Repair to Skilton Road Bridge
The original Skilton Road Bridge in Watertown, Connecticut, is one of those classic stone structures that you find in small east coast towns. Constructed in 1865, the stacked cobbled stones and classic arch are typical in these historical structures.
But by 2011, this fairy tale structure was in desperate need of repairs. Despite efforts to reinforce the arch and walls of the historic landmark, the stone masonry at the west abutment was beginning to fail.
In response, the Connecticut Department of Transportation (CDOT) launched a rehabilitation project for the bridge that was jointly funded by the CDOT and the town of Watertown. The new design had to bring the bridge up to code and extend its life, without sacrificing the history character of the original structure. It also had to be expanded from one lane to two in order to secure CDOT Local Bridge Program funding, and include a new sidewalk to increase safety for pedestrians.
The designers considered several potential schemes to rehabilitate the structure, and ultimately decided to install a new superstructure consisting of precast, prestressed concrete slabs that would bear directly on the existing arch bridge, thus preserving and protecting the entire historic structure. “The use of prestressed, precast concrete beams enabled the bridge to be widened while minimizing the impact on the existing bridge,” says William Andres, associate with Tata and Howard, the engineer for the project.
A key advantage to utilizing precast, prestressed concrete on this project was that the precast concrete slabs were cast while demolition and construction of new beam seats were being completed, which shortened the project timeline and minimized disruption. It also reduced the time needed to detour traffic around the construction site, and eliminated the need for scaffolding and formwork. “That made it made it less expensive than a cast-in-place design,” Andres says.
The Precast Solution
The precast concrete slabs were prestressed to resist the required construction loads and to carry the required in-place design loads. Instead of orienting them in the normal longitudinal direction, the slabs were installed transverse to the roadway, extending out roughly 5 ft beyond the existing stone masonry walls. Installation of the slabs took just 1 day.
“Typically, these types of precast, prestressed slabs are transversely connected using grouted shear keys with post-tensioning tendons to ensure that the slabs deflect together under load,” Andres says. However, due to the vertical geometry of this site, post-tensioning was ruled out because of concerns about alignment and deformations in the slabs during post-tensioning.
A mildly reinforced, cast-in-place concrete connection was adopted instead. “This provided the ability to transfer full shear and moment between slabs while allowing the each slab to rotate as necessary to follow the curvature of the vertical geometry.”
To ensure durability and decrease maintenance, a waterproof membrane was placed over the precast concrete beams and new storm drains were installed to protect the existing stone masonry from water damage. “The end result was a monolithic high-performance precast, prestressed concrete slab system that spans from start to end of bridge, Andres says. “It produced an excellent riding surface while protecting the stone arch below.”
Engineer of Record:
Precast Concrete Specialty Engineer:
Structural Precast Elements:
Key Project Attributes:
• Precast concrete slabs were set in 1 day, eliminating the need to construct staging and formwork on site.
• Waterproofing and storm drains will prevent damage from winter freezing-and-thawing cycles.
• The precast, prestressed concrete superstructure bears directly on the existing arch bridge, thus preserving the entire historic structure.