November 2002
Current and future testing options are designed to help instill confidence in Superpave
By Sandy Lender
Since Superpave’s inception more than a decade ago, researchers have searched for a simple performance test to give agencies and state departments of transportation (DOTs) confidence in the new pavement’s forecasted performance. A simple test — one that can be performed quickly and accurately by contractors and agencies — is the answer to bolster confidence in a pavement’s friction; potential for permanent deformation (rutting); resistance to fatigue and thermal cracking; and moisture sensitivity.
In the November 2001 “Performance Testing For Hot Mix Asphalt (Executive Summary)” from the National Center for Asphalt Technology (NCAT), the authors report that the Superpave system was intended to have a step that would “provide a method to analyze the mixture properties and to determine performance potential.”
The authors of the executive summary and researcher Ray Bonaquist, chief operating officer of Advanced Asphalt Technologies (AAT), Sterling, Va., said that most state agencies currently use a volumetric mix design method. They see the need, however, for the simple performance test to be used in addition to volumetrics. “For a number of years, engineers have had a concern that it may be possible to meet volumetric criteria but still produce mixtures that may not perform as well as we expect them to,” said Bonaquist. “They’ve been asking for a simple test to judge the relative performance of the mix.”
At an industry meeting earlier this year, Ray Brown, director of NCAT, discussed the variety of performance tests currently available. Brown put the NCAT stamp of approval on the simulative test performed by the Asphalt Pavement Analyzer (APA) from Pavement Technology, Inc. (PTI), Covington, Ga., when testing for a pavement’s rutting potential. “Six months from now, the recommendation may be different, but if you ask me today...rut testers are the way to go,” said Brown. “Rut tests show the most accurate correlation results with permanent deformation.”
Brown also listed the Hamburg and French testers as wheel tracking tests NCAT researchers recommend. But, he mentioned two basic drawbacks to using them: the need to compact excess samples when using the Hamburg wheel tracking device; and the lack of availability of the French rutting tester.
Widely accepted intermediate test
The APA from PTI — designed to test permanent deformation (rutting), fatigue cracking, and moisture susceptibility of asphalt pavements — performs the rutting test by simulating in-place traffic. Cylindrical core samples from a Superpave gyratory compactor (150 mm in diameter, 75 or 150 mm thick) or beam specimens (125 x 75 x 300 mm) are placed in an environmental chamber. Concave wheels apply a dynamic load atop pressurized pneumatic hoses. A typical test is 8,000 cycles and takes two hours and 15 minutes. Measurements are acquired while the test is running and displayed in numeric and graphical formats.
NCAT’s recommendation of the APA comes at a time when the Federal Highway Administration (FHWA) is still funding research for a simple performance test. “We need something today to predict how our mixes are going to perform before we make the investment to place them on the roadway,” said Wade Collins, vice president of PTI. “A lot of the DOTs kept waiting and waiting, and no simple performance test was forthcoming.”
Collins explained that his father, Ron Collins, formerly the state materials and research engineer with Georgia DOT, helped develop the Georgia wheel tester. When he retired from Georgia DOT in 1996, he joined forces with Don Brock, chief executive officer of Astec Industries, Chattanooga, Tenn., and started PTI. “The first product we built was the Asphalt Pavement Analyzer,” said Wade Collins. “We began to market the analyzer to the DOTs first. Basically, a lot of DOTs and contractors got impatient with waiting year after year with no performance test to accompany Superpave methodology, so they bought our analyzer. About eight or 10 DOTs have developed specifications with it.”
A test for the future
The National Cooperative Highway Research Program (NCHRP) has current research to identify simple performance tests (Project 9-19), and to develop equipment (Project 9-29). According to NCHRP: “Adoption of the simple performance test for routine use in the Superpave mix design method will require the production of commercial test equipment. This will include development of equipment specifications, equipment evaluation, ruggedness evaluation, and final procedure verification — all leading toward a national procurement for the state DOTs and eventual widespread adoption and use in the HMA industry.”
