UHPC Making Strides
Progress is continuing on implementing ultra-high performance concrete technology on the U.S. highway system, even while some challenges remain.
As in most of the developed world, the highway infrastructure in the United States is greatly dependent on concrete and steel. Following the infrastructure construction that occurred in this country in the mid-20th century, decades of wear and tear have focused attention on the need for durable, long-lasting structures.
Innovative materials with enhanced mechanical and durability properties make it possible to construct new infrastructure and rebuild aging highways and bridges with structures that last longer. Over the past two decades, significant advances in research on cementitious materials have led to development of a new class of market-ready materials with many times the strength and durability of conventional concretes.
These new concretes tend to contain high percentages of cement and silica fume, have low water/cement ratios, and include steel fiber reinforcement, all of which contribute to the advanced material behaviors. For instance, compressive strength in these new concretes is more than seven times that of conventional concrete, while tensile cracking strength is three times greater. This emerging technology, known as ultra-high performance concrete (UHPC), has the potential to affect the U.S. highway system significantly.
Challenges remain, however, limiting widespread implementation of projects using this new technology. Among them are the lack of design code provisions, inadequate industry familiarity with the product, and high initial costs. Addressing these issues will require significant knowledge transfer, industry support and buy-in, and greater reliance on life cycle costing.
The Federal Highway Administration (FHWA) initiated UHPC research in 2001 and since then has made major strides in introducing the concrete and transportation industries to this next generation of concrete technology. The first UHPC I-girder bridge opened to traffic in 2006 (Mars Hill Bridge in Wapello County, IA), a second UHPC superstructure bridge opened to traffic in October 2008 (Cat Point Creek Bridge in Richmond County, VA), and a UHPC decked girder bridge opened to traffic in November 2008 (Jakman Park Bridge in Buchanan County, IA).
"There is a strong push by bridge owners and bridge industry officials to develop and deploy innovative, higher performance materials that have the potential for making significant positive impacts on bridge performance while resulting in lower life cycle costs," says Ian M. Friedland, FHWA's technical director for bridge and structures research. "UHPC is one of these new, innovative materials with such a potential."
The Challenges to Widespread Deployment
As is frequently the case with established industries serving the public works sector, implementation of innovations occurs rather methodically. FHWA has identified five specific reasons for the slow pace of UHPC deployment.
First, unless industry sees a clear financial benefit, manufacturers are unlikely to invest in innovative technologies. Manufacturers who see a risk in using a new material are hesitant to modify current operations so that they can produce the innovative product efficiently. As would be expected, the costs of fabricating UHPC components thus are significantly higher than the costs of manufacturing conventional concrete components.
Second, owners (in this case government agencies) traditionally are justifiably risk-averse and tend to take measured responses when presented with innovative solutions to existing problems. Limited budgets and concern that a new approach will be less successful than a conventional one can reduce the desire to try creative solutions.
Third, the lack of design code provisions relevant to the advanced properties of these innovations is a clear hinderance. This gap effectively requires that all structural designs proceed along one of two paths. The designer can choose to make limited use of UHPC, in effect using the advanced properties of UHPC simply as an added safety factor. Alternatively, the designer can rely on research results, effectively requiring some level of demonstration testing prior to implementation.
Fourth, the limited number of applications of UHPC to date necessarily means that limited experience is available with regard to inspection, maintenance, and repair of UHPC structures. Although FHWA researchers and others expect these structures to perform well once deployed into the highway system, UHPC is not immune to damage from overheight or wayward vehicles, or unanticipated structural loadings. Methods for inspecting UHPC for damage and for repairing UHPC components will need to be developed prior to widespread acceptance of this material by the highway industry.
Finally, the higher cost of the constituent materials in UHPC necessarily mean that it will have a higher per-unit volume cost than conventional and high-performance concretes. This increase is unlikely to be offset entirely through the use of more efficient structural designs. To compensate for the greater cost, designers need to use a life cycle costing approach that takes into account the enhanced durability of UHPC.
UHPC Research Studies
Research related to the development, properties, and application of UHPC is progressing despite these hurdles. Many university researchers have secured industry and government funding for UHPC-related studies. In particular, the most recent Federal transportation legislation — the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU) — contained funding designated specifically for UHPC research. Compared to funding in Europe and East Asia, however, UHPC research in the United States remains more constrained.
Cross-Section of a UHPC Decked girder (π-Girder)
The intent of FHWA's UHPC research is to (1) determine the properties of UHPC that would be demonstrated when used in the highway system, (2) explore the best applications for UHPC in the highway system, and (3) aid in the initial deployment of UHPC in transportation structures. The early phases of this research, now complete, focused on determining the basic mechanical and durability behaviors of UHPC and establishing how this material would behave when used in common prestressed concrete superstructure elements.
