BROOKINGS – When it comes to asphalt, how well the layers stick together affects the durability of the road, according to South Dakota State University assistant civil engineering professor Rouzbeh Ghabchi. That’s why tack coat – the substance that bonds those layers together – is important.
Ghabchi will help the South Dakota Department of Transportation develop guidelines on tack coat selection and application through a two-year, $75,000 grant from the Mountain Plains Consortium plus matching funds from the SDSU Department of Civil and Environmental Engineering. Ghabchi came to SDSU last fall from the University of Oklahoma, where he did postdoctoral research for two years after receiving his doctorate. His research on transportation infrastructure materials focused on asphalt pavement.
“You want all the layers to behave together as a system,” he said. That will make the highway last longer and save taxpayers money. Ghabchi is working with SDDOT engineers to select the tack coat products that he and a graduate student will examine.
More than 80 percent of the state’s highways are made of asphalt, according to the 2016-17 SDDOT Fact Book.
“Many different tack coat products are on the market and each product has different properties,” explained Ghabchi. In addition, their behavior can be different depending on the type of asphalt surface – whether it’s applied to new or existing concrete or asphalt. Traffic and adverse environmental conditions must also be considered.
“We want to select the right tack coat and apply it in the right amount,” noted Ghabchi, who compared bonding asphalt layers to repairing a broken vase. “If you use too little glue, it will come apart; if you use too much, then the pieces can slip.”
Most tack coats are emulsified asphalt products that are sprayed on a prepared road surface, according to Ghabchi. When the coat changes color, the hot mix asphalt can be applied.
“If you have 8 inches of asphalt, for example, it may be applied as two 4-inch layers,” he said. Each layer of asphalt must be properly compacted. “If it has too many voids, it will not have the durability that you need.”
In addition to sealing the layers together, the tack coat also waterproofs the joints, explained Ghabchi. That’s important because when water penetrates the layers and freezes, it causes cracking and potholes.
The guidelines the SDSU researchers develop for tack coating on highways in South Dakota will also be applicable to asphalt roads in areas with similar materials and environmental conditions, such as North Dakota, Minnesota and Nebraska.