U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Date: Spring 1997|
Issue No: Vol. 60 No. 4
Date: Spring 1997
The Federal Highway Administration's (FHWA's) applied research program for using composite materials in highway structures is composed of two parts _ bridge structural applications and roadside structural applications. Bridge structural applications include concrete reinforcing components, all-composite bridge decks (i.e., the bridge roadway surface itself), and composite cables and primary structural members. Roadside structural applications include all auxiliary roadside structures (e.g., signs, lighting systems, barrier and rail systems, and crash cushions).
This research program is applied. That is, the research is directed toward specific applications or toward finding solutions for specific problems. It includes the development of the technology for using composite materials on our nation's highway infrastructure system.
Literature of the 1840s on the mechanical properties of civil engineering materials listed extensive properties of wood and iron. Steel was also mentioned but almost as an afterthought for, according to the literature of that era, this material was deemed exceedingly expensive. Because of its high cost relative to other structural materials, steel was considered by engineers of the time to be unsuitable for large civil engineering structures.
About four decades later, by 1884, John A. Roebling designed and completed the construction of the Brooklyn Bridge over the East River in New York City. At this time, the Brooklyn Bridge was the longest suspension bridge in the world, and it was fabricated entirely out of steel. The age of steel construction had arrived.
Methods of analysis and design, however, lagged. Typical structural construction methods of that period were more of a "brute force" nature with factors of safety typically ranging from 18 to 20. (Factors of safety are the ratio of the yield stress [estimated failure point] to the design stress.) The Brooklyn Bridge was only one such large structure of that era designed and built with such overly generous factors of safety.
Today, isotropic materials such as steel and aluminum and non-isotropic materials such as wood and reinforced concrete are commonly employed in civil engineering structures. Analysis methods for these materials to determine the applied load reactions (deflection, strain, stress) and for the prediction of the different modes of failure under load (ductile failure, brittle fracture, buckling stability) are much improved. This is evident by the fact that today factors of safety typically range from 2 to 4 for large civil engineering structures, and for aircraft/aerospace structures, safety factors can be as low as 1.5.
Today, for anisotropic fiber-reinforced polymer (FRP) composite materials, such factors of safety are usually much more conservative. Analysis methods are not as well-developed, and reliable failure-prediction models have yet to emerge. Analysis procedures for composite beam, plate, and shell structures have largely evolved from isotropic formulations and take into account either no interlaminar shear effects or very simplified interlaminar shear assumptions.
In effect, today, composite materials for structural design and the associated design methodologies are in the same state of evolution and advancement as steel had been relative to the construction materials of the mid to late 1800s. Candidly stated, today, we are in the iron age of FRP composites _ the 1840s of structural engineering relative to FRP-composite materials!
Because of the lack of practical analysis, design, and failure-prediction methodologies and also because of the promise that FRP-composite materials present for solving many existing problems in our roadway infrastructure system, FHWA is studying, evaluating, and developing the knowledge base of FRP-composite materials for bridge and auxiliary roadside structures. Because of this need, FHWA has designated composite materials design methodology, fastening techniques, and mechanics of composite materials analysis research as an FHWA High-Priority Research Area. The objective of this program of high-priority research entitled Structural Composites and Adhesives is to: Develop definitive guidance and criteria for the design, fabrication, and performance of fiber-reinforced plastics and adhesives used in the construction and rehabilitation of bridges and auxiliary highway structures.
