By Bridget McCrea
What do the U.S. Army Corps of Engineers, Tulsa District, and the Frenchtown Charter Township Resort District Authority in Monroe, Michigan, have in common? After careful review of the material options available on the market for their bridge and seawall replacement projects, both groups selected precast concrete during the engineering and design process.
In October 2013, the U.S. Army Corps of Engineers, Tulsa District, announced plans for the $15.6 million replacement of the aging Highway 151 Bridge over Keystone Dam. Kiewit Infrastructure South Company of Omaha, Nebraska is handling the construction work and the precast concrete aspect of the project was split between two companies: Heldenfels Enterprises Inc. of San Marcos, Texas, (for the prestressed girders) and Tricon Precast Ltd. of Houston, Texas (for the post-tensioned deck panels).
By using precast concrete bridge elements, the time on site it typically takes to form and cure elements is eliminated. Elements can also be constructed simultaneously, reducing lag time waiting for one element to be completed before starting the next. That speed of construction factored into every element of the bridge, which was erected in 1964 and had reached the end of its useful life. And while the bridge isn’t considered “high traffic” by DOT standards, Daniel Morales, P.E., a senior structural engineer for the Corps, says it does carry a high volume of traffic across the Keystone Dam on a daily basis. Because of this, the project was put on a tight, 13-month timeline.
“We really wanted to cut down the construction time, the down time and the amount of traffic that had to be disrupted because of the project,” Morales said. As part of that mission, he said the Corps leveraged accelerated bridge construction techniques and materials such as precast concrete. The original bridge, which was being demolished at press time, was built with steel girders. The new one will be made of precast, prestressed and/or post-tensioned pieces that will instill internal redundancies into the structure.
“That’s just one of several technical benefits that we’re gaining with the new system,” Morales said. The bridge’s position over a body of water – plus the inclement winter weather conditions that the crew had to deal with between November and April – make it a perfect backdrop for factory-poured and assembled precast pieces.
“The site is very windy, and the winter weather exacerbated those poor conditions for several months,” Morales said. “Cast-in-place would have been more difficult to manage in this setting, not to mention the additional quality control measures that we would have had to deal with versus having everything made in a controlled, lab-type environment.”
Tearing it down
Keith Rice, project engineer with the Corps, says the Keystone Dam Bridge project included the demolition of the existing steel infrastructure. “We cut down the existing pier caps right down to sound concrete,” he said. “We did that to get to the sound concrete and because the new precast girders are of a different depth than the (existing) ones.” To maintain the same height across the roadway deck, he added, precast pedestals were used to replace the pier tops (and the precast girders will be placed on top of those pedestals). The structure will also include a prefab catwalk that runs from pier to pier.
“Everything was designed to be ‘placed’ out there on the bridge once we had the original structure demolished,” Rice said, “with the thinking that the construction will go quicker and smoother as a result.” He said the project is on schedule despite a few hang-ups that were created by the cold and stormy winter season. “The freezing weather slowed us down a bit in terms of saw-cutting the concrete deck off,” Rice said, “and we had an issue with water on the roadway deck freezing up, but overall we’ve managed to stick to our original timeline.”
Morales concurs and says the team is now anticipating the arrival and installation of the precast concrete pieces. “The part of the project that I’m most anxious about is the installation of the deck panels, and we’re not quite there yet. That’s what I consider to be the biggest and most quality-control (sensitive) aspect of this development,” he said. “There has to be a lot of quality control during that stage and I’m interested to see how all of that goes.”
Shielded from Lake Erie
About 900 miles northeast of Tulsa, the Frenchtown Charter Township Resort District Authority has also made precast concrete its material of choice for a very important project. In the bid/award phase at press time, the project encompasses the construction of a new seawall to protect homes situated on Detroit Beach from Lake Erie’s high water. Nearly 3,400 ft in length, the new precast concrete wall (with steel soldier piles) replicates another, similar shoreline project that was completed in the area about two years ago.
