Wrapping it Up
Fiber Reinforced PolymersA Cost-Effective Solution for Extending the Life of Our Aging Infrastructure
|by Ronald J. Watson
Ronald J. Watson is president of R.J. Watson, Inc., East Amherst, N.Y. This firm specializes in the application of FRP materials used to strengthen and rehabilitate structural elements on bridges, buildings, and other structures. He is the current chair for Subcommittee D04.34 on Performed Joint Seals and is also active in Subcommittee D04.32 on Bridges and Structures.
|Amazingly, a material wrapped like wallpaper around bridge columns
and beams can shore up highway infrastructures that are deteriorating
or threatened by seismic events. Ron Watson describes these materials
and the ASTM activities that standardize them.
Fiber reinforced polymers (FRP) are materials consisting of high strength fibers immersed in a structural matrix such as epoxy or other durable resin. The most common fibers used in FRP technology are glass, carbon, and aramid (more commonly known by its trade name Kevlar).
The use of FRP dates back to the first applications of fiberglass in the early part of the 20th century. The marine, aerospace, and automotive industries have made significant use of FRP due to the lightweight, high-strength and non-corrosive properties of these materials. It wasnt until the mid-1980s that bridge engineers began to realize the potential of FRP for columns in areas prone to seismic activity.
Research conducted at the University of California at San Diego (UCSD) revealed that FRP applied externally to reinforced concrete columns provided a significant increase in shear strength, enhanced the column ductility, and inhibited rebar lap splice failure, which is common in seismic events (see the sidebar below). The process most commonly used to apply FRP to these test columns has been the wet lay-up procedure. This procedure consists of high strength fibers that are matted or woven into a fabric and then immersed into an epoxy matrix. The fiber is then applied to the column in much the same fashion as wallpaper is hung.
An additional benefit of FRP retrofit is an increase in axial or vertical load capacity of the column. Testing conducted at the Georgia Department of Transportation Laboratories located in Forest Park, Ga., showed that a two-layer application of glass FRP on standard concrete cylinders increased the axial capacity by a margin of 135 percent.
With the research showing clear benefits in protecting structures from the harmful effects of strong ground motions, bridge engineers decided to implement FRP materials on actual bridge columns. One such project was the I-5/Highway 2 Interchange in Los Angeles. In the fall of 1991, 12 six-foot (1.8-m) diameter and three four-foot (1.2-m) diameter columns ranging from 18 (5.5 m) to 50 feet (15.2 m) high, were wrapped with glass FRP. An engineering analysis was performed to determine the number of layers of glass fabric to be applied (on projects to date the number of layers of FRP has varied from one to 16). Additional layers were required at the upper and lower plastic hinge areas of the column, which also resists rebar lap splice separation. The project was completed in three weeks at a cost that was significantly less than a comparable steel jacket retrofit. However, it wasnt until a little more than two years later that the FRP column wrap system was really subjected to a significant test.
On Jan. 17, 1994, the Northridge earthquake, with a Richter scale magnitude of 6.7, rocked southern California. This massive temblor resulted in the collapse of the I-5/Highway 14 Interchange northwest of Los Angeles. Columns on the Santa Monica Freeway just east of downtown that had not yet been retrofitted exhibited classical failure modes resulting from inadequate confinement steel.
The columns retrofitted with FRP on I-5 were undamaged by the Northridge earthquake. This catastrophic event field-verified what the researchers in the lab already knewthat FRP can effectively strengthen columns to withstand the harmful effects of strong ground motions.
California is not the only state to realize the benefits of FRP to strengthen their bridges for earthquakes. The latest American Association of State Highway and Transportation Officials (AASHTO) Seismic Bridge Design Code now requires that 39 out of 50 states have to design their bridges for the possibility of earthquakes. As a result, many states are now evaluating their existing lifeline structures for susceptibility to earthquake damage.
A case in point is the Pennsylvania Department of Transportation (Penn-DOT) District 4 headquarters in Scranton, Pa. During a routine evaluation of the I-84 bridge east of town, it was determined that the six five-foot (1.5-m) diameter columns supporting this 468-foot (143-m) four-span continuous steel plate girder structure had inadequate confinement steel. Previous retrofits consisted of taking threaded steel rebar and tightening them with turnbuckles around the base and top of the columns. This additional reinforcement is then covered with gunited concrete. While this is an effective seismic retrofit technique, it is also very costly.
When the option of using FRP to retrofit the columns was proposed to the PennDOT engineers, they quickly approved it due to the cost savings, speed of application, and non corrosive features of the FRP.
An engineering analysis was performed by HDR Engineering out of Pittsburgh, which determined that eight layers of glass FRP were to be applied in the upper and lower shear zones of the columns with two layers being applied in the flexural zones. The project started in July of 1993 and took about a week to complete at a total cost of $70,000 to the State of Pennsylvania. Broken down into a price per square foot of material installed, this project resulted in an $8.00 figure. This is not an exorbitant amount of money when one considers the importance of this lifeline structure, especially in the event of an earthquake that eventually will happen, even in Scranton, Pa.
