Leading the Way in Unique Segmental Bridge Construction
When the Florida Department of Transportation (FDOT) needed a bridge built over the Wekiva River, they had a unique bridge construction method in mind: a segmental bridge. Not only are segmental bridges aesthetically pleasing, but they offer other benefits as well:
- Minimal traffic and environmental impact during construction
- Economical to build
- Durable final structure
Even with all of these benefits, though, they are rare.
“A study in 2012 concluded that there were 604,426 bridges in the United States. Of those, only 0.08% were of the segmental bridge type,” says Garrett Jones, PE, engineer and assistant project manager at Superior Construction. Though these projects are few and far between, they are gaining popularity. In the last 45 years, segmental concrete bridge technology has grown in the US and around the world due to a few factors:
- Competitive initial cost
- Low maintenance costs
- Speed of construction
- Safety under extreme events
- Industrialization of the construction process
- Innovations in construction equipment
- Aesthetic appeal
What exactly is a segmental bridge? A segmental bridge is cantilevered, which means that rather than being built from one side to another with supports between, or one side to another with a truss or arch, they are supported at one end or in the middle and are held up by tensile suspension. With a segmental bridge, the cantilevered bridge is built in sections (or segments) out of cast-in-place or precast concrete.
Segmental construction can be used in a variety of difficult site conditions:
- Piers can be placed on small footprints
- Superstructures can go over natural hazards and community landmarks
- They may be used on a small radius such as curved highway access ramps as well as large-radius bridges
In the case of Wekiva Parkway Section 6, FDOT required the bridge to be segmental due to environmental limitations and impact over the environmentally sensitive Wekiva River. They wanted the main channel spans to be a three-span continuous unit consisting of segmental concrete box girders. Superior was required to keep all construction activities and equipment within the footprint of the proposed bridges. No trestles, falsework, or temporary supports were allowed in the river during construction. “These parameters, along with many others, essentially eliminated competition and assured the owner that they would receive a quality product from a quality design-build team,” says Garrett. “The teams that carried out the life cycle of the proposal had to get creative with the design and constructability efforts in order to attempt to gain an advantage over their competition and, ultimately, win the job.”
Because of the lack of space to set up a precast yard, Superior chose to construct the project using the cast-in-place method. “With this method of construction, the bridge was cast-in-place up-station and down-station. It’s hollow inside. It required a lot of post-tensioning,” says Garrett. “It’s not a typical bridge. There aren’t many segmental jobs that are cast-in-place in the country. There’s just a few that you can count on your hand. It’s a unique job.”
But Garrett and the Superior team were up for the challenge. “I, personally, like segmental bridge construction because of the challenge,” says Garrett. “They are considered unconventional and the amount of expertise in segmental bridges pales in comparison with conventional bridges.”
Fortunately, several members of the Superior team had experience with segmental construction. With the right planning, experience, and specialty equipment, Superior was able to successfully complete the bridge.
And it’s extremely durable thanks to some new innovations, including a flexible filler in lieu of grout. For some background, post-tensioning (PT) systems provide reinforcement to the concrete in the form of steel cable and bar tendons. The tendons are stretched by jacking and are anchored to produce compressive forces in the member. “The PT system is a primary structural component whose failure could result in total collapse of the member,” says Garrett. “Protection of the PT system is essential. The protection of the PT system is directly related to the overall durability and lifespan of the structure.”
Typically, grout fills the void within ducts, bonding to the PT steel and acting like rebar does in concrete — providing transfer of the prestressing force to the structural member in a bonded PT system. However, the steel can corrode over time, especially in the salt-laden air, leading FDOT to change its policy on post-tensioning in 2016. For this project, then, the flexible filler replacing the grout is a microcrystalline wax that provides the same strand protection as grout but makes maintenance easier — tendon strands can be replaced, as the wax provides zero bond of the strand to the surrounding concrete. Superior pumped the filler into the ducts using positive displacement pumps in combination with vacuum pumps. The filler is preheated prior to injection to 212–240 degrees F and must be uniform temperature. It is injected from the low points with vents included in the high points.
The use of the flexible filler has made Superior leading experts in the industry. In fact, Garrett, along with Jerry Pfuntner, PE, SE, PEng, of Finley Engineering Group and Robert Bennett, PE, of RS&H — two of Superior’s partners on the project — presented at the American Segmental Bridge Institute (ASBI) 33rd Annual Convention on the topic of flexible fillers. “We discussed the project’s success, where this new technique is headed in the future, as well as provided perspectives from the design, construction, and inspection points of view,” says Garrett.
Garrett says the segmental bridge is both the most challenging and most rewarding part of the Wekiva Parkway Section 6 project. “It’s not a cookie-cutter conventional bridge that you would typically see. A lot more detail goes into it. A lot more specialty equipment. We are using form travelers that we had fabricated overseas. We’ve got specialty equipment for the post-tensioning. It’s a challenge pulling strand, stressing, using the flexible filler,” says Garrett. “But the most rewarding is seeing it come to life from beginning to end over time. Putting that plan together on paper and walking out into the field and seeing it being built. It’s rewarding just to see that from start to finish.”