Magazine

Cathodic Protection Stops Corrosion in Its Tracks

Kermit Bright, P.E., and Richard McGuire, P.E.

When a Kansas City office garage, built in 1971, began experiencing severe floor-slab deterioration only seven years after its completion, owners could not afford to continue to let it deteriorate to the point of disrepair, tear it down and build new. The reason: The garage served as the ground floor of a sprawling multi-story office complex that houses many commercial tenants.
At that time, little was known about state-of-the-art corrosion-protection systems in structures made of reinforcing steel in a structural concrete slab. Owners immediately began seeking solutions to minimize further damage. They tried a series of solutions unsuccessfully, until a group of engineers suggested a then-new technique called cathodic protection (CP). This technique employs electrical current and mitigates corrosion completely. First used by visionaries more than 15 years ago, this system has been undergoing refinements ever since. Still thought to be by far the best, it is, in fact, the only solution that arrests the corrosion process entirely. But getting to that point involved trial and error.
Trial and Error
As mentioned, corrosion at the garage had taken place as early as 1978. By 1981, extensive deterioration forced major remedial work to be conducted. This consisted of extensive jack-hammering, cleaning and removal of deteriorated areas. Next, the entire garage was overlaid with latex modified concrete, a technique that was being used at the time by the highway industry for bridge protection. The theory was that when the concrete cured, the latex would form a barrier impervious to water and salt.
By 1986, however, it was obvious that the repair had not performed well. Studies of the garage found that "delamination" -- the breaking down of concrete caused by corrosion -- had accelerated at an alarming rate. These methods only further documented that most anti-corrosion techniques only slow, not stop, the deterioration of the reinforcing steel and surrounding concrete.
Cutting-Edge Corrosion Mitigation
The Kansas City-based engineering firm Structural Engineering Associates (SEA) was hired to conduct an in-depth condition analysis at the garage, and in 1989, its engineers recommended using cathodic protection. While CP requires a more expensive initial outlay than other solutions, it also is more effective. It is recommended for buildings with a service life exceeding 10 to 20 years.
To understand the basis of CP, first it must be understood that the corrosion process that takes place in concrete is electrochemical in nature. Corrosion occurs between anodic and cathodic sites on steel rebar. According to Corrpro Companies, an Ohio company specializing in the process, four basic elements are required: an anode where corrosion occurs and current is emitted; a cathode site where no corrosion occurs and to which current flows; an electrolyte, which is a medium capable of conducting electric current by ionic current flow (i.e., soil, water or concrete); and a metallic path, a connection between the anode and cathode that allows current return and completes the circuit.
Corrosion of reinforcing steel rarely is a problem on structures that are built with good quality concrete with adequate depth of cover. However, it is well-documented that the introduction of chloride ions from de-icing salts can induce corrosion.
CP is implemented by introducing an anode field, such as catalyzed titanium anode mesh, and overlaying it across the concrete. Protection begins when a common AC electrical current is converted to a DC current at a rectifier, or transformer. The DC current feeds electricity to the field of anodes contiguous with the steel bars buried in the structural concrete. At some point, the bars connect to a wire that runs back to the rectifier, thus completing the circuit.
In effect, the CP process reverses the polarity of the galvanic process. According to the laws of physics, steel reinforcing bar cannot corrode when receiving current; it can corrode only while it is emitting electrons. By definition, if all the anode sites are forced to function as current-receiving cathodes, then the entire metallic structure would be a cathode and corrosion would be eliminated. Maintaining a constant current achieves this, and corrosion is ceased. And, finally, this solution is long-term: Anodes have a life in excess of 40 years in existing buildings and 100 years in new buildings.
CP was installed in two of the six levels of the Kansas City office garage in 1989 as part of the first phase designed by SEA engineers. Five subsequent surveys conducted every other year after that installation confirmed positive results: concrete delamination growth was completely checked.
Cost Analysis
While CP is more expensive initially, it pays for itself over time. In 1989, other techniques costing $2 to $5 per square foot for new construction and $5 to $10 per square foot for existing structures proved to perform only temporarily. Comparatively, for a relatively modest increase, $8 to $12 per square foot at the time, CP completely solved the problem. Today, more cost-effective methods are used in new construction, but at about $12 per square foot, CP remains ideal for rehab projects with a long future service life and moderate to severe corrosion exposure.
High Efficiency
Over the years, CP also has become more efficient. For example, advances in computer software permit further automation in monitoring systems. Meters record amps, volts and other data, and operators linked to the system by phone line can take remote readings, pinpoint operations and operate controls remotely. In the Kansas City garage case, when considering some 80 systems for technicians to monitor, remote operations expedite maintenance greatly. Continuous monitoring permits precise adjustments to be made in response to seasonal variances in temperature, saving property owners money.
Summary
The fourth and final phase of CP installation in the Kansas City office garage will be completed this spring. Efficiencies gained in protecting the integrity of the structure, as well as efficiencies gained in maintenance, more than add up to a positive long-term solution.

Kermit Bright, P.E., is President of SEA; Richard McGuire, P.E., is Senior Project Manager of SEA. Memberships include IPI, ICRI, SWRI and ACI. For more information, go to
www.seassociates.com.

Article Abstract from June, 2005




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