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.