There are over 54,000 bridges in the United States that need to be repaired or replaced, according to the 2018 Deficient Bridge Report published by the American Road & Transportation Builders Association. This represents almost 9% of all bridges in the United States, including historic landmarks such as the Brooklyn Bridge.
Repair work on this many bridges would take decades if work continues at the current pace of conventional bridge construction. The Accelerated Bridge Construction (ABC) method speeds up the construction process while delivering other benefits to those involved in the project and those living near the construction site.
In this post, we take a closer look at the ABC method for bridge construction and answer the question: “Is ABC bridge construction the best way to build a bridge?”
What is Accelerated Bridge Construction (ABC)?
The U.S. Department of Transportation Federal Highway Administration (FHWA) defines ABC as “bridge construction that uses innovative planning, design, materials, and construction methods in a safe and cost-effective manner to reduce the onsite construction time that occurs when building new bridges or replacing and rehabilitating existing bridges.”
ABC bridge construction requires changes in the structural solutions that are used for building:
Prefabricated Bridge Elements and Systems (PBES)
PBES are structural components of a bridge that are built offsite or nearby, but outside of traffic areas. Once complete, they are moved to the site for quick installation.
Prefabricated elements include:
- Deck elements
- Beam elements
- Pier elements
- Abutments and wall elements
- Other elements such as deck closure joints or overlays
Prefabricated systems include:
- Superstructure systems such as an arch span with deck and full-width beam span with deck
- Superstructure/substructure systems such as rigid frames with parapets
Structural Placement Methods
Structural Placement Methods are the equipment, technologies, and procedures used in ABC. They include:
- Self-Propelled Modular Transporters (SPMTs) – A vehicle that is able to carry very large structures and structural components from offsite locations and place them in the proper position. Placement can usually be done in anywhere from several minutes to hours, reducing the overall time traffic is shut down when compared to instances in which all construction is done onsite.
- Slide-In Bridge Construction (SIBC) – A new bridge is built on temporary supports, often in parallel to the existing structure. Once complete, the old bridge is shut down and demolished, and the new bridge is slid into place. This process can usually be completed in 72 hours, drastically reducing the “down” time of the bridge during construction.
Ultra-High Performance Concrete
The prefabricated bridge components used in ABC typically demand field-cast connections that are both durable and easy to build. Ultra-High Performance Concrete (UHPC) fills this need, and is already being used by many construction companies for the seams in current projects.
UHPC is a class of concrete that is exceptionally strong and durable. It has ten times the compressive strength of regular concrete. Additional strength can be achieved with various mixing techniques, with 50,000 psi being achievable.
It has shown durability and resistance to natural forces not seen in previous concrete products. Pre-stressed solutions and integrated fiber allow for longer spans and thinner panels without supports. UHPC is very adhesive, making it ideal for repairs and reconstruction of existing bridges when pieces need to be modified or retrofitted.
Additional benefits include:
- Impact resistance
- Dimensional stability
- Freeze/thaw resistance
- Chemical resistance
- Aggressive environmental resistance (impervious to water)
- Longer useful life (estimated longevity exceeding 75 years versus 15-25 years for traditional concrete)
Is ABC bridge construction the best way to build a bridge?
The ABC bridge construction method has been used for over 20 years. The benefits of this methodology, combined with government support and recent research, demonstrate that the ABC bridge construction method is the best way to build a bridge.
The benefits of this approach include:
- Reduced time required to build a bridge – Using prefabricated elements that are constructed offsite speeds up the overall construction process. Multiple components can be built simultaneously in various locations and then assembled onsite more rapidly than if all construction occurred at the bridge site.
- Less onsite construction – Offsite construction directly translates to less onsite construction. An existing bridge that is being replaced can remain open for a longer period of time before being repaired or replaced.
- Improved safety for the public traveling near the bridge and for workers – Reduced onsite construction keeps the public safer. Bridge closures and the associated heavy machinery and re-routed traffic increase the incidence of accidents for both drivers and pedestrians. Less construction time onsite also reduces the number of accidents for construction workers, as accidents are more likely to happen while maneuvering within the construction zone.
- Reduced traffic delays – ABC construction keeps the bridge to be repaired or replaced open for a longer period of time. Replacement parts can be built offsite or the new bridge can be built in parallel to the existing structure. The older bridge can remain open until the new one is ready. Total closure time can be reduced to just a few days, during which time the old bridge is demolished and the new bridge is slid into place.
- Reduced impact of weather-related issues on the project timeline – Offsite construction provides more options to work around weather-related issues. Prefabricated elements that are sensitive to certain conditions can be built in climate-controlled areas.
- Lower environmental impact – Fewer road closures and traffic delays are good for the environment. Less idle time for cars sitting in traffic jams means less exhaust and emissions. Drivers are not forced to take long detours, wasting fuel.
- Increased durability – The components used in ABC construction, such as UHPC, are more durable and have a longer life. The bridges constructed from these materials last longer.
- Minimized impact to existing roadway alignment – The prefabricated structures used in ABC construction allow for better alignment with existing roadways.
- Lower overall cost – Conventional onsite bridge construction can cause traffic delays and detours that have an economic impact on the surrounding commercial activities in the region. These costs have been known to be higher than the cost of the bridge construction itself. This makes the offsite approach of ABC construction a better choice for the local economy.
FHWA supports the accelerated bridge construction method. Every Day Counts (EDC) is an FHWA program that identifies and utilizes innovative technologies to reduce project timelines, increase safety, reduce traffic delays, and lessen the environmental impact of construction at the state and local levels.
Every two years, the FHWA works with state and local transportation departments, governments, and other vested parties to identify the latest innovations to support. The EDC-2 Innovations which were released in 2013 and 2014 lists Accelerated Bridge Construction as an innovation that should be championed. FHWA’s support of ABC bridge construction should compel contractors to use this method as it is among the preferred approaches.
The Accelerated Bridge Construction University Transportation Center
The Accelerated Bridge Construction University Transportation Center (ABC-UTC) engages in ABC-related research. It is funded by the United States Department of Transportation.
Some of the objectives of ABC-UTC are:
- Advancing the Accelerated Bridge Construction method
- Discovering new knowledge around ABC
- Educating professionals on ABC methods
ABC-UTC conducts research, engages in education and workforce development, and transfers knowledge of ABC technology. The Center is led by Florida International University, and it partners with Iowa State University and the University of Nevada – Reno.
A recent experiment was performed at the University of Nevada, via Reno’s Earthquake Engineering Laboratory, which tested the combination of ABC methods. The tests were conducted in April 2018.
A 70-ton, two-span, 70-foot-long bridge underwent multiple earthquake simulations. Six different types of bridge connections that were combined into a single bridge were tested.
This was the first time tests were run on a combination of connections rather than on individual connections. All six connections performed as expected and according to design.
Additional data from hundreds of sensors in the connections will be further evaluated over the next few months, but the immediate results enable ABC-UTC to recommend these connections for use in real-world applications with confidence.
The ABC bridge construction method reduces costs, saves time, improves safety, and protects the environment. Contractors who are serious about winning bids for bridge repair and reconstruction will look to this method as the go-to approach given government support and the proven advantages.
Photo Source: Peter Buitelaar Consultancy and design by FDN in Eindhoven the Netherlands