I-FIT accurately predicts in-service pavement cracking

Committed to designing and building sustainable and flexible pavements that maintain their durability, safety, and smoothness, 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.”  The goal of the project was to identify the most cost-effective combinations of ingredient materials to improve pavement performance using various types and amounts of recycled materials with various asphalt binder grades. Asphalt binder is graded based on its susceptibility to winter and in-service cracking and summer shoring.

As part of this study, 12 pavement test sections were constructed. Eight of the sections were constructed in 2014 and 2015 and the other four were constructed in 2013. The earlier sections consisted of total-recycle asphalt (TRA) and were built to demonstrate the performance of rehabilitated  pavements containing up to 97% recycled materials.

The research project spanned over several years and has been chronicled in three reports, available on ICT’s website:

Construction and Performance Monitoring of Various Asphalt Mixes in Illinois: 2015 Interim Report

Construction and Performance Monitoring of Various Asphalt Mixes in Illinois: 2016 Interim Report

Utilizing Lab Tests to Predict Asphalt Concrete Overlay Performance

Imad Al-Qadi, Bliss Professor of Engineering at Illinois and director of ICT, who served as principal investigator of the study considers this project a very unique one. “The project explored a variety of parameters for numerous field test sections including 12 mix designs of the pavement asphalt concrete overlay. The mix designs included different materials and contents 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 testing and regular field condition assessment, the I-FIT was validated with field data over time.”

Jim Trepanier, Engineer of HMA, Aggregate and Chemical Tests at IDOT’s Central Bureau of Materials, served as Technical Review Panel (TRP) chair for this project.  Trepanier feels that with the help of the newly developed I‑FIT, the Department was able to assess various qualities and quantities of recycled  pavement materials used on IDOT construction projects.

According to Trepanier, this project also helped IDOT in several ways. It helped validate the Department’s methods for determining AC mixes. It also allowed IDOT to determine the best approach when using recycled products. Finally, it provided the data necessary for ongoing IDOT and ICT research aimed at developing a protocol for simulating long-term field aging in pavements. Ultimately, this procedure 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 design and/or production. The researchers reported that IDOT’s proposed Flexibility Index value is an appropriate threshold for controlling field cracking during the early life of AC pavements.  Most importantly, I-FIT results well correlated with field performance, and I-FIT accurately predicted AC crack susceptibility. It must be noted that additional data are expected to be collected from the test sections and will be used to inform I-FIT’s ability to estimate cracking performance beyond the early years of pavement life.

“We were very pleased  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.

Click here for more information about I-FIT research.