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.
What Was the Need?
In very cold temperatures, asphalt pavement shrinks and fractures as it pulls from its various restraint points. Low-temperature or thermal cracking is the most widespread distress found in asphalt pavements in cold climates like Minnesota’s.
Pavement designers select an asphalt binder performance grade (PG) based on expected seven-day average maximum and minimum temperatures that the asphalt pavement is expected to experience. A PG 58-28 binder, for example, is supposed to maintain good performance at maximum temperatures of 58 degrees Celsius (136 degrees Fahrenheit) and minimum temperatures of -28 degrees Celsius (-18 degrees Fahrenheit).
PG binder ratings and tests do not characterize asphalt mixtures precisely because PG does not account for aggregate types and gradations, recycled material in the mix, and plant and field aging of asphalt mixtures. Asphalt mixtures must be tested at relevant temperatures.
“The disc-shaped compact tension fracture energy provides a good basis as a reflective cracking performance predictor, as well as for thermal cracking,” said Eshan Dave, associate professor, University of New Hampshire Department of Civil and Environmental Engineering.
For MnDOT to fully implement performance-based specifications for asphalt mixtures, it must better understand mechanistic tests of the susceptibility of asphalt mixtures to certain kinds of stresses. The disc-shaped compact tension (DCT) test has emerged from a decade of study as the best tool for measuring fracture susceptibility of asphalt mixtures at low temperatures and other conditions. Recommendations from a 2015 pilot DCT low-temperature fracture study included repeatability testing trials for DCT testing of low-temperature fracture energy.
What Was Our Goal?
The goal of this research was to evaluate the DCT test to support its use in performance-based specifications for examining fracture properties within asphalt mixtures at cold temperatures. Researchers would develop a fracture energy database for Minnesota asphalt mixtures, refine the test as necessary, characterize the variability and reproducibility of test results by testing mixtures at multiple labs, recommend test specifications or parameters, and evaluate the potential for DCT testing of the susceptibility of asphalt overlay mixtures to reflective cracking.
What Did We Do?
Researchers identified pavement and overlay projects with asphalt mixtures that could be sampled for DCT testing. Then they compiled MnDOT data on asphalt pavement construction projects, mixture designs and DCT test results, developing a single database for this information to be updated continuously and managed by MnDOT’s Office of Materials and Road Research.
Using this database, researchers selected 16 asphalt projects representative of pavement and overlay mixtures, and gathered samples of mixtures from each of these projects for distribution to four testing laboratories. The team analyzed test results for all samples at all labs to assess repeatability.
The research team then collected field performance data on 15 selected asphalt overlay projects also in the database and ran over 100 finite element models of the mixtures to examine how their fracture energy test results may predict reflective cracking performance in the field.
What Did We Learn?
The DCT testing database included over 6,000 test results as of the end of 2018 and can be updated with new test results as they become available to optimize the use of DCT testing in performance-based specifications.
Typically, DCT testing requires holding a specimen at low temperature during testing for up to 12 hours; investigators refined the method to reduce the holding time to approximately 2.5 hours, in part by instrumenting specimens for interior temperature monitoring. Researchers also determined that reheating plant-produced loose asphalt mixtures to compact samples for DCT testing produced lower fracture energy results.
Researchers identified a fracture energy limit of 90 joules per square meter over which low-temperature fracture energy test results reproduce poorly and found that testing of 12 replicated specimens lowers variability and minimum and maximum fracture energy range values. Increasing binder content and the high- and low-temperature range of PG binders improves fracture resistance in mixtures, as does lowering recycled asphalt content.
“We need to continue to evaluate DCT for mix design. The researchers made good recommendations for overlay projects to combine thickness and fracture energy,” said Shongtao Dai, research operations engineer, MnDOT Office of Materials and Road Research.
Overlay fracture resistance, determined by combining overlay thickness and mixture fracture energy, correlates with improved reflective cracking performance in asphalt overlays. A fracture-resistance value of 50 joules per square meter found in this study may be used to minimize reflective cracking potential in overlays.
MnDOT is closer to implementing DCT test specifications for asphalt pavement. Turnaround time in DCT testing limits its use in quality assurance and control of asphalt mixtures during production, and MnDOT is researching alternative tests for this purpose. The DCT database may be valuable in calculating and evaluating other performance parameters for their ability in predicting field cracking performance.
Use of the DCT for overlay performance testing and specifications proved promising. Validation and field implementation, as well as further analysis of fracture energy, mixture composition and overlay thickness relationships to reflective cracking performance all warrant more research.
This post pertains to Report 2019-24, “Disc Shaped Compact Tension (DCT) Specifications Development for Asphalt Pavement,” published June 2019. For more information, visit the MnDOT project page.