If you want to find out what is happening to a bridge on cold winter nights when the temperature is expected to plunge to minus 20F, you can just call it up on a computer screen.
It is one of several state-of the-art advances built into a newly completed bridge in northwestern Vermont. In a state long known for its quaint churches and winding roads, the bridge, located just outside of Morrisville in the northwestern quarter of the state, is a bellwether for future technologies that can be used in the state.
The bridge has a number of fiber optic sensors encased in the concrete that will give specialists at the University of Sherbrooke, in Canada, a sense of how the bridge responds to strain and temperature, according to Tom Lackey, design engineer of the Vermont Agency of Transportation (VTrans).
The sensors were placed in the concrete and connected to a modem which is fastened to one of the abutments. Approximately 10 sensors were attached to the bridge, although some of them were lost when the deck was poured. The sensors also can reveal how it deals with stress when trucks travel across it.
“VTrans is curious about what is happening with the data,” Lackey said. “It’s a way of looking inside the bridge.”
But that’s not all there is to this bridge. Instead of traditional steel rebar, engineers used glass-fiber reinforced polymer (GFRP), a much lighter alternative that also reduces the possibility of corrosion. It is the first bridge in the state to use it. Even with pieces measuring 40 ft. (12.2 m), it is easy for workers to pick them up by hand. No longer does it require a crane to move batches of them, Lackey explained. According to VTrans Project Manager Ray Hayes, the new form of rebar weighs only one third that of the steel — less than 8 oz. (226 g) per ft. ( .3 m) compared with 24 oz. (680.4 g) for the traditional form.
GFRP stretches and contracts with the weight that travels across it and the sensors will indicate just how great the stretch is.
While it allows engineers to design a lighter product, GFRP does add approximately $50,000 to the price tag of the project. However, that price is being covered by a federal grant.
The only problem with GFRP is that the bars are coated with sand, so it will be more strongly anchored to the concrete. Because of that, it is rough on the hands when workers lift it.
The bridge also was built with a bare concrete deck. Hayes explained that almost all bridges in Vermont are paved, but with the bare concrete, it is much easier to see when it starts to deteriorate. Other states, such as New York, have used this technique for a number of years.
“The state wanted to use high-performance concrete because there is less chance of deterioration,” Hayes said.
The bare concrete demanded that a large number of saw cuts be made across to help with drainage. Every 1.5 in. (3.8 cm) there is a cut .25 in. (.64 cm) wide and .25 in. (.64 cm) deep. Hayes said that the cuts make tires “sing” when vehicles cross the bridge, but that it definitely prevents water from standing on the deck.
As an aesthetic touch, an ornamental concrete railing was added, recreating a look found in many bridges built back in the 1930s. “The exposed aggregate dresses it up,” Hayes said.
Finally, the bridge features integral abutments, which replace the typical bearings found at either end of the bridge. The new abutments allow the bridge to pivot slightly from the bottom, where it is better protected from the weather.
The bridge is entirely new, replacing an older, much narrower one across the Ryder Brook. Although the old one measured only 21 ft. (6.4 m) between the curbs, this one is 34 ft. (10.4 m) wide, so it is hoped that incidents in which wide trucks occasionally get stuck when trying to cross the span at the same time will be eliminated. That makes the new one “light years different” than the original, according to Mark Cote of Blow and Cote Inc., of Morrisville, the primary contractor. It also is 144 ft. (43.9 m) long and just 15 ft. (4.6 m) above the water and a farm drive that travels next to it.
In all, it is an innovative project that took two construction seasons to finish. Work on the new $1.4 million project began on June 21, 2001, and concluded on Aug. 30, 2002. During the first year, a detour was created, temporary bridge built, piles driven and abutments poured. After suspending construction for the winter, the remainder of the actual bridge was built this year.
Blow and Cote did most of the work on the project. The subcontractors were all from Vermont and included: A. Marcelino and Company, paving, South Burlington; F.R. Lafayette, guardrail, Essex Junction; L&D Safety Striping, Berlin; Rockaway, ledge blasting, Morrisville; Ralph Derrah, welded sheer connectors, St. Johnsbury; and A.D. Rossi, bridge joints, St. Johnsbury.
Cote downplayed the significance of the innovations included in the job.
“It’s pretty much the same as any other job. It’s all buried in concrete, so there’s nothing left to be seen,” Cote said.
Most of the work in the project was done on the superstructure well above the brook. At the same time, approximately 2,000 yds. (1,829 m) of dirt and 3,000 yds. (2,743 m) of rock were moved down below, primarily in the construction of the temporary road last year.
The contractors used equipment such as Liebherr excavators, International trucks, Allis bulldozers and Bucyrus cranes.
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