Tech Transfer Success: How the DAV Improved SuperpaveTM Design
The Dynamic Angle Validation (DAV) kit ensures uniformity and consistency between gyratory compactors in asphalt testing. Here's how it came about.
Mixing asphalt can be tricky business. To use the analogy of baking chocolate chip cookies, it is essential to balance the ingredients to get the desired taste. To make the cookie analogy work, think of a recipe as having three components: the fine (flour, baking soda, salt, sugar), the coarse (chocolate chips, brown sugar), and the sticky stuff (egg, vanilla, butter). Similarly, in asphalt mixes, there is the fine (sand or aggregate typically less than 2.36 millimeters, or that can pass through a number 8 sieve), the coarse (typically rocks or aggregate retained on a number 8 sieve), and the sticky stuff (asphalt binder, which can include modifiers and additives). In asphalt, the desired taste is a long-life pavement.
|FHWA's Dynamic Angle Validation kit enables highway engineers to verify the required angle of any manufacturer's gyratory compaction equipment, required for Superpave asphalt.|
|The Dynamic Angle Validation Kit is designed to ensure uniformity between gyratory compactors. This second-generation DAV 2.0 includes an added eccentric ring to create an internal movement to simulate asphalt mix.|
How these ingredients are proportioned together requires a meticulous baker and state-of-the-art utensils. In an asphalt laboratory, ingredients are combined, mixed, and then compacted using a SuperpaveTM (SUperior PERforming asphalt PAVEment) gyratory compactor. The gyratory presses the mix into its cookie shape, which is a cylindrical specimen 115 millimeters high with a diameter of 150 millimeters. During mix design, where the baker develops the recipe for production, the asphalt cookies are evaluated volumetrically and undergo additional performance testing. Dry "cookie" recipes can lead to cracking on the roadway and recipes that are too soft can lead to rutting.
The Superpave asphalt mixture design system requires gyratory compactors to provide a uniform compaction effort. This effort is based on the downward pressure inside the compaction mold, the speed and number of rotations, and the tilt or angle of the model. The most important component is the angle, which creates the shearing action. Today, the angle of gyration is set and verified internally to 1.16 °±0.02° under American Association of State Highway and Transportation Officials specifications. However, there was a time when internal angles could not be measured, and there was no way to ensure uniformity from machine to machine or manufacturer to manufacturer. The issue became a major hurdle in the 1990s during the national implementation of the Superpave system.
Enter the Dynamic Angle Validation kits, or DAVs. DAVs are used for ensuring consistency and volumetric quality between gyratory compactors. Asphalt "cookie" specimens from gyratory compactors are not only used for developing tasty recipes, but are also used during field production in quality assurance testing.
The original prototype gyratory compactor, developed during the Strategic Highway Research Program (SHRP), called for an external angle of gyration set to 1.25°. An external angle does not account for something called "machine compliance." As a gyratory compactor presses an asphalt specimen into shape, the asphalt specimen pushes back on the load frame, which causes it to comply (deform slightly). This issue of machine compliance was not realized during the initial technology transfer effort for the Superpave system in the early 1990s.
Under a pooled fund procurement, the Federal Highway Administration solicited manufacturers to produce a gyratory compactor that could meet an external angle of 1.25°. FHWA selected two companies as successful bidders for this procurement and asked them to provide "first article" units to FHWA for evaluation. Upon receipt of these units, FHWA, along with the Asphalt Institute, performed extensive evaluations on each unit and conducted extensive discussions with SHRP researchers and the manufacturers. After a slight redesign of one of the units, FHWA ultimately approved both units, enabling a pooled fund purchase to proceed for all State highway agencies.
As time went on, more equipment manufacturers became involved with producing Superpave gyratory compactors. However, each production model employed a unique method of setting, inducing, and maintaining the angle of gyration. Unfortunately, this meant that no single manufacturer's calibration system could be universally applied to all the different models that were commercially available.