The overall time frame for this program of large-scale, civil engineering, highway infrastructure research is at least 10 years. The total funding is estimated at $11 million. Although this dollar amount may appear very large to the reader, it is actually relatively small. The Civil Engineering Research Foundation (CERF), an organization affiliated with the American Society of Civil Engineers (ASCE), has estimated that in a 10-year period almost $900 million is required nationally to fully exploit FRP-composite materials and their applications in the construction-infrastructure marketplace.(1,2)
To the extent feasible, prior engineering research, both theoretical and experimental, from the aerospace and automotive industries is being used as the springboard or "starting point" of this research program. Because this is an applied research program, specific applications both in the bridge and the auxiliary roadside structures arena are to be developed and placed into service. One highly visible project related to this program is the advanced composite, cable-stayed bridge that will span Interstate 5 near San Diego. This congressionally mandated, two-lane, vehicle bridge, which is both publicly and privately funded, is currently under design by the University of California at San Diego.(3) Program Rationale
Why this emphasis by FHWA on FRP-composite structural analysis and design methodology? Perhaps the primary reason is related to the large number of existing bridge structures nationwide. Many bridge structures that are constructed from traditional structural materials, which include reinforced concrete, steel, wood, and even cast iron and stone, are old and deteriorated due to the severe outdoor corrosive elements. (For example, the effects of ice and snow are exacerbated by the application of salt, cinder, and sand mixtures on the roadways and also by freeze-thaw cycles.) Many others are structurally deficient due to steadily increasing truck loads, expanded seismic requirements, and/or upgraded safety requirements. All of these requirements lead to a large number of bridge structures nationwide that are in need of either repair, renewal, or replacement.
This is not to say that FRP-composite materials have no environmental problems. Techniques for improving the performance of these materials exposed to sunlight and ozone have been developed. The effects of long-term exposure to extreme temperature variations, various moisture conditions (rain, snow, and ice), variable and extreme wind conditions, wind-driven particles, and motor vehicle exhaust products on the performance of these materials are not fully understood. These effects must be determined and, where necessary, improved. However, because of the continual maintenance associated with corrosion of steel members, the potential replacement of steel reinforcing bars (rebars) in concrete bridge decks with FRP-composite grate systems is one active research program underway at the FHWA's Turner-Fairbank Highway Research Center in McLean, Va.
With a growing population in the United States, the national vehicle fleet, the total number of drivers, and the total number of vehicle-miles traveled each year on U.S. roadways have increased proportionally. Accordingly, the focus and needs of the nation's transportation infrastructure have expanded and changed. The focus has shifted from primarily new construction to repair, renewal, replacement, and new construction. All of this construction activity must be carried on at considerable cost to limited federal, state, and local budgets and is an increasing burden to the tax-paying public. Accordingly, the nation's needs now include a significant decrease in the cost of construction and maintenance, as well as a significant increase in the durability of U.S. transportation infrastructure systems.As previously stated, bridge deck deterioration of older bridges is a significant problem. Spalling of older concrete bridge decks, exacerbated by the effects of melted snow and deicing salt solutions seeping deep into the concrete deck surfaces, promotes chemical reactions with the internal rebars and results in corrosion (rust) and an attendant volume increase of the rebars. The result is a buildup of internal pressure causing material stresses that lead to spalling of the concrete. The use of FRP-composite reinforcing systems in concrete bridge decks is viewed as one short- to mid-term solution with all composite bridge decks as a potential longer term solution. Other incentives for using FRP-composite materials include the material's inherent high strength-to-weight ratio. The use of composite structural members, deck surfaces, and bridge railings can decrease the "dead load" associated with the bridge and allow for reduced-weight bridge designs or increased vehicle-load-carrying capabilities or both.
When used in roadside safety structures, the material's high energy absorption-to-weight ratio and the fact that high strength can be designed in the primary load-carrying directions make this material ideal for roadside structures under typical environmental loadings (wind, vibration, etc.) and also for vehicle collisions. The material's ability to transfer large quantities of kinetic energy from out-of-control vehicles efficiently into the structure, causing partial failure and deformation, makes this material highly attractive for use in these structures. Load transfer from the impacting vehicle into the roadside structure through predefined energy transfer paths can (potentially) reduce or eliminate occupant-compartment deformation and resulting vehicle-occupant injury.
FRP-composite materials have the potential to be "the civil engineering structural material of the 21st century." They have the potential to solve many of the problems that have plagued the highway structural engineer for most of the 20th century _ problems that have been addressed from a number of perspectives but have never been adequately solved.(4)
Composite Bridge Structures
Photo 3: This W-beam, steel guardrail system was effective in keeping the high-speed vehicle from leaving the road shoulder, but the ability of composite materials to transfer kinetic energy from the vehicle to the structure can potentially reduce or eliminate damage to the vehicle's occupant compartment and resulting injury to the occupants.
