|In the Hot-Mix
Aggregate Contributions to Hot-Mix Asphalt (HMA) Performance in Superpave
by Sam R. Johnson
|by Sam Johnson
Sam Johnson, registered professional engineer, is the manager
of Technical Services for Martin
|The aggregate properties of hot-mix asphalt were not studied under
the important Strategic Highway Research Program, but experts
have continued to work at defining and standardizing these properties.
The authors provide an overview of these efforts and an upcoming
ASTM-sponsored symposium on the subject.
THE STRATEGIC Highway Research Program (SHRP) was a five-year, $150 million research program initiated in 1987. The purpose of the research was to develop new tests and procedures that would improve the nations roads by improving the performance of hot mix asphalt (HMA) pavements. Of the $150 million, $50 million went toward the study of asphalt binders and HMA. However, the majority of the $50 million was used for asphalt binder research. Secondary products included the adoption of a gyratory laboratory compactor, volumetric mixture criteria, and development of an HMA shear test device. The results were a grading system for asphalt and mixture design criteria associated with performance titled Superior Performing Asphalt Pavements or Superpave.
Aggregates compose approximately 95 percent by mass of an asphalt mix. Aggregate properties and their relationship to performance were not researched under the SHRP Superpave Program. Instead, an expert task group (ETG) was selected to identify 1) which aggregate properties were critical to asphalt pavement performance, 2) which aggregate tests accurately measured these aggregate properties, and 3) set specification limits on each test. Unfortunately, at the time, the only basis from which the panel had to make these critical decisions was from experiences learned from the Marshall HMA mix design and construction control procedures. So through the Delphi process (a process used to evaluate opinion-based concepts) and the Marshall System experiences, the panel set specification limits on the aggregate tests, which they had identified.
The Mixture ETG identified seven tests thought to affect HMA performance. Four of the tests, for which the panel was able to reach an agreement regarding specification limits, became known as the Consensus Properties. The Consensus Properties are Coarse Aggregate Angularity (CAA), Fine Aggregate Angularity (FAA), Flat and Elongated (F&E), and Sand Equivalent (SE). The panel concluded that regardless of location within the United States, these tests should have the same specification limits, based only on traffic loading and environmental conditions. Other aggregate properties, which were agreed upon as affecting asphalt pavement performance but for which no consensus could be reached on specification limits due to the large geological variances throughout the country, became known as the Source Properties. The Source Properties include soundness, toughness, and deleterious materials. Each state specifies their own limits for the Source Properties based on past performance in their respective state.
The Consensus Properties have caused much conversation among the players of the hot-mix industry, due primarily to the lack of research and performance data. While most people agree that these properties affect pavement performance, the debate arises over the tests that have been identified to measure the value of these properties and the arbitrary specification limits. Are the tests actually measuring the desired properties? Not until recently has the much-needed research on aggregates relationship to hot mix asphalt performance begun to evolve. Two of the primary aggregate properties of concern are the Fine Aggregate Angularity (FAA) and the percent Flat and Elongated (F&E) particles. Neither of these properties were researched in terms of their relationship to Superpave HMA performance.
The National Aggregate Association had developed the fine aggregate angularity test to predict a fine aggregates influence on the workability and finishability of concrete. ASTM has since adopted it as ASTM C 1252, Test Method for Uncompacted Void Content of Fine Aggregate. It had never been considered as a predictor of HMA performance. However the ETG decided that it would be a good test procedure to separate manufactured, crushed fine aggregates from natural, rounded fine aggregates. As we gained knowledge of this test it was discovered that some 100% crushed manufactured sands did not meet the Superpave criteria. Conversely, some nodular silica sands exceed the void content requirement. Recent Superpave related research is showing that certain aggregate geologies (i.e., high-calcium limestones) that have historically performed very well in bituminous asphalt pavements have difficulty in passing this consensus requirement. Are we truly measuring angularity?
The Flat and Elongated specification(1) and test procedure has also generated much discussion. While Superpave specified D 4791, the terminology they used and the required accuracy did not match the procedure. Over the past two years much work has been done within ASTM to resolve these differences, while in other areas a great deal of research is under way investigating the use of video imaging as an alternate method for measuring this and other criteria. On the specification side of this test procedure there was a push to change the flat and elongated requirement from a maximum of 10 percent at a 5:1 ratio to a maximum of 20 percent at a 3:1 ratio. This was based on the opinion that the flat and elongated particles were detrimental to HMA and a more cubical particle was needed, although the research substantiating this change has not been completed. The belief that flat and elongated particles lie flat and break during compaction is not always factual. Strong, durable aggregate particles that are somewhat elongated, exceeding the 3:1 ratio, do not degrade during construction and may actually assist in achieving the required voids in the mineral aggregate (VMA). The major concern is that if these opinions were implemented they would cost the aggregate industry, perhaps needlessly, millions of dollars.
