What should have been a typical hot mix asphalt resurfacing project, quickly turned to a challenging laydown that required an innovative solution to prevent tearing and asphalt pickup during compaction in order to achieve the project's required density and smoothness specifications.
Donn Eide, general manager of Manatts' Newton (IA) Division has worked on his fair share of challenging projects over the past 24 years he's been with the asphalt contractor, but the 9.3-mile I-80 HMA resurfacing project (starting one mile east of the Kellogg interchange and ending one mile east of the Grinnell interchange) proved to be a bit more perplexing during compaction of the final surface mat.
Earlier this past summer, Manatts worked on adding a leveling/strengthening course to areas of the four-lane interstate road the Iowa DOT had specified prior to the placement of 2 inch intermediate binder course. The 20-year-old full-depth asphalt road showed some cracking, but was still structurally sound and had not required any full-depth patches over the years. Rutting of three-eighths of an inch was evident, and the 14,000 tons of leveling/strengthening course was intended to correct inadequate structural conditions.
"We had some problems achieving the density requirement when placing the intermediate course over areas where the leveling/strengthening course had been put down (due to the surface texture conditions) and we had to pay $20,000 in penalties, but we were able to get the engineers to back off the density requirement," Eide says.
Manatts tried to convince DOT engineers that milling three-quarters to an inch off the surface would have eliminated the ruts and the density problems experienced, and saved approximately $400,000. The paving and rolling crews could not achieve consistent density across the travel lanes because of the 3/8-inch ruts.
"Milling would have given us a consistent surface and eliminated the need for a leveling/strengthening course, but that's not what the engineers wanted," Eide says. "Because of the axle volume of traffic the road carries and is projected to carry, they felt a need to add more asphalt in certain areas to help strengthen the road."
The $4.8-million project specified a mix design and compaction density to support an annual 100M Equivalent Standard Axel Load volume of traffic. Manatts had 70 working days to complete the project, but had to stay off the road from June 18 to September 7, as well as Fridays to accommodate higher levels of vacation travelers.
On the intermediate mat, Manatts placed 40,000 tons of three-quarter inch aggregate mix, followed by 40,000 tons of surface mix — both were 2-inch mats. An additional 20,000 tons of asphalt mix was used to construct shoulders. The leveling/strengthening mix used a PG 64-22 AC binder, while the intermediate and surface mixtures used a PG 70-22 AC binder.
Manatts used a Caterpillar AP-1000B paver equipped with an 18-foot-wide screed, and its hopper was fed by a Cedarapids MS2 windrow machine. Compaction was achieved with a Caterpillar CB-634C and a Sakai SW900 breakdown vibratory roller following directly behind the paver. After initial compaction, a Hamm pneumatic roller and a Hamm oscillatory roller were used to achieve final compaction density of 95 percent. Mix for the project was produced at a nearby Cedarapids/Standard Havens 340 tons-per-hour counter flow portable plant. With mix leaving the plant at 320 degrees F, initial vibratory compaction was achieved before the temperature dropped to 280 degrees and final finishing compaction was achieved with the oscillatory roller before the mix dropped to 200 degrees.
The paving crew placed two 12-foot-wide travel lanes in each direction of the I-80 project, along with a 6-foot-wide shoulder along the median, a 10-foot-wide shoulder along the right lane, as well as ramp work and a couple of rest areas.
The density challenge
One of the challenges Manatts experienced during placement of the surface course was tearing and asphalt pickup while trying to achieve maximum density during compaction. For whatever reason, the breakdown roller was causing the mat to tear whenever it changed directions during its rolling pattern and the pneumatic rollers were picking up asphalt whenever the operators would travel over the tears made by the breakdown roller.
"We knew we had to achieve density, but the breakdown roller was causing some tearing problems in the mat and the rubber tires (pneumatic roller) seemed to pick up asphalt when rolling over those areas," Eide explains. "The mix seemed stable coming out of the screed and after the breakdown roller passed over it, but then it became tender. We tried making some temperature adjustments at the plant, but that didn't seem to help."
Eventually, the asphalt contractor's crews were able to solve the problem by using a small Bomag BW120AD 48-inch roller in between the breakdown and the pneumatic rollers. The roller helped seal the mat enough to prevent the rubber tires from grabbing the asphalt during its rolling pattern.
The solution seemed to be the right answer in solving Manatts' laydown challenges, while maintaining density and smoothness specifications. Manatts expects the project to generate additional smoothness bonuses, but also anticipates density problems experienced during placement of intermediate and surface courses, as well as a late completion penalty, to offset some of those incentives.
"We're limited by the times we can work on the project (only four days a week), and it also took a couple of days to figure out how to correct the problems we were experiencing during the final laydown of the surface course," he says.
Manatts was able to achieve all required specifications of the resurfacing project; it just took a little more innovative thinking to come up with a solution to the pesky challenges that arose during final placement of the surface course. And Eide will always wonder if those problems could have been prevented if DOT engineers would have approved his value engineering recommendation to mill the existing road prior to placement of the new HMA surface.