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Vermont is about to complete its bridge to the 21st century. It’s not a political slogan, but an actual bridge, reinforced with composite rebars and fitted with fiber-optic sensors that allow traffic and stress on the structure to be monitored from 50 miles away.
Neither of the two layers of the reinforcement bar, or rebar, grid that give strength to its concrete deck is made of steel. Instead, they’ve been built out of glass-fiber reinforced plastic, which engineers say has a distinct advantage over traditional steel: It doesn’t corrode. That’s a key consideration in a state where roads and bridges take a beating every winter from ice, snow and road salt.
The high-tech bridge sits between farmers’ fields and runs 144 feet across the Ryder Brook, a tributary of the Lamoille River, on heavily traveled Route 100 between Morrisville and Stowe. Some 7,000 vehicles a day are expected to cross it after it opens in the fall; they’re doing so now on a temporary bridge on the detour that dips down around the construction site. Thirty-four feet wide, it replaces a bridge that was just 21 feet from curb to curb and at least once has seen two large trucks become wedged between its rails and back traffic up for miles.
The polymer costs a bit more than traditional steel. It will add about $50,000 to the bridge’s $1.4 million price tag — with the difference covered by a federal grant. But it’s expected to at least double what otherwise would be expected to be a 40-year life span for the bridge’s deck.
Phil Phelps, construction foreman for the general contractor on the job, Blow & Cote Inc. of Morrisville, said his crew considered the plastic rebar a mixed blessing. “It’s rough,” he said of the material’s texture. “It’s hard on your hands. But in fact putting it in is quite easy because it’s light.”
Brahim Benmokrane, a professor of civil engineering at the University of Sherbrooke in Quebec, will be monitoring the bridge to see how well the plastic rebar holds up over time. The monitors also will allow the team to track temperatures at the bridge, the stresses that are being put on the structure and even the weight of individual vehicles traveling over it.
Benmokrane said he expects such sensors to become more common in bridges and buildings in the coming years, with the main purpose being “structural health monitoring. If it needs repairs, if there is something going wrong, you can know it.”
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