AAT was assigned the task of specifying and selecting a simple and affordable machine to perform flow time, flow number, and dynamic modulus tests on asphalt concrete specimens. Interlaken Technology Corp., Eden Prairie, Minn., and ShedWorks Inc., College Station, Texas, were selected to design and build prototype machines.
Under Project 9-19, Arizona State University and the University of Maryland developed three candidate simple performance tests with draft test protocols that are currently going through in-depth field verification, Bonaquist said (see “Candidate Simple Performance Tests”). AAT’s role is to take the draft protocols and develop detailed purchase specifications for equipment, then purchase and evaluate two devices.”
Bonaquist said that four manufacturers responded to the request for proposals. AAT engineers chose the designs presented by Interlaken and ShedWorks.
Bonaquist seemed confident that his company will meet the April 2003 deadline for reporting the results of the equipment tests. “The overall product of our project,” said Bonaquist, “is a detailed purchase specification that can ultimately be used by state highway agencies or other people who are interested in purchasing a simple performance test device.
“There still are two things left to be done,” continued Bonaquist. “One is the work that we’re doing on the equipment specification. The second part is the work still being done by the Arizona State University and the University of Maryland in Project 9-19 to validate the three tests using in-service performance data. Our project will make specific recommendations with respect to the equipment. The test, and how it’s conducted, will be developed by the (universities). It is my understanding that this work should be completed by the end of the year (2002) at the latest.”
Once researchers complete the equipment evaluation in Project 9-29, NCHRP panel members will evaluate the results. “We have several state bituminous engineers, a couple of contractors, a couple of people from an academic background, and a liaison from the Federal Highway Administration,” Bonaquist stated. “It’s a very good cross section of people who have a major stake in using the equipment… Once we (AAT) provide a final report to the panel, they decide the final disposition of what happens.”
He explained that the panel members can either reject the report and have the researchers try again, or they can move on to the next level. Bonaquist said that the next level possibly involves recommending that the new equipment and test be considered by a wider audience, perhaps through the Transportation Research Board’s Superpave Mixture and Aggregate Expert Task Group. “If that group is happy with it, then it would most likely be forwarded to one of the AASHTO subcommittees to be potentially balloted as a provisional kind of standard.”
Bonaquist said it is a long path for the equipment and test to reach completion, but the result will be a long-needed simple performance test for contractors and agencies to have confidence in mix performance predictions. “It’s inevitable that some type of a test is going to be added to the volumetric mix design method,” he said. “The work that’s being done here and under the project by the Arizona State University and the University of Maryland is very good. It’s important to start getting this information out.”
| Candidate Simple Performance Tests Ray Bonaquist, chief operating officer of Advanced Asphalt Technologies, Sterling, Va., explained that NCHRP Project 9-19 places a major emphasis on rutting performance, but the three candidate tests recommended for simple performance test consideration cover more than rutting. 1. Creep Test “The first one is a very simple creep test,” explained Bonaquist. “It uses the flow time, which is the time it takes the mixture to go into a tertiary flow condition. That’s related to rutting only.” 2. Repeated Load Test “What is used here is the flow number, which is very similar to the flow time in the creep test,” said Bonaquist. “It’s the number of cycles of load applied to the specimen before it reaches this tertiary flow condition.” 3. Complex Modulus, Frequency Sweep Test “That one has been related to both the rutting and fatigue cracking behavior of the pavement,” said Bonaquist. “You need a mixture that has a certain stiffness at high temperatures to resist rutting. But at intermediate temperatures, you would not want the stiffness to be too high, otherwise it would crack.” Bonaquist explained that, at press time, the dynamic modulus and dynamic load tests were being proposed for the future pavement design method, or the AASHTO 2002 structural design. “So there’s a lot of coordination going on in these material property tests between the mixture design part of it and the pavement structural design.” |
Sandy Lender is editor-at-large for AggMan.