When officials at the Vermont Department of Transportation decided to replace two bridges on State Route 73 due to their deteriorated shape, they wanted the work done as quickly as possible. The construction team used Accelerated Bridge Construction methods and precast concrete beams to not only meet the deadlines but maximize the incentive bonus for finishing early.
The two bridges, both single-span, 20-foot-wide structures, were deemed functionally obsolete due to their deterioration and small width. Bridge 15, which crosses Brandon Brook, was constructed in 1929 in an historic district and was considered historic, but Tropical Storm Irene caused heavy bank erosion in 2011. Bridge 16, which crosses Corporation Brook, was constructed in 1929 of cast-in-place concrete T-beams.
VTrans conducted an engineering study to evaluate alternatives for replacement, which recommended the use of precast concrete NEXT (New England Extreme Tee) beams. They also set a road-closure maximum time of 60 hours for Bridge 16 and 84 hours for Bridge 15 to minimize user costs. The additional time for Bridge 15 was designed to provide adequate time to maneuver the crane into position to install subfoundation pours at the site, which included a 90-degree turn for the approach road.
NEXT beams were chosen due to their need for “a short-construction duration, reducing disturbance to the public, and the relatively low construction and maintenance costs,” according to VTrans. General contractor Schultz Construction worked with precaster J.P. Carrara & Sons Inc. to create a construction plan that met the tight schedule and other requirements.
Bridge 15 features four 12’2” wide NEXT beams spanning 74’3”, while Bridge 16 features three 11’3” wide NEXT beams spanning 72’1”. Both bridges also have precast concrete approach slabs, wingwalls and two-piece abutments.
“The precast concrete beams allowed us to use ABC methods, which helped meet the schedule,” says Kevin Ture, project manager for Schultz. The additional time allotted for Bridge 16 was more than adequate, as the construction team managed to maneuver the crane into position to install all of the subfoundation piles before the road was closed, speeding demolition and construction of the new bridge.
In both cases, once the piles were in place and the road was closed, the existing bridge was demolished and the piles were exposed and graded. A two-piece precast concrete abutment was set onto the piles with a pile cavity, which was filled with 12-hour rapid-set concrete. The abutments were back-filled, the beams were set onto the abutments, and closure pours were placed between the beams.
The NEXT beams featured curtain walls with 3 inches of compressible foam on their ends to align with the abutments. “There was very little tolerance, about ¼ inch,” Ture says. “I was very skeptical that they would fit, but they fit perfectly. J.P. Carrara did a wonderful job.”
Time was a Big Factor
The project had heavy liquidated-damage fees of $1,000 per hour for going beyond the allotted Bridge Closure Period (BCP), as well as incentives for finishing early. Schultz met the full incentive bonuses for finishing early: Bridge 15 (84-hour BCP) reopened in 72 hours, while Bridge 16 (60-hour BCP) opened in just 52 hours.
“This was the first time we had used NEXT beams, and they were super,” Ture says. “J.P. Carrara did a great job. Both bridges are on a skew, which complicated the form work, but they handled the challenges without a problem.”
Ture notes that he was particularly impressed with the “hands-on” approach taken by the Carrara family in conducting their business. “Company president Joe Carrara and his brother, Vice President J.P. Carrara, spent countless hours with us onsite coordinating erection procedures and CPM schedules down to the minute. Every time I visited the plant to check in, their father Paul (who is supposed to be retired) had tools in his hands and was working with crews in the precast plant to ensure the quality was up to his high standards. Without this team cooperation, we could not have accomplished what we did.”
The two bridges were part of a $6.4-million, four-bridge construction project that has been completed. Now the construction team is using a similar system to replace a bridge in Castleton, Vermont. The bridge spans railroad tracks, requiring a 28-day shutdown to deal with complications, but otherwise will follow a similar approach, Ture says. “The design is very similar, and we expect we can achieve the same results.”