Ben Graybeal, Federal Highway Administration
(Photo courtesy of New York State DOT.)
The use of modular bridge deck components has the potential to produce very high quality and extremely durable concrete bridge decks. Advanced cementitious composite materials, whose mechanical and durability properties far exceed those of conventional concretes, present an opportunity to significantly enhance the performance of field-cast connections thus facilitating the wider use of modular bridge deck systems. Ultra-high performance concrete (UHPC) represents a class of such advanced cementitious composite materials. Of particular interest, UHPCs can exhibit both exceptional bond when cast against hardened concrete and can significantly shorten the development length of embedded steel reinforcement. These properties allow for a redesign of the modular component connection, facilitating simplified construction and enhanced long-term system performance.
Field-Cast UHPC Connection Details
The concept of using the advanced properties of UHPC to significantly modify the design of connections between precast concrete components is not new. Research and deployments in this area date back to at least 1995 in Denmark and Sweden.(1) More recently, the concept of using the properties of UHPC to redesign the connections between modular bridge components has been recognized in North America. As of late 2010, field-cast UHPC connections between prefabricated bridge components have been implemented in nine bridges in Ontario, Canada, and two bridges in the United States. These bridges use a range of details to connect a variety of different precast concrete modular bridge components, including adjacent box beams, full-depth precast deck panels, and deck bulb-tee girders. The connection designs used to date have tended to mimic non-contact lap splice connections with a female-female shear key profile.
The photograph shows the casting of UHPC into the longitudinal connection between the top flanges of deck bulb-tee girders on the Route 31 Bridge over the Canandaigua Outlet in New York State. This bridge was opened to traffic in 2009.
The two fundamental differences between the field-cast UHPC connection concept and conventional modular component connection concepts are simplicity and performance. This connection concept allows for small, simple connections without requiring the use of post-tensioning or the use of large volumes of field-cast concrete.
Ultra-High Performance Concrete
Advances in the science of concrete materials led to the development of the next generation of cementitious materials, namely UHPC. As a class, these concretes tend to have high cementitious materials contents, very low water-cementitious materials ratios, compressive strengths above 22,000 psi (150 MPa), and sustained tensile strength resulting from internal fiber reinforcement.(2) Tensile behavior of UHPC stands in contrast to that of conventional concrete. The discrete steel fiber reinforcement included in UHPC components allows the concrete to maintain tensile capacity beyond cracking of the cementitious matrix. The inelastic straining of the component is resisted by the fiber reinforcement that bridges the tight, closely-spaced cracks. The mechanical behavior exhibited by UHPC allows for the full development of discrete embedded reinforcing bars in exceptionally short distances; thus facilitating the redesign of closure placements and other field-cast connections between modular components.
Physical Testing Program
The Federal Highway Administration (FHWA) through its Structural Concrete Research Program recently completed an experimental study focused on the performance of field-cast UHPC deck-level connections between precast modular bridge components. This study was part of Transportation Pooled Fund Project 5(217), which is being completed in partnership with New York State Department of Transportation (DOT) and Iowa DOT. A summary of the study and results can be found in FHWA-HRT-11-022.(3) The full study results are in NTIS-PB2011-101995(4) available at www.ntis.gov.
Bridge deck components simulating both longitudinal and transverse connections were fabricated and tested under both cyclic and static wheel patch loadings. The loading program was designed to allow for the assessment of three critical behaviors. First, cyclic loading below the cracking load allowed for the assessment of the cracking performance of the field-cast UHPC and the bonding performance of the UHPC to the precast concrete interface. Second, cyclic loading generating stresses above the static cracking stress of the specimen allowed for the assessment of the cracking performance of the system, including whether there was any uncontrolled, progressive cracking or any interface debonding. Finally, the loading program allowed for the assessment of the static overload performance of the system.
Test Results
The results of this test program, in combination with the experience gained through deployments of field-cast UHPC-filled connections, have demonstrated the viability of this connection system for precast modular bridge deck components. The structural behavior of the UHPC-filled connections tested in the FHWA research program fulfilled the requirements for deck behavior. The study also demonstrated that the non-contact lap spliced reinforcement in the transverse and longitudinal UHPC-filled connections was not susceptible to debonding from the UHPC under cyclic and static loadings. The most severe cyclic test concluded with the metal fatigue failure of a series of straight, uncoated No. 5 (16M) steel reinforcing bars, which were lapped over a 5.9 in. (150 mm) length in a non-contact lap splice configuration. No evidence of the reinforcing bars debonding from the field-cast UHPC was observed.
Future Implementation
The concept of using field-cast UHPC to facilitate the use of prefabricated modular bridge components is gaining interest around the United States and Canada. The Ontario Ministry of Transportation and the New York State DOT are continuing to use this technology as appropriate projects arise. Iowa DOT is also working toward a first application. Other states are also considering the benefits of this technology as they move toward increased usage of modular components and other accelerated bridge construction technologies.
Further Information
For further information, readers are encouraged to contact the author at 202-493-3122 or [email protected].
References
1. Hansen, L. and Jensen, B., “A New Building System Using Joints of Ultra High-Strength Fibre Reinforced Concrete,” Proceedings of the International Conference on Creating with Concrete, University of Dundee, Scotland, 1999, pp. 543-552.
2. Graybeal, B., Material Property Characterization of Ultra-High Performance Concrete, Report No. FHWA-HRT-06-103, Federal Highway Administration, August 2006, 186 pp.
3. Graybeal, B., Field-Cast UHPC Connections for Modular Bridge Deck Elements, Report No. FHWA-HRT-11-022, Federal Highway Administration, Nov. 2010, 8 pp.
4. Graybeal, B., Behavior of Field-Cast Ultra-High Performance Concrete Bridge Deck Connections Under Cyclic and Static Structural Loading, NTIS Report No. PB2011-101995, Nov. 2010, 106 pp.