Category:774 Cathodic Protection: Difference between revisions

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.)

774.4 Maintenance Procedures

The bridge number shall be stenciled on endpost of the bridge. Directly under the bridge number, the words "CATH. PROT." shall be stenciled in black letters approximately 3 in. high.

All maintenance work that may disturb the cathodic protection system shall be discussed with the district office. The maintenance personnel should have at their disposal a complete set of design drawings for the cathodic protection system.

Conduit System Repair

All conduit system components, including wiring, pull boxes, junction boxes, access fittings, expansion fittings and conduit supports, shall be inspected for damage and repaired as soon as possible. (See Appendix for conduit notes)

Anode Repair If the anode system is damaged, the assigned electrician, should check the system and make necessary recommendations for repair. District personnel should contact the manufacturer of the cathodic system for necessary repair materials on an as needed basis. Photos and an updated bridge layout showing the repaired area shall be kept to aid in future system evaluations. In addition, the following rules shall apply.

Mounded and Slotted Systems - When replacement sections of anode are required, they shall be spliced with sound existing in-place anodes, using factory approved kits.

Elgard Mesh System - When replacement sections of anode are required, it shall be replaced with a two inch overlap on sound in-place mesh. A current distributor bar or bars shall be resistance welded to the replacement mesh and to sound in-place mesh. The ends of the existing mesh at the splice shall be cleaned and tied to the replacement mesh.

Deck Repair Contact must be made with the district office before overlay and/or deck repairs are done on a cathodically protected bridge. Repairs to the wearing surface should have no adverse effect on the cathodic protection system. Anodes shall be inspected and an electrical continuity test performed on the effected area. When repairing the wearing surface, the debonded area shall be saw cut 1 in. plus or minus. The material in the area shall be removed by lightweight air or electrical hammer and chisel or point. If anodes or supply wires are damaged, they shall be repaired. The appropriate splice kit and procedure shall be used, see anode repair. If sound concrete is removed to allow room for a splice, remove by chipping. In addition, see the following notes for different type systems.

Slotted Systems (Type 2) - All slots should be inspected and an electrical continuity test performed on the effected area.

Surface System (Type 3) and Mounded System (Type 4) - Additional care is required when making saw cuts and removing concrete to keep from damaging the surface anode system.

Rectifier Maintenance The rectifier unit shall be checked to insure the number and direction (NBL, SBL, etc., if applicable) and the word "SURFACE" along with the type (TYPE 1, 3, or 4) is stenciled on the outside of the front door of the rectifier cabinet in black letters approximately 3 in. high. The following maintenance procedures shall apply:

Guarantees and warranties shall be used where appropriate.

Maintain all components as specified by the manufacturer.

Maintain anti-corrosion material on components where corrosion may form.

Care shall be taken to provide a relatively moisture free cabinet.

The rectifier shall be clean of dust and other foreign particles.

774.5 Field Data Records

774.5.1 Procedure

The field data collector should have at his disposal a complete set of design drawings for the cathodic protection system.

The Cathodic Protection Record consists of three sections. At the top left of the form is various structure identification information. At the top right of the form are: Section A, the Routine Cathodic System Evaluation, and Section B, the Four Hour Depolarization Evaluation. It is desirable that the routine evaluation be performed monthly for the first year of operation and then every two months thereafter. The four hour depolarization evaluation shall be performed annually in the spring. Upon completion a copy of only the Spring evaluation forms (with sections A and B filled out) shall be forwarded to the Maintenance Division by June 1, each year. The following describes the procedures for completing the form.

775.5.2 Routine Cathodic System Evaluation: Section A

The Routine evaluation is completed to insure that the cathodic systems are operating at adequate protection levels that will provide the maximum life of the structure.

General Information

Record the Bridge Number, District Number, Date, Observer's Name, and Rectifiers' Serial Number. Record the air temperature, and deck surface condition (wet or dry).

Cathodic System Condition

Visually evaluate all field wires, and rectifier components. The observer shall make comments on the overall appearance of the system and record any problems.

Electrical Readings

Indicate rectifier mode, constant current or constant voltage. If constant voltage is indicated, list those zones. (All new systems should be set to constant current mode.)

Indicate the meter used in evaluation process, external hand held or built in meter.

Fixed Design Parameters

Zone Size (ft²) is calculated from plan dimensions.

Example, 25 ft. wide X 60 ft. long = 1500 ft²

Current Parameters are established from the cathodic system design phase. A lower limit of 0.5 milliAmps / ft² of zone and an upper limit of 2.0 milliAmps / ft² of zone area.

Example:

Lower Limit: (1500 ft²) x (0.5 mAmp/ft²) ¸ (1000 mA/Amp) = 0.75 Amps

Upper Limit: (1500 ft²) x (2.0 mAmp/ft²) ¸ (1000 mA/Amp) = 3.00 Amps

Rectifier Settings

Input Voltage (volts) is the DC voltage for each zone measured to the nearest 0.1 volts.

Input Current (Amps) is the current to each zone read from either the external hand held or built in meter. These readings should fall between the upper and lower current parameters established in the Fixed Design Parameter Table. If the input current does not fall between the limits, see Trouble-Shooting Checklist.

Input Current Density (milliAmps/sq. ft.) is equal to the Input Current (Amps) divided by the zone size (ft²) multiplied by 1000.

Reference Cell Parameters

Potential Parameters (millivolts) are derived from either an E-log I test or a four hour depolarization test. Copy these values from the most recent four hour depolarization evaluation sheet. If this is a new cathodic protection (CP) system, where a depolarization test has not yet been performed, then copy the values from the initial E-log I. These parameters are to be re-computed after every four depolarization evaluation test. See Four Hour Depolarization Evaluation, Section 5.

Reference Cell and Probe Readings

On Potential (millivolts) is a reference cell voltage reading, for each zone, taken with the system on using either the external hand held or built in meter. Electrically connect the reinforcing steel (system ground) to the positive terminal of the voltmeter and the reference cell lead to the negative (ground) terminal of the voltmeter. Note, these readings will be negative, example, -365 millivolts. Compare each zone reading with the lower and upper potential parameters from Reference Cell Parameter Table. If the reading is not in the given range, see Trouble-Shooting Section 6.

Instant Off Potential (millivolts) is the reading taken immediately after the protective current is turned off. This reading should be taken from about 100 to 1000 msec after shutting off the current . Measuring the potential incorrectly could lead to significant error. (see alternate procedure below).

Probe Reading (+ or –) This reading is a voltage polarity which indicates the direction of current flow. This is accomplished by connecting the positive lead of a voltmeter to the probe and the negative lead to the system negative (reinforcing steel). The probe is connected to the reinforcing steel through a 10 Ohm shunt resistor. A negative (–) reading indicates that the reinforcing steel is corroding. A positive (+) reading indicates that the reinforcing steel is protected. Note: A number value is not needed, only the polarity of (+) or (–) is to be recorded.

Alternate Procedure for Obtaining Instant Off Potential (millivolts). The Back EMF (BEMF) reading is the potential generated by the polarization of the anode and the cathode. This voltage has to be overcome by the rectifier before it can force current through the cathodic protection system. BEMF is measured across the rectifier output terminals. The positive lead of the multimeter is connected to the positive output terminal of the rectifier and the negative lead is connected to the negative terminal of the output waveform. A multi-meter such as a Fluke 87 can be used for the purpose. The minimum reading should be measured using a 1 millisecond window. On the filtered rectifiers, the current must be interrupted to make the measurement.