As the winter months wind down, Texas Department of Transportation (TxDOT) district offices throughout the state will complete planning for the hectic summer seal-coat season. Best applied when the weather is warm, seal coats are by far the most popular preventative maintenance treatment used in Texas.
A recently completed research project, sponsored by TxDOT and conducted by the Texas Transportation Institute (TTI), examined methods for improving the safety and durability of seal-coated roadways. The project developed guidelines for how and when to vary the amount of asphalt being applied across the width of a pavement.
“A roadway does not wear evenly across the surface because traffic is usually channelized,” says Paul Krugler, research engineer with TTI. “The difference in surface texture may result in new seal-coat aggregate not bonding adequately outside of the roadway wheel paths, or the asphalt may rise to the surface in the wheel paths. Either situation diminishes roadway quality, and the loss of skid resistance in the wheel paths can be a safety issue. Asphalt rising to the surface in the wheel paths is commonly known as flushing, and it’s what we’re trying to prevent by varying the amount of asphalt sprayed on the roadway.”
Transversely varying asphalt rates (TVAR) works by designing the asphalt shot rate specifically for the wheel path conditions. If the wheel paths are worn down or exhibiting flushing, the proper asphalt rate is somewhat lower. Then, the asphalt shot rate is increased for the areas outside of the wheel paths. Additional asphalt is needed there to fill the greater surface texture and still have adequate asphalt available to ensure proper bonding of the aggregate. The goal of TVAR is an even coating of aggregate that is well-bonded across the entire width of the roadway but without causing flushing. This bonding optimizes skid resistance and makes the roadway safer.
Last summer, researchers interviewed TxDOT maintenance personnel throughout the state to gather information on how often and what criteria they used to set these transversely varied asphalt rates. The result of the project was a comprehensive guidebook with step-by-step instructions on the TVAR process. For the implementation phase of the project, the research team is hitting the road this spring, teaching five workshops on TVAR.
In addition, roadways that underwent TVAR seal coats last year will be evaluated in the future for effectiveness in eliminating reoccurring flushing. “We can adjust the numbers in our guidebook if necessary,” says Krugler.
“The whole concept of TVARs is to put more asphalt on the road,” says Paul Montgomery, who served as the project director and is the TxDOT director of maintenance for the Lufkin District. “We want more asphalt on the road, but we don’t want to get flushing in the wheel paths when we do it. By using the set of guidelines developed by TTI, our construction inspectors and maintenance crews can achieve this goal and optimize our seal-coat applications, while also making the roadways safer for the traveling public.”