Fiber Reinforced Polymer Composite Bridge Technology
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Fiber Reinforced Polymer Composites Applications in USA
DOT - Federal Highway Administration
Benjamin Tang, PE
Published in the First Korea/U.S.A. Road Workshop Proceedings, January 28-29, 1997
Word Version (.doc, 92 kb)
History on Fiber Reinforced Polymer Composites
"Although the concept of fiber reinforced materials can be traced back to the use of straw as reinforcement in bricks manufactured by the Isrealites in 800 B.C., and in more recent times to the use of short glass fiber reinforcement in cement in the United States in the early 1930's, fiber reinforced resin matrix materials (or fiber reinforced composites as we know them today) were not developed until the early 1940's". In this paper the term composites is used in the context of advanced fiber reinforced polymers (FRP's).
After World War II, US manufacturers began producing fiberglass and polyester resin composite boat hulls and radomes (radar cover). The automotive industry first introduced composites into vehicle bodies in the early 1950s. Because of the highly desirable light weight, corrosion resistance, and high strength characteristics in composites; research emphasis went into improving the material science and manufacturing process. That effort led to the development of two new manufacturing techniques known as filament winding and pultrusion, which helped advance the composite technology into new markets. There was a great demand by the recreation industry for composite fishing rods, tennis rackets, ski equipment and golf clubs. The aerospace industry began to use composites in pressure vessels, containers, and non-structural aircraft components. The US Navy applied composites in mine sweeping vessels, crew boats and submarine parts. The domestic consumers began installing composite bath tubs, covers, railings, ladders and electrical equipment. The first civil application in composites was a dome structure built in Benghazi in 1968, and other structures followed slowly.
The Composites Institute, that tracks the domestic economy, reported the total consumption figures for 1994 and 1995 at more than three billion pounds each year. Of the nine industries reported, the transportation industry claimed 31% of the total market, thus, being the biggest consumer of composites in both years. The preliminary figures for 1994 was 945.6 million pounds followed by 1995 with 984 million pounds. It is predicted that orders for composites in the transportation sector will exceed one billion pounds for 1996.
With the increasing demand for composites, new and improved manufacturing processes such as pultrusion, resin transfer molding, and filament winding were developed and implemented in the early 1990s. With these enhancements in place, the current focus is to rebuild the US transportation infrastructure using FRP composites for maintenance and rehabilitation of existing bridges as well as new construction.
Civil Structural Applications Using Composites
Many pedestrian bridge projects have been constructed throughout the United States using pultruded composite structural shapes. In states vulnerable to high seismic activity, concrete bridge columns are being retrofitted with a composite filament winding to increase ductility. Plate bonding using thin composite laminates to strengthen concrete and steel bridge members has been demonstrated. Composite prestressed piles are being applied to civil and marine structures in some of the coastal states.
The first US advanced composite vehicular bridge superstructure was dedicated into service on December 4, 1996 in Russell, Kansas. The deck panels were shop-fabricated with composite honeycomb cells which are sandwiched between two face sheets. These panels were then joined together with epoxy in the field. Demonstration bridge projects are being developed in other states such as Delaware, West Virginia and California. Continued research projects using composite reinforcing bars in concrete slabs are being studied in New Hampshire, Washington, D.C. and Michigan. Composite prestressing tendons and stay cables are being developed in Pennsylvania, Michigan, South Dakota and California.
There are three basic manufacturing techniques in producing composite structural products, with many variations and patented processes: 1) The pultrusion process involves a continuous pulling of the fiber rovings and mats through a resin bath and then into a heated die. The elevated temperature inside the die cures the composite matrix into a constant cross-section structural shape. 2) The filament winding process can be automated to wrap resin-wetted fibers around a mandrel to produce circular or polygonal shapes. 3) The layup process engages a hand or machine buildup of mats of fibers that are held together permanently by a resin system. This method enables numerous layers of different fiber orientations to be built up to a desired sheet thickness and product shape.
