U.S. Department of Transportation
Federal Highway Administration
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Focus
| Accelerating Infrastructure Innovations |
Publication Number: FHWA-SA-97-029
Date: November/December 1997
Like their colleagues in the United States, European highway researchers have set their sights on developing a mechanistic system for asphalt pavement design. The task of developing that system is the responsibility of the Forum of European National Highway Research Laboratories (FEHRL), whose members are involved in developing European standards for asphalt pavement design.
The European standards will be based on physical properties directly related to pavement performance. Because the Superpave system takes the same approach to mix design, European researchers are drawing on the work conducted by the Strategic Highway Research Program (SHRP) and the Federal Highway Administration (FHWA) in the United States.
To get up to speed on the Superpave performance models, the members of FEHRL invited Matt Witczak, professor of civil engineering at the University of Maryland (UM) and principal investigator on FHWA's Superpave models contract, to meet with them earlier this year.
Witczak's presentation focused on the status of the original Superpave models developed under SHRP, which were subjected to a rigorous and thorough evaluation and analysis in the first phase of the FHWA models contract. He described how the second phase of the contract would concentrate on developing the most accurate means to characterize the behavior of materials in asphalt pavements, as that information is critical to the successful refinement of the models.
At a follow-on meeting with engineers from the United Kingdom's Transport Research Laboratory (TRL), the discussion centered on fatigue cracking, reflective cracking, and finite element models. Field data collected by TRL indicate that the vast majority of load-associated fatigue cracking appears to initiate at the surface of the pavement and propagate downward.
"This is in total agreement with the recommendations contained in the models evaluation report [prepared by the UM team], and in contradiction to the common belief that fatigue cracking generally starts at the bottom of the asphalt layer and propagates up to the surface," says Witczak.
In addition, the TRL engineers reported that findings of preliminary field studies indicate that reflective cracks (i.e., cracks in an asphalt overlay that are caused by distress in the underlying pavement) form at the surface of the new layer, rather than at the bottom, as is generally thought. "Neither TRL nor I have any explanation, from a mechanistic concept, as to why this may be happening," says Witczak. "But it will be important to follow the progress of the TRL study."
Researchers at TRL are also working on developing a three-dimensional finite element model that will take into account nonuniform contact stresses produced by tires. This will allow researchers to more precisely model how tires affect the way loads are imposed on pavements. The TRL work is identical to that recommended by the UM models team.
Katherine Petros, FHWA's manager for the Superpave models contract, says that the FEHRL meeting "was an excellent way for us to provide a status report on what is happening with the Superpave models in the United States and to learn what European researchers are doing in the area of mechanistic asphalt mix design. Much of what we learned confirmed that we are on the right track with the Superpave models work in the United States."
For more information or to request a copy of Witczak's report on his presentation in Europe, contact Matt Witczak at the University of Maryland (phone: 301-405-0305; fax: 301-405-6230) or Katherine Petros at FHWA (phone: 202-366-1340; fax: 202-366-9981; email: katherine.petros@fhwa.dot.gov). Additional information on the models project is available on the Web at www.ence.umd.edu/superpave/.
European highway agencies are already using test methods and equipment similar to some of the components of the Superpave system. According to a survey by FEHRL, four devices or specifications developed by SHRP and enhanced by FHWA may contribute to asphalt research and development in Europe:
As for the Superpave mix analysis and performance prediction procedures, FEHRL members are waiting to see what comes out of the models project being conducted by FHWA and the University of Maryland team.
The Forum of European National Highway Research Laboratories (FEHRL) was established in 1989 to encourage cooperation and collaboration among highway infrastructure researchers in Western Europe. Since then, the membership has expanded to include Eastern Europe.
The organization's goals include providing input on highway standards and policy, creating and operating a safe and efficient highway network, and improving the competitiveness of the European highway industry. As part of that mission, FEHRL is working on European standards for highway infrastructure.
To help its members follow the implementation of the Superpave system and other technologies developed or evaluated under the Strategic Highway Research Program (SHRP), FEHRL has appointed a representative to the Transportation Research Board (TRB) SHRP Committee. The organization's current representative to the TRB-SHRP Committee is Ian Jamieson, director of the laboratory of the National Roads Authority of Ireland.
The organization's membership includes highway agencies and research laboratories in 23 countries.
For more information on FEHRL, check its Web site (http://fehrl.lcpc.fr/).
