Fiber Reinforced Polymer Composite Bridge Technology
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A Successful Beginning for FRP Composite Materials in Bridge Applications
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(Published in the FHWA Proceedings, International Conference on Corrosion and Rehabilitation of Reinforced Concrete Structures, December 7-11, 1998, Orlando, FL.)
Benjamin Tang, P.E. and Walter Podolny, Jr., Ph. D., P.E.
Corrosion decay has been a continued challenge for bridge engineers, but a new material technology that delivered the stealth aircraft can eliminate corrosion from our bridges. The high strength, high fatigue resistance, lightweight, and corrosion resistance of composites are highly desirable characteristics for bridge applications. Of more than 80 bridge projects using fiber reinforced polymer (FRP) composite materials in the world, the U.S. has a modest beginning with 31 projects, 27 of which were built within the last 4 years. This paper will focus on some of the initial successful bridge applications in the U.S. using FRP composites. The discussions will also include the advantages, characteristics, concerns, and future needs to advance the composite technology into the civil infrastructure. These new materials are applicable to both construction of new structures and maintenance and rehabilitation of existing bridges. This is an exciting time for civil and structural engineers to be involved with the FRP composite technology. The FRP technology for civil infrastructures has demonstrated some initial success. The current focus for the FHWA is to advance the FRP composite technology to rebuild the American transportation infrastructure in new bridge construction as well as to rehabilitate and maintain existing bridges. The rebuilding of the Nation's highway system presents a tremendous market opportunity well into the 21st Century.
(corrosion, corrosion resistance, fiber reinforced polymer composites (FRP), bridges, durability, high performance materials, composites technology, FRP bridge decks, reinforcing elements, cable, tendon, laminates, concrete pier repairs, retrofit)
Background in Nurturing New Technology
Corrosion decay has been a continued challenge for bridge engineers. Bridges constructed of steel members or reinforcements must be properly protected for their long- term durability. When the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991 was signed into law, it had not only recognized the corrosion problem faced by our Nation's bridges but also provided Federal funding in bridge painting and the application of calcium magnesium acetate (a noncorrosive deicing salt)(1). For 1997, ISTEA provided $2.5 billion for the Highway Bridge Replacement and Rehabilitation Program; the majority of the funds were expended to replace or rehabilitate bridges caused mainly by corrosion deterioration. When the States' matched and unmatched estimated funds are included, the amount doubled to a staggering $5 billion.
For new bridge deck construction, epoxy coated rebar has been shown to provide durable corrosion protection and to prevent premature spalling or corrosion-induced cracking on more than 20,000 bridge decks in America. When both the top and bottom mats of rebar are epoxy coated, a concrete reinforced deck can be expected to have a 50-year life. High performance concrete with added corrosion inhibitors will further extend the design life. Stainless steel reinforcement shows a potential corrosion-free service life of 75 or more years. For existing bridge decks that are both reinforced with black reinforcing steel and contaminated with chloride ions, cathodic protection systems are routinely used as a rehabilitation technique to extend their service life.
In spite of the high success achieved in alleviating corrosion-induced deterioration in newly constructed and rehabilitated existing salt-contaminated bridges, there is still a need to evaluate new technologies for bridges of the 21st Century. It is believed that the space age technology that delivered the stealth aircraft can eliminate corrosion from our bridges. The proposed solution is simply to use high performance, nonmetallic materials in our bridges. These new breeds of nonmetallic materials developed in the last 70 years are called fiber reinforced polymer (FRP) composites. The most common fibers are made of glass, aramid, and carbon. The resin matrixes that bind the fibers together to form composites are those of the polyester, vinyl ester, and epoxy groups. These composite materials have been applied in almost every type of consumer, industrial, marine and military defense products worldwide. The Composites Institute reported that the U.S. has consumed more than three billion pounds of FRP composite products each year in nine industrial markets since 1994(2).
FRP Composites in Civil Structures Are Gaining Momentum
While the defense industry has taken this technology to new heights in the development of the stealth aircraft, civil engineers are beginning to gain confidence and experience in applying this technology to civil structures. In October 1996, the Federal Highway Administration organized and sponsored a scanning tour on advanced composites in bridges to three select European countries and Japan. The purpose of the tour was to assess the state-of-the-art in the use of composites in bridge construction. The technical findings from the scanning team are basically categorized into new bridge construction, strengthening of existing bridges, and seismic retrofitting of bridge piers. The scanning report states that the U.S. composite materials bridge technology has developed concurrently with those of the world and is neither behind nor significantly ahead of the countries visited(3).