Characteristics of Composites
The mechanical properties of composites depend on many variables such as fiber types, orientations, and architecture. The fiber architecture refers to the preformed textile configurations by braiding, knitting, or weaving. Composites are anisotropic materials with their strength being different in any direction. Their stress-strain curves are linearly elastic to the point of failure by rupture. The polymeric resin in a composite material, which consists of viscous fluid and elastic solids, responds viscoelastically to applied loads. Although the viscoelastic material will creep and relax under a sustained load, it can be designed to perform satisfactorily. Composites have many excellent structural qualities and some examples are high strength, material toughness, fatigue endurance, and light weight. Other highly desirable qualities are high resistance to elevated temperature, abrasion, corrosion, and chemical attack.
Some of the advantages in the use of composite structural members include the ease of manufacturing, fabrication, handling, and erection. Project delivery time can be short. It took the Russell county engineer one day to install the deck panels in the first vehicular composite bridge. Composites can be formulated and designed for high performance, durability and extended service life. They have excellent strength-to-weight ratios. If durability can be proven to last 75 years, composites can be economically justified using the life-cycle cost method.
Some of the disadvantages in the use of composites in bridges are high first cost, creep, and shrinkage. The design and construction require highly trained specialists from many engineering and material science disciplines. The composites have a potential for environmental degradation, for examples, alkalis' attack and ultraviolet radiation exposure. There are very little or nonexistant design guidance and/or standards. There is a lack of joining and/or fastening technology. Because of the use of thin sections, there are concerns in global and local buckling. Although the light weight feature may be an advantage in the response to earthquake loading, it could render the structure aerodynamically unstable. In manufacturing with the hand layup process, there is a concern about the consistency of the material properties.
What Is Really in Composites?
As the name implies, advance fiber reinforced polymer composites is made of fiber reinforcements, resin, fillers, and additives. The fibers provide increased stiffness and tensile capacity. The resin offers high compressive strength and binds the fibers into a firm matrix. The fillers serve to reduce cost and shrinkage. The additives help to improve not only the mechanical and physical properties of the composites but also workability. The discussions that follow immediately will explain the basic functions and behaviors of the constituents.
The fiber is an important constituent in composites. A great deal of research and development has been done with the fibers on the effects in the types, volume fraction, architecture, and orientations. The fiber generally occupies 30% - 70% of the matrix volume in the composites. The fibers can be chopped, woven, stitched, and/or braided. They are usually treated with sizings such as starch, gelatin, oil or wax to improve the bond as well as binders to improve the handling. The most common types of fibers used in advanced composites for structural applications are the fiberglass, aramid, and carbon. The fiberglass is the least expensive and carbon being the most expensive. The cost of aramid fibers is about the same as the lower grades of the carbon fiber. "Other high-strength high-modulus fibers such as boron are at the present time considered to be economically prohibitive".
The resin is another important constituents in composites. The two classes of resins are the thermoplastics and thermosets. A thermoplastic resin remains a solid at room temperature. It melts when heated and solidifies when cooled. The long-chain polymers do not chemically cross link. Because they do not cure permanently, they are undesirable for structural application. Conversely, a thermosetting resin will cure permanently by irreversible cross linking at elevated temperatures. This characteristic makes the thermoset resin composites very desirable for structural applications. The most common resins used in composites are the unsaturated polyesters, epoxies, and vinyl esters; the least common ones are the polyurethanes and phenolics.
Since resins are very expensive, it will not be cost effective to fill up the voids in a composite matrix purely with resins. Fillers are added to the resin matrix for controlling material cost and improving its mechanical and chemical properties. Some composites that are rich in resins can be subject to high shrinkage and creep and low tensile strength. Although these properties may be undesirable for structural applications, there may be a place for their use.
