RoadScience: Corrosion — the enemy within
“A lot of the systems work for a couple of years, and if no one goes out to maintain them, they can work imperfectly,” Krauss said. “It can be too much of a burden for the DOTs to maintain and keep track of them. State agencies that are more aggressive implementing bridge deck cathodic protection usually have a good electrical group with technicians who understand electrical systems.”
Krauss has visited existing cathodic protection systems. “Oftentimes they are not well-maintained or are not working,” Krauss said. “Individual zones won’t be working; different zones may be partially working, while other zones may not be working at all. It becomes a challenge to the owning agency.”
This was borne out by a 2008 report by the Virginia Transportation Research Agency. In Survey of Cathodic Protection Systems on Virginia Bridges by Michael C. Brown, Ph.D., P.E. and Stephen R. Sharp, Ph.D., research scientists for the Virginia Transportation Research Council, the researchers found that bridge CP systems in the commonwealth were successfully deployed, but not always.
“The Virginia [DOT] has used CP systems on 12 reinforced concrete bridge structures,” the authors write. “Although CP systems deployed in concert with VTRC research studies remained in service, the researchers found little evidence that the transfer of the responsibility for maintenance and monitoring of those systems to the appropriate VDOT field personnel has occurred. As a result, little information has been gathered regarding the performance of these systems beyond the initial evaluations during the research studies. In addition, this led to little or no maintenance of CP systems being conducted at these sites.”
VDOT’s bridge inspection reports were found to include only minimal information about the status of the systems in place on the structures evaluated in the field survey, they said. “It appears that visual observations have been made about the outward appearance of the systems.
However, routine electrical measurements to confirm the operational status of these systems were not reported, thus it is not clear from the records that sufficient cathodic current has been applied to ensure protection of the structure(s). This points to a policy need for consistent routine maintenance and monitoring of CP systems to document their performance.”
Even if CP systems are working, they will need to be periodically fine-tuned. “Bridge deck cathodic protection systems need to be adjusted,” Krauss said. “Bridges change over time, so to be effective, the owner needs to adjust the amount of current being applied over time.”
Remote monitoring not a panacea
The advent of cellular phone technology has enabled real-time reporting of the status of active systems in the field, in which a field unit will “dial-in” its status to another system, which logs the status or issues an alert if the system is not performing as specified. But these monitoring systems too can misbehave.
“Remote monitoring systems can pull up a system status on a computer,” Krauss said. “The problem is that technology also can be prone to failure or lack of maintenance. It can be done, but it can be at a high expense. As DOTs are strapped for money, for them to put remote monitoring systems in service and maintain them poses the same problems they will have with the cathodic protection system itself. In the future such a system may become more reliable and less expensive, and that may help CP systems become more acceptable.”
Physical repair of CP systems can involve wiring repairs, or replacement of anodes. “The systems can be damaged for a variety of reasons,” Krauss said. “We have seen birds get into control boxes and build nests. Vandalism is a big issue; vandals see wires and want to rip them out or cut them.”
When used, cathodic protection may be favored for more costly structural elements like precast/prestressed girders, and piers and columns, rather than the deck.
“The decks on a lot of bridges are expendable,” Krauss said. “They are easily replaced, and concentration of cathodic protection will be on supporting elements that are not replaceable. In those areas there is increased interest in corrosion protection, but instead of going totally with active systems, there is a movement toward passive systems that will require less maintenance than the active systems These can be applied to deck supporting elements like girders, ends-of-beams at joints, and piers and pier caps. Leakage at joints is one of the worst offenders; chloride solution then gets down through the joint and we start to get beam corrosion. It can result in a very expensive fix.”
Passive CP systems may include arc sprayed zinc, activated zinc sheets, or discrete anodes. These are often combined with improved joint systems and coatings.
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