United States Department of Transportation
i
Intermittent application of cathodic protection : interim report.
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2005-05-01
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[PDF-1.58 MB]
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Abstract:Oregon’s coastal highway includes over 120 bridges, most of which are reinforced concrete (RC) bridges. Over 40,000 m2 (430,566 ft2) of bridge surface has been repaired and protected from further corrosion damage using thermal-sprayed (TS) zinc anodes in impressed current and sacrificial cathodic protection (CP) systems. In addition, thermal-sprayed titanium, conductive carbon paint, and zinc-hydrogel anodes are being evaluated in demonstration projects on coastal RC bridges.
Field and laboratory studies were conducted to evaluate commercial corrosion rate monitoring devices (CRMDs) suitable for use in intermittent CP (ICP) field operation on Oregon's coastal RC bridges. These studies showed that there was good agreement between mass loss and linear polarization resistance (LPR) measurement of rebar corrosion rates when the Stearn-Geary constant B was estimated using harmonic distortion analysis (HDA). There was good agreement between laboratory LPR corrosion rate measurements for rebar in concrete and measurements made using three commercial CRMDs (Gamry RPX1 LPR, SmartCET LPR, and SmartCET HDA). Since ICP operation uses depolarization and protection current measurements to monitor the CP system performance, it is necessary to install the CP system with conductive coating anodes isolated from the rebar. The studies showed TS zinc anode CP systems on RC bridges exhibit long-term increases in CP system circuit resistance and decreases in anode bond strength that would benefit from the application of ICP. However, there was no evidence that TS titanium and carbon paint ICCP anodes or zinc hydrogel SACP anodes would benefit from the application of ICP.
A two year field study is recommended for an Oregon coastal RC bridge with multiple TS zinc anode CP zones to assess the effectiveness of ICP compared to present Oregon DOT CP practices for protecting coastal bridges from corrosion damage. Four ICP test zones are proposed along with two impressed current CP (ICCP) control zones. The four zones include: (1) current-interrupt ICCP, (2) corrosion-rate controlled ICCP, (3) constant-voltage CP, and (4) sacrificial CP.
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