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Advanced High-Performance Materials for Highway Applications: A Report on the State of Technology
Chapter 5, Candidate Asphalt Concrete Materials
Warm-Mix Asphalt Concrete
WMA refers to technologies, originally developed in Europe, that are aimed at allowing the production and placement of HMA at lower temperatures. WMA is produced and mixed at temperatures roughly between 212 and 284 °F (100 and 140 °C), about 68 - 104 °F (38-58 °C) lower than an conventional HMA. This is achieved by using techniques that reduce the effective viscosity of the asphalt binder, allowing full coating of aggregates and subsequent field compaction at lower temperatures. The techniques to reduce the effective viscosity of the asphalt binder include:
It should be noted that producing HMA at lower temperatures is the desired product to achieve the benefits, not the particular technology that is used to produce the WMA mix.
WMA is being used in all types of AC, including dense-graded, stone matrix asphalt, porous asphalt, and mastic asphalt. It is also being used in a range of layer thicknesses. WMA sections have also been constructed on roadways with a wide variety of traffic levels, from low to high.
WMA technology could have a significant impact on transportation construction projects in and around non-attainment areas such as large metropolitan areas that have air quality restrictions. The reduction in fuel usage to produce the mix would also have a significant impact on the cost of transportation construction projects.
The benefits of these technologies include worker safety, energy savings, air quality improvements, improved constructability, and longer performance due to reduced aging of the asphalt binder during the construction process. These technologies continue to be investigated.
European countries are using WMA technologies to reduce energy consumption - burning fuels to heat traditional HMA to temperatures in excess of 300 °F (149 °C) - at the production plant. The lower production temperature of WMA results in the added benefit of reduced emissions from burning fuels, a cooler working environment for workers, and less fumes and odors generated at the plant and the paving site. Specific benefits related to the paving process include:
The WMA cost is considered to be similar to the cost of conventional HMA.
Current Status of Usage
The European countries continue to increase the use of WMA. The consensus of the European countries using WMA is that WMA should provide equal or better performance than conventional HMA. Over 40 State highway agencies have constructed demonstration projects, and several agencies are using WMA on a regular production basis. The FHWA Office of Pavement Technology is actively involved with WMA technologies and is working in cooperation with FHWA Turner-Fairbank Highway Research Center's Bituminous Mixtures Laboratory to develop and monitor WMA demonstration projects and research and also to advance the knowledge and state of practice of these materials and technologies.
WMA is considered a viable technology and is beginning to be used in "production" paving in the United States. Because of the many advantages of WMA, its usage is growing in the United States and it is expected that the use of WMA will become standard practice. However, there are many elements of WMA that still need to be investigated.
European Asphalt Pavement Association. The Use of Warm Mix Asphalt: EAPA Position Paper. June 2009.
National Center for Asphalt Technology has published the following reports, which are available at http://www.ncat.us/info-pubs/technical-reports.html:
National Cooperative Highway Research Program:
Prowell, B. D., and G. C. Hurley. 2007. Warm-Mix Asphalt: Best Practices. National Asphalt Paving Association. Lanham, MD.
Warm-Mix Asphalt: European Practice. 2008. FHWA-PL-08-007, FHWA-HPIP, Federal Highway Administration, Washington, DC. http://international.fhwa.dot.gov/pubs/pl08007/index.cfm
Perpetual Asphalt Pavement Systems
A perpetual asphalt pavement system is defined as an HMA pavement that is designed and constructed to last for an extended time period (50 years or more) before requiring major structural rehabilitation or reconstruction. The system is designed such that the strain levels experienced at critical locations under traffic loadings are held below critical threshold values, thereby limiting the development of key structural distresses such as fatigue cracking and rutting. The perpetual asphalt pavement may only need periodic surface renewal (e.g., thin overlay, mill-and-fill) when surface distresses, such as transverse and longitudinal cracking or raveling, have reached unacceptable levels.
Perpetual asphalt pavements use multiple layers of durable asphalt mixtures, with each layer designed to accommodate the specific demands and constraints of the project (e.g., fatigue resistance, rutting resistance, safety, noise). The typical perpetual pavement cross section consists of the following layers (TRB 2001; Newcomb 2002):
The concept of perpetual asphalt pavements is derived from the successful performance exhibited by many full-depth and deep-strength AC (FDAC and DSAC) pavements built by a number of highway agencies since at least the 1960s. The thick asphalt layers associated with these pavement structures provided good fatigue resistance, which often resulted in service lives that far exceeded the original 15- to 20-year lives for which they were designed (TRB 2001). Combined with the many advancements in asphalt mixture technology (e.g., modified binders, Superpave binder, and mix design) that have occurred over the years and the recent development of the Mechanistic-Empirical Pavement Design Guide (MEPDG), the FDAC and DSAC of old continue to evolve to meet the needs of modern highways.
