Researchers determined that natural soil amended with locally sourced materials performed well in bioslopes and bioswales. This practice will allow MnDOT to avoid hauling in costly commercial materials for stormwater management installations.Continue reading Using Regional Materials to Manage Stormwater Runoff
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.
What Was the Need?
A number of factors lead to cracking and other damage in asphalt. Cold temperatures cause pavements to contract, triggering internal tensions that lead to low-temperature cracking. Aging asphalt binder grows brittle and under loading pressure generates bottom-up, or fatigue, cracking. A variety of causes may contribute to top-down cracking, such as mixture properties, construction practices, tire design and loading.
MnDOT, in partnership with the National Center for Asphalt Technology, and four other state transportation agencies are part of a pooled fund study to develop mixture performance testing focused on cracking. This group, termed the Cracking Group, installed eight different pavement cells at MnROAD in the summer of 2016 to examine pavement performance and testing approaches for low-temperature, top-down and fatigue cracking.
The group’s approach does not embrace every potential test, including some examinations other agencies and research organizations have found potentially valuable in predicting cracking behavior of asphalt pavement materials.
What Was Our Goal?
MnDOT sought to investigate the viability of testing methods not included in Cracking Group studies. These tests would be conducted on asphalt mixtures sampled during construction of the test sections at MnROAD to help in material selection, quality control and forensic investigation of paving materials.
“This was a knowledge-building, data-gathering study that will help fill out our materials library database to correlate test results of asphalt materials to field performance.”
—David Van Deusen, Research Operations Engineer, MnDOT Office of Materials and Road Research
What Did We Do?
Preliminary testing focused on the eight MnROAD cells, pulling cores from the existing pavement before reconstructing new sections. Researchers tested these cores to refine methods for proposed tests. The team then gathered details on the binders and mixtures used in the 2016 reconstruction to use in its planned tests.
Researchers ran three tests on the eight asphalt mixtures and one test on the five asphalt binders used in the pavement mixtures at MnROAD. The asphalt mixture tests were:
- Bending beam rheometer (BBR) test of mixtures to obtain creep stiffness and strength of asphalt mixtures. This approach uses small beam specimens useful in forensic investigations.
- Low-temperature semicircular bend (SCB) test to measure fracture energy in mixtures. Currently there is no national standard test for fracture energy, but based on previous pooled fund work, MnDOT implemented the disk-shaped compact tension (DCT) test. The SCB results will be used to tie in the previous work and compare to the DCT.
- Dynamic modulus test of mixture resilience that uses smaller cylindrical specimens, a benefit in forensic studies.
To obtain asphalt binder strength, researchers used a variation of the BBR test for mixtures.
What Did We Learn?
The four tests proved to be viable options for materials selection testing, quality control and forensic examination of samples from existing asphalt pavements. The SCB and dynamic modulus can be run with research equipment. These tests yielded repeatable results and identified differences in the eight mixtures that are expected to impact performance. In particular, the BBR test of mixture has potential for being a practical field screening test.
The BBR test of mixtures measures strength and creep of ½-inch-thick asphalt mixture specimens compared to an indirect tensile test of strength on 2-inch asphalt pucks, and the test produces similar results. The dynamic modulus test uses the same configuration as the indirect tensile test, but instead of applying vertical compression to a 6-inch asphalt core, it applies pressure on a 1.5-inch puck diametrically, yielding similar results on an asphalt mixture’s resistance to loading.
The SCB test, an alternative to the DCT test, provides similar results in measuring the fracture energy of asphalt pavement mixtures. Either of these two newer tests is viable for MnDOT use. The binder BBR strength test represents a viable alternative to the direct tension test that, due to complex sample preparation and expensive equipment, is not frequently used.
“These test methods produce repeatable, consistent results, are simple to perform and differentiate between mixtures. They could provide critical information on the evolution of pavement performance since they can be used for forensic analyses.”
