“Let’s talk for a moment about the importance of tack coat,” said Wayne Jones, senior regional engineer with the Asphalt Institute, during his presentation “Introduction to Thickness Design” at the 2024 National Pavement Expo in Tampa. “If you think of a pavement in terms of structure, those layers form a single beam. In a building, when a load is applied, the steel beams flex. The same thing happens with asphalt.”
Read more of Jones’ insight in this article, How Thick Should Your Pavement Be?.
In beam theory, Jones said, one factor that affects the strength of a beam is its height. “If we have five 1-inch beams, the height is cubed in that equation,” he said. “So, if I have five 1-inch layers of asphalt (1x1x1), that strength is 1 per layer times five layers, so 5 total. But if I glue those five 1-inch layers together (5x5x5), the strength is 125.”
That’s the role tack coats play in improving the strength of the overall pavement. “If those beams aren’t glued together with tack, they’re not nearly as strong,” Jones said. He shared an example of a colleague in Montana who tested the concept on multiple layers of 1-inch-thick plywood. “Unbonded, with three layers, there was ½-inch deflection with just a 60-pound load. But when they’re glued together, he put 160 pounds on there with only ¼-inch deflection. That’s why that tack coat is so important.”