Other requirements that were implemented that have a major effect on the aggregate industry are the Restricted Zone (new grading requirement) and minimum VMAs. While Superpave gradations are typically on the coarse side, the Restricted Zone was initially inserted as part of the gradation requirement in an attempt to eliminate the potential use of rounded sands that are believed to increase the possibility of rutting. The restriction was later changed to a recommendation after it was found that several in-place mixtures were performing exceptionally well with gradations that passed directly through this zone and did not contain rounded sand.
The VMA specifications currently being used were based on the Marshall system of asphalt design. These specifications may not be practical for Superpave. For instance, a 12.5-mm Superpave mix is required to have a minimum VMA of 14 percent. If the aggregate grading of this mix passes below the restricted zone, the coarsest allowable grading, the aggregate surface area can be as low as 22 ft2/lb. [4.5 m2/kg] of mix. That same 12.5-mm mix passing above the restricted zone, a fine graded mix, can have as much as 36 ft2/lb. [7.4 m2/kg] surface area. Yet, the same VMA is required of both mixes. The result here is that the coarse graded mix will likely result in a much higher average asphalt binder film thickness on the aggregate surface, over 10 microns, resulting in the potential for premature rutting and flushing of the pavement. As a result, some states are placing a maximum, along with the previous minimum, VMA requirement on their mixtures. Research is needed to show that the VMA specification should not only vary with the nominal size aggregate in the mix but the coarseness of the mix as well, allowing the coarser mixes to possibly have lower VMA requirements.
Other tests being utilized include Superpave shear tests, triaxial tests, unconfined compression tests, and indirect tensile tests. Originally, the Superpave shear tests were expected to provide the measure of HMA performance. However, correlation of the Superpave shear test with performance is continuing. Constant rate-of-strain, constant rate-of-stress and cyclic triaxial tests are being conducted to determine HMA shear strength, creep, and resilient modulus values for various HMA asphalt, aggregate, and gradation combinations. Indirect tensile tests are also being utilized to determine tensile creep and resilient tensile modulus.
The implementation of Superpave has resulted in anomalous performance results. In a number of instances changes have been made to the material and mixture criteria to overcome the anomalies. In addition, laboratory research and accelerated pavement testing (APT) have been undertaken to better understand the factors introduced in the Superpave system affecting pavement performance. For example, several NCHRP projects on aggregate properties and mixture performance have been completed or are in progress.
The National Pooled Funds Study No. 176, Validation of SHRP Asphalt Mixture Specifications Using Accelerated Testing, has addressed Superpave requirements for gradation, fine aggregate angularity, and voids in the mineral aggregate.
Significant performance data was generated at WesTrack, an accelerated pavement performance test in Nevada. Factors of the WesTrack experiment included gradation, asphalt content, and construction quality. A group of WesTrack test sections were replicated, and the performance of the initial and replicate sections were in agreement. Other test tracks and APT (Accelerated Pavement Test) facilities are also being used to study HMA performance.
A number of states, concerned with the need for performance data, have investigated the utilization of laboratory wheel tracking devices (WTD). Studies utilizing the WTDs have been made to determine the effect of aggregate type, aggregate gradation, fine aggregate angularity, asphalt grade, asphalt modifiers, mixture compaction and asphalt content on HMA performance. Significant WTD data is available from this research as well as evaluation of specific project mixtures. There is, in general, a correlation of WTD and APT and actual pavement performance.
Many states have mandated Superpave implementation by this year. However, it is only now that the actual research on aggregate and aggregate properties as related to Superpave HMA performance is evolving. New and helpful data is becoming available to evaluate aggregates contribution to HMA performance; with this performance information Superpave aggregate requirements can be properly evaluated. This data is being generated with tests related to strength and performance that is offering some insight. Unfortunately, while some of the research conclusions are supporting Superpave other research is challenging it. The truth is that we are still very much learning about Superpave. //
Talk to the Editor: Maryann Gorman
|by Thomas D. White
Thomas D. White, professor and head of the Civil Engineering Department at Mississippi State University, is involved with research on aggregates, asphalt and mixtures specifically related to Superpave criteria and performance.
|by John J.
John J. Yzenas, Jr., is the technical services manager for The Levy Company in Portage, Ind. He is chairman of Subcommittee D04.51, Bituminous Aggregates Tests and Membership Secretary of C-9 on Concrete and Concrete Aggregates.