By Eric Carleton, P.E., and Mason Nichols
You may recall the small, stackable alphabet building blocks you played with as a kid when you were learning your ABCs. But ask an engineer or road builder about ABCs today and he or she will tell you the letters refer to accelerated bridge construction.
The Federal Highway Administration defines ABC as “bridge construction that uses innovative planning, design, materials, and construction methods in a safe and cost-effective manner to reduce the on-site construction time that occurs when building new bridges or replacing and rehabilitating existing bridges.” (1) As Tom Macioce, chief bridge engineer for the Pennsylvania Department of Transportation explained, ABC is all about speed.
“Accelerated bridge construction minimizes the amount of time we are impacting the traveling public,” he said. “It’s important to the public that we get in and get out as quickly as possible.”
As with any construction project, the materials chosen for the work help dictate the outcome. The only building material that can consistently and efficiently meet the FHWA’s ultimate goal of reduced on-site construction time is precast concrete. Engineers have recognized that many of the same benefits precast concrete brings to the building industry, including increased quality control and service life, are crucial to ensuring the long-term success of ABC projects.
“We feel like if you’re casting a component and building it in a controlled environment, that component may very well have more durability to it,” said Dave Juntunen, bridge development engineer for the Michigan Department of Transportation.
Corey Rogers, engineer of bridge field services for MDOT, agreed, adding that precast provides additional advantages over other materials. “With a precast operation, your connections are usually a high-strength grout that gains strength really quickly so you can move on to your next stage within a matter of 12 hours or a day,” he said. “So time savings is a big thing, because you don’t have the cure times associated with cast-in-place concrete.”
Precast beam sections have been used successfully in bridge construction for a half century. But the effort to fully employ precast concrete components throughout the bridge structure began taking shape more recently through FHWA’s “Highways for LIFE” program. This program, combined with the 2009 state-based “Every Day Counts” initiative, provided the perfect catalyst. Still, expanding the use of precast beyond beams to other critical bridge elements required additional research and full-scale demonstrations to overcome concerns and biases in the design and construction communities.
In 2007, the FHWA addressed these questions in a report entitled “Innovative Bridge Designs for Rapid Renewal: ABC Toolkit.” The report was published in 2013, with supplementing information added in 2014. Selected concerns include:
- Higher initial costs.
- Lack of access for equipment or the need for large staging areas unavailable in urban locations. The use of smaller elements for superstructure and substructure that can be assembled on-site could overcome access issues.
- Lack of ABC standards that can be adopted regionally by states and prefabricators.
In 2015, many if not all of these questions have been answered, resulting in increased ABC installations across the U.S. Finn Hubbard, senior vice president for engineering firm Fish & Associates, presented on the “ABC Toolkit” initiative at the Transportation Research Board’s 94th Annual Meeting earlier this year. During his presentation, Hubbard indicated that eight ABC projects throughout the country have resulted from the initiative, including work in Arizona, California, Kentucky, Maine, Missouri, Rhode Island, Wisconsin and Michigan. Four peer-to-peer exchanges are scheduled later this year to disseminate successful ideas and lessons learned from each project.
States which have implemented ABC programs have benefitted considerably from sharing ideas, information and guide specifications with other DOTs. According to Juntunen, the ability for engineers to network has also resulted in significant strides in ABC techniques.
“I really applaud the FHWA and their ‘Every Day Counts’ effort, making it possible for us to go to workshops and get peer reviews with other states,” he said. “It would be foolish for 50 states to be inventing these details, so we’re learning from each other all of the time.”
One venue for sharing ideas and cutting-edge techniques that has generated success is the National Accelerated Bridge Construction Conference. This year, the annual event will be held December 7-8 in Miami, Fla. For more information on the conference, visit 2015abc.fiu.edu.