Creating a Universal Angle Validator
Recognizing the need for a way to validate the angle of gyration on any device, a team of three researchers at FHWA began to brainstorm. Tom Harman, then the lead for the Asphalt Pavement Team at FHWA's Turner-Fairbank Highway Research Center; Tom Brovold, who, at the time, sold his own version of a portable gyratory compactor; and Paul Fuchs, who had been an onsite contractor under FHWA's Nondestructive Evaluation Validation Center, sketched out a rough idea of what a universal fixture might look like that could dynamically measure the angle of gyration internally and work with any device.
Using this idea to produce a single, small conceptual device, the team worked to demonstrate consistent calibration with the same uniformity of compaction between two different devices. As with all eventual successful technologies, the first few attempts failed, and there were many design iterations. However, over the course of several years, the team developed and validated a functional internal device, which is now known as the DAV.
The team realized it had something novel, and they had the added advantage of using Brovold's private gyratory company to fully realize the technology. At the time, FHWA's device was the only proven technology. The creation of the DAV spurred competitors to develop similar devices. Other companies designed at least two devices–and even produced the same results–but their approaches were different.
FHWA aimed to get the internal calibration technology into the hands of State departments of transportation and asphalt suppliers. To do so, the DAV kits needed to be commercially available. In addition, the inventors wanted to secure the intellectual property rights. The next step was to file for a patent, which was successfully awarded in 2002.
The successful patent led to an exclusive license agreement between FHWA and Brovold, who already owned a company that manufactured and sold gyratory compactors. A royalty arrangement for the use of the technology was established, with a nominal licensing fee with the intent of giving recognition back to the laboratory and researchers while also making the use of the technology affordable and attractive at all levels of industry.
Improving Asphalt Testing
The invention of the dynamic angle measuring device has dramatically changed the practice of asphalt testing. Most transportation departments and asphalt suppliers are now using gyratory compactors to verify the volumetric data and compaction consistency. A version of the DAV, or a similar device, is used to calibrate gyratory compactors used in the United States.
The Nebraska Department of Transportation (NDOT) has been using the DAV technology for more than a decade. Two of the main advantages are convenience and consistency. Because the DAV acts as a mix inside the mold, it does not need existing hot mix. In other words, no mix has to be continually hauled to various branch labs. Further, says Jody Paul, the highway quality assurance manager at NDOT's Bituminous Aggregate Laboratory, "The DAV gives you your true angle reading. Consistently. From my gyratory to the branch lab to the next branch lab, it is all consistent."
Robert Rea, NDOT's assistant materials and research engineer, says, "The DAV allows contractors and Nebraska labs to tighten up their verification testing tolerance and precision between laboratories, which means producing more consistent test results. Therefore, it reduces the amount of test variability on a paving project."
|The DAV, shown here, is a Technology Transfer program success story.|
FHWA's Technology Transfer program played an important role in the process of getting the DAV kits patented, licensed, and commercialized. The Technology Transfer program is designed to provide support services in moving research from laboratory to market and ensures that the technologies produced are developed and disseminated with the user and adopter needs guiding the process. In addition, the program seeks to market products and demonstrate to State and local highway agencies that the FHWA research products are valuable, practical, and constitute best practices.
"The overall impact of the DAV technology and its successful adoption by the industry are a Technology Transfer success story," says Jack Jernigan, the director of FHWA's Research and Technology Program Development and Partnership Team.
Mary Huie is the manager of the Technology Transfer program at FHWA's Turner-Fairbank Highway Research Center. She holds a B.S. in civil engineering from the Catholic University of America in Washington, D.C.
Thomas Harman serves as the director of FHWA's National Highway Institute and was the Director of the Center for Accelerating Innovation. He has a bachelor's degree in civil engineering from the University of Maryland and a master's in civil engineering from the University of Illinois.
Eileen S. Nelson is a program manager working as a contractor with FHWA's Technology Transfer program. She holds a B.A. in history and curricular studies, as well as a minor in women's studies, from the former Randolph-Macon Woman's College in Lynchburg, VA.
For more information, contact Mary Huie at 202–493–3460 or Mary.Huie@dot.gov.