Researchers ran a sophisticated low-temperature asphalt cracking performance test at multiple labs to study the test, its variability and repeatability, and its additional promise in studying reflective cracking susceptibility of overlays. Results put MnDOT closer to implementing test specifications for low-temperature cracking test for pavement mixes.Continue reading Low-Temperature Cracking Test Produces Repeatable, Reliable Results
The Minnesota Department of Transportation is working with other state agencies in a pooled fund study to improve methods for testing crack resistance of asphalt mixtures. To expand options further, MnDOT asked researchers to evaluate alternative tests with standard lab equipment. The new tests produced repeatable results. Methods include the semicircular bend (SCB) test in a nontypical configuration, a dynamic modulus test of smaller asphalt mixture samples, a bending beam rheometer (BBR) test of mixtures, and a BBR of asphalt material for binder selection.Continue reading Testing Methods for Crack Resistance in Asphalt Materials
Research showed that lower asphalt binder mixtures are susceptible to premature cracking. The current use of coarse-graded mix designs should be adjusted to narrow the gradation difference between larger and smaller aggregates in the mixes. While the research suggests such mixes should be used sparingly in Minnesota, it did not provide definitive data suggesting the practice should be stopped altogether. The practice may continue on a limited basis.
What Was the Need?
Introduced in 1993, Superpave has successfully helped transportation agencies in northern regions design asphalt pavements that are less susceptible to thermal cracking. When tested, Superpave-compliant designs were found to resist both rutting and thermal cracking.
Gradation-based design approaches have also allowed for the use of coarse-graded, low asphalt binder mixtures. These mix designs establish a maximum aggregate size and reduce the range of usable gradations. Such coarse-graded designs meet MnDOT specifications because the maximum aggregate size falls within the acceptable gradation range. However, the reduced fine aggregate content made possible by the use of coarse aggregates leads to a mix that, while still within specifications, offers less surface area to be coated by the asphalt binder and can encourage unwelcome permeability in the field. To win low-bid competitions, contractors have embraced these low-binder, coarse-graded designs to reduce binder and aggregate costs.
Transportation engineers noticed that these pavements seemed to “gray out” or lose their dark color more quickly than previous asphalt designs. These pavements also seemed to grow somewhat more brittle and were less able to rebound from loading. Such asphalts are thought to be prone to quicker failure than mixes with finer aggregate and more binder. Road designers typically attribute thermal cracking and potholing in low-binder asphalt to the increased permeability that leads to water incursion and freeze-thaw damage.
What Was Our Goal?
The goal of this project was to determine how well low-binder asphalt pavements per-form and whether current designs make sense in terms of cost–benefit and durability. Researchers would identify any relationship between reduced bitumen use and potential for cracking, and would suggest changes to specifications for coarse-graded asphalt pavement mixtures to prevent such cracking issues.
What Did We Do?
Researchers worked with MnDOT to identify 10 pavement locations in Minnesota that used 13 coarse-graded, low-binder asphalt mix designs. Investigators extracted data on cracking, roughness and other factors for these sites from MnDOT’s pavement management system. The research team then visited the sites and inspected the pavements.
Researchers developed a coring plan, and field samples were cored for volumetric analysis to determine the binder, aggregate, air void level and other properties of each mixture. They also tested permeability and dynamic modulus, and conducted fracture energy testing to determine cracking resistance.
Investigators used performance modeling to analyze the test results of pavement proper-ties and project pavement durability. Then they compared the projected performance to actual field performance. From this assessment, they drew recommendations for modifying specifications for MnDOT low-binder, coarse-graded asphalt mixtures.
What Did We Learn?
This study suggests MnDOT should reduce its use of coarse-graded asphalt mixtures, but the findings did not provide sufficient data to justify prohibiting the use of coarse- graded, low-binder asphalt designs.
Low-binder mixtures were prone to thermal and transverse cracking. Their high permeability left them vulnerable to premature moisture and freeze-thaw damage. Field and laboratory testing and modeling demonstrated that coarser mixtures produce excessive cracking in a short period of time. Thin overlays of 3 inches or less crack more quickly than thick overlays of 4 to 6 inches. Mechanistic-empirical simulations showed that low-binder asphalt mixtures were significantly inferior to higher-binder mixtures in terms of thermal cracking.