According to a data base recently compiled by the Composites Institute, there are more than 80 bridge projects worldwide using FRP composites materials(4). The U.S. has a modest beginning with 30 projects, 26 of which were built within the last 4 years. The remaining discussion in this paper will focus on some of the initial successful bridge applications in the U.S. using FRP composites. The discussion will also include the advantages, characteristics, concerns, and future needs to advance the composite technology into the civil infrastructure. These new materials are applicable to both construction of new structures and maintenance and rehabilitation of existing bridges. In new bridge construction, the FRP composite materials may be used in the entire structure (pedestrian and vehicular), or they could be used as structural members or components.
FRP Composite Materials for Bridge Applications
The first pedestrian FRP bridge was built by the Israelis in 1975. Since then, others have been constructed in Asia, Europe, and North America. Many innovative pedestrian bridges have been constructed throughout the United States using pultruded composite structural shapes which are similar to standard structural steel shapes. Because of the light-weight materials and ease in fabrication and installation, many of these pedestrian bridges were able to be constructed in inaccessible and environmentally restrictive areas without having to employ heavy equipment(5). Some of these bridges were flown to the sites in one piece by helicopters; others were disassembled and transported by mules and assembled on site. The advancement in this application has resulted in the production of second generation pultruded shapes of hybrid glass and carbon FRP composites that will increase the stiffness modulus at very little additional cost. The recognition of providing high quality fibers at the most effective regions in a structural member's cross section is a key innovation to the effective use of these high performance materials.
With the knowledge gained from working with pedestrian bridges, many researchers took the next step toward designing FRP composite vehicular bridges. Many deck systems were developed and tested beginning in the early 1990's. The first U.S. all-composite vehicular, public bridge was placed in service on December 4, 1996 in Russell, Kansas. It was a wet lay-up manufacturing method, a technology transfer from the defense industry. The FRP deck panels were shop-fabricated with composite honeycomb cells sandwiched between two face sheets. It took the Russell County work crew one day to install the bridge superstructure. Since then other demonstration projects using different geometrical variations of fiberglass composite deck systems have been constructed in States such as West Virginia, Delaware, Ohio, Idaho, Virginia, Maryland, and New York.
FRP composites have a high tensile strength; however, in almost all of the demonstration bridge projects constructed to date, the design has been driven by the stiffness requirement rather than strength. There is still much room for improvement and advancement of the composite deck systems in order to capitalize on its material strength. The key to successful application of the deck superstructure system is to optimize its geometric cross section and to establish well-defined load paths.
Because most of these deck systems are sealed and enclosed, they are inaccessible for field inspection. To ensure the composites' integrity, sophisticated nondestructive evaluation/testing (NDE/NDT) devices and fiber optic sensors have been incorporated into some of the composite deck systems to monitor the in-service condition of and the presence of moisture in the bridge deck. With time the effectiveness of the monitoring systems and the long-term service performance of composites can be ascertained.
The modular panel construction of bridge deck systems enable quick project delivery. A bridge built of composite materials can be constructed and put in service in a relatively short time and at a competitive cost. Its lightweight materials and ease of construction provide tremendous labor and traffic control cost savings to offset a higher first cost. An FRP deck could reduce the weight of conventional construction by 70 to 80 percent. This technology has demonstrated that a bridge structure can be replaced and put into service in a matter of hours rather than days or months. When a driver can cross a bridge on his way to work the next morning without realizing that it has been replaced the night before, it is innovative technology put to good use.
