U of M researchers are helping the Minnesota Department of Transportation (MnDOT) evaluate a repair method that could reduce traffic interruptions caused by infrastructure repairs by bringing real-world bridge girders into their lab.
The salting of bridge and roadway surfaces during Minnesota winters can create highly corrosive conditions that result in damage to bridges. Such was the case with the Trunk 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. MnDOT repaired the damaged girder ends in 2013 by encasing them using a system of steel dowels, additional shear reinforcement, and sprayed concrete. MnDOT was able to make the repairs without traffic interruption.
Now, the bridge is being replaced, and U of M professor Carol Shield and her team of researchers are evaluating the effectiveness of the 2013 repair. The researchers’ goal is to determine if the repair strengthened the corrosion-damaged girders to a level similar to noncorroded girders. If proven effective, MnDOT could use this type of repair to lengthen the useful life of existing bridges and save travelers time and frustration caused by repair-related traffic delays.
When the southbound lanes of the bridge were taken out of service this spring, four prestressed girders were removed from the structure and brought to the U’s Theodore V. Galambos Structural Engineering Laboratory for testing.
“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 [are testing] the four girders and comparing their strengths to determine if the repair actually returned the girders to the strength they had prior to the corrosion-related damage.”
Once the girders arrived at the lab, new concrete decks were cast on them. The concrete deck ensures that the girder has sufficient flexural strength to guarantee a failure will occur near the repair under investigation. MnDOT provided the design and calculations for the required deck depth, width, and concrete strength.
The first girder end was tested in May. An external shear reinforcement system was placed on the “non-test” end to force the shear failure to the girder end of interest. Once the deck concrete reached sufficient strength, the girder was tested. Load was applied, increasing step-by-step so damage could be monitored. As the load increased, crack patterns were documented to help understand the behavior of the repaired girder under load. The girder finally failed—and as expected, the result was a shear fracture.
Testing of the other girder ends and subsequent data analysis has continued through the summer and early fall. Ultimately, a comparison of failure loads for the repaired and unrepaired girder ends will help determine if this repair method is able to safely restore strength to deteriorated girders.
(Adapted from an article previously published in CEGE, a publication of the Department of Civil, Environmental, and Geo- Engineering at the University of Minnesota.)