Keeping It Together: An In-Depth Examination of Various Tack Coats

Similar to frosting in a layered cake, which holds the multiple layers together, interface bonding in road pavement structures ensures that the asphalt layers remain in place for longer periods of time. 

The current interface bonding procedure includes the use of tack coats. According to the Asphalt Institute, a tack coat is a light application of bituminous materials to an existing surface using a distributor to provide sufficient bonding between pavement layers. The most common tack coats on the market are hot asphalt products and emulsified asphalts. Hot asphalt products are a blend of petroleum-based asphalt binder and sometimes a modifier. Emulsified asphalts (emulsions) are products made of asphalt cement, an emulsifying agent, and water.

In a previous research collaboration between the Illinois Department of Transportation (IDOT) and Illinois Center for Transportation (ICT), an interface shear test device (ISTD) was developed to characterize the bonding strength of pavement interlayer in the laboratory under Project R27-100, Best Practices for Implementation of Tack Coat: Part 1 – Laboratory Study.

However, the ISTD was developed for a hydraulic testing frame—something that is customized and not easily accessible. Therefore, there was a need for a simplified design of the shear fixture for local agencies and contractors—many who do not have servo-hydraulic loading devices—to perform routine performance tests on tack coats.

A close-up of a tack coat application

ICT and IDOT teamed up again in Project R27-SP34, “Evaluation of Various Tack Coat Materials Using Interface Shear Device and Recommendations on a Simplified Device,” to evaluate the properties of various tack coat materials, and provide recommendations for a simplified shear testing device that can be integrated into the testing frames at IDOT.

The project was conducted under the direction of a Technical Review Panel (TRP) chaired by James (Jim) Trepanier, HMA Operations Engineer at IDOT’s Central Bureau of Materials. According to Jim, as night paving is becoming more common the need for tack coats with reduced cure time has become a high priority. In addition, there are a number of tack coat options that have little or no cure time that are currently not listed as acceptable tack coats in the Department’s specifications.

“This project was initiated in order to evaluate the effectiveness of these products for possible inclusion in IDOT’s specifications,” Trepanier said. “The addition of these new tack coat products will increase productivity and shorten the time needed for construction, thus minimizing safety concerns and maximizing cost savings to the contractor, IDOT, and ultimately the tax payer.”

Dr. Hasan Ozer served as principle investigator (PI) on this project. Ozer is a research assistant professor in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign (UIUC).

“There are various types of tack coat materials in the market place used in original pavement construction and rehabilitation projects,” Ozer said. “However, it is important to select the right material and use the optimum application rate to maximize bonding between different layers.”

Also assisting with this project was graduate research assistant Jose Rivera-Perez, who is pursuing his master’s degree in civil engineering at UIUC.

In this latest study, the research team evaluated the interface shear strength of seven tack coat materials using the ISTD by testing the composite specimens consisting of concrete and hot-mix asphalt mixture (HMA). Four of the tack coats were hot-applied products and three were emulsion.

“The ISTD has been very useful in making the right choices when it comes to selection and application of tack coat materials,” Ozer said.

Overall, the research showed that, at the same application rate, hot-applied products have significantly higher shear strength than the emulsion products.

The research team also conducted a separate round of preliminary tests on a simplified shear testing device and found that it is important to control normal pressure build-up within the device. Therefore, recommendations were made to modify the simplified shear testing device to conform to the ISTD that has been used in this study and previous studies.

The final report detailing the results of the research project is available here.