Composites Add Longevity to Bridges
North Carolina uses fiber-reinforced polymers to rehabilitate bridges and extend their service lives.
The general need for bridge repairs across the Nation is widely reported. Whether subject to frequent application of road salt in the Northeast and Midwest or exposed to the naturally corrosive environment in the coastal States, bridges often suffer from severe chloride loading. Carbonation, sulfate attack, alkali-silica reactivity, and chloride reduce the lifespan of many structures. Statistics available through the Federal Highway Administration (FHWA) National Bridge Inventory indicate that many structures still are in need of repair.
North Carolina has the second largest State-maintained highway system in the United States, with more than 78,000 road miles, including 17,000 bridges. Despite the large inventory, the State's statistics for bridges over the last decade mimic the trend at the national level. Currently, about 30.7 percent of North Carolina bridges are considered deficient compared with 39.4 percent in 1992. The North Carolina Department of Transportation's (NCDOT) goal aligns with the one stated in FHWA's 1998 Strategic Plan, namely to decrease the percentage of deficient bridges to 25 percent by the year 2008.
Accordingly, engineers with the NCDOT Structure Design, Bridge Maintenance, Materials and Tests, and Research and Development units are collaborating to further reduce the bridge maintenance backlog. They are working on two fronts concurrently: durable design and innovative materials.
From a design perspective, durability is a given. North Carolina's structures are subject to a wide variety of chloride infiltration from applications of road salt in the Piedmont and Western regions to waterborne and airborne chlorides along the coast. Improvements in designing bridges for durability are therefore paramount to reducing the maintenance backlog.
For large structures over the coastal sounds or among the islands of the Outer Banks, where water chloride content can reach 17,000 parts per million, NCDOT engineers have used a mathematical model for systematically designing bridge components for a service life of 100 years. The concept is extrapolated to smaller bridges along the coast, by incorporating mineral and chemical admixtures, high-performance materials, and alternate concrete-reinforcing products.
From a bridge maintenance perspective, NCDOT is looking to innovative materials for bridge rehabilitation and extension of service life. One of the fastest-growing exploration areas is the use of composite materials, specifically fiber-reinforced polymers (wrap).
Paul Simon, bridge engineer in FHWA's North Carolina Division Office, predicts that “composite bridge components are on the horizon of revolutionizing bridge materials. Within 10 years, the use of wrap products could be routine in bridge construction and reconstruction.”
Chatham County Bridge
Bridge maintenance engineers often are confronted with rapidly deteriorating concrete caused by expansion of the corroding reinforcing steel and subsequent cracking and spalling (chipping away) of the overlying concrete cover. Exposed surface area increases the chloride ingress, which in turn, perpetuates and accelerates degradation. Such was the case with the westbound bridge on U.S. 64 over the Haw River in Chatham County, NC.
Built in 1982, this two-lane structure is approximately 214 meters (700 feet) long. Each of the bridge's ten supporting piers is roughly 9 meters (30 feet) tall and has three 0.9-meter (3-foot)-diameter columns on top of spread footings. But the similarities among the columns end there, as their condition varied widely when inspected in 2002.
Many exhibited no visible signs of degradation, and core samples revealed chloride content well below the commonly accepted corrosion threshold of 0.5 kilograms per cubic meter (1.5 pounds per cubic yard) of concrete. In contrast, several others showed excessive spalling and chloride content as high as
2.1 kg/m3 (6 lbs/yd3) at a depth of 50 to 75 millimeters (2 to 3 inches) below the surface of the column. Inconsistent and inadequate concrete cover contributed further to the variations in condition.
During the inspection in 2002, the aggregate rating of the columns was “fair.” The maintenance personnel issued a Prompt Action Notice, however, to address several severely degraded columns. Overall, 15 columns required repairs. Since the deteriorated area represented only 30 percent of the total length of the columns, however, maintenance personnel sought rehabilitation rather than total replacement.
Moreover, NCDOT was intent on minimizing traffic disruptions on this newly widened four-lane facility. In response to these needs, NCDOT research personnel applied for a grant through FHWA's Innovative Bridge Research and Construction program. The grant application specifically requested $95,000 for this inaugural use of fiber-reinforced polymers in the NCDOT bridge program. Upon obligation of the FHWA funds, NCDOT selected an advanced composite system of fiber-reinforced wraps. An engineer with Fyfe Co. LLC, of San Diego, CA, Sarah Witt, recommended “unidirectional glass fibers embedded in 100 percent epoxy matrix to provide additional confinement to the existing bridge columns.” She added, “The application will take advantage of the system's very high strength-to-weight ratio relative to more traditional repair techniques.”
