When it comes to roads and bridges, civil and structural engineers face a difficult task. They have inherited a mature road network that is in need of significant repair, replacement, and improvements to meet current and projected demands. According to a 2013 report from the American Society of Civil Engineers, 32 percent of major American roads are in poor or mediocre condition, costing every motorist about $324 per year in repairs and additional operating costs.
Investment levels remain insufficient to properly build and maintain our nation’s infrastructure, which affects long-term performance goals. Engineers are in the unenviable position of making tough decisions on pavement preservation practices to balance public needs with available funding.
However, with this challenge comes the opportunity and the need to embrace cost-effective innovations. The asphalt pavement industry has long been interested in research and technological innovations to deliver smooth, long-lasting, and high-performing roads for the driving public. Thinlay asphalt overlays are one example of this sort of innovation.
As agencies adopt pavement management plans to extend pavement life through preservation and maintenance, it becomes increasingly important to implement measures that provide the greatest benefit at the best cost for the longest amount of time. To achieve this, pavement structure must be considered when thinking about pavement preservation options.
Alhough pavement preservation tends to focus on the functional benefits received from preserving the pavement surface, the benefit received from protecting the pavement structure is real and has long-term benefits. Properly designed and constructed perpetual (or long-life) asphalt pavements are substantially thick structures on a solid foundation where the substructure remains distress free indefinitely. These pavements are engineered to confine distresses at the surface level where quick, timely action can restore ride quality while preserving the road structure for the long term (see Figure 1).
With the ability to mill and overlay the top layer of a pavement, asphalt overlays provide a smooth, well-maintained surface and a high-performing ride while the pavement substructure remains crack- and distress-free, effectively enhancing the pavement’s longevity.
Recently, the use of thinner asphalt overlays designed specifically for pavement preservation has gained acceptance. Thinlays — a suite of asphalt mixes that can be placed as thin as 5/8 inch — are a reduced cost option that provide a safe, smooth driving surface with improved ride quality. For road owners, Thinlays can improve the structural capacity of structurally sound pavements, extending pavement life while reducing future maintenance costs and serving long-term preservation needs.
Thinlay mixes are a standard asphalt mixture designed with a smaller nominal maximum aggregate size and a binder selected to help optimize flexibility, durability, and rut resistance. The proper selection of aggregate, binder, and the mix-design approach are crucial to the success of a Thinlay, and application-specific designs are being developed and tested in several states.
A frequent problem for pavement preservation is choosing the wrong treatment for a pavement’s condition or trying to obtain a little more life from a pavement that needs more substantial repair. A Thinlay should be used only after careful evaluation of existing pavement conditions, taking time to ensure that distresses are confined to the pavement surface. Optimal uses for thin asphalt overlays include correcting pavement surface deficiencies such as raveling, longitudinal cracking outside of the wheel path, and transverse cracking. Other distresses may indicate a deeper structural problem that should be addressed.
The engineering viability of using Thinlays has been demonstrated through research, field studies, demonstration projects, and long-term performance tests and analysis. Significant advances in materials, mixture design, and construction of surface layers during the last few decades have increased the range of thin asphalt overlay applications, especially for preservation. Research conducted by the Federal Highway Administration shows that thin overlays perform better than other pavement preservation treatments under more pavement conditions and traffic levels.
When used appropriately, thin asphalt overlays are safe, reduce costs, and are sustainable through the incorporation of recycled materials. Construction is fast, with less impact on drivers, and there are additional environmental benefits, such as noise reduction and conservation of natural resources through incorporation of reclaimed asphalt pavement. In addition, thin asphalt overlays provide a smooth driving surface, improving ride quality and reducing fuel consumption compared with rough pavements, and can modestly improve pavement strength.
Because resurfacing can be done during off-peak hours, keeping roads fully open during rush hour or other high-traffic-volume times, asphalt overlays help reduce inconvenience to drivers. New or newly rehabilitated asphalt pavement can be opened to traffic as soon as it has been compacted and cooled.
Thinlays also allow a pavement owner to incorporate new surface innovations as they develop or as traffic needs and conditions change. They can also be designed to be more sustainable through the use of reclaimed and recycled materials and warm-mix asphalt.
Warm-mix asphalt is the omnibus term for a group of technologies that allow producers of hot-mix asphalt to lower the temperatures at which the material is mixed and placed. Typically, warm-mix asphalt is produced 15° F or more below typical mix temperatures, but reductions of 50 to 100°F have been documented. Such drastic temperature reductions decrease fuel consumption and production of greenhouse gases. In addition, constructability benefits have been documented with warm mix.
Originally a European development, warm-mix asphalt has been embraced by the U.S. asphalt pavement industry and the FHWA, which named warm mix to its first round of Every Day Counts initiatives. In January 2014, U.S. Secretary of Transportation Anthony Foxx predicted, “By 2020, we’ll save $3.6 billion by using warm-mix asphalt — another example of how innovation will help us create more capacity.”
As of 2013, warm-mix asphalt comprised more than 30 percent of the asphalt mixture market. Because of its rapid adoption and construction benefits, the Construction Innovation Forum recognized warm-mix asphalt with its NOVA Award in 2012.
In addition to reduced fuel consumption during production, warm mix helps create a more comfortable work site because of the lower placement temperatures. It also helps extend the paving season, allowing pavements to be placed at lower ambient temperatures without sacrificing quality.
Compaction is critical to the long-term performance of any pavement. Warm-mix asphalt improves the workability of a mix at a paving site, which aids in proper compaction of the materials. Plus, because the temperature differential between the air and the product is smaller than that of conventional hot-mix asphalt, longer haul times are possible with warm mix. A study of the use of warm-mix asphalt to aid in delivering construction materials to disaster zones found that certain warm-mix technologies allowed mixes to remain workable after a 10-hour haul.
An additional benefit of warm mix is that it may aid in the use of recycled materials in asphalt pavements. Nearly 100 percent of asphalt pavement removed during resurfacing or reconstruction of a road is put back to use in new roads. When used in new asphalt pavement mixtures, the asphalt binder in reclaimed asphalt pavement is reactivated, conserving natural resources and saving money.
Balancing the needs of road owners and the public is no easy task, but innovations such as Thinlays and warm-mix asphalt offer sustainable, cost-effective ways to manage aging infrastructure while ensuring that drivers get the highest level of performance available.
- ASCE, 2013, 2013 Report Card for America’s Infrastructure, Washington, D.C., www.infrastructurereportcard.org/a/#p/roads/overview.
- Wiser, L., 2011, TechBrief: Results of Long-Term Pavement Performance SPS-3 Analysis: Preventive Maintenance of Flexible Pavements, Report FHWA-HRT-11-049, Federal Highway Administration, McLean, Va., www.fhwa.dot.gov/publications/research/infrastructure/pavements/ltpp/11049/index.cfm.
- 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, Tenn., www.serri.org/publications/Documents/MSU%20Project%2070015%20-%20Full%20Scale%20Test%20of%20Hot-Mixed%20Warm-Comp%20Asp%20for%20Emerg%20Paving-Rpt%20No%2070015-11-30%20August%202012%20(Howard).pdf.
Audrey Copeland, Ph.D., P.E., is vice president for Engineering, Research and Technology at the National Asphalt Pavement Association (www.asphaltpavement.org). Kent Hansen, P.E., is director of Engineering for NAPA.