Award-winning ABC project in Pennsylvania
Widespread collaboration among U.S. DOTs has resulted in highly successful bridge replacement projects across the country. For Pennsylvania, the use of precast and ABC techniques is critical, as the state currently contains the highest number of structurally deficient bridges in the U.S.
Outside of Pittsburgh, one such bridge carried traffic from State Road 288 in Lawrence County over Wampum Run, and was an important artery for local residents and the trucking industry. Shutting the bridge down meant creating a 22-mile detour that would significantly impact the nearly 5,000 drivers that traversed it every day.
To minimize traffic disruption, the bridge was targeted for ABC replacement. According to Macioce, the 78-foot-long replacement bridge took only seven days to install, and was built entirely of precast concrete substructure elements. Precast products used in the new bridge included pile caps, wingwalls, cheekwalls, backwalls and approach slabs. The bridge deck also consisted of modular precast units, which were connected together using ultra high performance concrete.
The SR 288 bridge was recognized last year at the National Accelerated Bridge Construction Conference in Miami, Fla. It received top honors in the precast bridge elements and systems category for its extensive use of precast and expedited project timeline. Thanks to the success of the project, precast will continue to play an important role for the more than 500 bridges that will be replaced in Pennsylvania using ABC techniques over the next 3 1/2 years.
From four months to four days
Like in Pennsylvania, ABC techniques have played a vital role in bridge construction and rehabilitation projects throughout Michigan. For one particularly busy interchange, construction crews used a special slide-in method to reduce overall construction time from more than four months to only four days.
In this approach, the new superstructure is assembled on temporary foundations next to the existing bridge. Once the new bridge is completed, high-powered jacks are used to help move it into place. This process was used at the Interstate 96 and Michigan State Road 50 interchange near Grand Rapids. According to Roger Safford, region engineer for MDOT, the interchange’s location meant reducing closure time was a top priority.
“The largest carpool lot in the state is located in the shadow of this interchange,” Safford said. “I can’t emphasize enough the impact of technology on this location.”
For this project – only the third of its type ever completed in the state – engineers specified precast concrete box beams and MSE wall panels. The 4.5-million-pound bridge took between 6 and 7 hours to slide from the temporary substructure onto the permanent substructure. The success of the project and other ABC endeavors in the state has resulted in a new perspective on bridge work for MDOT engineers.
“We currently evaluate every bridge project now for ABC, and it’s not a question of, ‘Let’s justify this as ABC,’” Rogers said. “We have to justify for not being ABC.”
While advancements in techniques and increased adoption of precast concrete products has helped improve ABC, more remains to be done to further enhance the process. Researchers at U.S. universities and engineers across the country continue to seek improvements in design, standards and construction methods. But no matter what the future holds, durable, time-saving precast concrete products will continue to be an important part of the equation. Refining the techniques that make ABC successful are complex, but the choice to specify precast should be as easy as A-B-C.
Eric Carleton, P.E., is NPCA’s vice president of technical services.
Mason Nichols is the managing editor of Precast Solutions magazine and is NPCA’s external communication and marketing manager.
Sidebar – Going Strong
The Winter 2013 issue of Precast Solutions magazine covered MDOT’s first-ever foray into ABC (precast.org/new-approaches) in 2008. Construction crews prepared the Parkview Avenue Bridge over U.S. 131 in Kalamazoo County for precast bridge elements and systems construction with the hopes of finishing the project in 12 weeks. Thanks to the use of a variety of precast elements, the project was completed on time and considered a success.
Several sensors were placed inside of the Parkview Avenue Bridge to monitor its performance. According to Bruce Campbell, senior project manager with Parsons, the bridge has lived up to expectations.
“We did a physical inspection of the bridge last summer, and the only issue we found is that one of the cast-in-place wingwalls had some cracking,” he said. “All of the precast elements are performing very well.”
(1) Accelerated Bridge Construction: Experience in Design, Fabrication and Erection of Prefabricated Bridge Elements and Systems (FHWA-HIF-12-013)