Measure End Results to Eliminate Thermal Segregation

Building better, longer lasting roads in the United States is becoming an ever more urgent requirement. As we consider not only heavier loads and increases in the shear numbers of miles driven, but also the budgetary constraints on all Departments of Transportation, it is crucial to invest in a reliable road system that will serve us well into the future and will cost much less to maintain in the long run. Our Interstate and urban highways are the lifeblood of our economy, and even at current funding levels, our highways are getting worse. To address this problem, many states are now testing temperatures “behind the paver” in an effort to increase pavement quality.

To improve the quality of pavements in the United States, including bridges, concrete and asphalt applications, the Transportation Research Board invested $150 million dollars in its Strategic Highway Research Program (SHRP), which was active from 1988 to 1993. The final product of the SHRP asphalt research program is Superpave, which stands for Superior Performing Asphalt Pavements.

Superpave represents an improved system for specifying the components of asphalt concrete, asphalt mixture design and analysis, and asphalt pavement performance prediction.

More than 94 percent of our roads are constructed using asphalt. Conservative estimates from the Transportation Research Board project that if Superpave procedures achieved only a 25 percent increase in highway service life, between $1.3 billion and $2.1 billion would be saved by reducing maintenance-related delays and vehicle wear and tear. What’s more, traffic accidents, injuries and fatalities carry a real dollar cost, and better overall road conditions also mean improved safety. However, even after adopting Superpave, premature failures continued to occur because while new material requirements dominated the research results, there had been no focus on field construction practices, or new construction technologies that are available. More research on construction practices and technologies were needed.

Engineers typically design highways to last 15 years or more, and perpetual asphalt pavements are designed to last 50 years. But some roads are failing earlier because of potholes, cracks, raveling and other problems. These premature failures unnecessarily waste millions of taxpayer dollars and threaten the strategically critical highway system.

One cause of premature failures in asphalt is caused by non-uniform aggregate gradation, asphalt content, and air voids in the asphalt mixture. Physical segregation (non-uniform, non-consistent) has long been recognized as a defect that agencies simply will not permit, and elimination of physical segregation is high on the agencies lists of priorities. Specifications and test procedures have been adopted to eliminate physical segregation. Now, states are beginning to address another major culprit: temperature segregation.

Segregation study

In 1996 a graduate student from the University of Washington, who had worked on paving crews in the past, conducted studies of segregation problems in paving operations for his master’s thesis, under the direction of his advisor, Dr. Joe Mahoney. The student, Steve Read, studied the phenomenon known as “truck fans” — spot segregation and end of load segregation, sometimes called cyclic segregation.

The purpose of the study was to determine the cause and a potential solution to the problem. Read’s thesis states, “When a pavement rehabilitation project is affected by this phenomenon, the expected life of the overlay can be reduced by roughly half of the 12 to 15 years that the Washington State Department of Transportation normally expects. There has been no way to predict which projects will be affected by cyclic segregation … it may not manifest itself during construction, but will show up in a project as long as two years after completion.”