The construction of a perpetual asphalt pavement depends on the specific design used. In some instances, the design may include a dense aggregate base on which the various asphalt layers will be placed (DSAC design). In others instances, the asphalt layers may be placed directly onto a prepared subgrade (FDAC design). Placement and compaction of the asphalt layers generally follow normal asphalt paving practices, with proper consideration given to the type of mix being placed.
Maintenance and rehabilitation of perpetual pavements is generally limited to preventive maintenance applications, such as crack sealing and traffic- and climate-appropriate surface treatments. Periodic resurfacing activities will be required, but such work is largely confined to the top portion of the pavement.
Perpetual asphalt pavements are generally most suitable for use on highways with moderate to high traffic volumes. They are not commonly used on lower volume roadways because of their significantly higher initial construction costs. Also, their use in urban environments where underground utilities are present is generally not recommended.
Perpetual asphalt pavement systems provide a durable, safe, smooth, and long-lasting roadway without frequent expensive, time-consuming, traffic-disrupting reconstruction or major repair. Because of their long life with only periodic minor interventions, the system is also environmentally friendly.
The initial cost of a perpetual asphalt pavement may be anywhere from 10 to 25 percent more than a conventional HMA pavement, depending on the specific designs being compared. However, while the initial construction costs are higher, the overall life-cycle costs of a perpetual asphalt pavement are considerably lower when the extended pavement life and lower frequency of maintenance/rehabilitation activities are included.
Perpetual asphalt pavements are being constructed by a number of State highway agencies, including those in Arkansas, California, Colorado, Delaware, Illinois, Kentucky, Michigan, Minnesota, Mississippi, Ohio, Oregon, Texas, Washington State, and Wisconsin. In addition, Canada and several European countries also have extensive experience with perpetual asphalt pavements.
For More Information
Newcomb, D. E. 2002. Perpetual Pavements: A Synthesis. Publication No. APA 101. Asphalt Pavement Alliance, Lanham, MD.
Newcomb, D. E., and K. R. Hansen. 2006. "Mix Type Selection for Perpetual Pavements." Proceedings, International Conference on Perpetual Pavements, Columbus, OH.
Transportation Research Board (TRB). 2001. Perpetual Bituminous Pavements. Transportation Research Circular No. 503. TRB, Washington, DC.
Porous Asphalt Pavement
Porous asphalt pavements are specially designed pavements that use porous AC (PAC) mixes to laterally or vertically drain storm water runoff (Iowa LTAP 2007). PAC mixes have traditionally been used as surface courses on new asphalt pavement structures or as part of HMA overlays placed on existing pavements. Sometimes referred to as open-graded surface courses (OGSC) or porous/permeable friction courses (PFC), these surfaces are designed to facilitate storm water runoff and thereby prevent the development of water films that would otherwise decrease friction and increase splash/spray and hydroplaning potential. The high air void content inherent in the mix is also effective at absorbing tire-pavement noise emissions.
Recently, PAC mixes have been incorporated into full drainable pavement systems that substantially reduce runoff and promote natural infiltration of water into the soil. In this system, a somewhat thicker (2- to 4-in. [51- to 102-mm]) PAC layer is placed on top of a thin (1 to 2 in. [25 to 51 mm]) choke stone layer (typically 0.5-in. [13 mm] chips) and a thick (10 to 12 in.) aggregate recharge bed/reservoir course (with 1.5- to 2-in. [38- to 51-mm] stone), lined with a geotextile filter fabric. The PAC layer may consist of an OGSC/PFC layer and an asphalt-treated permeable base (ATPB) layer containing even higher voids. Storm water flows through the PAC surface into the aggregate recharge bed where it is stored and allowed to infiltrate into the soil between rainfalls (FPO 2008).
The PAC mixture consists of open-graded crushed aggregate (0.5- to 0.75-in. [13- to 19-mm] top size, with small amounts of sand or dust) and an asphalt binder (typically, 5.5 to 6.5 percent by weight of mix). In recent years, polymer-modified asphalt binders have been employed, which enable the use of higher air voids for better drainage (resulting in significantly less stripping in underlying asphalt layers) and higher binder content for improved adhesion between aggregate particles (resulting in less raveling) (APA 2002). In addition, fibers have been incorporated to control drain down of the asphalt binder from the aggregate during the construction process.
