To address concerns about the long-term performance of asphalt concrete mixtures that use higher amounts of asphalt binder replacement, such as reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS), the Illinois Center for Transportation (ICT) and the Illinois Department of Transportation had teamed up, conducting a research study—Testing Protocols to Ensure Performance of High Asphalt Binder Replacement Mixes Using RAP and RAS—that resulted in the development of the Illinois Flexibility Index Test, or I-FIT. The test ensures that asphalt concrete mixtures have the flexibility necessary to resist premature cracking.
Following this study, a number of machines were manufactured to perform the I-FIT using various load frames. This led the two research partners to develop an experimental program aimed to compare various I-FIT configurations under a special project, Evaluation of I-FIT Results and Machine Variability using MnRoad Test Track Mixtures, which was completed in Summer 2017.
“Although the I-FIT protocol was proven to successfully discriminate between AC mixes, the machine-dependent effects on the results were not thoroughly analyzed before,” says Hasan Ozer, research assistant professor in the Civil and Environmental Engineering Department of the University of Illinois at Urbana-Champaign, and Principal Investigator on the project. “We developed a method to evaluate variability among different loading frames and quantify machine-dependent effects.”
The researchers compared two custom designs with varying loading and fixture configurations using eight asphalt concrete mixtures with different design characteristics. The difference in fracture energy, slope, and overall patterns of load-displacement curves was analyzed for each mixture using each of the custom designs.
The results obtained showed that the variation in device configurations does not have a significant effect on the I-FIT results as the researchers observed that all results obtained were within one unit of flexibility index for each of the tested machines and mixtures.
According to Thomas G. Zehr who led the Technical Review Panel overseeing the research study on behalf of IDOT’s Central Bureau of Materials, the findings of this project further emphasize the credibility of the I-FIT test by demonstrating that the procedure is repeatable with varying machine types and that different machines give statistically similar test results.
“While IDOT has purchased and committed to the servo hydraulic machine, this study shows that contractors, industry partners, and other state DOTs can choose to use other types of machines from different manufacturers and get similar results.” adds Zehr.