An alternative bridge repair research project has received a 2019 State Government Innovation Award. The Minnesota Department of Transportation (MnDOT) Bridge Office received the honor for its use of a novel repair method and subsequent testing. The project included an evaluation by U of M researchers that confirmed the effectiveness of the method.
MnDOT estimates it has saved at least $1.3 million to date through its use of the technique, which repairs the damaged ends of reinforced concrete bridge beams. These beams typically perform well for decades, but when they show distress, it’s often at the beam ends under expansion joints. Water and road salt can degrade the concrete and corrode the reinforcing steel in these areas.
During a routine repair of the Nine Mile Creek Bridge in 2013, MnDOT construction inspectors encountered two concrete beams with severe deterioration. Needing to rethink their plans, MnDOT decided to adapt a method that the Michigan DOT had developed for lower levels of deterioration. The method places steel reinforcement cages around the damaged beam ends and encases the ends in concrete through a process known as “shotcrete.” This process sprays a concrete mix through a hose and adds water at the exit nozzle, projecting the mixture at high velocity.
The repair performed well for three years. When the bridge was selected for replacement, a unique opportunity for testing the repair method fully was made possible. During the scheduled bridge replacement in 2017, MnDOT salvaged two pairs of beams, each containing one repaired beam and one beam in good condition.
After being removed and cut to appropriate test lengths, all four girders were brought to the U’s Theodore V. Galambos Structural Engineering Laboratory. There a team led by structural engineering professor Carol Shield began a MnDOT-funded evaluation.
Shield’s team cast a new, high-strength concrete deck for each of the girders, simulating the beams’ field configuration. Each beam was loaded by a hydraulic ram pushing down on the beam. Beams were tested until they failed by applying a load in 25,000-pound increments—up to almost 500,000 pounds—on each girder.
The researchers found that the repaired girders were actually slightly stronger than the undamaged ones. “It was very satisfying to see that the repair performed as good as or better as beams that were in good condition,” says Paul Pilarski, MnDOT bridge construction engineer.
The shotcrete repair method costs only about $10,000, compared with $300,000 for traditional repairs that require removing a portion of the bridge deck, and it eliminates several weeks of lane closures. There’s also an advantage for worker safety, since the repairs take place under the bridge deck. And by extending the service life of bridges, fewer bridge replacements—and less concrete—are needed annually. “Less concrete use means a much lower annual carbon footprint for the DOT to meet transportation needs,” Pilarski says. “Cement production is at least 5 percent of the world’s carbon footprint.”
Since the testing, MnDOT and the City of Saint Paul have used the shotcrete method to repair three additional bridges, including one on Interstate 94 in downtown Minneapolis.
The Minnesota State Government Innovation Awards recognize the work of state government entities and encourage an environment of experimentation and innovation in Minnesota. The 2019 awards were administered by the Public and Nonprofit Leadership Center at the U’s Humphrey School of Public Affairs with support from the Bush Foundation. The bridge project was one of the top four innovations this year in state government.
“We look for ways to be cost-effective,” says Kevin Western, MnDOT state bridge engineer. “If we have even two of these a year, we’re going to see pretty substantial savings.”