Modular UHPC Superstructure and Deck
Four FHWA research studies on UHPC are underway. The first is a project to develop a modular precast, prestressed concrete decked girder system applicable to typical highway bridges around the United States. The intent is to combine the increased strength and enhanced durability of UHPC into a packaged bridge with components fabricated offsite at a precast/prestressed plant and then quickly and efficiently transported and assembled onsite.
Many bridges in the U.S. highway system span 20 to 35 meters (65 to 115 feet) and are composed of superstructures and decks that are nearing the ends of their design lives. Reconstruction to replace these bridges likely will exacerbate traffic congestion because of lane closures. Development of modular components that allow for overnight replacement of superstructures and decks will be a major leap forward.
Between 2004 and 2006, FHWA led the effort to design, construct, and test a first-generation UHPC superstructure and deck modular component. In conjunction with FHWA, researchers from the Massachusetts Institute of Technology designed the first-generation component to be fully structurally optimized, with global flexure and shear being the driving factors. Given the lack of experience with components of this type, less emphasis was placed on fabrication considerations and localized structural behaviors in this first-generation component than on structural optimization.
Proposed UHPC Two-way Ribbed Precast Bridge Deck
FHWA designed a second-generation UHPC decked girder module to address the shortcomings of the first-generation component, namely difficult fabrication and insufficient local flexural strength. The first-generation module's test program demonstrated that the component expressed acceptable global structural behaviors, but that larger cross-sectional dimensions were necessary to alleviate local stresses and to allow for effective fabrication. The second-generation module has rounded fillets, thicker webs and deck, and no overhang blockouts.
Deteriorated Bridge Decks
The second FHWA research project focuses on developing an alternative to replacing deteriorated bridge decks when their superstructures remain viable. "Deterioration of bridge decks is among the most significant maintenance and bridge rehabilitation issues for the U.S. highway system," says FHWA's Friedland.
In recent years, U.S. researchers have pushed to develop modular deck components that can be assembled onsite during brief whole or partial bridge closures. The advanced properties of UHPC open the possibility of designing a deck component that is as strong and robust as existing cast-in-place concrete decks while also having significantly greater durability. The first phase of this effort is complete. The component that FHWA developed is a two-way ribbed precast deck element.
FHWA's study focused on analytically determining the viability of this solution in terms of structural behavior. The study demonstrated that a UHPC bridge deck panel can exhibit capacities equivalent to conventional concrete decks while being 30 percent lighter. The enhanced mechanical and chemical bond of UHPC to discrete reinforcement also allows for reduced connection sizes, thus potentially simplifying installation of precast deck elements.
Quantification of UHPC Properties
The third FHWA research effort focuses on quantifying the advanced structural and durability properties of UHPC. The existing body of knowledge on the tensile fatigue behavior of UHPC is limited. For U.S. design codes to allow general use of UHPC in transportation structures, researchers need to demonstrate that the material's advanced tensile mechanical properties are applicable over a wide range of environmental and mechanical stressors.
One phase of this research involves investigating the postcracking tensile fatigue of UHPC subjected to various stress and strain ranges. Another phase will investigate the influence of chloride-laden water on steel-fiber reinforcement of cyclically stressed cracks. Within this overall research, another part of the project will investigate the durability and structural response of a full-scale UHPC modular deck component stressed both environmentally and structurally, and then will compare those results to a similarly stressed conventional concrete component.
The fourth FHWA research project focuses on addressing issues of immediate importance to the industry tasked with fabricating UHPC components. One topic under investigation is the use of concrete cubes instead of cylinders when determining compressive strength. Compressive strength is a commonly captured structural concrete property, but obtaining this property for very high strength concretes is problematic due to the high testing machine forces necessary and the lack of an inexpensive means to ensure parallelism of the cyclinder ends.
Testing cubes to determine compressive strength is rare in the United States, probably because cylinder molds are cheaper and easier to use than cube molds. But using cubes can relieve the need to prepare the ends of a cylinder for testing through a time-consuming and expensive grinding process. Cubes are fabricated with parallel ends that thus can be tested with minimal preparation.
Another topic is the potential use of ultrasonic inspection techniques to determine the setting state of UHPC within a closed, restraint-inducing formwork. This research is ongoing but so far has demonstrated that properties of ultrasonic waves can be monitored as a surrogate for UHPC strength and shrinkage.
First Deployment: Iowa
The Mars Hill Bridge in Wapello County, IA, is the first UHPC bridge on a public road in the United States. This three-girder bridge spans 33 meters (108 feet) and has 1.07-meter (3.5-foot)-deep prestressed girders. The designer modified the girders from the standard Iowa bulb-tee design (a prestressed I‑girder designed to span longer distances than conventional prestressed I-girders) by using thinner flanges (the shallower tops and bottoms) and a narrower web (the thin part of the beam connecting the flanges). The designer also eliminated the normal steel shear reinforcing bars because testing demonstrated that the UHPC, with its steel-fiber reinforcement, was sufficient to carry the design loads.