Specifically, NCDOT chose a glass fiber product and two epoxies to saturate the fabric. The first epoxy was used for most of the project, while the second one treated the bases of three columns immersed in river water, to enable underwater curing of the wrap system. In the fall of 2002, representatives of the supplier and NCDOT's Bridge Maintenance and Research units met to define the project scope. They identified spalled or cracked concrete over a collective column length. Intact concrete extending 1 meter (3 feet) beyond each spall was earmarked for wrap to ensure that the degraded portion of the column was fully encapsulated. Four of the 15 columns warranted the glass wrap for their entire lengths.
By the late fall of 2002, the NCDOT Bridge Maintenance forces began the important work of preparing the columns. Inordinate rainfall and resulting fluctuations in the river's water level disrupted the preparatory operations. Debris common to the Haw River damaged scaffolding on more than one occasion, adding potential safety risk to the project staff.
Despite the environmental challenges, crews completed the preparations in less than 1 month. They carefully examined all spalled areas and removed surrounding concrete to an area and depth adequate to ensure that all remaining concrete was sound. Exposed reinforcing steel was wire-brushed by hand and painted with a moisture-cured urethane coating.
The entire area of the concrete spall was treated with three products. The concrete was primed with a two-part 100 percent epoxy primer. The concrete spall then was filled with a combination of a 100 percent solids urethane epoxy modified polymer, a filler, to create a three-part patching material specifically suited for vertical surfaces. This combination provided a highly impermeable patch that was sufficiently malleable to fill small voids.
In mid-May of 2003, the certified contractor for the project began installing the fiber-reinforced polymer. After using a leveling adhesive to smooth large variations in the patch material surface, the contractor began to wrap each deteriorated column. For this process, the glass fiber fabric is cut to length and “wetted-out” using a machine specifically tailored to optimize saturation.
The crew then transported the fabric to the column via a boom truck and boat, and applied the fabric in a continuous nonspiral wrap. The precut length of each piece of fabric provided a two-layer wrap with a minimum overlap. During the 48-hour curing window, the crew applied a coat of gray paint directly to the glass wrap. The timing of the paint application enhanced the bond between the wrap and the paint. The gray color offered a reasonably close match to the surrounding concrete, making it blend better with surrounding materials.
The onsite inspections included a daily diary of activity and verification of resin mixes and mixing procedures, fabric saturation, and glass wrap and paint applications. In addition, the inspecting firm collected a minimum of two test panels from each day of production work. Two individual laboratories tested these witness panels in accordance with American Society for Testing and Materials (ATSM) D-3039 to determine the conformance of the glass wrap system and components to material specifications.
John Levar, president of the inspection firm, Advanced Structural Technologies, says that the inspector's role is not simply “to verify that the materials were fabricated and installed properly, but also to establish a baseline condition of the repair so that future NCDOT inspections will have adequate information to successfully evaluate the structure and the repair technique.”
Although high water levels in the river delayed the underwater wrap operations, almost the entire installation was completed within 2 weeks. A typical rehabilitation approach such as using shotcrete or concrete jackets would have required a much greater mobilization effort, far longer construction times, and probable lane closures.
Discussing his first experience with the glass fiber-reinforced polymer wrap, NCDOT Bridge Maintenance Supervisor Kevin Smith described the material as “impressive and certainly an easier solution than encasing the columns in concrete.”
To replace this bridge would cost more than $2 million and take years to complete. The fiber-reinforced polymer wrap solution restored the columns' original strength at a price tag of approximately $120,000.
This project demonstrates the practicality of using fiber-reinforced polymers in bridge maintenance. At this site, composite materials provided an environmentally friendly, cost-effective, and rapid solution to a challenging maintenance issue. Perhaps most importantly, the fiber-reinforced polymer wraps were applied without lane closures and associated traffic delays. For these reasons, public interest in this rehabilitation project has been strong and favorable. The project was featured by local radio, three television stations, and two major newspapers.
“This project exemplifies the Department's commitment to improving efficiency,” said NCDOT Transportation Secretary Lyndo Tippett. “Not only is the use of this material a major accomplishment for NCDOT, but also it is an advantage for the citizens of North Carolina. Savings like this allow us to apply more money toward other pressing maintenance projects in the State.”
