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Advanced High-Performance Materials for Highway Applications: A Report on the State of Technology
Chapter 4, Candidate Asphalt Binder Materials
Shell is marketing Shell Thiopave® to modify asphalt binder properties that would improve the performance of the extended AC mixtures. The Thiopave modifier consists of small pellets of sulfur modifier that are added to the asphalt mixture during the mixing process. The Thiopave melts rapidly on contact with the hot mix and is dispersed throughout the asphalt mixture during the mixing process.
During the energy crisis period of the 1970s and early 1980s, the use of liquid sulfur as an extender of asphalt binder properties in hot-mix asphalt (HMA) was investigated. However, once the energy crisis was over, the interest in sulfur-extended asphalt binder subsided. In addition, during this period, sulfur-extended asphalt mixtures were produced using hot liquid sulfur that emitted a significant amount of fumes and odors unpleasant to workers. The transportation and supply of hot liquid sulfur were also problematic.
To replace the use of hot liquid sulfur for asphalt mixture production, solid sulfur pellets, known as Sulfur Extended Asphalt Modifier (SEAM) and recently renamed Shell Thiopave®, were further improved by Shell in the late 1990s. Thiopave is both a binder extender and an asphalt mixture modifier. The manufacturer reports that Thiopave can improve the performance of sulfur-extended asphalt mixtures, reduce construction costs and production temperatures, and provide more friendly conditions for sulfur-extended mixture production. Recent technological improvements in sulfur production, coupled with an increase in sulfur abundance, have led to resurgence in the exploration of the use of sulfur as an asphalt mixture modifier.
While the Thiopave pellets contain some additives designed to reduce odor and fumes during mixing, temperature control of the mixture and good ventilation practices are still required. Thiopave mixtures are typically produced at a target mixing temperature of 140 ± 5 °C (284 ± 41 °F). The mixtures must be produced above a temperature of 120 °C (248 °F) so that the sulfur pellets will melt and the sulfur will be dispersed throughout the asphalt mixture. Above mixing temperatures of 145 °C (293 °F), the potential for harmful emission generation greatly increases and could be problematic for workers involved in both the mixing and compaction processes.
The sulfur-extended asphalt can be used in warm-mix asphalt (WMA).
According to Shell, Thiopave can significantly alter the performance properties of the mix. The change in these performance properties is dependent both on the percentage of virgin binder that is substituted with Thiopave and the amount of time the specimen is allowed to cure prior to performance testing. The most notable impact of the addition of Thiopave to an asphalt mixture is an increase in the stiffness of the mixture.
Literature has shown that the addition of Thiopave materials can have a positive impact on laboratory mixture performance. The addition of Thiopave has been shown to significantly increase the Marshall Stability and deformation resistance of asphalt mixtures in the laboratory after a 2-week curing period. The Thiopave material also had little negative impact in areas that were thought to be problematic, such as fatigue cracking resistance, low temperature cracking resistance, and moisture susceptibility.
No cost data are available. Costs are dependent on the global supply of sulfur and the state of the energy crisis.
Current Status of Usage
Thiopave-modified asphalt has been used in many countries and there are many pavement sections in Canada. In the United States, Shell has been involved in pavement sections in Los Angeles and Port of Oakland, the NCAT Pavement Test Track, and a section near Kansas City, Missouri. Shell will be involved the Louisiana Transportation Research Center/Louisiana DOT Accelerated Load Facility research project during 2010. Several other trials and projects are scheduled for 2010.
David Timm et al. 2009. Evaluation of Mixture Performance and Structural Capacity of Pavements Using Shell Thiopave®,"National Center for Asphalt Technology, Auburn University, Auburn, AL.
Bio-Derived Asphalt Binders
Vegetable oil formulations (from soybean, corn, sunflower, and canola) are being investigated as possible modifiers for asphalt binders. Vegetable oil - based modifiers are considered renewable resources and are beginning to be used in other countries. These products include rejuvinators (extender oils), bio-polymers, and resin-like synthetic binders.
The bio-derived binders are considered applicable to a range of asphalt binders and uses.
Similar to other asphalt binder modifiers and extenders, the vegetable oils improve specific properties of the asphalt binders and allow for partial replacement of the asphalt binders with the vegetable oils.
These materials have not been implemented in the United States, and no cost data are available.
Current Status of Usage
The bio-derived binders are commercially available in Europe and Australia, but have found little use in the United States.
King, Gayle, and King, Helen. Asphalt Alternatives: Asphalt Contractor Options in a Market of Escalating Petroleum Prices and Dwindling Asphalt Supply. Report No. SR-198, National Asphalt Pavement Association, Lanham, MD.
