Ongoing study seeks to improve hot-mix asphalt performance

In an ongoing effort to maximize performance of hot-mix asphalt (HMA) containing higher amounts of recycled materials, IDOT and ICT started a project entitled Construction and Performance Monitoring of Various Asphalt Mixes (R27-161) in 2014. This effort is part of a larger goal that aims to improve pavements built using recycled materials to enhance their sustainability. A second interim report on the project was recently published. 

At the beginning of the study, five active IDOT rehabilitation projects were selected to be monitored; two projects were constructed in 2014 and three were constructed in 2015. All projects included composite pavements consisting of a Portland cement concrete pavement that had been overlaid with HMA; a new HMA overlay was placed in the five projects using eight different HMA types.

The HMA incorporated varying amounts of reclaimed asphalt pavement (RAP), reclaimed asphalt shingles (RAS), and various grades of asphalt binder as determined during initial development of the research project. The overall asphalt binder replacement (ABR) of the mixes varied from 15% to 48%. All or part of the ABR came from RAP. The amount of RAS used in the mixes varied from none to 5%. The primary goal of this project was to determine the impact of the amount and types of recycled materials used in HMA on pavement performance and durability.

Researchers are excited about this project. David Lippert, one the researchers on the project, states, “Very few projects have this level of data collection and performance tracking over time. This project started with recording the existing pavement condition prior to any work. Once work started, samples of each component material of the mix were collected, then the resulting HMA plant mix was sampled. The construction was then monitored, post-overlay pavement surveys were conducted, and cores were taken over time. This level of detail is rare and will be of great interest to the highway community.”

The first interim report, published in early 2016, documented the construction, material tests, and early-life performance of mixes used in the first two projects that were constructed in late 2014. The report also included information on three additional projects with significant ABR values that IDOT constructed in 2013 and was monitoring internally.

The second report was aimed at documenting the construction and early performance of the remaining three projects built in 2015 and to provide an update on the performance of the two projects constructed in 2014, as well as the other IDOT-monitored projects constructed in 2013.

This second interim report includes manual distress surveys, ride quality data (smoothness and rutting), details of various laboratory tests on the HMA surface and binder courses, and baseline conditions and early performance trends that can be used to measure future performance.

Performance of the new HMA overlay in test sections where the existing HMA surface was left in place showed substantially less cracking than the sections where the new overlay was placed on either bare concrete or where the existing HMA surface was milled to concrete prior to placement. Results of the remaining work to be done on the project are eagerly anticipated by IDOT and industry professionals. A final report is set to be published in late 2017 or early 2018.

Looking to the future beyond this project, Lippert states, “Due to the level of data detail, IDOT may want to revisit these projects after this research ends in 2017 and conduct additional crack surveys. This would capture performance around the critical 6- to 7-year time frame when most pavements will more firmly establish performance trends. From what is being learned on this project, there may be a desire to establish a similar experiment, but with minimum Illinois Flexibility Index (I-FIT) values rather than asphalt binder replacement (ABR) or type of binder replacement material (RAP or RAS).”

Imad Al-Qadi, Founder Professor at UIUC and Principal Investigator presented the preliminary outcome of this study at the 2017 Association of Asphalt Paving Technologists (AAPT) meeting in March 2107. He showed a good correlation between field crack development and the ICT-developed I-FIT results; hence, the ability of predicting pavement cracking in the future. He demonstrated that I-FIT is a quick and easy performance test used for assessing the susceptibility of mixes to rapid crack propagation. He suggested that high RAP/RAS may reduce rutting potential, but may increase cracking potential. Al-Qadi also emphasized the importance of using high-quality leveling binder and the Balance Mix concept in the HMA design.

Based on results from this project, IDOT hopes to take a big step forward  in quantifying sustainable materials in pavements and maximizing their benefit. Jim Trepanier of IDOT served as Technical Review Panel chair for the project. He called the project “a unique opportunity for IDOT to see and quantify the effects of including varying amounts and types of recycled products with various grades of asphalt binder on the performance of pavements on real life IDOT construction projects.  It is also a great opportunity to correlate the new Illinois Flexibility Index Test (I-FIT) to field performance for a wide range of asphalt mixtures in terms of flexibility.  Findings from this research should provide the necessary guidance to the department to improve specifications and have a better understanding of what asphalt binder grades are more effective with the various types and amounts of recycled RAP and RAS.”