New Opportunities for Building Bridges Better: The Evolution of UHPC

As State and local agencies seek innovative materials for long-lasting and resilient bridges, ultra-high performance concrete (UHPC) has emerged as a solution that offers unmatched strength and durability in many applications. Over the past two decades, the use of UHPC has been gaining momentum in projects across the country and transforming the way agencies look at extending the service lives of bridges.

In 2003 the Federal Highway Administration (FHWA) began working with States to consider UHPC deployment, which was developed in Europe in the 1990s. Since then, more than 400 bridges across the United States have incorporated UHPC, mostly for field-cast connections between prefabricated components or for preservation and repair. These successful early deployments led to a growing interest in broadening the use of UHPC to a new application—construction of the bridge’s primary structural components.

Creative UHPC Solutions

UHPC has exceptionally high durability and compressive strength compared to conventional concrete and offers a tensile strength that is approximately twice as high. These advanced properties can offer significant advantages in bridge design, construction, and preservation, including for primary bridge elements such as the girders, decks, or the piles that are driven in the ground to support the substructure. Importantly from a structural engineering standpoint, UHPC offers sustained post-cracking tensile strength, meaning that after it cracks it can still resist loads.

The Iowa Department of Transportation (DOT) has piloted various UHPC technologies and applications over the past couple decades, including the first use of UHPC for a bridge’s structural members. The Mars Hill Bridge in Wapello County, IA, which opened to traffic in 2006, was a collaborative effort between Iowa DOT, Iowa State University, and FHWA to replace an aging rural bridge with one using UHPC components. Due to the lack of formal U.S. design specifications for UHPC at the time, the Iowa DOT used guidelines developed in France to determine the design capacities of the bridge beams.

This first U.S. deployment of UHPC on a highway bridge provided valuable experience in design, testing, and fabrication methods, but before UHPC for primary structural components could advance to everyday use, bridge owners would need to gain more familiarity with the new material and, because its properties vary from those of conventional concrete, formal guidelines would need to be developed.

A few years after the Mars Hill Bridge project, round one of FHWA’s Every Day Counts program (EDC-1) kicked off with the inclusion of prefabricated bridge elements and systems (PBES) for accelerated bridge construction. Because PBES are connected onsite, they require high-quality, durable connections. FHWA added UHPC to EDC rounds three and four as an innovation that could improve the strength, simplicity, and durability of PBES connections.

Ben Graybeal, FHWA’s Bridge Engineering Research Team Lead, said that UHPC connections for PBES gained momentum quickly in States around the country, and the EDC program helped a lot in spreading the word. By December 2018, 33 States demonstrated, assessed, or institutionalized the technology.

“The entry point was connections, and we went from there to promoting UHPC for bridge repair and preservation during EDC-6,” said Graybeal. “Bridge owners across the country are now beginning to use UHPC in overlays to repair deteriorated bridge decks, in link slabs to replace failing expansion joints, and in other structural element repair scenarios. Preservation and repair provided another relatively easy entry point for more owners to get accustomed to the technology.”

Because of its strength and durability, UHPC can be used for preservation and repair in situations that normally use conventional concrete or repair mortars, and in some cases those that use structural steel. Additionally, UHPC repairs are long lasting and resilient, requiring less maintenance and fewer follow-up repairs than conventional methods. Some applications, such as bridge deck overlays and replacing expansion joints with UHPC link slabs, can extend the service life of bridges well beyond that of traditional repair strategies and are more cost-efficient than bridge replacement.

Structural Design with UHPC

As bridge owners around the country became more familiar with and gained experience using UHPC, interest in UHPC for structural design applications began to grow, so FHWA began a multi-year effort to draft the needed guidance.

In October 2023, FHWA published a report on Structural Design with UHPC that included a draft version of design guidance based largely on FHWA research and informed by work from other research groups across the U.S. and around the world. The American Association of State Highway and Transportation Officials (AASHTO) Committee on Bridges and Structures used the draft guidance as a basis for its Guide Specifications for Structural Design with Ultra-High Performance Concrete, 1st Edition, which was released in March 2024.

“The new structural design guidance offers a path forward,” said Graybeal. “The goal is to get to the point where designing structures with UHPC is commonplace where appropriate—when the UHPC solution fits the need—and the structural design guidance is an important part of that.”

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Cover photo: The first bridge in the United States to use UHPC for primary structural elements was the Mars Hill Bridge in Wapello County, Iowa.