Large-scale precast concrete products and innovative construction methods are key to the construction of 100-year bridge in North Carolina.
By Mason Nichols
In August 2019, a tropical storm began developing in the central Atlantic Ocean. For days, the storm rapidly intensified, eventually gaining enough power to merit a Category 5 classification. With a maximum wind speed of 185 mph, Hurricane Dorian was a monster, packing enough punch to decimate anything in its path.
After battering the Bahamas, Dorian shifted toward the East Coast of the United States, eventually making its way to the Outer Banks, a series of barrier islands in North Carolina. While the storm had decreased significantly in intensity, it still posed ample threat to residents, business owners and visitors. The islands experienced significant flooding and damage from Dorian, including to state Highway 12, the main roadway linking all the islands of the Outer Banks.1
Damage to Highway 12, unfortunately, is nothing new for the area. Many sand dunes are located directly adjacent to the roadway since the islands are so narrow. During strong storms, the dunes are often pushed onto the highway, effectively cutting off access to broad portions of the Outer Banks. To help counteract this issue near the Cape Hatteras National Seashore, the North Carolina Department of Transportation (NCDOT) set out to build the Rodanthe Bridge using the only building material strong and resilient enough to withstand the harsh marine environment – precast concrete.
Up against it all
Constructing a bridge is rarely a straightforward task, and the $145 million, 2.5-mile-long Rodanthe Bridge is no exception. According to Adrian Price, P.E., project manager for Flatiron Construction, the general contractor on the job, the bridge is being built over the Pamlico Sound, which contains a maximum water depth of just 4 feet. Traditional construction methods, which include the use of barges, causeways or temporary work bridges, weren’t viable due to the water depth.
Moreover, the bridge’s location guarantees it will constantly be subjected to saltwater, high winds and an assortment of other environmental factors like submerged aquatic vegetation (SAV). A strict project requirement called for a construction approach that minimized negative impact on SAV in the area.
As NCDOT, Flatiron and design partner RK&K took these considerations into account, it became clear that precast concrete products – combined with a unique construction method – would offer the best means for addressing these issues and meeting the bridge’s 100-year service life requirement. Price and the team at Flatiron partnered with Coastal Precast Systems of Chesapeake, Va., thanks to an existing relationship, as well as Coastal Precast’s proximity to the project site.
David Morgan, P.E., director of engineering for Coastal Precast, explained the benefits of the connection.
“We have a long history with Flatiron,” he said. “We helped them develop their plans regarding the logistics of getting the precast pieces to the job site. It’s quite a process, and our staff is very knowledgeable with the Outer Banks area and how to get our product there.”
Plenty of precast
A variety of precast concrete products were specified for the project, including nearly 4,000 prestressed deck panels, approximately 400 Florida I-beams and roughly 300 54-inch-diameter cylinder piles. Altogether, Coastal Precast will ship more than 4,500 products to the project site by the time the work is completed in fall 2021.
The team is using a “top-down” construction method to ship and install products since traditional construction methods are not an option. This approach uses a specialized rail system – in conjunction with the new bridge itself – to complete sections incrementally across a 2.5-mile span. Using this innovative method, Flatiron builds a section of the bridge, then uses each completed portion to deliver additional products to the rail system and install the next section. The rail system features two tracks running in tandem on either side of the structure. When pieces are delivered to the site, they are loaded onto gantry cranes and carried out to where they will be installed.
Due to their size, the cylinder piles are the most difficult component. The longest pile manufactured to date is 162 feet long, a size that requires planning and coordination across the entire manufacturing process, from production to shipping and installation. According to Bert Richardson, project manager for Coastal Precast, each of the piles is being cast with 10,000-psi concrete that includes 5% silica fume, 25% fly ash and 4 gallons of CNI, a special anti-corrosion admixture that’s formulated to withstand the fierce marine environment. As Morgan explained, that combination is built for the long haul.
“This is really the Rolls-Royce of concrete mix design,” he said. “You have the highest performance as far as the properties of the concrete to resist the sulfates and the chlorides in the saltwater.”
Coastal Precast also is using self-consolidating concrete for the piles and the rest of the products specified for the project. The production team can manufacture pieces free of voids and minimize the amount of finishing work required – a crucial step given the demands of the construction schedule. Richardson noted that delivery requirements call for Coastal Precast to ship six piles to the project site per week, with each pile achieving a release strength of 6,500 psi.
“Our biggest obstacle in producing these is release strength, when we can cut the prestressing strands,” Richardson said. “To get to the 6,500 psi and cycle the forms, we had to accelerate the curing process.”
The production team at Coastal Precast purchased and installed a steam plant, which included a boiler and associated temperature-reading devices and controllers, to meet the demands. Each of the piles is steam cured at 155 degrees F for 12 hours, speeding up the process enough to meet the six-pile-per-week requirement. Coastal Precast also bought an additional five acres to store product at their plant.
Quite a trip
After the piles, girders, deck slabs and other precast products are delivered to the job site, they are routed to the rail system for a special installation method using a “pile tripping frame.” Flatiron partnered with Deal, an engineering and equipment manufacturer based in Italy, to design the highly specialized equipment, as well as in house on technology that would assist in the process.
“For those who are familiar with the project, the 54-inch precast pile driving operations get the most fame,” said Sean O’Neal, project engineer for Flatiron. “We designed and implemented a custom pile leads system to drive the massive piles. When the operation is active, you can’t drive by the bridge without it grabbing your attention.”
The piles are driven with an impact hammer, an approach that only affects the environment within the pile’s footprint. By installing the precast piles, Flatiron was able to reduce the size of the foundations, which also minimizes environmental impact. When the project is completed in 2021, the Pea Island National Wildlife Refuge also will benefit from another positive outcome.
“Beyond the transportation considerations, a major purpose of the bridge construction is to return private land to the refuge, allowing the land to return to its natural state,” O’Neal said.
By specifying a wide variety of precast concrete products, the parties involved were able to accelerate the construction process and minimize disruption to wildlife. But the biggest advantage of installing precast on the Rodanthe Bridge will be in the strength and resiliency of the finished structure.
“For projects in marine environments, precast is the obvious choice due to long-term durability,” Price said. “There’s also less maintenance involved.”
Plus, as Richardson explained, cast-in-place concrete construction would have exposed the building materials to consistent salt spray, necessitating the use of stainless steel reinforcement, which would have increased costs significantly.
Ultimately, the Rodanthe Bridge is primed to provide residents, tourists and other Outer Banks visitors with a safe, long-term solution to traversing the islands that will not be impacted by severe weather events. By specifying precast concrete products and implementing the top-down construction method, the environment – and the people who enjoy it – will benefit mutually from this important work.
Mason Nichols is a Grand Rapids, Mich.-based writer and editor who has covered the precast concrete industry since 2013.