FHWA Presents Workshops Based on ICT/IDOT Tack Coat Research

A project completed by ICT in 2013 on behalf of the Illinois Department of Transportation (IDOT) is still making an impact in the pavement world. The project, “Best Practices for Implementation of Tack Coat” (R27-100), was published as two reports: Part 1 (laboratory study) and Part 2 (field study). A previous ICT/IDOT project, “Tack Coat Optimization for HMA Overlays” (R55), was a predecessor to the best practices study.

The two reports in the R27-100 project outlined a number of best practices aimed at optimizing tack coat application rates by factoring in performance and cost effectiveness in lab and field experiments. The findings from the project led to several revisions in IDOT’s tack coat specifications.

According to Imad Al-Qadi, principal investigator for the project and director of ICT, “Tack coat between layers is critical in achieving the desired performance in pavements. A low tack coat application rate can cause deterioration in pavement, while a high rate can cause slippage between layers. Hence, an optimal tack coat application rate is desired and important. ICT has worked closely with IDOT in developing an optimal application rate based on a comprehensive study that involved theoretical investigation and advanced modeling, development of a testing device, extensive lab testing, full-scale accelerated load testing simulating field traffic and, finally, in situ field validation. This process allowed us to identify an accurate tack coat application rate that will significantly contribute to increasing pavement service life.”

IDOT Technical Review Panel (TRP) Chair Derek Parish says of the project: “IDOT began implementing a special provision in District 4 based on the findings of the first project, in 2007. Since then we have been continually evaluating the specification in the field and have been making adjustments to the specification. These changes are based on the second-phase project recommendations, our experience with implementing the specification in the field, and working with industry to ensure the constructability and minimizing initial cost to IDOT.” Parish is the District Materials Engineer for IDOT District 4 in Peoria.

He adds that this research has led to other innovations as well. “There is a substantial learning curve for both IDOT and industry personnel  in implementing  the specification efficiently.  Some tack coat producers have made adjustments to greatly reduce cure time.  Additional best practices, such as pre-wetting to reduce cure time, have also been identified.  Because we have been using the specification with the emphasis on a higher application rate, concentrating on the applied residual AC rate, and eliminating poor performing products, we have eliminated slippage failures and greatly reduced rutting at intersections at a minimal increase in additional cost.”

Very early on, the tack coat project was recognized as one that could have a big impact. It was selected as an AASHTO Sweet 16 High Value Research project in 2013—only one of four projects selected in the Midwest and one of 16 across the entire United States to be singled out for its importance and impact. Since then, the project, along with similar studies in other states, has led to nationwide changes in tack coat application and prompted the Federal Highway Administration (FHWA) to organize several tack coat workshops.

FHWA, in conjunction with the Asphalt Institute, will present a workshop on tack coat best practices for Illinois. The workshop is scheduled for December 14, 2015, from 12 noon to 4:00 p.m. at the iHotel and Conference Center in Champaign. This date was selected to coincide with the 56th Annual Illinois Bituminous Pavement Conference that evening and the following day at the iHotel and Conference Center.

According to Jim Trepanier, HMA Operations Engineer for IDOT, “IDOT has implemented a new tack coat specification based largely on the findings of ICT research projects on tack coats. While currently a statewide special provision, the new tack coat specification will be part of IDOT’s new 2016 Standard Specifications for Road and Bridge Construction. There were implementation challenges with the new, more stringent specifications, which were largely handled through education at various meetings and training classes.” Trepanier served as TRP chair for the Phase I study (Project R55).


Two versions of the interface testing fixture developed for the ICT/IDOT tack coat study.


An example of slippage cracking from a non-optimal tack coat.