Program ScopeThe scope of the Composite Bridge Structures research program includes new bridge construction (both all-composite and partially composite structures), rehabilitation, and repair of existing bridge structures using FRP-composite materials. The scope also includes development of associated adhesive and mechanical connection technologies. This program includes all significant types of modern bridge construction _ e.g., deck-and-girder bridges, prestressed segmental bridges, as well as orthotropic plate decks (both composite reinforced concrete and all-composite decks).(5,6)
This program has developed along two parallel research paths: (1) Material Characterization and Design Methodology Development, directly funded by FHWA, and (2) Construction Applications, cooperatively funded by FHWA and the private sector. The first path is thought to be of direct value to the designers, the owners, and the maintainers of new and retrofitted bridges _ i.e., bridge design firms and the federal, state, and local departments of transportation (DOTs). Thus developed the rationale for primary funding by FHWA, augmented by secondary funding (perhaps) by state DOTs.
The second research path, Construction Applications, is of greater benefit to the FRP-composite raw material producers and finished-part fabricators although the designers, owners, and maintainers of these structures will also benefit. As a result, this research path is to receive significant fiscal support from both the raw material producers and finished-part fabricators because it directly results in marketable products. Also, from a practical standpoint, construction projects are expensive to implement, and those that involve new material applications involve some element of risk. Thus, with cost-sharing, cost and risk are shared by both the private and public sectors.
Material Characterization and Design Methodology Development
Much of the prior composite material technology (i.e., design, analysis, and testing techniques; material properties characterization) originated from the aerospace and automotive engineering communities in the 1960-1990 time frame. In both of these communities, composite applications have been developed to survive specific environmental and loading conditions.
Such is not the case with highway bridges and auxiliary roadside structures, where conditions are usually much more varied and highly ill-defined. However, given that this body of research does exist, FHWA is _ both through research underway and through planned future research _ adapting and augmenting this knowledge base by funding research, using a program of contracts, fellowships, cooperative agreements, grants, and in-house staff research projects.
This research path concentrates on the characterization of material properties, the effects on these properties due to environmental conditions typical of the 50 states, the effects of loading other than static (both impact and long-term), and the further development of reliable design and failure-prediction methods. A part of this program will provide users with commonly accepted test methods to measure FRP component and structural behavior. This will provide state and local DOTs, FRP component fabricators, and bridge builders with a means to evaluate the behavior of both component members and overall structural performance both during and long after fabrication. Although subject to change as this program of research and associated knowledge base matures, the following is a list of individual studies completed, underway, and planned for this research path:
The development of FRP-composite bridge structural applications is, of course, the ultimate objective of this research program. This development is open-ended; improvements will be made throughout the useful life of this "new" material (to civil engineering structural applications). To ensure a continuous and relatively rapid pace of improvements, FHWA is cooperatively sponsoring university and private sector development of practical composite structures and structural elements, including bridge decks, primary structural members and shapes, suspension cables, reinforcing rebars/grating/grates, and prestressed tendons. FHWA is also testing the adequacy of fasteners and other joining techniques. Although subject to change as our knowledge base develops, individual studies completed, underway, and planned for this research path include:
Photo 4: Research and testing has shown that FRP wrapping of existing reinforced-concrete bridge columns is very effective for increasing a bridge's column's resistance-to-failure during seismic disturbances. Application of this new technique to reinforced concrete bridge structures in California and other earthquake prone areas of the world is currently underway.
Composite Roadside Structures
The scope of the roadside structures composite research program includes all auxiliary structures that are found along the roadway _ signs, roadway lighting systems, rail and barrier systems, and crash cushions. At present, this program is focused on developing an FRP-composite rail (or roadside barrier) system. However, within the last few years, several kinds of FRP-composite roadway lighting systems have been developed by private sector manufacturers and are currently marketed nationwide.
