Cathodic Protection of Reinforced Concrete Bridge Elements: a State-Of-The-Art Report
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SHRP-S-337 Cathodic Protection of Reinforced Concrete Bridge Elements: A State-of-the-Art Report ELTECH Research Corporation 625 East Street Fairport Harbor, OH Strategic Highway Research Program National Research Council Washington, DC 1993 PUBL. NO. SHRP-S-337 ISBN: 309-05265-3 Contract C-102D Program Mar.ager: Don M. Harriott Project Manager: Marty Laylor Consultant: dohn P. Broomfield Program Area Secretary: Ann Saccomano Copyeditor: Katharyn Bine Brosseau January 1993 key words: anode bridge cathodic protection chlorides corrosion maintenance Strategic Highway Research Program 2101 Constitution Avenue N.W. Washington, DC 20418 (202) 334-3774 The publicalion of this report does not necessarily ind!cate approval or endorsement by the National Academy of Sciences, the United States Government, or the American Association of State Highway and Transportation Officials or its member states of the findings, opinions, conclusions, or recommendations either inferred or specifically expressed herein. ©1993 National Academy of Sciences 2M/N'A[/I93 Acknowledgments The research described herein was supported by the Strategic Highway Research Program (SHRP). SHRP is a unit of the National Research Council that was authorized by section 128 of the Surface Transportation and Uniform Relocation Assistance Act of 1987. This report is a compilation of work by ELTECH Research Corporation, Corrpro Companies, Inc., and Kenneth C. Clear, Inc. Each of these organizations is acknowledged for their effort in writing this report. °oo 111 Contents Acknowledgements .................................................. iii List of Figures ..................................................... vii List of Tables ....................................................... ix Abstract ........................................................... 1 Executive Summary .................................................. 3 Introduction ........................................................ 7 Does it Work ..................................................... 11 History of Cathodic Protection ......................................... 13 Introduction .................................................... 13 Cathodically Protected Structures .................................... 14 Conductive Overlays .............................................. 15 Slotted Systems ................................................. 17 Distributed Anodes with Concrete Encapsulation ........................ 19 Conductive Coatings .............................................. 20 Other Anode Systems ............................................. 21 Probes and Criteria .............................................. 22 Alternate Cathodic Protection Power Sources ........................... 23 Effect of Cathodic Protection on Bond ................................ 24 Service Life of Cathodic Protection Systems ............................ 25 Cathodic Protection System Costs .................................... 26 Summary ...................................................... 27 Condition Survey ................................................... 31 Tests Used for Assessing Bridge Structure Condition ..................... 31 Condition Tests Used for Assessing the Option of Cathodic Protection ........ 35 Remedial Action ................................................... 39 Incompatible Concrete Materials .................................... 39 V Electrical Continuity ............................................. 40 Scarification .................................................... 41 Sandblasting ..................................................... 42 Concrete Bonding Agents .......................................... 43 Epox) Injection ................................................. 43 Anode Systems ...................................................... 45 Conductive Coke-Asphalt ........................................... 45 Distributed FHWA Conductive Polymer Anode Sy,,_tems ................... 47 Catalyzed Titanium Mesh Anodes .................................... 50 Conductive Carbon-Based Coatings ................................... 51 Sprayed Metallic Coatings .......................................... 52 Other Anode Systems ............................................. 54 Design Aspects .................................................... 57 Current Requirements and Distribution ............................... 57 Anode Considerations ............................................ 59 Monitoring Devices .............................................. 60 Power Supply ................................................... 67 Hydrogen Embrittlement .......................................... 68 Operation and Maintenance ........................................... 71 Types of Recitifier Operation ....................................... 72 Test Requirements and Monitor Schedule ............................. 73 CP System Failures .............................................. 76 Research and Development Needs ...................................... 79 References ........................................................ 81 vi List of Figures Figure 1. Current distribution due to anode position ........................ 61 Figure 2. Relative position of selected reference electrodes ................... 64 Figure 3. Junction potential error ...................................... 66 vii List of Tables Table 1. Available cathodic protection systems and estimated costs and lives ...... 28 Table 2. Concrete bridge component deterioration chart ..................... 32 Table 3. Advantages and disadvantages of various power supplies .............. 67 ix Abstract This report describes the evolution of cathodic protection of reinforced concrete bridges and its current state-of-the-art. It discusses how cathodic protection works, and the effectiveness of the technique. An extensive history of cathodic protection of reinforced concrete is included that covers all aspects of completed projects. There are sections on condition surveys, remedial action, anode systems, design aspects, operation and maintenance, and research and development nerds. All of the various types of anode systems are characterized in detail as they have been designed and constructed. The ancillary equipment common to all systems such as power supplies and monitoring equipment are reviewed. Research and development needs are addressed to bring to light the essential research work that is required to further the development and use of cathodic protection of reinforced concrete structures. Executive Summary Since the first cathodic protection systems were used in the 1970s, many advances have been made in the system components and test procedures. Techniques are available today to assess the extent and rate of corrosion, as well as the performance of cathodic protection systems. Although there is no doubt that cathodic protection mitigates the corrosion of reinforcing steel in concrete, the effects of passage of current through concrete is still not fully understood. Presently there must be a compromise between the complexities of concrete and cathodic protection, and the need for a simple and functional cathodic protection system. This report is intended to be a concise review of the history of cathodic protection and its current status for protection of highway structures. Examination of several structures has shown that cathodically protected areas have less corrosion than nearby unprotected areas or structures. Also, extensive studies have proven that cathodic protection stops corrosion of the embedded reinforcing steel. Thus, there is no question that properly installed and activated cathodic protection systems halt corrosion and corrosion induced damage in salt-contaminated above-ground reinforced-concrete structures. Cathodic protection was fu'st used in 1824 to protect steel in seawater. The first uses on concrete began prior to 1955 on prestressed concrete water pipelines. These applications involved the use of the soil or water to apply and distribute the current, and are not pertinent to this report. However, the above-ground concrete field developed from these efforts. Cathodic protection controls the corrosion of steel in concrete by applying an external source of direct current to the surfaces of the embedded steel. This is referred to as impressed current cathodic protection. Such a system requires the following components: • External DC Power Source (Rectifier) • Current Distribution Hardware (Anode) • Conducting Electrolyte (Moist Concrete) • Protected Metal (Reinforcing Steel) • Completed Circuit (Wiring) • Evaluation and Control Devices (Probes, Reference Cells, Controllers) 3 Cathodic protection has been used primarily as a rehabilitation technique to extend the life of structures lJaatalready are deteriorated. Although this is cost effective, studies have shown that installation of cathodic protection prior to extensive corrosion-induced damage can be even more cost effective because the high cost of concrete removal and repair is avoided. More than 275 structures have been cathodical]y protected in North America. Missouri has over 100 installations and Ontario has nearly 50. A recent survey indicated that 90% of the installations were operating satisfactorily as de:_igned. Several types of anode systems have been developed. The ftrst was a conductive coke-asphalt overlay for bridge decks that operated successfully. Then slotted systems were developed and used. Next, distributed anodes with concrete encapsulation were used.