—Mihai Marasteanu, Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering
All tests found sample performance highly dependent on temperature. Fracture resistance does not correlate directly with other tested values; two mixtures that share similar creep stiffness, for example, may not have similar fracture resistance. Results indicate the eight mixtures tested may perform similarly, although one with high recycled asphalt content and another with a highly modified asphalt binder may be outliers. Based on the laboratory test results, mixtures with performance-graded binders do not differ markedly when one is mixed with recycled asphalt materials. As is the case with all pavement field studies, time is required for the mixes to begin to distinguish themselves from one another in terms of field performance.
MnDOT will share test results from this study with the Cracking Group team and include them in the overall examination of the MnROAD test cells. Researchers recommend comparing results to observed distresses and core tests periodically from these pavement cells to correlate field conditions and tested mixture performance over time. MnDOT will consider some of these testing methods and findings in its continuing effort to develop a performance-based balanced mix design approach for asphalt pavement.
This post pertains to Report 2019-03, “Investigation of Cracking Resistance of Asphalt Mixtures and Binders,” published January 2019. For more information, visit MnDOT’s Office of Research & Innovation project page.
Researchers developed procedures for selecting asphalt pavements for thin whitetopping based on site examination and lab testing. Test results do not offer definitive indications of how overlaid asphalts will perform, but procedures offer recommendations on pre-overlay pavement treatment, testing protocols and design considerations for bonded concrete overlay of asphalt.
“This research established a procedure for testing pavement cores. However, more performance data on whitetopping is needed to correlate pavement performance and asphalt properties,” said Tim Andersen, Pavement Design Engineer, MnDOT Office of Materials and Road Research.
“These procedures address collecting field data and testing pavement core samples in the lab. They also provide useful guidance for pavement repair and design considerations for overlays,” said Dale Harrington, Principal Engineer, Snyder and Associates, Inc.
What Was the Need?
Many counties throughout Minnesota have used bonded concrete overlays to rehabilitate asphalt pavement. Though not widely used by MnDOT, a bonded concrete overlay, or whitetopping, normally involves milling a few inches of asphalt off the damaged surface and placing 4 to 6 inches of concrete over the asphalt pavement. A well-bonded overlay can add 20 years to a pavement’s service life.
Bonded whitetopping performance has not been care-fully tracked, and correlation of its performance with the underlying pavement condition is not well understood. Be-fore MnDOT can expand its use of bonded whitetopping, materials engineers wanted to better understand what asphalt pavement conditions are best suited to this type of overlay, how asphalt behavior influences the concrete top layer and what underlying pavement characteristics affect the expected lifetime and performance of bonded white-topping.
What Was Our Goal?
This project sought to develop an integrated selection procedure for analyzing existing, distressed asphalt pavement to identify good candidates for bonded whitetopping and establish design considerations for a site-specific, effective concrete overlay. By testing pavement core samples in the lab, investigators wanted to identify asphalt pavement properties that correlate with distresses in concrete overlays that are 6 inches or less. They also sought specific recommendations for managing trans-verse cracking in asphalt to avoid reflective cracking into concrete overlays.
What Did We Do?
Researchers began with a literature review of approaches to selecting pavements for bonded whitetopping. The results of this review were used to develop testing procedures to identify the volumetric properties of existing asphalt pavements. Researchers applied these procedures to 22 pavement cores from six concrete overlay sites in Iowa, Michigan, Minnesota and Missouri. Selected projects entailed 4-inch to 6-inch overlays in fair to good condition that were built from 1994 through 2009. Data about mix design, asphalt condition, pavement thickness, overlay thickness, site conditions and other details were available for each site.
The research team compared roadway data with falling weight deflectometer measurements from pavement cores to evaluate field performance and design recommendations suggested by the selection procedure. To refine the procedures, investigators evaluated volumetric asphalt characteristics for their potential influence on premature overlay cracking due to stripping, slab migration and reflective cracking. Finally, the team developed a detailed selection process that includes steps to identify and test asphalt pavements with potential for bonded whitetopping, repair asphalt before overlays and establish design considerations for overlays based on the test results from the selected asphalt pavement.
What Did We Learn?
The selection procedure, which is based on recommended practices from the National Concrete Pavement Technology Center, has six steps:
- Perform a desk review of available site data, including design, repair and environmental conditions.
- Obtain pavement core samples.
- Conduct site visits to examine existing conditions.
- Obtain additional core samples for testing, when necessary.
- Prepare preliminary cost and materials estimates, if practical.
- Provide design recommendations.
Investigators tested pavement cores for air voids, density, stiffness, fatigue, aging, strip-ping potential and other distress parameters. Results were inconclusive in terms of identifying asphalt properties that lead to specific bonded concrete overlay failures or to long-term performance of bonded whitetopping projects. The pavement cores showed wide variation in material properties, but few of these distresses. Researchers framed the recommendations for testing volumetric properties in the format of MnDOT’s Pavement Design Manual, giving the agency an easily adoptable core testing protocol.
The selection procedures include information about the impact of transverse cracking, rutting, longitudinal cracking and other distresses on concrete overlays, and provide recommendations for treating various distresses before whitetopping. Design considerations for whitetopping are also provided based on site conditions and the results of core, ground penetrating radar and falling weight deflectometer testing.
Tested overlay sections should be evaluated over time to determine if life expectancy is met or if asphalt stripping, slab migration or reflective cracking has decreased overlay life. Because volumetric tests failed to provide conclusive relationships between asphalt properties and overlay distress, further research is needed to identify mechanistic or field tests that could correlate asphalt properties with concrete overlay performance. Once this additional research is completed, the selection procedures identified could be refined and placed in the design guide. A life-cycle cost analysis of overlays would also be useful for decision-makers considering bonded concrete overlays of asphalt.
This Technical Summary pertains to Report 2017-24, “MnDOT Thin Whitetopping Selection Procedures,” published June 2017.
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.
Transportation agencies have long placed high importance on the thickness of their concrete roadways, making it a major focus of control and inspection during construction. While it is commonly believed thicker concrete pavements last longer, there is little data to support this claim.
“One big reason for the lack of data on the relationship between concrete pavement thickness and performance is the destructive nature of these measurements,” says Lev Khazanovich, a former professor in the University of Minnesota’s Department of Civil, Environmental, and Geo- Engineering. “Concrete thickness is typically assessed by coring—a destructive, expensive, and time-consuming test that only offers widely spaced measurements of thickness.”
In a MnDOT-funded study, U of M researchers set out to fill this knowledge void by leveraging recent advances in the nondestructive testing of pavements that allow for large-scale, rapid collection of reliable measurements for pavement thickness and strength. They conducted four evaluations on three roadways in Minnesota using ultrasonic technology to collect more than 8,000 measurements in a dense survey pattern along with a continuous survey of observable distress.
“We found that both pavement thickness and stress measurements are highly variable, with a half-inch of variation in thickness about every 10 feet,” Khazanovich says. “Interestingly, three of the four surveys averaged less than design thickness, which is contrary to typical accounts of contractors building slightly thicker slabs in order to avoid compensation deductions.”
Data analysis showed that exceeding design thickness did not seem to increase or decrease pavement performance. However, a measurement of pavement strength and quality known as “shear wave velocity” did produce valuable findings. “A drop in the shear wave velocity strength measurement corresponded to an increase in observable pavement distresses such as cracking and crumbling,” Khazanovich explains. “This was especially apparent when we were able to easily identify locations of construction changes, where significant changes in shear wave velocity matched up with observable distress.”
The results of this study illustrate the importance of material quality control and uniformity during construction, since alterations in pavement strength and quality may significantly influence pavement performance. In addition, researchers say that despite inconclusive thickness results, it is still important that pavement has significant thickness to carry its intended traffic load over its service life. Finally, the study demonstrates that new methods of ultrasonic shear wave velocity testing are useful for identifying changes in construction and design that could lead to higher rates of pavement distress.