Building Connections That Last
FHWA is encouraging the use of ultra-high performance concrete to join prefabricated bridge elements and improve their performance.
The Federal Highway Administration, during the first two rounds of its Every Day Counts (EDC) initiative, EDC-1 and EDC-2, promoted prefabricated bridge elements and systems to help improve quality and durability and shorten onsite construction time. Prefabricated bridge elements, however, need to be connected onsite, and the overall benefit of using prefabricated bridge elements is limited by the quality and durability of those connections.
To address this issue, FHWA is promoting ultra-high performance concrete (UHPC) in EDC-3 and EDC-4 to improve the strength, simplicity, and durability of prefabricated bridge element connections. This innovation builds on the experiences of early adopters, starting in 2009 and continuing today with implementation by dozens of States across the country.
“Ultra-high performance concrete is an advanced construction material that offers performance that far exceeds expectations for conventional concretes and grouts,” says Ben Graybeal, FHWA’s team leader for bridge engineering research in the Office of Infrastructure Research and Development. “Although UHPC can be expensive, targeted use to grasp specific opportunities has proven to be highly valuable to bridge owners. In the U.S. bridge market, using UHPC to cast simpler, more durable connections between prefabricated bridge elements is the ‘killer app’ that has emerged as the entry point for owners, consultants, and contractors to begin using this new class of concrete.”
What Is UHPC?
UHPC is a fiber-reinforced concrete with mechanical properties that exceed those of conventional concrete.
The common ingredients of UHPC include portland cement, supplemental cementitious materials such as silica fume and fly ash, fine aggregate, superplasticizer, steel fibers, and water. These ingredients are used to varying degrees in conventional concrete. What greatly differentiates UHPC from conventional concrete, however, is the amount of cementitious material, the type of aggregate, and the fiber content. According to FHWA’s Ultra-High Performance Concrete: A State-of-the-Art Report for the Bridge Community (FHWA-HRT-13-060), UHPC can have 2.5 times more cementitious material than conventional field-cast concrete.
Most commonly, only fine aggregates such as sand and glass powder are used in UHPC, creating a gradation that, when combined with the cement, provides a dense matrix with a discontinuous pore structure that prevents moisture from penetrating the concrete and significantly reduces permeability compared to conventional concrete. Steel fibers are not commonly used in bridge concretes, but UHPC often has steel fibers amounting to 2 percent by volume. At 2 percent, the amount of steel fiber in UHPC is comparable to the amount of reinforcing steel bars often used in conventional field-cast bridge concrete.
FHWA defines UHPC as having a water-to-cementitious ratio less than 0.25, a compressive strength greater than 21.7 kips per square inch, ksi (150 megapascals, MPa), and sustained post-cracking tensile strength greater than 0.72 ksi (5 MPa). At 21.7 ksi (150 MPa), UHPC is more than four times stronger in compression than most conventional field-cast concretes. More important, the tensile strength of UHPC is higher than conventional concrete, and UHPC retains its tensile strength after cracking. The direct tension cracking strength of steel fiber reinforced UHPC can be as high as 1.2 ksi (8.5 MPa), or more than three times the tensile strength of most conventional field-cast concretes. Thus, compared to conventional concrete, UHPC can make stronger connections between prefabricated bridge elements. For more information, see Ultra-High Performance Concrete: A State-of-the-Art Report for the Bridge Community and Design and Construction of Field-Cast UHPC Connections (FHWA-HRT-14-084).
Advantages of Using UHPC
Highway agencies use prefabricated bridge elements to help improve the quality and durability of bridges and to accelerate onsite construction. The offsite fabrication of bridge elements allows for careful use of high-quality materials in a controlled environment. These superior bridge elements, however, are only as good as their connections.
Connecting prefabricated bridge elements typically requires splicing reinforcing steel bars within the connection, and the bars carry the tension forces from one element to the other. Long splice lengths and hooks in the reinforcing steel bars are used to help transfer the forces through conventional concrete.
However, the strength and bonding characteristics of UHPC enable designers to eliminate the hooks and make the splice lengths shorter, with the concrete taking a greater and more efficient role in transferring the forces through the connection. In other words, with UHPC the connections can be smaller, and the reinforcing steel details can be simpler.