Mill Creek Bridge
Although the Chatham County site has become North Carolina's best-known use of composite materials, NCDOT recently placed in service a bridge deck made entirely of a glass fiber-reinforced polymer. Located roughly 16 kilometers (10 miles) northeast of Charlotte on S.R. 1627 over Mill Creek in Union County, the bridge incorporates a pultruded fiber-reinforced polymer deck. The bridge deck system enabled rapid replacement of the aging bridge superstructure.
The NCDOT team placed five individual glass fiber-reinforced polymer deck panels into position and made them composite with underlying steel stringers through in situ placement of shear studs and grout. State Bridge Design Engineer Greg Perfetti reflects that the glass fiber-reinforced polymer deck has “proven to be a viable, light-weight, corrosion-resistant alternative to a conventional reinforced concrete deck, and the ease with which it can be placed fulfills the need for structural options that minimize the impact on the motoring public.”
The bridge was subjected to multiple controlled load tests as part of an NCDOT-sponsored research project. The research out of the University of North Carolina at Charlotte employed roughly 80 gauges and instruments to collect data on deflections, distribution factors, strains, and levels of composite action. The research afforded NCDOT an opportunity to affirm the design protocol for a glass deck and to learn how it behaves as a part of a fully operational bridge.
Macon County Bridge
Another bridge, located in Macon County, NC, soon will include
fiber-reinforced polymer materials. Construction of a new 49-meter (160-foot)-long bridge on S.R. 1470 over the Cartoogechaye Creek is expected to begin in early 2004 and will entail substituting fiber-reinforced polymer reinforcing for conventional mild steel reinforcing in both the concrete bridge deck and approach slabs. Additional research will be conducted on this bridge and the fiber-reinforced polymer components to validate the design procedures and assumptions made for the project.
A Bright Future
All three of these projects were funded in large part by FHWA's Innovative Bridge Research and Construction program. The total contribution allocated from this program reached $700,000 for these three projects alone. To date, NCDOT has received seven such grants totaling $2.3 million.
These fiber-reinforced polymer projects exemplify the objectives of the FHWA program. John Hooks, the FHWA program manager, emphasizes that the Innovative Bridge Research and Construction program “has spurred innovation by providing funds to offset the cost of demonstrating high-performance and innovative materials in 288 bridge projects across the Nation. Solutions developed and validated under [this program] will help engineers extend the life of existing bridges and build new bridges better.”
This certainly has been the case in North Carolina, particularly in the exploration of further uses for fiber-reinforced polymer products. And the future for using composite materials by NCDOT is bright. State Bridge Maintenance Engineer Lin Wiggins foresees a “much wider use of composites in our maintenance activities . . . [They are] particularly useful in the repair of girders damaged by overheight vehicles.”
In fact, fiber-reinforced polymer is increasingly being considered to correct construction deficiencies, rehabilitate difficult-to-replace substructure members, repair collision damage to existing structures, reduce life cycle costs, and minimize traffic delays. To support these endeavors, the NCDOT research program currently is facilitating a 2-year, $160,000 research project by North Carolina State University. Dr. Sami Rizkalla at the university summarizes the study as a “testing of damaged girders, including a variety of fiber-reinforced polymer strengthening techniques . . . resulting in design guidelines, selection criteria, and construction specifications” suitable for bridge design and maintenance practitioners.
Upon completion of this research project, expected in 2005, the findings will be distributed across the Nation.
The North Carolina Department of Transportation is committed to building safe, durable, cost-effective bridges. Exploiting cutting-edge technology, such as that offered by fiber-reinforced polymers, provides a potential avenue through which existing deficient bridges may be replaced with structures of significantly greater longevity.
Rodger D. Rochelle, P.E., is the State research engineer with NCDOT and manager of the NCDOT research program. He has a bachelor's degree in civil engineering and a master's degree in structural engineering, both from Duke University. He is a member of the American Association of State Highway and Transportation Officials (AASHTO) Research Advisory Committee and the Transportation Research Board (TRB) Subcommittee on Bridge Aesthetics, an associate member to the National Science Foundation Industry/University, Cooperative Research Center for the Repair of Buildings and Bridges with Composites, and a certified public manager.
For additional information on the use of composites in North Carolina, please contact Rodger Rochelle at 919-715-4657 or firstname.lastname@example.org.