Researchers with the Minnesota Department of Transportation (MnDOT) (St. Paul, Minnesota, USA) and the University of Minnesota (Minneapolis, Minnesota, USA) have successfully tested a new, cost-effective method for repairing damaged bridge girders that they believe can restore them to their original shear strength or even make them stronger.
According to MnDOT, the findings1 will help transportation agencies avoid lengthy traffic closures and more costly techniques when repairing bridges.
“This innovative method works remarkably well,” says Carol Shield, a professor in the University of Minnesota’s civil, environmental, and geo-engineering department. “The amount of damage the crew repaired was pretty extensive. In the end, the strength of the repaired damaged girders was slightly more than the strength of the undamaged girders.”
While the method appeared to work well in the field when previously used, researchers say the technique had not been scientificy analyzed until their recent study.
Leaking Expansion Joints
MnDOT explains that the salting of bridges and roadways during cold and snowy Minnesota winters can create highly corrosive conditions, which can damage the underlying structure. Such was the case with the Highway 169 Nine Mile Creek Bridge near the cities of Edina and Minnetonka, where leaking expansion joints caused corrosion to elements responsible for the strength of bridge girders: shear reinforcement, prestressing strands, and the surrounding concrete.
The prestressed, reinforced concrete girder bridge was built in 1975 by Hennepin County and acquired by MnDOT in 1988. In the years following the acquisition, the expansion joints were replaced, but corrosion damage was already widespread from prior joint failures. As such, district maintenance engineers and bridge inspectors were concerned with concrete degradation on the bridge and poor concrete in the bridge deck.
According to MnDOT, reinforced concrete bridge beams can bear heavy loads well for decades. If beams show distress, it is often at beam ends located under the expansion joints, which are necessary for thermal expansion. Expansion joints consist of flexible neoprene seals that are prone to tears from roadway objects and material buildup. Water and salts leaking through these tears can corrode the reinforcing steel embedded in concrete elements under the joint, such as the beams and supporting structure. Thus, deterioration at the beam end can jeopardize the safe function of the entire bridge.
New Repair Method
During a 2013 repair on the Nine Mile Creek Bridge, crews encountered two locations of severe beam deterioration. To repair these areas, MnDOT used a novel method developed in Michigan that involved removing deteriorated concrete and cleaning the area, placing steel reinforcement cages around the damaged beam ends, and then encasing the beam ends with concrete.
The repair concrete was a specific form of concrete placement known as shotcrete—a mix of sand, aggregate, and cement applied with a hose that is wetted at the nozzle before the mixture is sprayed at high velocity onto the repair surface. When the desired thickness of the concrete placement is reached, the placement is troweled and shaped to finish to the desired cross section. The beam end repairs were made by MnDOT in October 2013 without traffic interruption, thus allowing the bridge to continue its function to the public.
Several years later, the bridge was scheduled for replacement. The repaired girder ends appeared to be in good condition, but the repair technique had not been studied for strength. As a result, he bridge replacement presented MnDOT with an ideal opportunity to evaluate the repair method for use on other damaged girder ends.
The Nine Mile Creek bridge consisted of 49 spans. Each span was 60-ft (18 m) long and contained at least eight prestressed concrete beams. For the study, MnDOT identified two pairs of beams, each containing one repaired beam and one beam in good condition. The good condition beams were not located under expansion joints, but they were very similar in design to the repaired beams.
When the bridge’s southbound lanes were taken out of service in spring 2017, the four prestressed girders were removed from the structure and brought to the University of Minnesota’s Theodore V. Galambos Structural Engineering Laboratory for testing. During removal, crews separated the existing bridge deck from the supporting beams. The beams were cut from 60 ft (18 m) to less than 38 ft (12 m) to fit in the school’s lab.
Once specimens were in the laboratory, the researchers cast a new, high-strength concrete deck for each of the four girders to recreate the beams’ field configuration for effective testing. Each beam was then loaded by a hydraulic ram pushing down on the beam. Investigators applied a load in 25,000-lb (11,340 kg) increments to almost 500,000 lb (226,796 kg) on each girder.
“Repairs were done in the field by an experienced contractor, in a colder environment and season, in a swamp,” says Paul Pilarski, a regional bridge construction engineer with MnDOT’s bridge office. “The fact that we tested good girders alongside repaired girders gives us a high level of confidence in this method.”
The researchers traced cracks with colored markers on the beam ends, recording details about the beam condition each step of the way. Failure was photographed, data was recorded, and the next beam was tested to evaluate the effectiveness of the reinforced shotcrete repair method. According to the agency, all repairs conducted in field conditions have the potential to adversely affect testing results. However, when the beams broke in the lab, the shotcrete repair did not separate from the bonding surface.
“Two of the girders have ends that were repaired by MnDOT, and two girders have ends that never needed to be repaired,” Shield says. “We [tested] the four girders and [compared] their strengths to determine if the repair actually returned the girders to the strength they had prior to the corrosion-related damage.”
Ultimately, the repaired reinforced concrete beam ends were found to be at least as strong as similar beams that were in good condition and had not needed repair. The initial repair methods and subsequent testing of the prestressed beam ends were demonstrated in a video created by the research team.
Using these methods, severely deteriorated beam ends can be repaired with reinforcement cages and shotcrete for $5,000 to $10,000, according to the agency. The alternative to this type of repair involves constructing a new beam, closing traffic, removing the bridge deck over the damaged beam as well as the beam itself, and recasting the bridge deck and barrier—an intrusive replacement that would cost hundreds of thousands of dollars and more than a month of bridge lane closures.
According to MnDOT, presentations on the method at various U.S. transportation conferences have impressed transportation engineers from around the country and have increased confidence in their ability to maintain aging infrastructure. MnDOT says it will continue to refine its repair methods with the shotcrete treatment based on best industry practices, and it plans to continue using the beam end repair method when similar conditions are encountered elsewhere in the state.
Source: MnDOT Research Services, www.mntransportationresearch.org.
1 “Affordable Bridge Girder End Repair Method Restores Concrete Beams,” Crossroads: A Minnesota Transportation Research Blog, July 18, 2018, https://mntransportationresearch.org/2018/07/18/affordable-bridge-girder-end-repair-method-restores-concrete-beams/ (April 9, 2019).