Composites add longevity to bridges: North Carolina uses fiber-reinforced polymers to rehabilitate bridges and extend their service lives

Public Roads, Nov-Dec, 2003 by Rodger D. Rochelle

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 oil 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 II.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/[m.sup.3] (6 lbs/[yd.sup.3]) 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 cohuuns7 She added, "The application will take advantage of the system's very high strength-to-weight ratio relative to more traditional repair techniques."