as reclaimed asphalt pavement (RAP) or recycled asphalt shingles (RAS), these additives reduce the need for virgin asphalt binder and aggregates, which can help save costs as well as increase the sustainability profile of a pavement. Fibers, including recycled cellulose fibers, are added to some mixes, particularly open-graded mixes, to strengthen them.12
Ground tire rubber (GTR) is incorporated into some mixes, usually blended with the asphalt binder, to improve performance characteristics and to reduce pavement noise.13 Similarly, polymers in the form of polymer-modified asphalt binders, are used to improve constructability or performance characteristics.14
A special class of improved production processes and additives are the various technologies used to produce warm-mix asphalt (WMA). The average temperature at which asphalt paving mixtures are produced is from 280°F to 320°F, however, WMA technologies can lower production and placement temperatures by 30°F to 120°F, reducing the emissions generated and energy needed to produce a pavement.15 WMA can have constructability benefits, such as improved workability and a longer time to achieve compaction.16 WMA additives can extend haul times for emergency repairs as part of a disaster recovery program.17
WMA can be produced through a foaming process, which injects a small amount of water into a mix during production causing the asphalt binder to foam and coat the aggregates effectively at a lower temperature. It can also be produced through a variety of organic or inorganic chemical additives, waxes, or surfactants.16 There have even been warm-mix waxes developed from recycled plastic water bottles.18
Arámbula, E., C.K. Estakhri, A.E. Martin, M. Trevino, A.de F. Smit, and J. Prozzi (2013). Synthesis of Current Research on Permeable Friction Courses: Performance, Design, Construction, and Maintenance. Report FHWA/TX-12/0-5836-2. Texas A&M Transportation Institute, College Station, Texas.
Michel, L., G. Burke, and C.W. Schwartz (2003). Performance of Stone Matrix Asphalt Pavements in Maryland. Journal of the Association of Asphalt Paving Technologists. Vol. 72, pp. 287–314.
Rungruangvirojna P. and K. Kanitponga (2010). Measurement of Visibility Loss Due to Splash and Spray: Porous, SMA and Conventional Asphalt Pavements.International Journal of Pavement Engineering. Vol. 11, No. 6, pp. 499–510.
Asphalt Institute and Eurobitume (2011). The Bitumen Industry — A Global Perspective: Production, Chemistry, Use, Specification, and Occupational Exposure, 2nd Edition IS-230. Asphalt Institute, Lexington, Kentucky.
Von Quintus, H.L., J. Mallela, and M. Buncher (2007). Quantification of Effect of Polymer-Modified Asphalt on Flexible Pavement Performance. Transportation Research Record: Journal of the Transportation Research Board, No. 2001. Transportation Research Board of the National Academies, Washington, D.C.
Prowell, B.D., G.C. Hurley, and B. Frank (2012). Warm-Mix Asphalt: Best Practices, 3rd Edition, QIP 125. National Asphalt Pavement Association, Lanham, Maryland.
Howard, I.L., B.A. Payne, M. Bogue, S. Glusenkamp, G.L. Baumgardner, J.M. Hemsley Jr. (2012) Full Scale Testing of Hot-Mixed Warm-Compacted Asphalt for Emergency Paving. SERRI Report No. 70015-011. Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee.