The asphalt industry is careful about adopting new materials or techniques. It has to be: people’s lives depend on the materials or techniques working as advertised. At the same time, no innovation means no improvements, which doesn’t work either. Improving pavement requires rigorously thinking through the risks of new ideas and finding the best field projects to test them. Enter the idea of installing a highly modified Superpave mixture in one thick lift. Howard Anderson, a state asphalt engineer for Utah with more than 30 years of experience, and Lonnie Merchant, a Region 2 UDOT materials engineer, worked together to implement a UDOT thick lift project of their own in Utah. Anderson was drawn to the idea because he viewed applying two layers as a waste of time. Instead, he wanted to find a good way to do one layer without sacrificing quality. For five years, Anderson and others researched projects done elsewhere and did lab tests in Utah to solve the problems they faced. Their goal was to reduce construction time, eliminate problems caused by multi-lift paving such as tack coat and delamination between layers, and put the asphalt down in a location that would make quality shortfalls obvious. However, the most challenging part of the project was producing high-quality asphalt with 1% air voids to achieve their goals. Constructing stable, durable HMA depends on the pavement’s density. Compaction determines density, but the thicker the lift, the harder it is to get the correct density. In general, the asphalt industry has gotten its best results when in-place air voids are limited to a 3%-8% range. That means successful pavement with 1% air voids is something out of the ordinary. According to Anderson, normal mix designs in the U.S. are usually 4%. The Utah average is 3.5%. What determines the specifications for most air voids? When they are below 3%, pavement tends to rut and form ripples. The ripples are also called shoving or wash-boarding. When air voids are above 8%, air and water can get into the pavement and cause cracking, oxidation, raveling, and water damage. Raveling occurs when traffic wears away aggregate particles from the asphalt cement. It indicates a poor quality mixture or asphalt hardening. Anderson was familiar with rutting risks when air voids are below 2%. However, he also knew that binding technology had changed. Polymer-modified binders would provide a stronger glue than was available in the past, which meant going below 4% air voids was no longer an unrealistic goal. Even with the smaller air void percentage, laying quality asphalt would still be possible. In 2016, Anderson worked with Clark Allen, a UDOT central lab technician, to run Hamburg rut tests that used 76-34 polymer modified binder instead of the standard UDOT choice, 64-34 polymer modified binder. It is two grades higher and has twice as much polymer as the standard. Allen tested binder contents of 4.8%, 5.8% and 6.8%. The results were tested with extra weight after the first test was successful. The material would not rut, and Anderson learned that when the binder content approaches 7%, the asphalt acts more like the polymer than regular asphalt. It doesn’t become sensitive to over-asphalting. Having higher binder content minus the rutting risk meant thicker asphalt lifts could be compacted than was possible previously. Two lifts separated by a tack coat could be replaced with one lift, and the increased binder would lubricate compaction. Anderson attended the WASHTO Conference in April 2016, held in Salt Lake City, and he presented his idea at the conference. After additional presentations, Staker Parson Materials & Construction decided to use the idea as part of a larger project. They had agreed to mill two inches off 20 miles on I-80 and replace it with two inches of stone matrix asphalt totalling 40,000 tons. A port of entry with a six-inch lift was part of the project spec. The project site is between Nevada and Utah, and it is located in the middle of the Bonneville Salt Flats. It’s hard to imagine a more difficult place for using asphalt. Most asphalt that has been laid at this location begins having problems days after being laid. Summer temperatures are often higher than 100 degrees Fahrenheit. Winter temperatures are often below 10 degrees Fahrenheit. Heavy truck traffic headed for California ports is a day-in, day-out reality. The annual daily traffic count is 7,600 vehicles per day. Of that total, 51% are trucks. SOLVING THICK-LIFT CHALLENGES: A SUCCESS STORY 10
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