Text Size:   A+ A- A   •   Text Only
Find     
Site Image

Patterson Bridge Project
Design Technology
 
In an attempt to preserve Oregon's coastal bridges, the Bridge Preservation Unit of the Oregon Department of Transportation has implemented a system known as Impressed-Current Cathodic Protection.
 

Cathodic Protection involves a complex electrochemical process designed to arrest the corrosion of the reinforcing steel embedded in concrete structures. The process involves the application of a sacrificial anode to the surface of the bridge, which is then wired electrically to the reinforcing steel within the bridge. The anodic material applied to the surface must be a metal that is more reactive with oxygen than the iron in the reinforcing steel. Typically, the anodic material is aluminum or zinc. The dissimilar metals establish a weak electric field, causing hydrogen and sodium ions in the concrete to be attracted to the cathode (iron), and oxygen and chloride ions (the cause of the corrosion) to be attracted to the anode (zinc). The naturally existing electrical flow inherent in this chemical reaction is then enhanced by installing an electric current throughout the bridge.

The following is an example of the process, or steps, involved in applying a Cathodic Protection System:

1. Repair the reinforcing steel and deteriorated concrete, including the architectural detailing such as bush - hammered inset panels, curved brackets, dental bands and column capitals.

2. Establish electrical continuity of the reinforcing steel throughout the entire structure. Weld pieces of steel rebar together if necessary.

3. Arc - spray a zinc coating on the concrete surface, which acts as an external anode.

4. Establish a negative electrical connection to the steel by wrapping copper wire around reinforcing steel and installing a PVC junction box over the exposed area.

5. Establish positive electric current sites by installing brass plates where t he electric current will be distributed to the zinc anode.
 
Illustration of the ion flow between elements in a Cathodic Protection System. Source: John B. Vrable, National Cooperative Highway Research Program Report 180: Cathodic Protection For Reinforced Concrete Bridge Decks, Laboratory Phase (Washington DC: Transportation Research Board, 1977).
 
When the entire system is in place, the electric current causes ions to flow from the anodic material zinc (the arc - spray coating) to the cathodic target iron (the steel rebar embedded in concrete). The concrete acts as a medium for the ions to flow from one source to the other, eventually sacrificing the anodic material.