Connections made with conventional concrete can deteriorate because of water and chloride infiltration, which eventually causes freeze-thaw damage to the concrete and corrosion of the reinforcing steel. In contrast, UHPC samples exposed to marine conditions at Treat Island, ME, were in excellent condition after 15 years, despite undergoing approximately 100 freeze-thaw cycles per year. The chloride penetration in the samples was one-third that of typical high-performance concrete. For more information, see “Marine Performance of UHPC at Treat Island” in Ultra-High Performance Concrete and Nanotechnology in Construction at www.uni-kassel.de/upress/online/frei/978-3-86219-264-9.volltext.frei.pdf. According to A State-of-the-Art Report for the Bridge Community, the dense matrix of UHPC stops water from penetrating into the concrete, which prevents the freeze-thaw damage and steel corrosion that can cause conventional concrete to deteriorate.
In short, the benefits of using UHPC for connections are improved strength, simplicity, and durability. The material cost of UHPC can be high, but the cost benefits in terms of the connection details, field assembly, and long-term performance are significant.
Offsetting UHPC’s Higher Cost
The high cement content, tight gradation control of the ingredients,and steel fibers make UHPC significantly more expensive than conventional concrete. Although many highway agencies understand the strength and durability advantages of UHPC over conventional concrete, the higher material costs have so far discouraged transportation agencies from routinely using UHPC to replace the conventional concrete used in bridges.
With connections between bridge elements, however, the situation is different. The need for strength and durability is pronounced, and the relative quantities of UHPC needed for connections are small. This is why the FHWA EDC initiative has focused on UHPC connections. Also, smaller connections with less concrete and reinforcing steel are used with UHPC. This helps to offset the higher material costs. When transportation agencies consider life-cycle costs, the durability of UHPC further offsets initial costs by reducing the need for future repairs.
Locations of U.S. Bridges with UHPC Connections
The 3.5-mile (5.6-kilometer)-long Pulaski Skyway illustrates these life-cycle cost considerations. The skyway carries U.S. 1 and U.S. 9 between Newark, NJ, and Route 139, which connects to the Holland Tunnel into New York City. The New Jersey Department of Transportation currently is replacing the deck on the skyway using precast deck panels attached with UHPC connections. In addition to decreasing the cost of future repairs, the use of UHPC connections reduces the high costs of traffic control and the costs to users when lanes are closed for repairs. For more information on the Pulaski Skyway project and the use of UHPC, see the webinar recording at www.fhwa.dot.gov/innovation/everydaycounts/edc_4/uhpc_webinar_recording.cfm.
In summary, the cost of using UHPC for bridges is reduced by using it only for connections, and by using it to decrease the size and increase the durability of the connections.
Highway Agency Use of UHPC
The first highway project in the United States that employed UHPC was the Mars Hill Bridge in Wapello County, IA, constructed in 2005 (completed in 2006). The Iowa Department of Transportation and its partners on the project used UHPC to make the bridge’s concrete girders.
State Deployment of UHPC Connections (Demonstration Level or Higher)
Since 2005, transportation agencies in the United States have completed more than 100 bridge projects using UHPC. From 2005 through 2014, transportation agencies completed 52 projects using UHPC. In 2015 and 2016 alone, agencies completed another 48, which shows an acceleration in the application of UHPC.
The New York State Department of Transportation is largely responsible for the increase in the number of projects. Fifty-six of the 100 projects that used UHPC before 2017 were constructed in that State. FHWA has created an interactive map that presents these and additional project information (see www.fhwa.dot.gov/research/resources/uhpc/bridges.cfm).
Using UHPC for connections has been the most common application. Ninety-five of the 100 projects built before 2017 used UHPC to connect prefabricated bridge elements.
Through the EDC initiative, FHWA tracks the deployment status of UHPC connections by the 50 State DOTs, the District of Columbia, Puerto Rico, the Virgin Islands, and FHWA’s Federal Lands Highway Division. In January 2015, 12 highway agencies reported using UHPC connections, and by January 2017, the number had grown to 21 agencies. Also in January 2017, 14 additional agencies reported an interest in using UHPC connections.
Types of UHPC Connections
Agencies most frequently have used UHPC to connect precast deck panels together and to connect the panels to the girders. Shipping precast deck panels to the bridge site and connecting them onsite reduces the time to construct the deck compared with placing the bridge deck concrete at the bridge site. Offsite shop manufacturing also allows for the fabrication of high-quality deck panels. The bridge deck has extreme exposure to the environment and traffic, and the deck protects the rest of the bridge. Consequently, the durability of the deck panels and their connections is critical to the life of the bridge. For agencies that use protective wearing surfaces for these connections, the durability of UHPC can result in further savings in cost and time by reducing the need for protective wearing surfaces.
