Test Track Shares Results, Expectations
If you’ve been in the asphalt industry for any length of time, you’re familiar with the test track at the National Center for Asphalt Technology (NCAT) at Auburn University. Phase VII is underway now and results from Phase VI were released in July. The results show how the 46 sections on the 1.7-mile track performed under the 10 million accumulated equivalent single axle loads (ESALs).
During the first research cycle beginning in 2000, the team tested surface mixes. Structural experiments started in 2003 for the second cycle.
Now that NCAT has published Report 18-04, “Phase VI (2015-2017) NCAT Test Track Findings,” the industry has information, statistics, and verifiable data on porous friction courses with cellulose fiber, with synthetic fiber, and with ground tire rubber; on a surface mix containing bio-based rejuvenators with high RAP percentages; on high friction surface treatments; on the cracking performance of surface mixes containing different RAP percentages with PG76-22 and PG58-28 binders; on the reflective cracking potential of a triple chip seal and an open-graded interlayer (OGI); on different longitudinal joint performances; on different thin overlay performances; on the effect of tack coat rate on bond strength; and more.
There are seven participants in the cracking group experiment test sections alone. For this issue of AsphaltPro that looks at plant controls, checking out the materials that producers may someday be specified to feed to plants is highly productive.
Here are just a few of the conclusions, as written in July’s report, alongside upcoming tests, as discussed on the NCAT website. You can read the full 197-page report here.
Cracking Group Findings
For several years, a recurring topic at paving conventions has been cracking tests. While engineers in different freeze-thaw zones of North America have different views on which test should be labeled “best,” the NCAT Test Track and Minnesota DOT partner test site (MnROAD) have mixes on the ground undergoing specific testing.
In the Report 18-04, the authors share: “After two years of trafficking with 10 million accumulated ESALs, only one of the seven test sections in the top-down cracking group experiment on the NCAT Test Track had a substantial amount of cracking. Section N8 containing 20 percent RAP and 5 percent RAS with a PG67-22 binder has cracking in nearly 17 percent of the lane area.”
We all know this doesn’t mean Section N8 failed. It means Section N8 has provided valuable information.
“Limited coring showed that the cracking in N8 was confined to the surface layer with no evidence of debonding between layers,” the report states. “Structural analysis based on back-calculated asphalt concrete moduli also indicated that the wheel path cracking in this section has resulted in damage to the pavement structure.”
Three other test sections with low-severity cracking on the surface were cored, but showed the cracks had not gotten beyond the surface. Researchers saw no damage to the pavement structures in Sections N1, N2 or N5. Researchers found the following truths, quoted from the report:
- Based on the results of this project, the Energy Ratio (ER) method has several significant shortcomings. In its current procedure, it is not possible to properly characterize the variability of the ER parameter. The equipment cost and test complexity also render it impractical for routine use.
- The OT results (both the Texas method and the NCAT-modified method) ranked the mixtures largely in accordance with their anticipated level of field cracking. Results of the two test methods were highly correlated….However, one of the disadvantages of the OT methods is their relatively high variability.
- The SCB (Louisiana method) and Jc criteria was able to identify the mixture from N8 as susceptible to cracking, but it also indicated very similar results for four of the other mixtures, two of which have no signs of cracking to date.
- The I-FIT method yielded a relatively large spread of Flexibility Index results for the seven mixtures. This kind of statistical spread in results for different mixtures would allow users to better assess how to improve mix designs and adjust field mixtures….The IDEAL-CT data showed the same trends as the I-FIT data in most respects.
The good news is the sponsors will support the testing through Phase VII so we can learn more. “The laboratory test results presented in this interim report represent only about one fourth of the total testing plan associated with this study,” the report states.
Cracking Group Going Forward
NCAT’s website states: “The aim of the cracking group experiment is to develop and implement asphalt performance tests to predict cracking for common pavement distresses found in North America. There are several lab tests that claim to relate to one or more modes of cracking. However, most of these tests lack robust validation outside of the state where the method was developed.
“Both MnROAD and NCAT have developed test sections to validate laboratory cracking tests by establishing correlations between test results and measured cracking performance in real pavement test sections within a very short timeframe. This research will provide much needed data on where to set criteria for specifications.
“Test sections with differing asphalt mixtures that have a range of expected cracking susceptibilities have been constructed at MnROAD. A suite of laboratory cracking tests conducted on the mixes will identify which test results best correlate with field cracking as well as consider their variability, utility and practicality of implementation for both mix design approval and quality control testing. A crucial component of the study is regular performance monitoring of the pavement test sections for ride quality, distresses, strength and response.”