The FRP composites offer the potential to eliminate the problem of excessive dead load of long span bridges. Structural components for hybrid bridge construction such as FRP reinforcing elements, cable and tendon systems, and laminates have been successfully demonstrated in highway bridges. The reinforcing elements are fabricated into 1-D rods, 2-D grids or gratings, and 3-D fabric or cage systems. The University of West Virginia has designed and built the first U.S. all FRP composites reinforced concrete deck for vehicular traffic. Prestressing tendons are another one-dimensional elements that has been successfully demonstrated. Professor Grace of the Lawrence Technological University, Southfield, Michigan, has demonstrated that the carbon fiber composite prestressing tendons can be cost effective when combined with conventional materials and can also be designed to perform in a ductile manner(6). Cable and tendon systems are subject to fatigue, and under sustained loads will creep. Creep rupture is a major concern for glass fibers. Proper selection of fibers and adequate design criteria must be established to ensure the proper use of these products and are essential to the advancement of FRP's in prestressing applications. Laminates are pultruded with unidirectional fibers to form thin and narrow plates. The University of Maine determined that by adding about 2 percent FRP laminate into timber glulam beams, the strength is increased by more than 70 percent(7).
There is much work to be done in developing well-designed anchorages, connection details, and bonded joints in compsites for long-term durability. Bridge engineers are reluctant to rely solely on epoxy adhesive bonding technology to connect or join structural components. Electrical transmission towers out West have been built with connections that were snapped and locked together without the use of any fasteners. It is a tough challenge, but when adequate testing and performance data are available, bridge engineers will change their paradigm.
After the 1989 Loma Prieta earthquake, the California Department of Transportation mounted an extensive testing and development program to retrofit bridge piers using FRP composites. Having confirmed its performance in the laboratory, fiber wrap, filament winding, and pre-cured cylindrical half-shell systems were quickly developed, tested, and accepted as alternates to steel jackets for seismic retrofit of bridge piers in high seismic areas. Since then, thousands of concrete bridge piers that were designed with inadequate ductility, lap splices, and shear capacity have been successfully retrofitted using FRP composite wrap systems. The University of California at San Diego has determined that when a wrap system is properly designed and installed, the ductile capacity can be significantly increased to allow twice the deformation levels without any reduction in its capacity as compared to the as-built bridge piers(8).
These fiber wrap systems are also being used to repair deteriorated concrete piers, pier caps, and damaged beams. Under a small-scale research study, the FHWA repaired a damaged prestressed concrete girder with carbon fiber sheets. The repaired girder was tested to a higher capacity than that of the original girder. State highway departments such as those in Georgia, Florida, South Carolina, and Utah have successfully applied FRP composite materials to the repair of damaged or deteriorated beams in highway bridges. With this application, the condition of the deterioration in the concrete behind the composite materials remains uncertain. A good repair program should include an evaluation of the preexisting condition and structural integrity of the concrete to establish a baseline reference. After a structural member has been repaired, the in-service condition of the concrete substrate as well as the performance of the composites should be continuously monitored.
Challenges and Technical Issues
There are challenges with all new materials technology. Those challenges mentioned above should not be viewed as barriers, but as opportunities to study and improve the materials to ensure that the product will be durable and reliable. The main concern with FRP composites is long-term durability because the materials do not have sufficient historical performance data in bridge applications. There is a concern among bridge engineers for the long-term integrity of bonded joints and components under cyclic fatigue loading. There are concerns with improper curing of the resins and moisture absorption and/or ultraviolet light exposure of composites that may affect the strength and stiffness of the structural system. Certain resin systems are found ineffective in the presence of moisture. In the case of a glass fiber composite, moisture absorption may affect the resin and allow the alkali to degrade the fibers.
The high strength, high fatigue resistance, lightweight, and corrosion resistance of composites are highly desirable characteristics for bridge applications. Currently, these new materials are a direct technology transfer from the aerospace industry, and they are far more advanced than those required by civil applications. Most of the advanced composite materials that are cured at high temperature produce high quality components and possess excellent characteristics. In bridge applications, resins as the binders for the fiber and adhesives for joints and connections that can adequately cure at ambient temperature and still offer comparable quality and characteristics are more desirable and practical. More research is needed to develop the most effective and durable resin formulations. More efficient manufacturing and effective production methods for large volume panels and higher modulus materials are needed to make it more cost effective for composites to compete in the civil infrastructure. At the present time, the direct use of fiber composites from the aerospace industry is not cost effective as compared to conventional materials in bridge applications.