High Modified Asphalt Binders
High modified asphalt binders are asphalt cements that are blended with synthetic additives or chemical modifiers to enhance their physical properties for use in asphalt-aggregate mixes. The most common type of high modified asphalt binders are polymer-modified binders. Polymers are materials with long-chained molecular structures that, when mixed with asphalt cement (typically, at a rate between 3 and 6 percent by weight of the asphalt) and a chemical catalyst, dissolve and "cross link" with the asphalt to form a homogeneous binder material. Polymers include natural and synthetic rubbers (thermoplastic elastomers, such as styrene butadiene styrene [SBS] tri-block copolymer and styrene butadiene rubber [SBR] latex) and plastics (thermoplastic plastomers, such as ethyl vinyl acetate [EVA], ethylene glycidyl acrylate [EGA], and polyethylene).
Polymer-modified binders are seeing increasingly widespread use in HMA. The modified binder is more elastic and has improved low- and high-temperature stiffness (viscosity) properties that are better capable of meeting the performance requirements of the Superpave performance-graded (PG) asphalt binder specification (AASHTO M 320, AASHTO M 323), which are tied to the environmental and traffic conditions of the project site. Polymer-modified asphalt binders are typically specified and used in situations where the PG grade span (i.e., the low-temperature grade plus the high-temperature grade) is greater than 90 (e.g., PG 70−22). They exhibit the following binder-enhancement characteristics (WAPA 2002):
The construction and maintenance of pavements with polymer-modified HMA is similar to that of conventional HMA pavements. A number of highway agencies have constructed polymer-modified HMA pavements since their introduction in the late 1990s. Performance of these pavements has generally been good and has improved over the years corresponding to the advances in technology.
Polymer-modified asphalt binders are most commonly used in HMA mixes that are to be placed in high-stress applications. Typical locations include intersections with stop-and-go traffic, high-volume freeways and interstates, and high truck volume routes (D'Angelo n.d.). In addition, they are often used in areas of extreme climate (e.g., deserts or areas with very low temperatures).
Although more expensive than neat asphalt binder, the use of polymer-modified binder in HMA can provide markedly improved performance in terms of reduced rutting, reduced fatigue cracking, and reduced thermal cracking, particularly in high-stress and climate-sensitive conditions. Depending on the costs and performance characteristics specific to a locale, the life-cycle costs of mixes that incorporate polymer-modified binders can be significantly lower than those of mixes using unmodified binders.
Bahia et al. (2001) estimated that the cost per ton of a modified binder is between 50 and 100 percent greater than that of neat asphalt cement, translating to an increase of 10 to 20 percent in the in-place cost of HMA. D'Angelo (n.d.) estimated that polymer-modification can increase the cost of virgin binder anywhere from 30 to 100 percent, which consequently increases the price of HMA by 10 to 40 percent.
Polymer-modified asphalt binders have increasingly become the norm in designing optimally performing pavements, particularly in the United States, Canada, Europe, and Australia. Bahia et al. (2001) estimated that the use of modified asphalt binders in HMA was as much as 15 percent of the total annual tonnage of asphalt binder used in the United States. A later report by Tandon and Avelar (2002) indicated that 16 of 47 State agencies used modified binders. A 2005 survey by the Transportation Research Board (TRB) revealed that 34 States have established Superpave specifications covering modified binders.
For More Information
Bahia, H. U., D. I. Hanson, M. Zeng, H. Zhai, M. A. Khatri, and R. M. Anderson. 2001. Characterization of Modified Asphalt Binders in Superpave Mix Design. NCHRP Report 459. Transportation Research Board, Washington, DC.
D'Angelo, J. No date. Modified Binders and Superpave Plus Specifications. Federal Highway Administration, Washington, DC.
Robinson, H. L. 2004. "Polymers in Asphalt." RAPRA Review Reports, Volume 15, Number 11. Rapra Technology Limited, Shawbury, United Kingdom. Online at www.books.google.com/books?id=7d9tnYUlyXEC.
Tanden, V., and I. Avelar. 2002. Superpave Adoption by State Highway Agencies: Implementation Status and Assessment. Report Number DTFH-02-104-1. Federal Highway Administration, Washington, DC.
Transportation Research Board (TRB). 2005. Superpave: Performance by Design. TRB, Washington, DC.
Washington Asphalt Pavement Association (WAPA). 2002. WAPA Asphalt Pavement Guide. WAPA, Seattle, WA. Online at http://www.asphaltwa.com/.
Woo, W. J., E. Ofori-Abebresse, A. Chowdhury, J. Hilbrich, Z. Kraus, A. Epps Martin, and C. J. Glover. 2007. Polymer-Modified Asphalt Durability in Pavements. Report No. FHWA/TX-07/0-4688-1. Texas Department of Transportation, Austin, TX.