Auxiliary roadside structures have a primary and a secondary function. Roadside signs provide directional, regulatory, or cautionary information to motor-vehicle occupants. Lighting structures illuminate the roadway during darkness or inclement weather. Rail and barrier systems prevent a passenger vehicle from running off the road and rolling down a steep slope or dropping off a cliff. Crash cushions stop a passenger vehicle within a short distance and prior to a fixed roadside hazard. However, all these auxiliary structures have a secondary function that some would say is more important. They must be "crashworthy." They must be capable of mitigating or eliminating vehicle-occupant injury when wayward, out-of-control vehicles collide with such structures. In effect, they must be designed such that injury-producing forces-of-collision (impulsive forces) are not transmitted to the vehicle occupants, nor does the roadside structure (or parts of it) penetrate into the occupant compartment and injure the occupants, nor does the vehicle-occupant compartment deform to such an extent that occupants are injured by occupant compartment collapse (crush).
Auxiliary roadside structures also must sustain, on a daily basis, normal environmental and routine maintenance loadings without excessive deformation or collapse. For example, sign and lighting structures must endure high wind loads during stormy weather, ice and snow loads during inclement weather below the freezing point, and vibratory loads when mounted on bridges and overpasses. All roadside structures must tolerate loads applied by snowplows as snow and ice is pushed and packed against these structures during the clearing of roadways. They must be able to sustain minor impacts from tractor-driven mowers and other miscellaneous maintenance equipment. Many of these loads are not well-quantified. However, to remain functional, these structures must sustain these loads without substantial damage. That is, analogous to bridges and other roadway structures, these auxiliary roadside structures must be designed for normal in-service loads in which component members remain well within the material's elastic limit.
However, as previously mentioned, these structures also must sustain severe impact loads caused by infrequent, but potentially catastrophic, vehicle collisions. Although such collisions are highly infrequent for any one roadside object, the annual total is high. For example, one group of researchers has determined that during the early-to-mid 1980s, for each of the years, more than 900,000 vehicle occupants were involved in collisions with fixed roadside objects, both manmade and natural. Of these annual 900,000 "crash involvements," almost 9,000 occupants (roughly 1 out of 100) were fatally injured.(7) These vehicle collisions often result in impact loadings that are far beyond the elastic limit of the underlying structural material. During collision, such structures must "manage" the large amount of kinetic energy inherent in the colliding vehicle. To do so, they must be designed and crash tested to either prevent or mitigate the potential for vehicle-occupant injury and fatality.(8) Passenger vehicles, typical of today's U.S. vehicle fleet, range in mass from 800 kilograms (kg) to 2,000 kg.
To manage this kinetic energy of impact, two energy management approaches have traditionally been employed in auxiliary roadside structures _ the "breakaway" approach and the "energy-absorbing" approach. Many structures such as signs, lighting supports, and other pole-like structures are often designed to fail or breakaway upon vehicle impact. Structures that contain this breakaway feature are intended to do so with little change in the vehicle velocity and little resultant distress to the vehicle and its occupants. Other roadside structures, such as barriers, rail systems, and crash cushions are designed to absorb the impact energy of the vehicle without exceeding the ultimate failure strength of the underlying structural material. They are intended to either bring the vehicle to a "controlled" stop (as in the case of a crash cushion) or to redirect the vehicle in a controlled manner (as is the case of a barrier or rail system). The word "controlled" is meant to imply that both the potential for occupant injury and the severity of injury is minimized.
Roadside Rail System Development
Roadside rail systems are typically energy-absorbing and energy-dissipating. They must be designed and tested to prevent vehicle break-through during a collision. This is because catastrophic failure of the rail system and resultant vehicle penetration can result in more severe consequences to vehicle occupants than the vehicle-to-structure collision itself. As an extreme example, consider a roadside rail system located to prevent vehicles that are out-of-control from plunging down a steep roadside slope or over a cliff. Vehicle penetration of the rail system has a high potential for serious or fatal injury to vehicle occupants. Thus, roadside rail systems must contain and redirect encroaching vehicles. Such vehicles cannot be permitted to penetrate the barrier.
