In 2016, feedback from Minnesota drivers prompted Olmsted County’s Transportation Division to initiate a concrete pavement preservation project on County Highway 22 (also known as 37th Street NW) between the intersections of Trunk Highway 52 and County Highway 33. Road structure and pavement depth necessitated an unconventional rehabilitation approach: retrofitting 4 miles with 1-inch-diameter dowel bars instead of the more typical 1.25-to-1.5-inch-diameter dowels.
DBR: A Proven Strategy
Dowel bar retrofit (DBR) is a load transfer restoration process that delivers a renewed level of structural strength to Portland cement concrete pavement (PCCP). The process involves placing load transfer devices across transverse joints and/or cracks to prevent heavy faulting, pumping, and/or corner breaks. The load transfer mechanisms generally consist of two, three, or four round dowel bars placed parallel to the centerline in the primary wheel paths.
Retrofit techniques based on the use of epoxy-coated steel dowel bars were introduced in Georgia in the late 1980s and optimized for production operations in Washington State in the early 1990s. Since then, more than 7 million DBRs have been completed on numerous projects across the U.S.
Technology continues to evolve, with experimental designs and materials being developed and tested at various agencies and institutions. For example, DBR provides significant structural benefit on thin-section PCCP.
In 1998, Caltrans researched the effect of DBR on unreinforced 7-inch concrete installed in 1960. A section of Route 80 was retrofitted with three bars per wheel path using epoxy-coated steel and fiberglass dowels. Repeated testing using the falling weight deflectometer (FWD) measured load transfer efficiency at 30% before and 86% on average for areas retrofitted with steel dowels.
Furthermore, unlike the undoweled sections, retrofitted sections were minimally affected by daily and seasonal temperature changes, leading to better performance and longer life.
The use of 1-inch dowels has been proven effective as reported in the "Guide to Dowel Load Transfer Systems for Jointed Concrete Roadway Pavements" by Mark Snyder. Data presented in the guide shows that eight out of 11 states known for concrete paving reported using 1-inch-diameter (or smaller) dowels in thin-section concrete pavement. Dowels with a 1-inch diameter are well suited for thinner concrete pavement sections (those with a final thickness of less than 8 inches); their smaller size allows for the necessary concrete cover to accomplish effective load transfer at the pavement joint.
This procedure is particularly applicable in municipal and county road networks where aging undowelled thin-section concrete roads now need additional load transfer.
Using 1-Inch Dowel Bars in Olmsted County
When built in the mid-1980s, County Highway 22 was full-depth asphalt: 14.5 inches on grade. The road is heavily traveled, with daily traffic traveling to and from a sand and gravel pit. Over time, the heavy vehicles carved ruts as deep as 2 inches.
In 2011, expecting concrete to better resist rutting, the Transportation Division replaced the top 6.5 inches of asphalt with 6.5 inches of concrete. Because there were still 8 inches of asphalt to support the overlay, dowel bars weren’t installed.
Despite the asphalt base, though, some of the concrete above experienced faulting. That's when drivers began to express concern over poor ride quality.
"Dowel bar retrofit was the solution, but with a thinner-than-usual slab depth of 6.5 inches, we chose 1-inch diameter dowel bars rather than the usual 1.5-inch ones," says Transportation Supervisor of Construction & Traffic Scott Holmes. "We had MnROAD [a pavement test track owned and operated by Minnesota DOT] investigate the configuration, then chose to place three bars on the inside wheel path as well as the outside one for additional support, because the concrete slabs didn’t contain tie bars."
When the project went to bid in July 2016, molds and tooling to make appropriately sized expansion caps and chairs didn’t exist. However, the project’s size made investing in the manufacturing resources for producing them viable. Through coordination between the contractor and the material supplier, the components were designed, produced, and delivered in time to meet an aggressive schedule, according to Highway Materials LLC Co-founder Jake Steinberg.
“The resources are now in place to produce the quantities required to meet the demands of future projects, giving owners yet another tool to fix their thin-section and lower-volume concrete roads,” he says. His company ships the necessary components nationwide.
The use of 1-inch dowels can have applications beyond roads with thinner-than-usual pavement depths: Smaller dowels require fewer materials to produce and can therefore lower costs while providing the same level of performance for low-volume roads.
By September 2016 the project was complete with 72,558 square yards of pavement grinding, joint sealing, and pavement marking. Total cost was $1.2 million, with the DBR portion costing $860,000 (including slot-cutting, installation, patching, and saw and seal). A total of 26,882 dowels were used. The work is part of the county's rehabilitation program to optimize the life cycle of its concrete road network.
"We used dowel bars in projects in 2000 and 2004 and those are performing well," says Holmes.
Drivers notice the improvement, too. One e-mailed the county to say, "I want to congratulate the county and especially Diamond Surface’s crew. The company had the manpower and equipment to get the job done in an exceptional manner without undue traffic disruption."
The rehabilitation is expected to last 30 years, requiring minimal maintenance and ensuring a safe, smooth, and sustainable road.