For the past few years, the Utah Department of Transportation (UDOT) was regularly forced to patch and repair a 9-mile section of I-80 - going east and west - that begins at the top of the Wasatch Mountain summit and extends to Salt Lake City, UT.
Previously, the state used an open graded surface course (OGSC) that was made up of a thin layer of hot-mix asphalt, comprised of 1/2-in. aggregate, and provided a smooth and attractive ride for motorists. An OGSC surface treatment is designed to have very small voids so precipitation seeps through it and drains off the side of the road. This process often works well but not in this part of Utah. Extremely cold temperatures prevented the treatment from bonding correctly to the existing asphalt overlay. Each winter, precipitation froze before getting the chance to drain and then created major potholes upon thawing. So, the UDOT looked at other surface options.
"The OGSC lasted only two years, and it was failing," said Deryl Mayhew, UDOT resident engineer and overseer of the project. "We were patching potholes constantly because the surface was cracking severely. We have many freeze and thaw cycles here because it might be in the twenties in the morning and then up into the fifties by the afternoon. The OGSC really takes a beating during these cycles, so we decided to see if micro surfacing could do better because of its history of being a good product."
Utah-based Intermountain Slurry Seal won the Parley's Canyon I-80 micro surfacing bid. It consisted of six total lanes - three going east and three going west - over a stretch of nine miles. Intermountain Slurry Seal milled the previous surface and then used a full-size continuous micro surfacing paver to apply two layers, or more than 1.2 million sq. yds., of Type III micro surfacing treatment.
"Last winter we only observed about seven total potholes going both east and west," said Mayhew. "And honestly, those had more to do with the underlying asphalt surface that failed than the micro surfacing. With the OGSC we had more than 700 potholes per year for the last two years. The micro surfacing treatment has held up well and doesn't have the cracking that we saw with the previous treatment."
Intermountain Slurry Seal took on many challenges when they executed this high-profile job:
Challenge 1: The Existing Surface. Micro surfacing is ideal for protecting roads in the early stage of deterioration and serves as a surface treatment that extends the life of the existing asphalt. It doesn't provide structural stability to the road. With the previous surface in such bad shape, Intermountain Slurry Seal performed a 1 1/2-in. mill to remove the unstable OGSC layers before applying the micro surface. A fine head mandrel with milling teeth, only a 1/4-in. apart was used to create a smooth milling surface that appeased ongoing traffic and provided a better bond between the existing asphalt and the new micro surfacing treatment.
Challenge 2: UDOT Restrictions. With an average traffic volume of 51,000 vehicles going in each direction per day, the UDOT and Intermountain Slurry Seal set up general guidelines to keep traffic moving as efficiently as possible. During rush hours, the contractor was allowed only a single-lane closure, while off hours allowed for a double-lane closure to do the necessary milling and surfacing. Failure to comply with any of these lane closure limitations would result in a penalty of $1,500 for every 15-minute violation.
"We did all of the milling work at night due to traffic restrictions," said Rusty Price, general manager for the Utah-based Intermountain Slurry Seal branch. "We cut the road the full width every night so it would be ready for motorists the next day. That meant backing up the mills and doing smaller stretches than normal. On an average night, we would cut between 4,500 to 5,000 ft."