To redirect vehicles in a controlled manner, rail systems must be designed with "pathways" to convert much of the kinetic energy of the colliding vehicle into the energy of work done on both the vehicle, the rail system, and the soil in which the system is mounted. For metallic rail systems, this is accomplished through a combination of energy transfers: (1) ductile permanent deformation of the rail system and the vehicle, (2) coulomb friction losses between the rail system while in contact with the sliding vehicle, and (3) the work done by the post-rail ground connection as the multiple posts "plow or rotate" through the soil. For an FRP-composite rail system, it is envisioned that ductile deformation of the rail can be approximated by a series of progressive fiber failures in many (but not all) of the individual fibers comprising the composite rail member. Thus, in this manner, a ductile-like failure can be approximated and energy can be irreversibly consumed by a partial breakage of the FRP rail member. In this manner, FRP rail systems can exhibit a high degree of material "toughness," that is, large amounts of distortional energy can be absorbed without catastrophic or complete failure.
To accomplish this _ to transform this design concept into a fully functional roadside rail system _ the associated Composites Roadside Structures research program is developing along two sequential, but overlapping, paths: (1) Developmental Testing and Analysis, and (2) Manufactured Part Fabrication and Compliance Testing. Both paths are aimed at the development and field implementation of a first-generation roadside FRP rail system capable of safely redirecting passenger cars and light trucks with masses of up to 2,000 kg in oblique angle impacts up to 25 degrees at collision speeds of up to 100 kilometers per hour (km/h). Currently, all developmental efforts are composed of a mix of research studies funded by FHWA. This mix includes: staff research, onsite graduate research fellowships, and cooperative research agreements with local universities. However, for this program to be successful, it must also demonstrate manufacturing and marketing savvy, as well as a national (or regional) sales presence. Composite part fabricators and material suppliers with manufacturing "know-how" and companies that have the wherewithal to "open up" existing markets to new material technologies are necessary additions.
Developmental Testing and Analysis
What has been accomplished to date?
To lay a technology foundation from which composite roadside safety structures can be developed and deployed, FHWA has funded and has cooperatively conducted a focused program of developmental testing and analysis for the past three years. Work to date on the FRP-composite rail system has included dynamic drop-weight testing of small-scale coupons and prototype shapes and has been conducted at the Turner-Fairbank Highway Research Center.(9-12) Static testing to failure and hand fabrication of parts has been accomplished under a cooperative research agreement at a university located in Washington, D.C.(13) In addition, an ignition loss method to determine the volume fraction of composite materials with filler materials has been cooperatively developed.(14,15) This method has been brought to the attention of the American Society for Testing Materials (ASTM) for possible incorporation into their testing standards. Furthermore, pendulum testing of full-scale steel guardrails has been conducted at Turner-Fairbank's Federal Outdoor Impact Laboratory (FOIL). Additional testing of steel guardrails and composite rail system testing of full-scale prototypes using this pendulum is planned. Computer simulation of selected impact tests _ both drop-weight and pendulum _ using the simulation code DYNA3D, which was developed and maintained by the Lawrence Livermore National Laboratory (LLNL), is ongoing.(16) Currently, these simulations use only isotropic materials. However, work directed toward simulation of composite material parts and systems is planned. Numerous research reports and papers have been published on the work completed to date.
Manufactured Part Fabrication and Compliance
What remains to be accomplished?
To continue and finalize work on an FRP-composite rail system, FHWA intends to cooperatively sponsor much of the development. Private sector involvement, with one or more U.S. companies (parts fabricators, material suppliers, marketing organizations) participating, is envisioned.
Initially, finalization of the rail system geometry and materials (cross-section shape, fiber material and directions, matrix material, internal energy-absorbing materials) and the associated connection details will be the principal focus. This will be accomplished through the procurement of full-size hand-fabricated and partially manufactured parts, subsequent quasi-static testing to failure, and then followed up by dynamic pendulum impact testing. All testing will be instrumented to collect applied load and resultant strain data using automated data-collection equipment. Dynamic pendulum testing also will be photographically recorded using multiple high-speed cameras. Subsequent analysis and retesting is anticipated until the design or designs have been finalized.
