The Michigan Department of Transportation (MDOT) (Lansing, Michigan, USA) continues to invest in carbon fiber reinforced bridge beams, with further implementation achieved in recent months. “The aim is bridges that last a century with minimal maintenance,” says Paul C. Ajegba, the state’s transportation director. “This technology, developed here in Michigan, is becoming wildly popular all over the country.”
Since 2001, MDOT has collaborated with Lawrence Technological University (LTU) (Southfield, Michigan, USA) on researching carbon fiber reinforced polymer materials in concrete bridge beams. That research has now moved from the lab into the field.
Modern bridges are built to withstand the loads of thousands of vehicles every day and extreme temperature changes for decades. Prestressing concrete with high-strength materials is one method to address this. Traditionally, in this method, steel cables are installed inside forms before concrete is poured. The strands are then tensioned with a significant force, causing them to elongate.
Once the concrete gains strength but before it carries loads, the strands are released, compressing the concrete. Any subsequent loads would have to overcome this built-in compression to actually stress the beam. Prestressing also reduces or eliminates cracking from concrete shrinkage, allowing for thinner and longer spans.
Steel has been the go-to material for prestressing concrete for highway bridges, but it has drawbacks. It is prone to corrosion and deterioration under assault from extreme temperatures, water, and deicing chemicals—which are common conditions in Michigan. Preventing corrosion and repairing damaged areas requires time and money and can limit the structure’s lifespan.
That’s where carbon fiber comes in, MDOT explains. Carbon fiber strands have a tensile strength comparable to steel, but they resist corrosion and require less maintenance over time. Longer service life is the major benefit.
“Rusting of steel elements is the leading cause of deterioration in our bridges,” says Matt Chynoweth, MDOT’s chief bridge engineer. “Since carbon fiber is non-corrosive, we are eliminating that potential for damage. Using a material that will not corrode is a real game-changer.”
MDOT engineers are putting this high-tech material to use. In 2001, LTU, MDOT, and the City of Southfield worked together on the deployment of the nation’s first three-span carbon fiber pre-stressed concrete bridge. The Bridge Street bridge was outfitted with numerous sensors and will be monitored by MDOT until 2025. Since then, more than a dozen bridges have been built throughout the state using carbon fiber components.
Chynoweth notes that MDOT is deploying the materials strategically, using them on higher-volume routes.
The southbound I-75 span over the Sexton-Kilfoil Drain in Detroit already incorporates carbon fiber. At 140 ft (42.7 m), they are some of the state’s longest concrete beams. Currently, two bridges are being built with carbon fiber reinforced beams as part of MDOT’s massive I-94 modernization project in Detroit. Those bridges will use beams with newer 0.7-in (1.8-cm) diameter carbon fiber strands, allowing greater initial tensioning than steel.
MDOT and LTU have conducted years of research on these components, looking to optimize the design process. They’ve subjected beams to 300 freeze-thaw cycles, combined fire/loading events, severe weather, and other trials. Now, bridge designers say they have the information and specifications they need to predict how carbon fiber reinforced beams will perform under real-world conditions, as well as design tools for future projects.
One factor limiting the deployment is price, since carbon fiber elements can cost as much as three to four times more than comparable steel elements. But, based on the MDOT/LTU research, they are anticipated to last much longer than steel. Thus, it may prove to be cheaper over the long run. One of MDOT’s main suppliers, Tokyo Rope, has already built a fabrication facility in Michigan, which cuts down on some component costs and delivery times.
“We’ve calculated the ‘break-even point’ to be about 22 years, based on lifecycle maintenance,” Chynoweth says. “But since the data points only go back about 20 years, this is a theoretical estimate.”
Source: MDOT, www.michigan.gov.