History of the Federal Highway Administration trailer program
In the late 1980s, the Federal Highway Administration (FHWA) Demonstration Projects (DP) Program instituted a new project – DP 74 “Field Management of Asphalt Mixes.” The core of the project was a mobile testing lab that could be brought to the plant site to test full volumetric properties of the as-produced asphalt mix.
Historically, asphalt content and gradation had been used for quality control and acceptance of asphalt mixes. While it seemed that most mixes met specifications during production there were many problems with rutting on the nation’s highways. The DP 74 project promoted the concept of using mix volumetric properties for quality control and acceptance. Instead of just running asphalt content and gradation on the plant mix, the compaction of Marshall samples in the field for quality control (QC) was demonstrated. It had been assumed that if the as-produced mix had the same gradation and binder content as the original mix design then it would perform in the same way as the lab-designed mix.
The mobile lab would set up at a plant site and sample mix during production. The typical site visit would last four weeks, with the first two weeks for preliminary testing for aggregate properties and binders. The last two weeks were used for mix sampling. Asphalt content and gradation on each sample would be determined and then Marshall samples would be compacted. The gradation, binder content and Marshall properties would be compared to the original mix design.
Startup
During the first four years of the project, the trailer visited construction sites in the majority of the states. Close-out reports for each site visit were provided to the state and contractor. These reports provided the data on mix verification. The reports compared the as-produced mix to the originally designed mix offered recommendations on how adjustments could be made to provide a mix that would meet gradation, binder content and volumetric properties during production. At the same time, quality assurance programs were being implemented that centered on statistical specifications like percent within limits (PWL).
This demonstration project started many of the highway agencies on their way to using mix verification in the field and the adoption of volumetric quality control. These two concepts are standards now in the industry. Reevaluation of statistical quality control and adoption of PWL specifications were major successes of the original demo project.
Another major effort going on in conjunction with the mobile trailer program was the Strategic Highway Research Program (SHRP) and the development of the Superpave system. In 1993, SHRP completed research activities culminating in the Superpave system and implementation started. Working closely with American Association of State Highway Transportation Officials (AASHTO) and Transportation Research Board (TRB), it was felt that the FHWA mobile asphalt trailer program was ideally suited to assist in the Superpave implementation efforts.
The Superpave system included both a binder and mixture component and the mobile lab program expanded to incorporate both. In the mobile mix trailer, the new aggregate test procedures were incorporated and the gyratory compactor brought into the lab. The trailer still traveled to asphalt plant sites on active projects, but now instead of just demonstrating volumetric QC, an introduction of the Superpave system was included.
Comparisons of Marshall to Superpave were performed on the production mix. This gave many states their first look at what the new Superpave mix would look like using local materials. Data was collected on the variability of the new Superpave equipment in comparison to Marshall. Most importantly, training was provided to the agency and industry personnel in the mobile lab under real-world conditions in a production facility. As with any new procedure or system, adjustments had to be made and the mobile lab provided the ideal situation to evaluate the new equipment and test procedures in the real-world environment.
Binder, too
The second component of the mobile lab program was to include the binder portion of the Superpave system. For the binder portion, the lab wasn’t mobile, but the technicians were. A full set of the binder equipment was set up in one of the older mobile labs but permanently stationed at the FHWA ALF (Accelerating Loading Facility) site. The lab did extensive testing for DOTs and the mobile mix lab provided data on the new Superpave binder specification. Agencies would send in samples to be tested for comparison to their own lab testing. The technicians in the lab would visit the DOT to provide training and help in setting up their own labs. As new equipment manufacturers came on board, the binder lab would evaluate the equipment to determine if it would meet specifications and provide repeatable results. As with the Superpave mix specification, adjustments and refinements were needed to make the binder specification robust for use by the DOTs. The binder section of the mobile lab program provided much of the data needed to make these changes.
The binder component of the mobile lab program provided much of the development work for the Multi-Stress Creep and Recovery (MSCR) specification. Creep and recovery testing is a basic rheological test that was evaluated as part of the original SHRP research and the follow-up research in the NCHRP 9-10 project. It is through work in the binder lab that the final test procedure and validation work was carried out for the development of the AASHTO M332 binder specification.
Both the binder and mixture labs provided significant support to numerous NCHRP projects that were initiated to help with the final development and refinement of the Superpave program – including ruggedness testing of the binder and mix equipment, field data for QC procedures for Superpave mixes and training for thousands of DOT and industry personnel.
Mix performance
While the implementation efforts were underway for the Superpave design system and the continued effort on statistical quality control, there was one area that was not completed in the original SHRP research. Mix performance testing was always intended to be part of the Superpave system. While the SHRP researchers did have recommendations for mix performance testing, it was generally very expensive and cumbersome to undertake. AASHTO, through the NCHRP program, continued the development effort for performance testing and introduced the Simple Performance Tester (SPT). The FHWA mobile lab program worked closely with the NCHRP contractors in the evaluation and ruggedness testing of the SPT.
The equipment was purchased and placed in the mobile lab again to provide visibility of the SPT to the industry. With the equipment installed in the mobile lab, it was easy to transport to trade shows and actual paving projects to demonstrate how it worked. It was during this process that the name changed from the simple performance tester to the asphalt mix performance tester. There were complaints from the industry that the test really wasn’t simple and it ignored mixes that weren’t Superpave. Changing the name from SPT to the asphalt mix performance tester (AMPT) made the equipment seem more universal in use and more directly related to a performance design concept.
Today
The FHWA mobile lab program is still active today. While statistical quality control, mix verification, volumetric acceptance and Superpave all have become standard in the U.S., there is still more to be done. One of today’s major issues is performance mix design or balanced mix design. The FHWA mobile lab program has been collecting data on field projects for the performance-related specification project centered on the simplified visco-elastic continuum damage model (SVECD). This is a model developed by North Carolina State University for materials characterization. Extensive testing of a mix is done using the AMPT and the dynamic modulus and direct tension axial fatigue tests to characterize the mix properties which are then fed into a finite element program for predicting pavement performance.
The FHWA mobile lab program has been active for 30 years. During that time, the lab has visited projects in almost every state once, in many states, multiple times. Thousands of people have come through the lab to see new and innovative approaches to asphalt testing. Over that time there have been many changes in lab activities. From the original Marshall mix design and statistical specifications to Superpave implementation and now performance-related specifications, the mobile lab has been a valuable resource in providing demonstration and training for the latest technologies.
D’Angelo is an asphalt consultant based in the Washington, D.C. area.