The system of interconnected voids inherent in PAC mixture design results in porosity levels between 10 and 20 percent and permeability levels anywhere between 350 and 6,000 ft/day (107 to 1,829 m/day). Such permeability levels are orders of magnitude higher than the best soil permeability of about 12 ft/day (3.7 m/day) and are capable of quickly draining high-intensity rainfalls (FPO 2008).
The placement of a PAC mix is essentially similar to that of a conventional HMA mix. A key exception is that the mix is only lightly compacted (two to three passes of a static steel-wheeled roller), so that the material retains its open nature for drainage.
The maintenance of PAC surface mixes is also similar to that of conventional HMA pavements, except that surface or fog seals are not recommended as they can block the internal drainage structure. Moreover, periodic cleaning is generally needed to remove contaminants that could plug the pavement and reduce its porosity. Finally, it is generally recommended that winter maintenance activities should exclude the use of abrasives or grits to prevent clogging; if the PAC structure is part of a parking lot intended to recharge groundwater, deicing chemicals should not be used (FPO 2008).
The use of PAC mixes for lateral surface drainage applications is fairly unrestricted. OGSC/PFC pavements have been and continue to be used on many types of pavement facilities, from high-volume freeways and major airport runways to lower-volume roads. They are generally not suitable for intersections or locations with heavy turning movements, nor for areas prone to heavy snowfall or subject to a lot of dirt or debris (Cooley et al. 2009).
The use of PAC mixes as part of full drainable pavement systems is currently limited, as it is a new design concept that is in the early stages of testing in the United States. Such testing is taking place primarily in urban settings on low-volume, light-traffic facilities like parking lots and subdivision roads.
Benefits realized from the use of OGSC/PFC pavements are primarily associated with improved safety. They have been shown to improve wet weather frictional properties, reduce the potential for hydroplaning, reduce the amount of splash and spray, and improve visibility. Other benefits include resistance to permanent deformation, reduced tire - pavement noise levels, and smoother pavements (Cooley et al. 2009).
Benefits of full drainable pavement systems using PAC mixes include increased safety (due to a drained surface), potentially lower overall costs (due to the elimination or reduction in underground storm drainage systems), groundwater improvements (aquifer replenishment), and reduced environmental impact (due to reduced potential for flooding and erosion).
The cost of a PAC mix in relation to a conventional HMA mix varies widely. Mixes with minimal or no binder modifications are estimated to be between 5 and 25 percent higher in cost than conventional HMA, while those modified with polymers and fibers are estimated to be between 25 and 60 percent higher (FPO 2008; Root 2009).
OGSC/PFC pavements are used by about 14 States, with only a few States (e.g., Texas, Georgia) using them on a significant number of projects. PAC full drainable pavements are being used for low-volume, light-traffic, large-expanse facilities, such as parking lots and local roads. These have seen limited use to date, but are generating considerable interest in light of the current sustainability movement.
For More Information
Flexible Pavements of Ohio (FPO). 2008. Porous Asphalt Pavement. Technical Bulletin. Flexible Pavements of Ohio, Dublin, OH. Online at http://www.flexiblepavements.org/documents/PorousTBSep08v1.pdf.
Iowa Local Technical Assistance Program (Iowa LTAP). 2007. "Popcorn Ball Pavement: Pervious Concrete and Porous Asphalt." Technology News, January-February 2007. Iowa State University, Ames, IA.
Recycled Asphalt Shingles
Recycled asphalt shingles (RAS) are waste roofing shingles salvaged for use in pavement construction materials (either as an aggregate supplement or in the modification of cold and hot asphalt mixes) as well as in other construction applications. The waste shingles are obtained from the demolition of existing roofs (referred to as "tear off" or "post consumer" shingles) or from factory rejects and tab cut-outs (referred to as "factory scrap" or "post industrial" shingles) resulting from shingle production (Griffiths and Krstulovich 2002).
To achieve the desired application size, both shingle types are ground and shredded in two to three stages using crushers, hammer mills, or rotary shredders. For HMA pavement applications (see AASHTO MP 15-09, Standard Specification for Use of Reclaimed Asphalt Shingles as an Additive in Hot Mix Asphalt and AASHTO PP 53-09, Design Considerations When Using Reclaimed Asphalt Shingles (RAS) in New Hot Mix Asphalt), the desired size is typically less than or equal to 0.5 in. (13 mm); generally speaking, the smaller the shreds, the better they are incorporated into the mix (Vermont ANR 1999).
Roofing shingles are made of a supporting membrane of organic or fiberglass backing felt, a saturant of hot asphalt cement (for impregnating the felt), coatings of additional asphalt cement, and a coating of mineral fines (Griffiths and Krstulovich 2002). The compositional breakdown of asphalt shingles is as follows (Vermont ANR 1999):
The asphalt cement used for the saturant and coatings is harder than the asphalt binder used in HMA mixes. Thus, when it is combined with virgin asphalt binder, an increase in viscosity occurs.