After finalization of the design, all essential composite parts will be manufactured from tooling. A cooperative, cost-sharing agreement is envisioned for all remaining efforts with the private sector partners sharing the cost of tooling and parts fabrication and with FHWA bearing much of the cost of testing. In return for sharing the developmental costs, the cooperative agreement partners will retain patent rights and royalty-free right to manufacture.
The remaining effort will primarily concentrate on safety compliance testing to ensure that the composite-rail system is safe or crashworthy and, thus, can be installed by state DOTs on all highway systems, including federal-aid highways. Additional pendulum tests may be performed to ensure adequate strength prior to the initiation of compliance testing.
Compliance testing uses actual full-size passenger vehicles as the "bullet" or impacting vehicle. Both lightweight, 820-kg, passenger sedans and heavier, 2,000-kg, pickup truck testing is required. All testing is at impact speeds of 100 km/h. The required impact angles are 20 and 25 degrees for the passenger sedan and the pickup truck, respectively. Electronic data from the bullet vehicle and overall high-speed film coverage of each test is required. For each test conducted, a "line run" of the composite rail system, approximately 40 to 50 meters in length, with each end rigidly anchored to the ground is required for evaluation purposes. The composite rail system will be instrumented with strain gauges, at a minimum, to access strains and to calculate stresses. Overall safety evaluation criteria include (1) structural adequacy of the rail system, (2) an appraisal of occupant safety, and (3) potential harm to occupants of other oncoming vehicles in the traveled way.
What guidelines are used for the development and acceptance of FRP-composite structures for the highway infrastructure environment?
Existing specifications _ applicable to bridge and auxiliary roadside structures fabricated from steel, concrete, and timber _ are currently being used as the starting point. For bridge structures, these specifications include the following:
What "yardsticks" or "effectiveness measures" can be used to determine the relative success of FHWA's Applied Highway Infrastructure Research Program on Composite Materials?
There are at least two and they are interdependent: field implementation and private sector involvement _ and both must be in substantial quantity. This is an applied research program. Specific applications, both in the bridge and the auxiliary roadside structure arena, are to be developed and placed into service. Therefore, one measure of success is continued, successive reapplication and refinement of this new technology in subsequent structures. This requires a transfer of the engineering technology; however, much more is required. The involvement of private sector U.S. companies with manufacturing "know-how," marketing savvy, and national or regional sales organizations is required. Therefore, other measures of success are the involvement of engineering design firms with an understanding of composite structural design, the involvement of composite part fabricators and material suppliers with the know-how regarding the art-and-practice of composite manufacturing, and the involvement of companies with marketing organizations that have the wherewithal to open up existing markets to this new material technology. Used in this sense, involvement implies public sector and private sector sharing of ideas and a pooling of resources and a sharing or pooling of the development costs, personnel assets, proprietary ideas, and the risks associated with this new technology. And, with the sharing of the risks associated with this new technology comes the potential for future rewards. For the public sector, the reward is a competitive, U.S. presence in world markets. For the private sector, the rewards are patents, rights-to-manufacture, and ultimately profits. And, for the public-at-large, the rewards are jobs and an increasing standard of living.
Martin Hargrave is a research engineer in FHWA's Office of Safety and Traffic Operations, Safety Design Division. He is the manager of FHWA's Composite Roadside Structures program. Before joining FHWA in 1979, he worked for 17 years in varied engineering assignments for private sector companies. He received a bachelor's degree in mechanical enginering from the University of Alabama, and a master's in engineering from Pennsylvania State University. He has completed additional graduate-level courses in civil engineering at the Catholic University of America.
Eric Munley is a research structural engineer in FHWA's Structures Division. Since 1989, he has directed the research program in Composite Materials and Structural Adhesives. He received a bachlor's degree from the University of Connecticut in 1974 and a master's in engineering mechanics from Cornell University in 1993. He is a licensed professional engineer in Connecticut.
Thomas J. Pasko is the director of FHWA's Office of Advanced Research. His professional experience includes more than 35 years with FHWA and two years with the Pennsylvania Department of Transportation. He received both his bachelor's and master's degrees in civil engineering from Pennsylvania State University. He has completed additional graduate-level courses at Cornell University. He is a licensed professional engineer in Pennsylvania.