Tag Archives: bridge girder

Affordable Bridge Girder End Repair Method Restores Concrete Beams

By load testing part of a bridge that was removed over Nine Mile Creek, researchers have proven that an innovative and cost-effective method for repairing damaged bridge girders restores them to their original strength.

The findings will help MnDOT and other transportation agencies avoid lengthy traffic closures and more costly techniques when repairing other bridges.

“This innovative method works remarkably well. 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,” said Carol Shield, Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.

Background

The salting of bridge and roadways during Minnesota winters can create highly corrosive conditions that damage bridges. Such was the case with the Highway 169 Nine Mile Creek Bridge near 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.

During a 2013 repair, 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 called “shotcrete”—a mix of sand, aggregate and cement that is 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 in October 2013 and allowed the bridge to continue its function to the public.

MnDOT was able to make the repairs without traffic interruption.

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. The bridge replacement presented MnDOT with an excellent opportunity to evaluate the repair method for use on other damaged girder ends.

What Was Our Goal?

When the southbound lanes of the bridge were taken out of service in spring 2017, four prestressed girders were removed from the structure and brought to the University of Minnesota’s Theodore V. Galambos Structural Engineering Laboratory for testing.

Researchers examined and tested the beams to evaluate the effectiveness of the reinforced shotcrete repair method.

“Two of the girders have ends that were repaired by MnDOT, and two girders have ends that never needed to be repaired,” Shield said. “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.”

The fact that researchers tested good girders alongside repaired girders gave MnDOT a high level of confidence, said Paul Pilarski, Metro North Regional Bridge Construction Engineer, MnDOT Bridge Office.

Bridge girder ends can be repaired for only $5,000 to $10,000, using this new method.

2018-07-p2-image
Repaired and unrepaired girders were tested to failure in a laboratory. This repaired beam end remains firmly connected to the beam, even after the girder was crushed.

What Did We Learn?

All repairs had been done in field conditions that have the potential to adversely affect the results. But when the beams broke in the lab, the shotcrete repair did not separate from the bonding surface. 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 are demonstrated in a video created by the research team (testing starts at 3:30 min).

Using this method, severely deteriorated beam ends can be repaired with reinforcement cages and shotcrete for $5,000 to $10,000. 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 costs hundreds of thousands of dollars and more than a month of bridge lane closures.

What’s Next?

Results have been presented internally at MnDOT, at state and Midwest conferences in late 2017, and at the National Bridge Preservation Partnership Conference in April 2018. Presentations have impressed transportation engineers from around the country and have increased confidence in dealing with aging infrastructure. MnDOT will continue to refine repair methods with the shotcrete treatment based on best industry practices, and will continue to use the beam end repair method if similar conditions are encountered in the state.

This post pertains to Report 2018-07, “BR27568—Experimental Shear Capacity Comparison Between Repaired and Unrepaired Girder Ends,” published February 2018. More information can be found on the project page. (Part of this article was adapted from an October 2017 article by the Center for Transportation Studies. This project was featured in a KSTP-TV news story.) 

 

New Study: Experimental Shear Capacity Comparison Between Repaired and Unrepaired Girder Ends

MnDOT Research Services recently executed a contract with the University of Minnesota to begin work on a research study titled “Experimental Shear Capacity Comparison between Repaired and Unrepaired Girder Ends.”

The research will determine if a bridge repair to the TH 169 Nine Mile Creek Bridge near Edina and Minnetonka was sufficient to restore the original strength of a girder end in shear. Load testing to failure will be conducted on two repaired girder ends and two unrepaired girder ends that will be removed from the bridge. Objectives include a comparison of the failure load between the repaired ends and the unrepaired ends. The test results also will provide some answers to questions on whether shotcrete is a structural repair or if it is just a covering over of deterioration.

Carol Shield, professor at the University of Minnesota’s Department of Civil Engineering, Civil, Environmental, and Geo- Engineering, will serve as the research project’s principal investigator. Paul Pilarski, MnDOT bridge engineer, will serve as the study’s technical liaison.

According to the initial work plan in the contract, the project is scheduled to be completed by the end of March 2018.

Background

Over time, the south bound exterior girder ends on each side of Pier 4 and Pier 26 of the TH 169 Nine Mile Creek Bridge have suffered significant corrosion damage that exposed shear reinforcement, exterior flange prestressing strands, and the sole plate anchorages. Girder ends were repaired in September 201 3 by encasing a 4-foot length of the end using a system of dowels, additional shear reinforcement, and shotcrete. The bridge is scheduled for replacement in 2017.

There is interest in determining if the repair was sufficient to restore the original strength of the girder end in shear. Load testing to failure will be conducted on two repaired girder ends and two unrepaired girder ends that will be removed from the bridge. Objectives include a comparison of the failure load between the repaired ends and the unrepaired ends.

Project Objective

The ability to effectively repair corrosion damaged girder ends extends the useful life of prestressed concrete bridges. These repairs are significantly less expensive than replacing the bridge. Repairing bridges is also beneficial to the traveling public as travel is not interrupted, or interrupted for a significantly shorter time than for bridge replacement. Experimentally demonstrating that the repair restores the girders up to the design strength enhances the safety of the bridge and provides MnDOT with a documented substantiated repair method that can be applied to other bridge girders in a similar state.

Project Scope

When the southbound lanes of the TH 169 Nine Mile Creek Bridge are taken out of service, the contractor will remove four prestressed girders from the structure and deliver the south ends of them to the University’s Department of Civil Engineering Theodore V. Galambos Structures Lab. Two of the girders will have ends that have been repaired. The other two girders will be of the same shear design but will not have been repaired, nor show significant signs of corrosion. Once at the Galambos Laboratory decks will be cast on the girders. One end of each girder will be tested to failure using a setup designed to precipitate a shear failure. Failure loads between the repaired and original undamaged girder ends will be compared. The development of crack patterns under load will also be documented to further understand the behavior of the repair.

Assistance

MnDOT will make arrangements for transportation of the girder ends to the University’s Civil Engineering Building. MnDOT will request that the contractor provide weights of the cut girders prior to delivery. MnDOT will make arrangements with the contractor to take concrete cores from the short end of the cut girders and provide the existing bridge bearing pads. MnDOT will provide calculations for determining the required deck width and concrete strength to avoid a flexural failure.