Section N7 included a surface mix designed with 35 percent RAP treated with the bio-based rejuvenator Delta S from Collaborative Aggregates LLC, Wilmington, Massachusetts. See the article, “NCAT Forensics: Section N7 Contributes to Silo Storage Recs, RAP Content Suggestions” for a full story on the project, which was compared with two other surface mixes as part of the cracking group experiment. All three mixes used PG67-22, but Section N1 had 20 percent RAP and Section N8 had 20 percent RAP and 5 percent RAS.
Among the results, the report states: “Section N7 with Delta S had a ride quality that would be classified as good condition in the pavement performance measures recommended by the FHWA, and it was similar to those of the other two sections.”
More information is available in the full report and in the article previously published in AsphaltPro, but the bottom line is, “Overall, Delta S did not produce any negative effects on the ride quality and rutting performance of the mixture. The cracking performance of the mixture was acceptable at the end of the 2015 research cycle. Delta S could be considered an alternative in the design and production of asphalt mixtures with high RAP contents. Section N7, as well as Sections N1 and N8, will be kept in place for continuing traffic for another research cycle to allow for a thorough field performance evaluation.”
Rejuvenators Going Forward
NCAT’s website states: “Cargill is sponsoring parallel testing at the NCAT Test Track and the Minnesota Department of Transportation’s MnROAD test highway to determine how to best implement balanced mix design procedures in asphalt mixes containing rejuvenators and high levels of reclaimed asphalt pavement. As part of the project, test sections made with Cargill’s Anova rejuvenator and a 45 percent reclaimed asphalt mix will be compared to control sections with lower recycled contents, measuring factors such as pavement ride quality, cracking and rutting. By conducting experiments at both the Test Track and MnROAD, researchers will be able to monitor results in both northern and southern climate extremes.
“As part of the current research cycle, the Mississippi and Tennessee Departments of Transportation have both sponsored rejuvenating seal experiments,” the website reports. “NCAT conducted a preliminary screening study to evaluate seven rejuvenating seals to determine which products would be used on the Test Track sections. The products were ranked based on their rheological properties and friction test results. The Mississippi DOT selected ReGenXTM (Blacklidge Emulsions) and Delta MistTM (Collaborative Aggregates LLC) to be evaluated on its S3 Test Track section. The Tennessee DOT selected Reclamite® (Ergon) and an experimental product, e-Fog S (Ergon), to be evaluated on its S4 Test Track section. The field performance of these four rejuvenating fog seals will be evaluated for the entire two-year Test Track cycle.
“While not new construction, Section N7 with Delta S®, sponsored by Collaborative Aggregates LLC, continues to be monitored by NCAT following completion of the 2015-2017 research cycle.”
Balanced Mix Design Going Forward
NCAT’s website states: “Four test sections on the Test Track focus on the balanced mix design of asphalt mixtures. These sections are sponsored by the Oklahoma and Texas Departments of Transportation. Oklahoma’s balanced mix design approach requires the use of the Hamburg wheel tracking test to evaluate rutting resistance and the Illinois flexibility index to evaluate cracking resistance. The overall objective of Oklahoma’s experiments is to implement performance testing and criteria for balanced mix design. Texas recently developed a special specification for balanced mix design that requires using the Hamburg wheel tracking test to evaluate rutting and moisture damage resistance and the overlay test for assessment of cracking resistance. Their objective is to compare the field performance of asphalt mixes designed using the proposed balanced mix design approach versus the Superpave volumetric approach under accelerated loading conditions.”
Pavement Preservation Going Forward
NCAT’s website states: “Many of the same pavement preservation treatments installed in Alabama are being investigated in Minnesota using MnDOT materials and construction methods. The northern pavement preservation experiment sections are located on U.S. Route 169, a high-volume road, and County State Aid Highway (CSAH) 8 in Mille Lacs County, a low-volume road. This study includes treatment selection, test site selection and layout, scheduling, construction, performance monitoring, and data activities to quantify the life-extending benefits of pavement preservation treatments.”
Sponsors of the 2015-2017 Track
Alabama Department of Environmental Management
Alabama Department of Transportation
Colorado Department of Transportation
Federal Highway Administration
Florida Department of Transportation
FP2 for Pavement Preservation
Georgia Department of Transportation
Illinois Department of Transportation
Kentucky Transportation Cabinet
Michigan Department of Transportation
Minnesota Department of Transportation
Mississippi Department of Transportation
Missouri Department of Transportation
New York Department of Transportation
North Carolina Department of Transportation
Oklahoma Department of Transportation
South Carolina Department of Transportation
Tennessee Department of Transportation
Virginia Department of Transportation
Wisconsin Department of Transportation