Study Maximizes Tack Coat Effectiveness

Tack coat is a light layer of diluted asphalt, which is applied to hot mix asphalt (HMA) surfaces to ensure strong interface bonding between layers. Project R27-100 analyzed interface bonding between two HMA layers while certain variables in the tack, coat, pavement, and environment were manipulated. The results of the study are assisting IDOT in determining the more effective guidelines for tack coat application.

Principal investigators Imad Al-Qadi (University of Illinois at Urbana-Champaign), Enad Mahmoud (Bradley University) and William Pine (Heritage Research Group) examined several factors, including tack coat (type, application rate, curing time, application temperature, and asphalt residue content); pavement surface characteristics (asphalt content, aggregate type and gradation and surface texture); as well as environmental characteristics to determine under which conditions tack coat application was the most effective. Overall, the research team determined milling the surface of the pavement improved interface shear strength. Interface shear resistance was greater when the surface nominal aggregate size was increased from 4.75 mm to 9.5 mm. Increasing the temperature resulted in a reduction in shear strength.

Derek Parish, acting local roads engineer for IDOT’s office in District 4 served as the technical review panel chair for this project. He stated that this study has several benefits for IDOT. He stated that knowing the most effective application rate of tack coats will reduce the amount of time it takes to complete jobs and the amount of time lanes need to be closed. In addition, he notes proper application can greatly increase the lifespan of the pavement overlay and minimize costs for additional repairs.

Best Practices for Implementation of Tack Coat Part 1: Laboratory Study

Best Practices for Implementation of Tack Coat Part 2: Field Study

-February 2013