Category:774 Cathodic Protection

774.1 Definition, Policy and Design Guidelines

774.1.1 Definition

Cathodic Protection in bridge decks is defined as: the reduction or elimination of corrosion by making the reinforcing steel a cathode by means of an impressed DC current.

Corrosion is the electrochemical process by which iron returns to its natural oxidized state. This process requires four basic elements: an anode (where current flows from, and corrosion occurs), a cathode (where current flows to, and no corrosion occurs), an electrolyte (a medium capable of conducting electric current by ionic flow), and a connection between anode and cathode. Moisture and oxygen along with de-icing salts penetrate the concrete to break down the reinforcement's passive layer and feed corrosion. Rust occupies a larger volume than the original steel, producing bursting pressure on the concrete, resulting in the concrete spalling away from the reinforcement.

Cathodic Protection applies an external electrical current in a sufficient amount to overcome the internal current flow from the anodic areas, thus corrosion of the reinforcement can be eliminated. Cathodic protection can be successfully achieved when a sufficient amount of DC current flows from a sacrificial anode material through the electrolyte (concrete) to the surface of the reinforcing steel, causing it to become cathodic.

774.1.2 Policy

It is extremely important that each cathodic protection system be tested, adjusted and repaired by qualified personnel. This will assure that the system is operating properly and at maximum efficiency while providing effective corrosion control.

Records of the cathodic protection system shall be collected and maintained to provide data for evaluating system performance, documenting system modification, and trouble shooting the system.

774.1.3 Design Guidelines

For bridges selected for cathodic protection, an impressed current system with overlay will be used. This system divides the bridge slab into zones. Current design guidelines include;

1. The currently established maximum zone size is 5,000 ft².

2. Each zone receives a maximum of 10 amps of current from a rectifier controller.

3. The size of each zone is determined by a 4.0 milliamps per square foot maximum current input.

4. In slotted type systems, slots are to be sawed 3/4 in. by 3/4 in. (3/4 in by 1/2 in. when deck is scarified).

5. Anodes in any system type are to be placed no closer than 3 in. to slab drains or expansion device armor.

6. Each zone contains a minimum of one reference cell. Reference cell is positioned within one inch, but not in direct contact with top reinforcing and set in concrete which has a 5 lb/yd³ chloride content.

7. Each zone should contain one rebar probe. The rebar probe is normally delivered cast in a small beam with a chloride content of 15 lbs/yd³. This probe beam is set in the deck, in concrete that has no added chloride.

8. Each zone will have two system negative connections thermite welded to the top reinforcement and carried back to the junction box. The concrete patch for this area shall have a chloride content of 5 lbs/yd³.

9. The perimeter of all patched areas shall be initiated with a 1/2 in. deep saw cut.

See Appendix for sketches of reference cell, probes, and the system negative connection.

774.2 System Types

There are four types of systems that have been used on selected Missouri bridges. (See Appendix for general sketches)

Type 1 (Coke Breeze)

This system is no longer in use on MoDOT bridges.

Type 2 (Slotted)

This system consist of electrical conductors (anodes in slots) laid out in a grid system with the deck divided into separate zones. Anodes are placed on 8 to 16 in. centers and then covered with a conductive grout and the deck overlaid. See section on overlay for specific types. There are two slotted systems used by MHTD, they are;

Platinum System - consist of platinum wires set into sawed slots in the top of the concrete deck and filled with a conductive grout.

Platinum Wire and Carbon Strand System - consist of platinum (primary anodes) laid around the circumference of the grid and carbon strand (secondary anodes) laid longitudinal on 12 to 16 in. centers on the interior of the grid.

(There are a few bridges with the platinum anode still conducting current on the zones circumference, but all carbon strand anodes have burned and shorted out.)

Type 3 (Mesh)

This system consist of a conductive netting or mesh (anode) placed on top of the deck, divided into separate zones and then the deck overlaid. See section on overlay for specific types.

Raychem System - There are no Raychem systems working and haven’t been for more than 10 years.

Elgard System - a titanium mesh coated with metal oxide anode, which resembles chicken wire netting, placed on top of the concrete deck.

Type 4 (Mounded)

This system consists of electrical conductors (anodes) laid out in a grid system with the deck divided into separate zones. Anodes are placed on 8 to 16 in. centers and then covered with a mound of conductive grout and the deck overlaid. See section on overlay for specific types. There are two mounded systems used by MHTD, they are;

Platinum System - consist of platinum wires spaced on the top of the concrete deck and covered with a mound of conductive grout.

Platinum Wire and Carbon Strand System - consist of platinum (primary anodes) laid around the circumference of the grid and carbon strand (secondary anodes) laid longitudinal on 12 to 16 in. centers on the interior of the grid, and both covered with a mound of conductive grout.

(There are a few bridges with the platinum anode still conducting current around the zones circumference, but all the primary carbon strand anodes have burned up and shorted out. These systems are protecting an insignificant portion of the surface area of the bridge and should be turned off. The District can save a little on electric bills and can keep the rectifiers in case they can be used elsewhere or for parts.)

774.3 Overlays

Five types of overlays have been used on Missouri bridges with cathodic protection systems.

Asphaltic Concrete Was used in a 2 1/4 in. thick layer on the early coke breeze systems and in a 1 1/2 in. layer on some early slotted systems (Asphaltic overlays were not used after January 1986, but have had new surface courses added on some and systems and overlays are still performing).

Latex Modified Concrete, Low Slump Concrete, Silica Fume Concrete These overlays were used on slotted and mesh systems. Silica Fume, used on only one cathodic system deck, is not recommended for use on cathodic protected bridges, therefore no future use on cathodically protected bridges is planned. Gemcrete Thin Overlay (Gemcrete overlay is no longer available and the only bridge deck remaining with it is being rehabilitated in 2006.)