Long-Term Performance of CP Systems on Bridge Structures

    May 12, 2023 Corrosion CONTROLLED, Corrosion Essentials, Coatings

    BridgesCoverPhoto_5-12-23A five-year study to evaluate the long-term performance of various cathodic protection systems installed on bridge deck and substructure elements and on the underside of a roadway slab inside a tunnel was initiated in 1994. This work was a continuation of a portion of the research completed under the structures segment of the strategic highway research program. The performance of 10 of the CCP systems included in this study is discussed. These systems have been in service 5 to 15 years.

    The Situation 
    in 1988, the strategic highway research program mandated by the US Congress, launched multiple research efforts to study all aspects of reinforced concrete deterioration three of these SHRP programs (C-102-D, C102F, and C-102G) established numerous field sites to study the effectiveness of CP, electrochemical chloride extraction (ECE), and corrosion inhibitors (CI) in mitigating corrosion of reinforced concrete bridge structures. These programs were completed in June 1993.

    To obtain long-term performance data on the corrosion protection systems at the sites established or identified by the SHRP programs, the Federal Highway Administration (FHWA) initiated the subject research study in July 1994. Also included were laboratory concrete slabs that had been prepared by the SHRP efforts and then treated with ECE and CI.

    The primary objective of the overall effort was to determine the effectiveness of CP, CI, and ECE through a five-year evaluation of 31 bridge structures and one tunnel; 10 of those brick bridges become became the focus of this case study. CP systems were installed on conventionally reinforced bridge decks and substructure components and inside the tunnel.

    All pertinent information regarding the condition of the concrete bridge components prior to installing the CP systems, information pertaining to installation of the CP systems, structure drawings, and history of operation, where available, were obtained and reviewed in each case. The tests and survey techniques included but were not limited to:

    - Electrical Continuity Testing
    - Polarization/Depolarization Testing
    - Embedded Instrument Evaluation 
    - Anode to Reinforcing Steel Alternating Current (AC) Resistance

    The Resolution
    The five different CP systems used during evaluations included a conductive coke asphalt system, a conductive polymer mounded anode system, a titanium mesh anode with overlay system, and a conductive coating system.

    Results included: 
    - The three conductive coke breeze anode systems have been satisfactorily providing the required current to protect the steel, without any indication of anode degradation, for five, eight, and nine years, respectively.

    - The CP system components of the mounted conductive polymer system are functioning properly. However, the effectiveness of the system in mitigating corrosion is less than desirable based on the level of polarization achieved. This may be corrected by simple adjustments of the current output from the rectifier.

    - The titanium mesh anode CP systems on three bridge decks are performing satisfactorily after 6, 7, and 12 years of operation, respectively.

    - The titanium mesh anode system encapsulated in a shotcrete overlay in the Brooklyn Battery Tunnel is not providing adequate protection and is considered to be ineffective.

    - One conductive coating system reached the end of its service life in less than eight years. The other system continues to provide protection in the majority of areas, but it is considered to be deteriorating with time.

    - Some degree of embedded instrumentation malfunctioning was found at most sites. Particularly, voltage probes and molybdenum/molybdenum-oxide reference cells have been very prone to failure. In some cases, the problem is exasperated over the life of the system. As more embedded instruments fail, fewer sites will have sufficient operational instrumentation to facilitate proper monitoring. At sites with no embedded reference cells, potential wells and external reference cells were used. (The reliability of this method is questionable.)  

    - The AC resistance test and limit criterion for assessing the functionality of embedded reference electrodes needs to be reevaluated considering the findings of the study. II

    This study was presented at CORROSION99. It will appear in full in the book, Bridges – A Look Back: Case Studies in Cathodic Protection. For more information, go to https://store.ampp.org/.

    This article by Jean Broge was originally published online for Infrastructure Insights Magazine. Republished with permission. 

    Check out AMPP's Highways and Bridges articles, related courses, and certifications. Keep reading to find out more.

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