Research sponsored by the Federal Highway Administration (FHWA) is opening up a new world of ultra-high-performance concrete (UHPC), offering the opportunity to build longer-lasting bridges and accelerate the renewal of the nation’s highway infrastructure.

First developed in the 1990s, UHPC is an advanced cementitious composite material. Compared with conventional concrete materials, it tends to exhibit superior properties, such as exceptional durability, high compressive strength, usable tensile strength and long-term stability. UHPC does not contain coarse aggregate and so does not exhibit the early-age microcracking common in conventional concrete. It also has extremely low permeability. UHPC generally contains high cementitious material contents, low water-to-cementitious material ratios, compressive strengths above 21.7 ksi, and sustained tensile strength as a result of internal fiber reinforcement. These advanced properties enhance bridge durability, allow for longer spans or shallower girder depths and can be used to develop new structural forms that will facilitate accelerated bridge construction.

Recent FHWA research projects included one that evaluated the performance of field-cast UHPC connections linking precast concrete bridge-deck components. This study was part of Transportation Pooled Fund Project 5(217), which is being conducted in partnership with the New York State Department of Transportation and the Iowa Department of Transportation.

While the use of modular bridge-deck components can produce higher-quality, more durable bridge decks, the required connections have often been lacking, diminishing the overall system performance. UHPC offers the opportunity to significantly improve the performance of these field-cast connections, allowing for greater use of modular bridge-deck systems.

As the study demonstrated, UHPC can exhibit an exceptional bond when cast against previously cast concrete. The use of UHPC also can significantly shorten the development length of embedded discrete steel reinforcement. These properties allow the modular component connection to be redesigned, simplifying construction and enhancing long-term system performance.

To date, field-cast UHPC connections between prefabricated bridge components have been implemented in nine bridges in Canada and two in the U.S. The projects have used several different details to connect precast concrete modular bridge components, including adjacent box beams, full-depth precast deck panels and deck-bulb-tee girders. The field-cast UHPC connections have allowed for simple connections without requiring the use of post-tensioning or large volumes of field-cast concrete. Performance of the connections to date has exceeded what is normally expected of a field-cast connection. More information on the field-cast UHPC connection project can be found in “Field-Cast UHPC Connections for Modular Bridge Deck Elements,” which will be available online at www.tfhrc.gov/structur/index.htm.

Another pair of recent FHWA research projects evaluated a UHPC pi-girder cross-section prototype developed for use in short- and medium-span highway bridge applications. The girder was developed and optimized to take advantage of the advanced mechanical and durability properties of UHPC. The projects demonstrated that the concept of decked UHPC modular girders for bridge construction is viable.

For more information, download “Structural Behavior of a Prototype UHPC Pi-Girder” (Pub. No. FHWA-HRT-09-068), which is available at www.fhwa.dot.gov/publications/research/infrastructure/­structures/09068, and “Structural Behavior of a 2nd Generation UHPC Pi-Girder” (Pub. No. FHWA-HRT-09-069), which can be found at www.fhwa.dot.gov/publications/research/­infrastructure/structures/09069.