New Structural Concepts Using FRP Composites
More than 25 years ago, a paper(9) was published in Finland that addressed the feasibility of elevated tubular tunnel highways utilizing advanced materials such as high-strength steel, high-strength concrete, and plastics, materials that are currently available or being developed. A tubular bridge constructed of composite materials would protect the environment from traffic and the traffic from the environment. Safety would be increased because the roadway surfaces would be kept dry and free of fog and ice at all times. Maintenance would be reduced because snowplowing and deicing chemicals would not be required.
The tubular monocoque bridge girder would have greater stiffness as a result of its large moment of inertia. Its elliptical cross section would be reasonably aerodynamically advantageous. Traffic within the enclosed tube would be unaffected by wind gust on the vehicles. A tubular highway could be constructed of successive suspension spans of 1,000 to 1,500 meters (3,000 to 5,000 feet).
Another advancing tubular highway concept in recent years is that of the submerged floating tunnel, or "underwater bridge." A number of proposals have been made for environmentally sensitive projects applying this concept in Norway, Italy, Japan, and Switzerland. The concept is simply a tubular superstructure that is sufficiently submerged under water to provide adequate clearance for marine vessels to pass over it. Support for the superstructure would be provided by underwater piers at appropriate spacing, or alternatively, the superstructure could be anchored to the bottom of the lake or sea by cables.
Exciting Times for Civil and Structural Engineers
This is an exciting time for civil and structural engineers to be involved with the FRP composite technology. Although most engineers are not trained to work with fiber and resin composite materials, they can quite easily pick up the knowledge. Education and training should be provided to the critical mass of practicing engineers. Research studies and results of the composite technology and its durability should be published in civil and structural engineering journals. Technical papers should be presented in conferences and workshops where civil and structural engineers participate. Until recently, most of the papers have been published in the materials science and testing, manufacturing and trade journals, which are not read by bridge designers. Information and knowledge must be openly shared with civil engineers, bridge designers, and owners. Professional organizations should be dedicated and/or established to direct the technical advancement of the composite technology if it is to have a future.
There is still more work needed in material science development and production. Design and construction specifications are important protocol for engineers, and they need to be developed. Bridge owners must know how to inspect, maintain, and repair FRP composite bridges and these procedures must be established.
We need to involve more knowledgeable and experienced engineers to help develop standards, design guidelines, and specifications. When we have an approved design standard specifications for FRP composites, the FHWA anticipates that the FRP composite technology will launch the U.S. into a new era in bridge construction and rehabilitation.
Opportunities for Rebuilding the American Transportation Infrastructure
The FHWA, for the past 20 years, has researched and confirmed the use of FRP composite technology as a potential bridge building material. The FRP technology for civil infrastructures has demonstrated some initial success. The current focus for the FHWA is to advance the FRP composite technology to rebuild the American transportation infrastructure in new bridge construction as well as the rehabilitation and maintenance of the existing bridge inventory. The rebuilding of the Nation's highway system presents a tremendous market opportunity well into the 21st Century.
Benjamin Tang received a B.S.C.E. from the University of Maryland and a M.S. in Structural Engineering from the University of Illinois at Urbana. He is a registered professional engineer in Maryland. He has been with FHWA since 1977 and serves in the Bridge Division, Office of Engineering, as the technical expert and review authority for all structural matters in the Federal-aid bridge program. His work experience includes many long span bridge projects such as cable-stayed, tied arch, steel truss, segmental concrete structures, and movable bridges. Currently, he is actively pursuing the use of high performance materials technology for bridge applications.
Walter Podolny, Jr. has been with the Bridge Division, Office of Engineering, FHWA, for the past 27 years. He is a registered professional engineer in four States and a Fellow of the American Society of Civil Engineers and the American Concrete Institute. Dr. Podolny is the author or co-author of over 75 technical papers, reports, and articles; including the books, "Construction and Design of Cable-Stayed Bridges" and "Construction and Design of Prestressed Concrete Segmental Bridges." Both books are published by John Wiley & Sons and have been translated into Japanese and the latter into Chinese.
He has served as an advisor to the Kuwait Ministry of Public Works, as a member of the Governor's Board of Inquiry on the 1989 Loma Prieta Earthquake, and as Chairman of an International three-man design review panel for the Yangpu Bridge in Shangai, China. He has also provided technical assistance to the Mayor of Guatemala on the Incienso Bridge and is the recipient of the Secretary of Transportation's Silver Medal for Meritorious Achievement.
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