WRI Achievements in Asphalt Research
Asphalt is the petroleum residuum that remains after petroleum is distilled into various transportation fuels and other products. In the United States, asphalts come from approximately 200 crude oil sources. Each of these crude oils has a different chemical composition and will react differently in pavement over time. By identifying fundamental properties that contribute to roadway performance, WRI research has laid the foundation for predicting and specifying asphalt materials for long road life.
The Interstate Era Introduces the Need for Asphalt Research
The idea for an asphalt chemistry program in Laramie began with John Ball and William Haines of the U.S. Department of the Interior Bureau of Mines, which was a precursor to WRI. At the time, construction of the federal interstate highway system was in high gear. Haines believed, however, that neither the federal Bureau of Public Roads nor the state transportation departments had any significant understanding of the materials they were building with. He convinced the U.S. Department of the Interior and Department of Transportation of the need for the research, and the precursor to WRI’s asphalt research group was formed.
In 1963, Dr. J. Claine Petersen organized the first asphalt research conference, emphasizing how the chemical and physical properties of an asphalt affect its performance as a pavement cement over time. In 1990, the name of the conference was changed to honor Petersen’s efforts. Today, this annual event is approaching its fifth decade.
In 1973, the Arab Oil Embargo underscored the truth of Haines’ belief that asphalts were not well understood. When OPEC changed its price structure, the market distinctions and product specifications that traditionally defined asphalt were eroded, and poorly performing asphalts were introduced into American roadways.
The need to develop new methods for specifying asphalts based on their performance characteristics became urgent. This work, begun through the federal Strategic Highway Research Program from 1987 to 1993, continues today under the U.S. Department of Transportation Federal Highway Administration.
SHRP at WRI, 1988-93
At the time the Strategic Highway Research Program (SHRP) was initiated, the primary test method (specification) for asphalt measured the refinery’s processing of the crude oil. The result was consistent processing—not a consistent product. The overall mission of the Strategic Highway Research Program SHRP was to develop an improved asphalt purchase specification so that asphalts of the same specification grade would perform similarly in roads, i.e., roadway lifetimes would be predictable.
The mission of the SHRP work at WRI was to study the fundamental compositional features of a wide variety of petroleum asphalts and to develop chemical and rheological analytical methods to measure differences among asphalts. During SHRP, WRI also began a comprehensive study of asphalt oxidation as it occurs naturally in roadways. Oxidation of asphalt, which is unavoidable in roads, changes their physical (performance) properties, but not in ways that are predictable from one asphalt to the next.
The Strategic Highway Research Program (SHRP) led to the development of the Superpave® binder specification by which an asphalt’s high and low service temperature characteristics and short-term aging may be reasonably predicted. WRI developed a base of information on the composition of a wide variety of paving asphalts and adapted several analytical methods for asphalt analysis. Two tests developed under the SHRP contract are now used in the Superpave® binder specification, released by AASHTO (the American Association of State Highway Transportation Officials) in 1993: dynamic shear rheometry to classify asphalt physical properties and a pressurized oxidation test to simulate the natural aging of asphalt in roads.
By the end of the Strategic Highway Research Program, long-term aging and its contribution to fatigue and moisture damage were still not predictable. WRI’s fundamental research, however, laid the foundation for correlating asphalt properties with long-term roadway performance.
WRI Research under ISTEA*, 1992-1998
*Intermodal Surface Transportation Efficiency Act, 1991
During this FHWA contract, WRI worked to correlate the readily measurable chemical properties of asphalts with their performance (physical) properties in roads. This research led to greater understanding of the effects of chemical changes, particularly oxidation, on the physical properties of asphalt in roadways.
WRI refined analytical methods (e.g. rapid size exclusion chromatography method, non-aqueous potentiometric titration, automated flocculation titration) and reduced the time required to perform them. We developed a new, large-scale asphalt plant mix simulation test requested by FHWA to reduce the testing time required for the Superpave specification.
The work also included a study of the effects of asphalt composition on crumb rubber as a modifier. We found that that the performance properties of crumb rubber modified asphalts (CRMA) were highly dependent on asphalt composition. Also, WRI designed, coordinated construction, and monitored road test sections to compare the performance of shale oil modified asphalt with polymer-modified asphalt. This test was carried out in Grand Teton National Park.
WRI Research Under TEA-21* 1999-2006
*Transportation Equity Act for the 21st Century, 1998
The research goals under WRI’s second Fundamental Properties of Asphalt contract were to
- Increase understanding of the multiple mechanisms of moisture damage by studying the behavior of asphalt at aggregate surfaces (thin films of asphalt)
- Develop practical moisture damage tests
- Convert the microdamage–healing tests developed during the ISTEA–FHWA contract into a user-friendly (rapid and straightforward) test
- Develop test methods to predict the long-term stability of polymer-modified asphalts, and
- Work with state highway departments to construct validation sites.
Although WRI began work on a rapid test sequence to predict moisture damage susceptibility, moisture damage remains one of the most difficult distresses to understand, predict, and to protect against. Work on a rapid method to predict the long-term stability of polymer-modified asphalt was also begun. Development of a rapid method to predict fatigue failure using the microdamage–healing method was started at the Texas Transportation Institute under a WRI subcontract.
Validation of new methods in actual roads, which is imperative to proving the utility and calibration of new predictive laboratory test methods, began in 1999. Under this initiative, when partnering states build new road sections as part of their construction programs, the validation effort is integrated by using asphalts of the same performance grade from several (preferably four or five) crude oil sources. WRI is correlating the new test methods with observed road performance by applying the test methods to construction materials and monitoring the roadway performance. Between 1999 and 2006, WRI worked cooperatively with the Federal Highway Administration, state transportation departments and their contractors to construct comparative pavement validation sites in Wyoming, Nevada, Arizona, Kansas, and Minnesota.
WRI Research under SAFETEA-LU* 2007-2012
* Safe, Accountable, Flexible, Efficient Transportation Equity Act - A Legacy for Users (SAFETEA-LU), 2005
In 2007, WRI began work on two five-year Federal Highway Administration-sponsored programs, Fundamental Properties of Asphalts and Modified Asphalts III, and the Asphalt Research Consortium. Both are advancing national goals of improving the performance, useful life, and economics of asphalt pavements.
The Fundamental Properties program correlates the chemical and physical properties of asphalt binders, aggregates and additives with their performance properties as “in-service” pavements.
The Asphalt Research Consortium is working cooperatively to address key issues in asphalt performance, especially to promote technologies that can be put into service right away. The participants are Western Research Institute (lead organization), Texas A&M University, University of Wisconsin-Madison, University of Nevada-Reno, and Advanced Asphalt Technologies, with direction and input from the FHWA and the Binder, Mixture and Construction, and Models Expert Task Groups.