ICT researchers work with IDOT to improve performance of various asphalt mixes using recycled materials

Like knights of old, pavement engineers have long sought their holy grail: a suite of tests that would ensure good hot-mix asphalt (HMA) pavement performance. Two recently published ICT reports present the progress made in that quest.

The first report, Testing Protocols to Ensure Performance of High Asphalt Binder Replacement Mixes Using RAP and RAS, was prepared as part of ICT/IDOT Project R27-128. It documents the development and establishment of the Illinois semi-circular beam (IL-SCB) test and the determination of a flexibility index (FI). The test and index together are known as the Illinois Flexibility Index Test (I-FIT).

That’s where ICT/IDOT Project R27-161, Construction and Performance Monitoring of Various Asphalt Mixes, comes into play. As part of that study, five IDOT pavement projects were constructed with eight different HMA mixes. The HMA mixes used varying amounts of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS), and various grades of asphalt binder. The asphalt binder replacement (ABR) 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%.

Pavement performance is a key sustainability issue when using recycled materials. Therefore, the basic goal of the research project is to determine whether there are performance tradeoffs related to the amount and types of recycled materials used in HMA. True sustainability gains are made when high amounts of low-cost recycled or reclaimed material can be used to replace more expensive virgin materials without any negative impact on pavement performance.

It is hoped that FI test results at construction can be related to pavement performance years later. Although a number of tests are being conducted on the mixtures, the key performance tests are Hamburg wheel tracking for measuring stability and the I-FIT for measuring flexibility/durability. Mix samples were taken during construction of each project section and are being followed up with annual cores for testing. Detailed crack surveys are conducted annually, as well as profile testing for smoothness and rutting.

The Illinois Department of Transportation is taking the first step toward implementation of the I-FIT by constructing 11 pilot projects this year using the department’s new I-FIT specification. The specification requires I-FIT testing and compliance for mix design verification as well as for plant-produced mixtures. On a national level, there is a new AASHTO provisional standard for the I-FIT, which for now is being referred to as IL-SCB.

A 2015 interim report was published to document the construction, material tests, and early life performance of the mixes under study. The report covers the construction of two projects. Also included in the report is information about the performance of IDOT’s first use of total recycle asphalt (TRA) mixtures, which were placed in 2013.

Another interim report for the R27-161 project will be published in Fall 2016 to document the construction of the projects built in 2015 and to update the performance on the remaining projects. The final report for the project is anticipated to be available in December 2017.