The I-FIT Accurately Predicts Pavement Cracking

Committed to designing and building flexible pavements that are durable, safe, and smooth, the Illinois Department of Transportation (IDOT) has been using recycled asphalt materials in asphalt concrete (AC) mixes for the past three decades. Recycling these materials, in such a way that their inclusion does not diminish long term performance, can help enhance sustainability and protect the environment by limiting the consumption of natural resources. 

Asphalt binder from reclaimed asphalt pavement (RAP) and reclaimed asphalt shingles (RAS) sources have been used to replace virgin asphalt binder. However, recycled asphalt binder, especially from RAS, is highly aged or oxidized, making rehabilitated and newly constructed AC pavements vulnerable to premature cracking.  Thanks to the Illinois Center for Transportation’s (ICT) Illinois Flexibility Index Test (I-FIT), developed in collaboration with IDOT, materials vulnerable to premature cracking can be identified before paving.

In an effort to maximize the performance of recycled material in AC mixes, IDOT, in collaboration with ICT, initiated a research project entitled “Construction and Performance Monitoring of Various Asphalt Mixes” (R27-161). The goal of the project was to identify the most cost-effective combinations of ingredient materials to improve pavement performance for mixes using various types and amounts of recycled materials with various asphalt binder grades.

US Route 52–Laraway Road to Gougar Road in Will County, IL is one of the five test sections investigated in 2015. Work on this section included surface HMA removal/replacement, curb/gutter removal and replacement, and addition of pavement markings.

As part of this study, a series of five experimental pavement test sections were constructed in 2014 and 2015. The study also included the construction of three total-recycle asphalt (TRA) pavement sections along with a comparison section constructed in 2013 to demonstrate the performance of pavements that are rehabilitated with mixtures using recycled material levels up to 97%.

The research project spanned over several years and has been chronicled in three reports. The first interim report, published in early 2016, documented the construction, material tests, and early-life performance of mixes used in the experimental projects constructed in late 2014. The report also included information on the projects with significant asphalt binder replacement (ABR) values that IDOT constructed in 2013.

The second interim report, published in early 2017, analyzed the construction and early performance of the experimental projects built in 2015. It also included updates on the performance of the projects constructed in 2014, as well as the other IDOT-monitored projects constructed in 2013.

The final report, published in late 2017, provided updated field performance results for all of the projects. The report also includes the results from testing core samples of each of the mixes evaluated over the life of the study.  Finally, this report compared laboratory test results with field performance results to better understand the impact of these asphalt mixes and their corresponding underlying conditions on performance.

Imad Al-Qadi, Bliss Professor of Engineering at Illinois and director of ICT, served as principal investigator of the study, and considers this project a very unique one. “The project explored a variety of parameters for numerous field test sections including 11 various mix designs of the pavement asphalt concrete overlay. The mix designs included different materials and content of recycled materials, with three designs consisting of 97% recycled materials,” Al-Qadi said.

“The sections were monitored before construction, during construction, and periodically for three years afterwards,” Al-Qadi added. “In addition to lab validation and accelerated field testing validation, Illinois Flexibility Index Test (I-FIT) was validated with field data over time.”

The project was conducted under the direction of a Technical Review Panel (TRP) chaired by Jim Trepanier, Engineer of HMA, Aggregate and Chemical Tests at IDOT’s Central Bureau of Materials. Trepanier feels that with the help of the newly developed I‑FIT cracking test, the department was able to assess various qualities and quantities of recycled HMA blends used on IDOT construction projects.

“This research provided a rare opportunity to evaluate various combinations of recycle types and amounts with various asphalt binder grades for HMA mixtures used on actual IDOT construction projects using the newly developed I-FIT cracking test,” Trepanier said.

According to Trepanier, this project also helped IDOT in several ways. It helped validate the Department’s current Flexibility Index (FI) threshold for as-produced HMA. It also allowed IDOT to determine what grades of asphalt binder are best suited to counteract hard, aged residual asphalt binder in recycled products. Finally, it provided the data necessary for ongoing IDOT and ICT research aimed at developing a protocol for long term aging and associated FI threshold that will be used to evaluate proposed asphalt additives and modifiers.

In light of the findings of this study, the research team first recommended that the I-FIT be adopted for use as a specification requirement in AC mixture design and/or production. They also recommended the adoption of IDOT’s proposed minimum FI criteria. Lastly, the research team contends that using virgin aggregates with recycled aggregates would result in better production control of the AC mix and less absorption, which could reduce the cost of mixes with high levels of recycled material.

“We were very pleased that the increasingly widespread I-FIT, developed at ICT, was able to accurately predict the development of cracks in the field and distinguish the ability of paving materials to perform,” Al-Qadi said. To see details regarding I-FIT research, click here.

More details regarding the results and recommendations of this research project are available here.