The use of increased amounts of asphalt binder replacement, such as reclaimed asphalt pavement and recycled asphalt shingles (RAP and RAS) in asphalt concrete (AC) mixtures reduces demand for virgin aggregates and asphalt binder, which potentially brings both environmental and economic benefits. But with those benefits come concerns and considerations about long-term pavement performance.
To address those performance concerns, the Illinois Center for Transportation (ICT) undertook a project on behalf of the Illinois Department of Transportation (IDOT) to develop a test method to identify AC mixes prone to overall cracking, including thermal, reflective, and fatigue, a mix characteristic that is associated with aged and hard asphalt. The ultimate goal was to develop a test by which IDOT will obtain mixes known to perform well from a cracking standpoint, and the contracting community will have the freedom to develop an economical mix based on available materials.
ICT efforts led to an important milestone with the development of a practical and reliable test to determine cracking resistance of AC mixes. The method centers around the use of the semi-circular beam (SCB) fracture test. Although the SCB device is not new, the project resulted in a complete transformation of previous SCB protocols to arrive at a modified test performed at intermediate temperature.
According to Matt Mueller, recently retired Engineer of Tests with IDOT’s Bureau of Materials and Physical Research, who served as the project’s Technical Review Panel chair, “The results of the ICT research include the complete transformation of previous SCB protocols to arrive at an intermediate temperature test, development of a flexibility index that takes into consideration the post-peak response, in addition to the fracture energy, in order to better define mix toughness.”
Imad Al-Qadi, Founder Professor of Engineering and UIUC and ICT/ATREL director, who served as the principal investigator, said that this is a new approach that will allow agencies to control pavement crack development before they place pavements. The approach is theoretically sound and has been validated with field results in Illinois, as well in other states, in spite of the short period since it has been developed.
These research findings, adds Mueller, “will allow IDOT to develop a single cracking performance specification, named the Illinois Flexibility Index Test (I-FIT) to identify it as a test validated to Illinois weather and loading conditions.”
IDOT is planning a dozen experimental feature projects for their early 2016 lettings that will allow contractors to use higher levels of RAP and RAS than standard projects use, as long as the plant-produced mix meets a minimum flexibility index.
A recent article in the ICT newsletter, Testing Protocols to Ensure Performance of High Asphalt Binder Replacement Mixes Using RAP and RAS (R27-128), discussed the research for this project in further detail. The project’s final report is available here.