Most of the high-cracking mixtures showed low fracture energy in testing, suggesting the value of fracture energy testing and modeling. Disk-shaped compact tension testing showed that higher permeability mixtures correlate reasonably well with lower fracture energy. Eight of the 13 mixtures were more permeable than recommended, and six significantly so. Typical volumetric properties poorly predicted cracking.
To better project pavement performance, researchers recommend that MnDOT maintain volumetric testing-based specifications, but add performance testing-based specifications and testing designs for fracture energy, fracture resistance, modulus and other parameters. For Superpave designs, investigators suggest using a narrower aggregate gradation range, reducing the gradation gap between minimum and maximum aggregates in mixes.
Although the research validates MnDOT engineers’ anecdotal concerns, the pavements evaluated were mostly overlays, which are known to be susceptible to transverse cracking because of flaws in underlying pavement layers. MnDOT may weigh the results and adjust specifications, but would likely require further study of coarse-graded mixture performance before ruling out its use or identifying situations in which coarse-graded mixtures may be the best option. Additional research could consider the use of nonuniform lift designs for asphalt pavements, varying mixes for each lift in the structure rather than using a single, uniform mix for every layer in the full depth of the pavement.
This post pertains to Report 2017-27, “Impact of Low Asphalt Binder for Coarse HMA Mixes,” published June 2017.
The nation’s two largest pavement test tracks are planning their first-ever co-experiments.
The Minnesota Department of Transportation’s Road Research Facility (MnROAD) and the National Center for Asphalt Technology (NCAT) began discussing a formal partnership last year and have now asked states to join a pair of three-year research projects that will begin this summer.
Representatives of the test tracks are meeting next week in Minneapolis at the 19th Annual TERRA Pavement Conference. They said the partnership will develop a national hot mix asphalt cracking performance test and expand the scope of existing pavement preservation research at the NCAT facility in Auburn, Alabama, to include northern test sections in Minnesota.
MnROAD plans to build test sections at its facility and also off-site on a low- and high-volume road, which may include concrete test sections if funding allows. These Minnesota test sections will supplement 25 test sections built by NCAT on an existing low-volume haul route in 2010 and an off-site high-volume test road planned for this summer in Alabama to assess the life-extending benefits of different pavement preservation methods. Both agencies have also been developing performance tests to predict the cracking potential of asphalt mixes, and they will now work together on that research as well.
“We will collect and analyze the data in similar ways, and I think we’ll have a greater appeal nationally, as we cover a range of climate conditions,” said MnROAD Operations Engineer Ben Worel.
Participation in the pavement preservation study is $120,000 per year for the initial research cycle, which will drop to $40,000 after three years; the cracking study will run three years at $210,000 per year. Alabama will be the lead state for this effort.
State departments of transportation are asked for commitment letters this month if they are interested in joining either study, even if they do not have SP&R (State Planning and Research) dollars available at the time. Participating agencies will get to design the scope of the research and be kept advised of the ongoing findings, so they can benefit early from the project. Initial planning meetings will be done through a series of webinars in March and April of this year with participating agencies.
At a January 8 webinar, speakers said the research will help states determine how long pavement preservation treatments will last.
“Many DOTs have really well-designed and well-thought-out decision trees, where they can take pavement management data and end up with a rational selection of pavement alternatives. But the issue of extending pavement life is the really big unknown, because references provide a broad range of expected performance,” NCAT Test Track Manager Buzz Powell said.
Another benefit is that states can learn how pavement treatments hold up in both hot and cold climates.
“It’s 14 degrees right now in Mississippi. It rains about every three days, freezes and then thaws,” said Mississippi Chief Engineer Mark McConnell. “So we need to know how pavement preservation is going to work in the north as well.”
For additional information, contact Ben Worel (firstname.lastname@example.org) at MnROAD or Buzz Powell (email@example.com) at NCAT.