The three major types of fillers used in the composite industry are the calcium carbonate, kaolin, and alumina trihydrate. Other common fillers include mica, feldspar, wollastonite, silica, talc, and glasses. When one or more fillers are added to a properly formulated composite system, the improved performance includes fire and chemical resistance, high mechanical strength, and low shrinkage. Other improvements include toughness as well as high fatigue and creep resistance. Some fillers cause composites to have lower thermal expansion and exotherm coefficients. Wollastonite filler improves the composites' toughness for resistance to impact loading. Aluminum trihydrate improves on the fire resistance or flammability ratings. Some high strength formulations may not contain any filler because it increases the viscosity of the resin paste. High viscosity resins may have a problem wetting out completely for composite with heavy fiber reinforcement. A filler should not be used with fiber volume greater than 50% for the sheet molding composite production method.
A variety of additives are used in the composites to improve the material properties, aesthetics, manufacturing process, and performance. The additives can be divided into three groups -- catalysts, promoters, and inhibitors; coloring dyes; and, releasing agents. Their roles are as simple as their names imply, and they need no further discussion here.
Professor Steenkamer and his coauthors at the University of Delaware stated it well when they wrote: "The development of a composite is a complex process that requires the simultaneous consideration of various parameters such as component geometry, production volume, reinforcement and matrix types and relative volumes, tooling requirements, process and market economics, etc. Every decision made during the product development process is intricately related to a set of three interacting decision's areas (i.e., materials, processing, and configuration)".
The development of the advanced composite technology is an engineer's dream for innovative design and application. The characteristics of a composite can be tailored and designed to meet any desired specifications. Most of the information and design data available on composites are in the aerospace applications, but they are protected under the guise of proprietary systems and/or military classified documents. Unlike conventional isotropic materials of steel and concrete, there are no readily available design charts and guidelines to help the structural engineer. When it comes to working with composites as opposed to conventional materials, as the author has discovered, the difference can be as dramatic as night and day.
The challenge in applying composites is for one to understand the behavior of not only the constituents in the composites but also the completed end product in the way they respond to an applied load. Since a separate design specification for composites bridges is not yet available, existing bridge design guidelines may have to be used with some caution. Under the current American Association of State Highway and Transportation Officials (AASHTO) LRFD bridge design specifications, the philosophy is one of a probability-based limit state approach. The four basic limit states that is applicable in bridge design with advanced composite materials are the service, fatigue and fracture, strength, and extreme event.
The service limit states dictate the level of deformation and crack width under normal service conditions for a bridge to perform satisfactorily during its service life. The fatigue and fracture limit states restrict the stress range under normal service conditions within an expected number of load cycles. They are to limit crack growth under repetitive loading and to prevent fracture during the design life of a bridge. The strength limit states are to ensure that both the global and local strength and stability are provided to resist the statistically significant load combinations as experienced by a bridge during its design life. Some overstress and structural damage may be inevitable, but the overall integrity of the structure will not be compromised. The extreme event limit states ensure the structural survival of a bridge during a major earthquake or unusual collision force.
Based on some of the design data obtained from completed composite bridge structures to date, the deflection and/or local buckling govern composite design. With the inherent low section modulus of a composite structural member and critical high stress demand in structural applications, a designer should consider the following features carefully in his design:
There has been a great deal of development in the composite technology. It is an exciting time for civil engineers to be involved with composites. Both the US Government and private institutions are funding many demonstration bridge projects to show that advanced composite materials can be applied to rebuilding our highway infrastructure.
There is much to be learned about composites. Manufacturers, bridge owners, government officials, academia, researchers and contractors need to work together. Trade secrets should be honored and respected to the extent that information is provided to bridge owners to understand and evaluate the behavior of their structures. The bridge owner has been and will continue to be held responsible for the safety of the traveling public during the service life of a structure. Current laws require bridge owners to inventory and rate their bridges. The owners need to know the pertinent information and data that are used in bridge design, manufacturing, and fabricating. The AASHTO, ASTM, and American Concrete Institute have established numerous technical committees to develop design specifications, guidelines, standards, testing methods and methodologies. The Composites Institute which represents the composite industry has been coordinating the industry's effort to develop product design manuals, improve manufacturing processes, and collect test data. Numerous universities are offering research and design courses in composites. There are ample opportunities for civil engineers to participate and contribute to this growing technology.
* Kevlar® is a registered trade mark of E.I. du Pont de Nemours & Co.
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