Agencies also have used UHPC to connect modular superstructure elements. With these elements, the girders and what will be the bridge deck are combined and fabricated offsite. The construction of a separate bridge deck on the girders at the bridge site is therefore eliminated.
Examples of modular superstructure elements include precast concrete girders or steel girders fabricated in pairs with the bridge deck attached; precast adjacent box girders; and deck bulb-tee girders. The connections between modular superstructure elements are deck-level connections, and the durability of UHPC has the same importance as it does for deck panel connections. With conventional concrete, the connections of adjacent box girders and deck bulb-tee girders can be susceptible to cracking along the length of the connection. Again, UHPC can provide a stronger connection that mitigates or eliminates this cracking.
In addition to superstructure elements, highway agencies have used UHPC connections for prefabricated substructure elements. Instead of casting the concrete for bridge abutments and piers in the field, the agencies have used abutments and piers composed of precast concrete elements that they connect in the field. Substructure connections generally have larger diameter reinforcing steel bars than superstructure connections. With conventional concrete connections, the larger bars require correspondingly larger and more complicated connection details. Here again, agencies have used UHPC to reduce the size and complexity of the connections.
FHWA’s Deployment Of UHPC
FHWA initiated UHPC research in 2001 and has taken an active role in delivering information on UHPC to highway agencies by providing technical publications and technical assistance to State DOTs.
To support the deployment of UHPC connections, FHWA published the Design and Construction of Field-Cast UHPC Connections (FHWA-HRT-14-084) in October 2014. This publication provides guidelines for the construction and testing of UHPC connections, and specific recommendations for designing and detailing the connections. For a full list of FHWA publications and research reports on UHPC, visit www.fhwa.dot.gov/research/resources/uhpc/publications.cfm.
In 2015, FHWA selected UHPC as one of the innovative technologies to deploy in EDC-3. Through the EDC initiative, FHWA offers UHPC workshops to State DOTs. The workshops provide an introduction to UHPC connections and cover the content of Design and Construction of Field-Cast UHPC Connections. The workshops include modules on the design, construction, and application of UHPC connections, as well as other applications of UHPC for bridges. Since March 2015, 23 highway agencies have hosted UHPC workshops. State agencies interested in hosting a UHPC workshop should contact their FHWA division office or Mark Leonard.
In addition, FHWA hosted six monthly public EDC webinars on UHPC, starting in March 2017 and ending in August 2017. The webinars featured an introduction to UHPC and presentations on the deployment of UHPC connections in Delaware, Georgia, Iowa, Minnesota (Hennepin County), New Jersey, and New York. FHWA recorded the webinars, and they are available for viewing at www.fhwa.dot.gov/innovation/everydaycounts/edc_4/uhpc_webinar_recording.cfm.
Through EDC, FHWA has projects in progress to update the agency’s guidelines for UHPC connections and to develop a construction checklist for UHPC connections. These two publications are scheduled to be available early in 2018 and will be accessible through the EDC Web page on UHPC: www.fhwa.dot.gov/innovation/everydaycounts/edc_4/uhpc.cfm.
“Whether part of a major river crossing or a grade separation, the FHWA investment made in UHPC is now helping owners build bridges better and faster,” says Joey Hartmann, FHWA’s director of the Office of Bridges and Structures. “FHWA will build on this success by continuing to support the development and deployment of a program of technology and innovation.”
Mark A. Leonard, P.E., is a structural engineer on the Structures Technical Service Team at FHWA’s Resource Center in Lakewood, CO. Leonard provides technical assistance, training, and review services in the areas of highway structure design, maintenance, preservation, and inspection. He began his employment with FHWA in 2012 and has 28 years of experience as a structural engineer with the Colorado Department of Transportation. He has a bachelor of science degree in civil engineering from the University of Notre Dame.
For more information on FHWA’s UHPC research activities, contact Benjamin A. Graybeal at firstname.lastname@example.org. For more information on FHWA’s EDC deployment of UHPC, contact Mark A. Leonard at 720–963–3747 or email@example.com.