Development of a Performance-Related Asphalt Mix Design Specification for the Illinois Tollway
By: William Buttlar, Behnam Jahangiri, Punyaslok Rath, Hamed Majidifard, Loreto Urra, James Meister, Henry Brown
Publisher: Illinois State Toll Highway AuthorityDownload PDF
In the past, traditional asphalt mixtures involved relatively simple combinations of virgin asphalt binder and aggregates to meet performance requirements. However, modern, heterogeneous asphalt mixtures exhibit more complex behavior due to a proliferation of new ingredients and because of the interactions that subsequently occur. As a result, recent asphalt mixes require advanced performance tests to account for the effects of the added components, increases in traffic loads, and the environmental conditions that prevail. In this project, fourteen different mixtures produced in 2018 on mainline and shoulders sections across Tollway system in Chicago were selected to characterize performance testing trends in current Tollway mixtures and to study the ability of the different performance tests to predict pavement performance. To this end, performance tests were performed on the collected plant produced mixtures, and later, on selected field-cored sections. The latter included both good and poor performing sections, which were determined based on an extensive survey of the Illinois Tollway in May of 2019. Evaluation of Tollway asphalt surface pavement management data indicated excellent overall performance vs. time, with minor amounts of several cracking forms developing gradually over time. These included transverse cracking, usually associated with reflective cracking on the mainline and/or thermal and block cracking on shoulders, and longitudinal cracking (typically along the construction joint between lanes). Rutting, on the other hand, was not observed to be a significant form of distress on modern Tollway sections. The Disk-shaped Compact Tension (DC(T)) test was chosen to be retained in the PRS for the design of crack-resistant mixtures due to its high degree of correlation with field results and its best repeatability. A systematic approach was developed, which allowed different reliability levels to be addressed in the specification, along with a consensus step to take advantage of local practitioner experience. Tailored Hamburg rut depth thresholds were established based on lift position relative to the pavement surface. Experimental results led to a new approach where the Hamburg stripping inflection point is used in lieu of the TSR test as the first step in moisture sensitivity verification. If a mix is determined to have stripping potential after analyzing Hamburg results, the TSR test can be then be employed as a final determination of stripping potential. For SMA mixtures with rut depths less than or equal to 4.0 mm, it is recommended that the SIP computation be waived and the mix categorized as non-stripping.