With the successful applications of FRP in the seismic retrofit arena, engineers soon realized the potential for non-seismic repairs and upgrades of highway bridges. One such area is the repair of reinforced concrete columns in the regions where high levels of deicing chlorides are applied to keep the bridge decks free from ice in the winter. The unfortunate side effect to this practice is the penetration of these chlorides into the concrete where it attacks and corrodes the reinforcing steel resulting in the cracking and spalling of the concrete surface.
A typical repair method for this problem is to first remove all loose and delaminated concrete followed by a tedious chipping procedure to facilitate the repair material flowing in and around the reinforcement steel in order to achieve a mechanical bond. Many bridge maintenance engineers perform this task on a regular basis only to find that in a short amount of time (typically two years or less) the repairs spall off due to continued rebar oxidation.
The use of FRP applied externally via the wet lay-up method to columns after the repairs are made achieves three additional benefits:
Provides confinement for the repair material to stay in place;
Strengthens the column by providing hoop strength to compensate for the section loss of the corroded rebar; and
Protects the concrete with an impervious barrier that will prevent de-icing chlorides from attacking the steel rebar.
Many states and provinces in North America are now employing this technique on a regular basis to prolong the life of their structures.
Another area where FRP are being implemented in bridge rehabilitation is on concrete beams. Back in the summer of 1996, the South Carolina Department of Transportation (SCDOT) was contemplating the damage to a concrete bridge beam at the I-85/I-585 Interchange in Spartanburg. This prestressed reinforced concrete beam was impacted by oversized vehicles resulting in significant damage to the point where beam replacement seemed imminent. The problem with beam replacement is that it results in expensive ancillary costs in terms of redecking the bridge above the beam and traffic control both above and below the bridge. The total cost for this replacement was estimated at $250,000.
At the same time an FRP repair and strengthening procedure was proposed whereby carbon FRP would be applied longitudinally to the underside of the beam to restore the flexural strength. In addition, glass FRP would be adhered to the beam in stirrup fashion to the ends of the beam to restore the shear strength.
The proposal was accepted and had to be approved by both the SCDOT and the Federal Highway Administration since it involved an Interstate Highway. The project was completed in approximately three weeks, and included re-forming the concrete beam with an approved repair material prior to the FRP application. The savings to the state was over $150,000, compared to the beam replacement.
The aforementioned repair and strengthening projects represent only a small portion of the work currently ongoing with FRP materials used on bridges. FRP reinforcement and prestressing materials for concrete are starting to be used in place of steel. Several states have installed pultruded FRP bridge deck sections that promise long term performance due to the non-corrosive features of these materials. Preformed FRP jackets are being used to repair concrete piles and columns on bridges in marine environments. Many other FRP applications are being considered as well.
At present ASTM has two committees that have developed test methods, specifications, practices and guides covering the use of FRP materials. Committee D20 on Plastics and Committee D30 on Composite Materials have numerous subcommittees dealing with the various test requirements ranging from acoustic emissions to delamination resistance. Subcommittee D30.02.02 on civil and marine applications of FRP materials is currently chaired by Professor Abdul Zureick of Georgia Tech University. His subcommittee is currently in the final stages of writing a specification on FRP prestressing tendons for prestressed concrete. Also in the development stage are documents covering testing guidelines for short and long-term loading, in-plane shear properties for thick composites, and durability standards.
Committee D04 on Road and Paving Materials Subcommittee D04.32 deals with bridges and structures. This subcommittee is currently chaired by Neal Bettigole, who has over 40 years of experience in the field of bridge engineering. Realizing the potential for FRP on bridges, Neals subcommittee is organizing an ASTM seminar entitled Fiber Reinforced Polymers to Strengthen and Rehabilitate Bridges. This seminar will be held on Dec. 5 in conjunction with the ASTM Committee Week in Orlando, Fla. Papers are being solicited on the use of FRP on bridge applications in the fields of research, design, installation, testing, materials, protective coatings, codes, and case histories. This seminar is designed to enlighten engineers and bridge owners to the numerous applications of FRP to rehabilitate, strengthen, and prolong the life of bridges. Bridge maintenance engineers, design engineers, general contractors, bridge owners, transportation authorities and any other technical people involved with the rehabilitation and maintenance of bridges will benefit from this information exchange.
The potential applications of FRP to strengthen and rehabilitate bridges are still in their infancy. FRP fabricators are now manufacturing structural shapes such as beams and girders out of FRP materials. One of the drawbacks for new construction has typically been the price of these structural members on a first-cost basis. However, advances in manufacturing processes are continually bringing the cost of FRP down to the point where they may be competitive with traditional materials such as steel and concrete in the very near future. The light weight, high strength and non-corrosive nature of the materials make FRP presently attractive to contemporary engineers on a life cycle cost basis especially in bridge retrofit applications. / /
Talk to the Editor: Maryann Gorman