The state of Iowa has long been a leader in concrete pavement technology. It’s not surprising that Iowans have embraced the use of concrete overlays–both whitetopping and concrete-over-concrete–to rehabilitate aging roadways. The history of concrete overlays in the area goes back more than 20 years, with more than double the number of whitetopping projects let in Iowa than the next closest state.
What it comes down to, according to Brian Ridenour, Allamakee County engineer, is that Iowa is making an effort to "get things done right."
Three Iowa counties–Allamakee, Buchanan and Dickinson–have completed overlay projects and have more lined up for the future. Allamakee County boasts 46 miles of whitetopping, including one of the first whitetopping projects in the state completed in 1982, and 12 miles of unbonded overlays. Buchanan County is planning its third concrete overlay project this summer.
According to Brian Keierleber, Buchanan County engineer, 80% of the county’s roads are concrete. Although he said there’s a time and a place for hot-mix overlays and other rehab solutions, the concrete roads in the county have yet to receive an asphalt overlay.
Dickinson County has been using concrete overlays since 1985. It has 32 miles of full-depth concrete pavement, 21 miles of whitetopping and 1.6 miles of unbonded overlays out of a total of 165 paved miles. It recently let an 8.5-mile whitetopping project.
Making the most of overlays
All three counties have developed methods and specifications for concrete overlays based on their own experience. In Dickinson County, whitetopping projects feature a variable thickness–5.5 to 7 in.
"The existing crown dictates the thickness of the overlay," said Dan Eckert, Dickinson County engineer. The county requires no milling or prepping and paves over the ruts in the existing asphalt.
"Some people are afraid the old asphalt is too deteriorated or oxidized to save," Eckert said. "You simply need to evaluate it—it’s not a big deal to research structural integrity."
Buchanan County has a whitetopping project lined up for this summer. According to Keierleber, the existing asphalt road is in very poor condition. The county plans no surface preparation, some patching and a 6-in.-thick concrete overlay. They chose the durable whitetopping option due to the large volume of truck traffic on the road, he added.
About 4 1/2 years ago the county constructed a concrete-over-concrete overlay, using 1 in. of asphalt as a bond-breaker topped with 6 in. of concrete. Keierleber reported the overlay is performing extremely well under commuter and farm traffic (2,300 average daily traffic).
Allamakee County employs techniques that speed construction of its overlay projects and lessen the impact of construction, such as the use of the maturity meter, using the shoulder as a temporary lane and a faster mix with more cement. The county typically uses overlays when widening narrow roads.
"We widen 2 or 3 ft with full-depth concrete, put in tie bars and overlay with 6 in. of concrete," said Ridenour. The county uses 15- to 20-ft joint spacing and skews the transverse joints to reduce noise and dynamic loading, standard in Iowa.
Projects with zip
All three counties have received benefits from the use of overlays. Eckert cited cost-saving advantages for Dickinson County. The cost savings stem from leaving the existing pavement in place and the ease of constructiblity.
"The contractor paves mainline with very little grading preparation work, which means that rain does not hold up the project," Eckert said. "We get it done fast, that’s what makes this type of project so appealing to both the contractor and owner alike."
Durability is another advantage.
"We’ve had tremendous success with concrete overlays," he said. "Our overlays that are 12 and 15 years old look absolutely fantastic, with very little cracking and no deterioration." With the county population projected to double by 2010, Eckert plans to continue using whitetopping for appropriate projects.
"The key is how long will the overlays really last?" he said. "Looking at the slow rate of deterioration of the 9-mile N14 overlay put down in the ’80s, they may very well last 35 to 40 years. Obviously, counties cannot afford to whitetop all their secondary roads. However, from a cost benefit standpoint, it has been my experience that whitetopping is a tremendous investment for our road program."
In January, Allamakee County let a 9-mile-long, $1.55 million rehabilitation project. According to Ridenour, his options were an asphalt overlay at $100,000/mile or an unbonded 6-in. concrete overlay with 1 in. of asphalt for $168,000/mile. Based on a life-cycle cost analysis, Ridenour chose the concrete option, planning on a 30-year pavement life.
"Concrete was well worth the extra cost in added life," he said. He also mentioned as oil prices increase, asphalt prices go up, making concrete more cost-competitive. Another benefit, according to Ridenour, is the skid resistance of concrete pavement.
Buchanan County’s Keierleber expects 40 years of performance out of his concrete overlays.
"Using the old pavement adds strength," he said.
The 99 counties in Iowa believe in working together to make paving projects more efficient and cost-effective. It is not unusual for two counties to join together to get better unit prices and share inspections, especially if projects are close geographically or even contiguous.
Buchanan County’s latest partnering project was a 7.1-mile PCC overlay where it partnered with one of its cities and adjoining Linn County. Dickinson County partnered with the town of Terrel (population 500) on rehabilitating N14.
"For years the town and county fought and the road deteriorated," said Eckert. Finally they made an agreement. The county administered the project and the town is to pay it back over five years, thus softening the burden on its relatively small budget.
"Partnering is a philosophy, an attitude, a team approach, and needs to be instilled in every working relationship," said Eckert.