According to Larry Smith, director of the Authority, the new seawall will be erected 30 in. in front of the existing structure. The latter is now 20 years old and was put in place following a major flooding of the shoreline at the time. The new structure will be built with a flair on top to force waves to roll back, dissipate and then slow the energy of the following wave.
Smith says the 20-year-old structure is in need of replacement because the walls and other components are deteriorating. “They’re in bad shape,” he said. Special tax assessment district funds are being used to fund the rehabilitation project, which will cost $6-$7 million to complete. “Flood protection is one of the primary goals for the Frenchtown Charter Township Resort District Authority,” Smith said. “The Authority has been putting seawalls up – and then replacing them and repairing them – since as far back as the 1950s.”
David K. Wehner, P.E., and Christopher Zangara, P.E., both of Maumee, Ohio-based Mannick & Smith, designed the seawall and specified precast concrete as the material of choice for the new structure, which will protect a total of 4,700 ft of shoreline. One of the final steps will involve cutting down the existing steel wall while leaving the lower portion in the ground.
“Essentially, we’re building a new wall in front of the existing wall, putting in the soldier pile precast concrete sections in front of it, and then filling in the space between the two walls,” Smith explained. “Then the contractor cuts the existing back wall down below grade so that it’s no longer visible, thus enhancing the property while preserving the surrounding earth and soil.”
Out for bid
Wehner says Mannik & Smith has been working with the township for about 15 years. For the Detroit Beach seawall project, the company served as both designer and engineer – putting the job out for bid and managing the construction aspect of the development. Once the job is awarded, the contractor will subcontract the precast concrete manufacturing portion via a different bidding process.
The project is expected to take 18 months to complete. Wehner says the design team considered steel as the primary material for the new seawall, but the corrosiveness of the surrounding soil and water prompted the designers to explore alternative options. “The tiebacks are always the weakest link on steel walls. They are why the existing structure was beginning to fail,” Wehner said. “There’s a lot of corrosion and oxidation on the seawall that’s there now – and the tiebacks are failing.”
Wehner said precast concrete was selected because the 8-ft panels can be constructed in a way that creates a smooth, flush wall surface. “Precast concrete looks good and we designed it to be very buildable,” he said. For example, the design is based on a self-supporting cantilever setup that requires no tiebacks or other forms of structural support. “The piles themselves go way down into the bedrock, which we’ll drill down into and pin with the concrete,” Wehner explained. “Then we’ll put the panels on it.” The project will incorporate some cast-in-place closure pours and recessed staircases, but is 95% precast.
The Authority’s decision to use precast concrete to build the new seawall also aligns with a six-year-old township ordinance that enhanced the municipality’s flood protection efforts. “In 2008, it was decided that anyone who wanted to repair their own walls in the area would have to use either steel or concrete,” Wehner said. “Earthen materials, wood or masonry are not allowed anymore due to a lack of durability with those options. Lake Erie gets high and the wave action is fierce. Steel or concrete are the only products we want in there.”
Finally, Wehner says precast provides a level of convenience and quality that can’t be replicated by cast-in-place alternatives. “The quality control is better and we can do more thorough testing,” he said. Easy to ship and install, the panels will include the seawall’s flair (which protrudes outward by 8 in.) in a monolithic manner. “That’s an advantage when you pour precast concrete in a plant,” says Wehner. “You can create any shape you want much easier than you would be able to do if you were out in the field using a cast-in-place wall.”
Once the seawall project is awarded and a contractor selected, Smith expects the planning and staging process to begin right away. “We’ve allowed 18 months total for this project, and that timeline depends on what kind of winter we have this year,” he said. “If we have a season like the one we just had, it’ll probably take the full 18 months from start to finish.”
Bridget McCrea is a freelance writer who covers manufacturing, industry and technology. She is a winner of the Florida Magazine Association’s Gold Award for best trade-technical feature statewide.