Highway 101 along Oregon's Pacific coast is linked by a series of graceful bridges constructed by renowned bridge engineer Conde B. McCullough in the 1930's. Six decades of exposure to ocean spray has taken its toll on the steel-reinforced portland cement concrete bridges, and many of the structures are now in need of extensive repair or even replacement. Not wanting to lose part of the State's architectural heritage, the Oregon Department of Transportation (DOT) has turned to cathodic protection to preserve McCullough's historic bridges and to avoid the huge cost of building new structures.

The bridges are deteriorating because of their location along the coast. Over the years, chloride ions from seawater have permeated the concrete, where they change the pH and therefore break down the protective film on the reinforcing steel bars. This allows the steel to corrode. Because steel expands as it corrodes, the concrete around it cracks and spalls as a result of the tensile stress.

Oregon DOT decided to implement cathodic protection systems to preserve the deteriorating historic bridges in 1990, after having had to completely replace the McCullough-designed Alsea Bay bridge. The new bridge cost Oregon taxpayers $43 million and meant the loss of a historic structure. Over the past 6 years, the highway agency has installed cathodic protection as part of the rehabilitation work on the Cape Creek, Yaquina Bay, and Depoe Bay bridges.

Oregon DOT first examined each structure to determine where corrosion had caused cracking and spalling. Crews then checked the reinforcing steel's continuity to determine its ability to carry an electrical current. They also repaired cracks, spalls, delaminations, and other defects in the concrete. Once the repairs were finished, Oregon DOT installed the cathodic protection system itself. An arc-sprayed zinc coating was applied to the bridges. Zinc coatings offer several advantages: they are very conductive, are relatively easy to renew when they wear out, do not significantly increase the load on the bridge, and do not obstruct architectural details in the concrete.

Applying the zinc coating was somewhat difficult. Enclosures had to be built around the bridges to trap zinc dust and fumes, which pose an environmental and health risk. The enclosures also ensured the warm, dry conditions necessary for a good bond between the zinc and the concrete.

The cost of rehabilitating the three bridges and installing cathodic protection systems was far less than replacing the bridges, reports Marty Laylor of Oregon DOT. At Cape Creek, the job cost $2.4 million. The Depoe Bay bridge system cost about $4 million, and the much larger Yaquina Bay bridge project cost $12.7 million. By contrast, it would have cost at least $70 million to replace the three structures.

But the State saved much more than money. "These bridges are almost works of art," says Laylor. "Replacement structures would not have possessed the architectural heritage of the original bridges. Oregon would have lost three irreplaceable landmarks."

Oregon's success is drawing interest from other States interested in preserving historic bridges. "We recently had a request for information from a New England State," Laylor says. Donald Jackson of the Federal Highway Administration notes that several States are using or considering cathodic protection for historic preservation purposes. In West Virginia, for example, cathodic protection is being used to protect a bridge that is more than 70 years old.

Cathodic protection requires a commitment to maintenance to ensure its success. To keep tabs on the systems, Oregon DOT employs remote monitoring devices on the bridges to supply data on voltage, current, and climate to DOT offices in Salem via modem. "The remote monitoring is primarily to ensure that the systems are working correctly and to tell us whether we need to modify or adjust the system," says Laylor. "We will also use the data in studies to determine the effectiveness of cathodic protection on a long-term basis."

Eventually, the cathodic protection systems will require new zinc coatings. Laboratory studies supported in part by Oregon DOT indicate that zinc coatings will last about 25 years. In hopes of finding a longer-lasting solution, the DOT is evaluating a titanium coating, which corrodes at a slower rate than zinc and may have a longer service life than sprayed zinc anodes.

Oregon DOT is also experimenting with another approach to cathodic protection. The State is planning its first field trial of a passive, or galvanic, cathodic protection system. Passive systems, which are currently being used in Florida, Virginia, and other States, do not require an external current. Instead, they rely on the electromotive force between the anode and the reinforcing steel. The Oregon project will test new zinc materials and evaluate their performance in the State's moderate climate.

According to Jackson, some engineers are intimidated by the electrochemical process; but as he points out, it is used to prevent corrosion in automobile bodies, hot water heaters, guardrails, and other familiar objects. "Cathodic protection is part of everyday life, not black magic," Jackson says. "It's an excellent tool for managing bridge rehabilitation. You know that structures with cathodic protection installed are not going to corrode any further. Managers can therefore focus their attention and resources on those structures with advanced deterioration."

Oregon DOT has produced a videotape on its use of cathodic protection on the historic Highway 101 bridges. The video was funded in part by FHWA's Local Technical Assistance Program and Office of Technology Applications. For more information, contact Marty Laylor at Oregon DOT (telephone: 503-986-2850; fax: 503-986-2844).

For more information on cathodic protection, contact Donald Jackson at FHWA (telephone: 202-366-6770; fax: 202-366-7909).

How Does Cathodic Protection Work?

Cathodic protection, a technology evaluated under the Strategic Highway Research Program (SHRP), controls the corrosion in the reinforcing steel. As steel corrodes, it loses electrons and reacts with oxygen and moisture. Cathodic protection supplies the metal with electrons from an artificial metallic anode, a negatively charged material that sacrifices itself to protect the positively charged reinforcing steel.

In most cathodic protection systems, an external power supply provides electrical current to the anode. Current passes from the anode and through the concrete, which is conductive because of its moisture content and impurities and thus acts as the electrolyte. From the concrete, the current reaches the steel reinforcing bars, which become the cathode in the system and are thus protected from corrosion.

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