"Two of the biggest challenges with the Wapello County bridge were the lack of a U.S. design specification and limited experience working with UHPC," says Norm McDonald, director of the Iowa Department of Transportation (Iowa DOT) Office of Bridges and Structures. "The design capacities of the bridge beams were determined using design guidelines developed in France and verified by testing a 22-meter [71-foot]-long full-scale beam. Although two local precast plants were certified to produce the beams, their bids were higher than expected due to production concerns, and a nonlocal precast company was selected to produce the beams. The project provided an excellent opportunity for the Iowa DOT and our project partners to gain valuable experience in design, testing, and fabrication methods involving UHPC."
The bridge opened to traffic in early 2006. "Our project was a great success as we were able to collaborate with industry, academia, and State transportation officials to advance the use of UHPC and demonstrate the great potential of UHPC in rebuilding the infrastructure of this country," says Brian P. Moore, P.E., Wapello County engineer and zoning administrator.
Second UHPC Structure: Virginia
In Richmond County, VA, the Cat Point Creek Bridge opened to traffic in October 2008 as the second UHPC bridge in the United States. This 10-span bridge contains one UHPC span of 24.8 meters (81.4 feet) with five 1.1-meter (3.7-foot)-deep girders. The prestressed girders included a one-for-one replacement of conventional concrete with UHPC. The tensile properties of UHPC allowed for the elimination of the traditional mild steel shear reinforcement that is normally cast into concrete girders. A local precast plant in southeastern Virginia fabricated the girders.
Third Deployment: Iowa
FHWA and Iowa DOT have designed a third bridge, which uses the second-generation UHPC modular decked girder component mentioned earlier. FHWA was responsible for the conceptual design of the UHPC component, while the Iowa DOT designed the remainder of the bridge. Buchanan County, in conjunction with Iowa DOT, constructed this bridge, which opened to traffic in November 2008. As opposed to the first two deployments, this bridge is the first one to make significant use of the advanced behaviors of UHPC through structural optimization of the bridge components.
The use of UHPC in the highway industry is progressing in the United States just as it is around the world. Other State departments of transportation (DOTs), such as Florida, Georgia, and New York, are investigating the use of UHPC on their highways. Superstructures with both longer and lighter prestressed girders are a possibility. Georgia, Iowa, and New York are contemplating using UHPC for precast modular deck components and as a cast-in-place cementitious material in joints. Some DOTs, such as Iowa's, also are considering using UHPC in specific areas where high durability is required, such as approach slabs between pavement and bridge decks.
Advanced cementitious materials exhibiting greater strength and durability clearly have a role to play in the construction and reconstruction of bridges and other critical highway structures. Research to date has demonstrated that the properties of UHPC will open many new avenues for engineers and bridge architects. However, certain challenges will continue to hinder the deployment of UHPC into the civil infrastructure. Most significantly, the high per unit volume cost of UHPC and the lack of codified design provisions are hurdles to be addressed. That is not to say, however, that UHPC doesn't have a clear role to play in addressing the challenges faced by the Nation's highway infrastructure.
"The needs and opportunities for UHPC certainly exist," says FHWA's Friedland. "We just need to address these technical and administrative impediments in a rational and systematic way."
- Graybeal, B. Material Property Characterization of Ultra-High Performance Concrete. Federal Highway Administration Report No. FHWA-HRT-06-103. August 2006. McLean, VA.
- Graybeal, B. Structural Behavior of Ultra-High Performance Concrete Prestressed I-Girders. Federal Highway Administration Report No. FHWA-HRT-06-115. August 2006. McLean, VA.
- Graybeal, B. and Hartmann, J. Experimental testing of UHPC optimized bridge girders: Early results. In: Proceedings of Precast/Prestressed Concrete Institute's National Bridge Conference. 2005. Palm Springs, CA.
- Garcia, H. and Graybeal, B. Analysis of an Ultra-High Performance Two-Way Ribbed Bridge Deck Slab. National Technical Information Service Report No. PB2007-112112. August 2007. Springfield, VA.
- Bierwagen, D. and McDonald, N. Ultra-High Performance Concrete Highway Bridge. In: Proceedings of Precast/Prestressed Concrete Institute's National Bridge Conference. 2005. Palm Springs, CA.
Benjamin A. Graybeal, Ph.D., P.E., is the structural concrete research program manager at FHWA's Turner-Fairbank Highway Research Center in McLean,VA. His duties include managing programs focused on high-performance concrete and UHPC. He holds bachelor's and master's degrees in civil engineering from Lehigh University and a Ph.D. in structural engineering from the University of Maryland.
For more information, contact Benjamin A. Graybeal at 202-493-3122 or email@example.com.