Parking Magazine - Repair of Cathodic Protection Systems by David Monroe, Evan A. Rowles, P.E., and Shane McCartney
Parking Magazine - Repair of Cathodic Protection Systems by David Monroe, Evan A. Rowles, P.E., and Shane McCartney
July 1, 2004
 Cathodic protection (CP) is one method of mitigating corrosion of steel reinforcement in concrete parking structures. This is crucial to extending the life of the structure because as steel reinforcement corrodes, it expands, causing cracking, delamination and spalling of the concrete and related leakage and deterioration.
The presence of chlorides in concrete allows an electrochemical reaction to occur, which can result in the corrosion of embedded reinforcing steel. Also, because parking garages can be exposed to large amounts of chlorides via road salt, they are especially susceptible to corrosion of the reinforcing steel. CP counteracts this reaction and, if properly designed and installed, can stop the corrosion process.
A Brief Look at Cathodic Protection
The science of cathodic protection was born in 1824 long before its theoretical foundation was established when Sir Humphrey Davy succeeded in protecting copper against corrosion from seawater by using iron anodes. From that beginning, CP has grown to have many uses in marine and underground structures, water storage tanks, gas pipelines,oil platform supports, and many other facilities exposed to a corrosive environment. Today, CP has become a viable alternative for protecting reinforcing steel in concrete structures from chloride-induced corrosion.
 The basic principle of CP is simple: metal corrosion is reduced through the application of a cathodic current. Cathodic protection basically reduces the corrosion rate of a metallic structure by reducing its corrosion potential and bringing the metal closer to an immune state. Corrosion is an electrochemical process that occurs when a metal naturally changes into its original metal oxide form. It is a chemical process because there is a chemical transformation of the metal, and it is an electrical process because a small electrical current passes when the metal changes to an oxide. This electrical current is similar to the current developed in a flashlight battery, where corrosion of a metal, such as zinc or cadmium, causes electrical current to flow.
In a titanium anode ribbon system, such as the one described here, the steel reinforcement is a cathode, and the titanium acts as an impressed current anode. In this type of system the titanium supplies the electrochemical charge instead of the steel reinforcement supplying the charge.
A complete CP system is composed of a network of metal anodes (which can be ribbon or mesh), sensors which ensure that the current is distributed throughout the structure, and equipment to monitor and control the current.
Case Study: Replacing Portions of a CP System
A recent project at the Mt. Lebanon South Parking Garage near Pittsburgh, Pennsylvania serves to illustrate one way to effectively repair CP systems. The six-story structure, located in a busy retail area, contains 305 parking spaces.
 The existing titanium anode ribbon CP system, installed in 1991, was designed primarily to protect the top mat of reinforcement steel and did not have the capacity to completely protect the bottom mat. Over the years, cracking and spalling of concrete slabs became evident as a result of corrosion of the bottom mat of reinforcing steel. The 1/2-inch ribbon was embedded into slots 12 inches on center, parallel to the span of each slab. Cross feeders were connected to those ribbons to supply current to them.
Atlantic Engineering Services was retained as the structural engineer for the project and Carl Walker Construction (CWC) was selected as the contractor to perform the work. CWC repaired columns, walls, beams and slabs. During the repair of the slabs, CWC also repaired and replaced sections of the titanium anode ribbon CP system that was installed thirteen years ago. Finally, 88,000 square feet of new cementitious overlay was applied at a depth of 1/4 to 3/8 inch thick, as protection for the ribbon system.
The structural repair and restoration began September 2, 2003 and is scheduled for completion by the end of June 2004. During the project, work was completed in phases so that the loss of parking spaces and revenue was minimized.
In order to carry out such a specialized project, a company with considerable expertise in cathodic protection systems was consulted. The consultant, CONCORR, Inc. was able to conduct the necessary testing and assessment, and offer advice on the best methods for repairing the CP system.
 The project required meticulous attention to detail, as illustrated byFigures 1-4. For each patch area, the new portions of the CP system were inserted into slots at roughly the top surface of the new ribbon and connected by means of cad welding with a 110-volt welder. Clips were placed on the reinforcing steel to prevent the ribbon from touching it (which would have caused a short in the system). Then, a reference cell was inserted into the top and bottom layers of reinforcing steel, allowing the system to be monitored. During the pouring of new concrete, each patch area had to be carefully monitored to ensure that a current was distributed throughout the system.
The titanium ribbon supplier, Corr Pro, provided guidelines for repair and monitoring of the system and also trained key CWC employees (who, in turn, trained others) to perform this delicate task.
Finally, a thin, breathable overlay was applied to help protect the existing anodes, provide a uniform wearing surface, and allow for the release of gas from the titanium ribbon system. In such a case, where the goal is to control corrosion using CP, it may seem wise to apply a waterproof membrane, such as polyurethane. However, this is not appropriate for this project, because as the ribbon corrodes, gas is emitted, and waterproof membranes do not allow for such outgassing.
Helpful Tips
Condition Testing
Initial testing can be performed through visual inspection, identifying and documenting any areas with rust stains, exposed reinforcing steel, cracks or spalls. A simple delamination survey by chain-dragging can also be performed. Drag a length of chain along the top surface of the concrete slab; a hollow sound indicates areas where delamination of the concrete around embedded corroding reinforcing steel has occurred. Before proceeding with any patching or repair, however, be sure to consult a qualified design professional experienced with corrosion of reinforcing steel and deterioration of concrete structures.
Who to Call
First, if the existing structure does not already contain a CP system, consult a structural engineer, who can conduct further condition testing using the appropriate technology. If the structure does have a CP system, it is best to first consult the system manufacturer who can provide guidelines for the monitoring and repair of the system.
Second, consult a corrosion expert who can offer further advice. Depending on the condition of the structure and the available budget, there can be a wide range of options for extending the service life.
Author Biographies
David Monroe is President of Carl Walker Construction, specialists in the construction of new parking structures and the repair and restoration of existing garages. In addition to his contracting work, he previously served as chairman of the ACI 362 Parking Structure Committee and was principal author of the ACI Parking Structure Maintenance Guide. He has over 30 years of contracting and manufacturing experience working with concrete structures.
Evan A. Rowles, P.E. is a Senior Project Engineer and Associate with Atlantic Engineering Services, a consulting structural engineering firm located in Pittsburgh, Pennsylvania, with offices in Jacksonville, Florida, and Silver Spring, Maryland. He has 20 years of experience with a wide range of building structures, and specializes in the repair and rehabilitation of concrete structures. He is a founding member of the International Concrete Repair Institute (ICRI) and immediate past president of the Pittsburgh Chapter of ICRI.
Shane McCartney is a graduate of the University of Akron, where he studied Construction Engineering and Management. He has ACI, PTI and OSHA certifications. As a Project Manager with Carl Walker Construction, his focus is on the repair and restoration of concrete parking structures. David Monroe, Evan A. Rowles, P.E., and Shane McCartney
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