The asphalt from post-consumer shingles is often in an irreversible, age-hardened state. Although this makes the grinding/shredding process easier as compared to post-industrial shingles, it can complicate the mix design of an RAS-modified HMA. Also, post-consumer shingles can potentially contain foreign materials (e.g., wood, nails) and asbestos, which can significantly add to the processing effort. Finally, as an additional consideration, post-consumer shingles usually contain a higher percentage of asphalt than post-industrial shingles because of the weathering loss of a portion of the ceramic-coated aggregate.
RAS-HMA formulation is based on two factors: climate and traffic (CMRA 2007). The target formulation is one in which the modified binder is stiff enough to resist pavement rutting in the summer months, yet soft enough to resist fatigue cracking due to repeated loading as well as cracking due to cold weather shrinkage of the pavement (Schroer 2009). Past research has indicated that RAS-HMA mixes with a maximum of 5 percent RAS (by weight of total mix) perform as well as conventional HMA mixes (CMRA 2007). However, higher percentages of RAS result in a significantly stiffer binder that is more susceptible to low-temperature cracking, although a softer virgin binder can counteract this effect. For example, recent research by the Missouri DOT indicates that when virgin asphalt comprises less than 70 percent of the total binder amount, a softer binder (PG 58-28 instead of PG 64-22) is needed to retain proper low-temperature viscosity (Schroer 2009).
The construction and maintenance of pavements with RAS-HMA is similar to that of pavements with conventional HMA. Several RAS-HMA test pavements have been constructed throughout the country, with short-term performance generally positive.
As mentioned earlier, highway pavement construction applications for RAS include aggregate supplement and modification of cold and hot asphalt mixes. In the most common application - RAS-HMA - the material can be used as a surface, intermediate, and/or base course for parking lots, lower-volume roads and highways, and paved shoulders for both highway and airport pavements. Testing for the acceptability of RAS-HMA on higher volume roads and highways and other airport pavement features is ongoing.
In addition to the conservation of asphalt and aggregate materials and the overall reduction in solid waste, major benefits of RAS-HMA include a savings in the cost of the mix and reductions in (a) the amount of energy required to produce the mix and (b) the amount of greenhouse gas (CO2) emissions associated with its production and placement (Robinette and Epps 2010).
The cost of RAS and RAS-HMA is dependent upon local market conditions. Because the price of asphalt binder and the price of processing asphalt shingles both fluctuate, the cost of RAS-HMA can also fluctuate. In general, however, the cost of processing the shingles tends to be offset by the savings in reduced amounts of virgin asphalt binder and fine aggregate. The cost of RAS-HMA is slightly to considerably less than the cost of conventional HMA (2 to 15 percent for mixes with RAS content less than or equal to 5 percent) (Robinette and Epps 2010).
Several States have experimented with the use of RAS-HMA, including Florida, Georgia, Indiana, Iowa, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Montana, Nevada, New Jersey, New York, North Carolina, Pennsylvania, Tennessee, and Texas (Vermont ANR 1999; Griffiths and Krstulovich 2002). Currently, 14 highway agencies have Standard Specifications or Special Provisions that allow up to 5 percent post-industrial and/or post-consumer shingles in HMA (Robinette and Epps 2010).
For More Information
Construction Materials Recycling Association (CMRA). 2007. Recycling Tear-Off Asphalt Shingles: Best Practices Guide. Construction Materials Recycling Association, Eola, IL. Online at http://www.shinglerecycling.org/sites/www.shinglerecycling.org/files/shingle_PDF/ShingleRecycling-BPG-DFK-3-22-2010.pdf.
Griffiths, C. T., and J. M. Krstulovich. 2002. Utilization of Recycled Materials in Illinois Highway Construction. Report No. IL-PRR-142. Illinois Department of Transportation, Springfield, IL.
Robinette, C., and J. Epps. 2010. "Energy, Emissions, Material Conservation and Prices Associated with Construction, Rehabilitation, and Material Alternatives for Flexible Pavement." Paper prepared for the 89th Annual Meeting of the Transportation Research Board, Washington, DC.
Schroer, J. 2009. "Missouri's Use of Recycled Asphalt Shingles (RAS) in Hot Mix Asphalt." Proceedings, 2009 Mid-Continent Transportation Research Symposium, Ames, IA.
Vermont Agency of Natural Resources (Vermont ANR). 1999. Recycled Asphalt Shingles in Road Applications: An Overview of the State of the Practice. Online at www.anr.state.vt.us/dec/wastediv/recycling/shingles.pdf.
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