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10 The surface-applied systems are primarily applicable to several configurations, thereby extending the service life of columns that are exposed to deicing salt spray and are not the system. used in tidal zones in marine environments. In tidal zones, the consumption rate of arc sprayed zinc can be very high and The ability to provide protective current is controlled by will not provide an acceptable service life, whereas the con- the resistivity of the concrete and the activation of the anode. ductive paints are not very durable in such an aggressive envi- The resistivity of the concrete can be maintained in a favor- ronment. The zinc mesh anode, when used in tidal zones, is able range by the presence of moisture in the concrete or used in combination with a bulk zinc anode. In the encapsu- exposure to high humidity. Zinc may passivate or stop acting lated category, the mixed metal oxide and zinc mesh anodes as an anode under certain conditions. To keep zinc active, are generally used. The anodes are encapsulated in a cemen- high alkalinity or halide ions (chloride, bromide, or fluoride) titious material most often held in place inside a fiberglass are required. The alkalinity of concrete is generally high, jacket. Mixed metal oxide anodes in a cementitious encapsu- although the surface is usually carbonated and pH is much lation without an outer jacket have also been used. The con- lower than that observed in the interior. The aluminum ductive rubber anode has been used in marine tidal zones zincindium alloy was developed to avoid or minimize pas- without any encapsulation. The anode is placed on the surface sivity in the application environment and provide a more of the concrete and held in place using fiberglass panels and negative potential to drive current through higher resistivity compression bands. The conductive saltwater present in the concrete. The majority of the zinc anode applications are in tidal zone improves the electrical contact provided by the the marine environment where sufficient exposure to mois- mechanical contact of the rubber with the concrete surface. ture maintains lower resistance and chloride ions are avail- able to keep the zinc active. In non-marine environments, materials specifically designed to keep zinc active can be used GALVANIC CATHODIC PROTECTION SYSTEMS for encapsulation. Galvanic cathodic protection is based on the principles of Similar to the impressed current systems, the configurations dissimilar metal corrosion and the relative position of spe- used in the galvanic systems can be categorized as follows: cific metals in the Galvanic Series. A more electronegative metal is placed in the vicinity of the metal to be protected and 1. Surface applied, is electrically connected to it. A typical installation is shown 2. Encapsulated, and in Figure 9. No external power source is needed with this type 3. Non-encapsulated. of system and much less maintenance is required. Zinc and aluminumzincindium alloy can be surface Zinc is more electronegative than low carbon steel, espe- applied using the arc spray technique. Zinc was also available cially in the presence of chloride ions, and is the anode ma- in the form of a foil with adhesive that could be applied to the terial of choice on reinforced concrete structures. It has been surface of the concrete. The surface-applied systems are gen- used in many different configurations such as arc sprayed, erally used on the underside of bridge decks and superstructure expanded mesh, perforated sheets, foil with adhesive, ribbon, elements such as beams, girders, diaphragms, and caps and pucks, and a solid rectangular mass (bulk anode). A typical substructure elements such as piles (above tidal zone), struts, installation of arc sprayed zinc and foil with adhesive is shown columns atop footers, etc. in Figure 10. The consumption rate of zinc is higher than that of the other primary anodes and limits its service life. How- Expanded zinc mesh encapsulated in a cementitious ma- ever, zinc anodes can be easily replaced or replenished in terial and contained in a fiberglass jacket that is installed Shunt Surface Applied Galvanic Anode System Negative (Structure Ground) Repair Concrete Reinforcing bars (Cathode) Continuity Bond Original Concrete FIGURE 9 Schematic of a galvanic protection system.

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11 FIGURE 10 Zinc foil and arc sprayed zinc systems. around piles is an example of an encapsulated system. A rep- resentation of such a system is presented in Figure 11. The majority of the installations of such systems are on marine piles in tidal zones. These jackets in most instances are not limited to the tidal zone but are also used to protect the area just above the tidal zones. More recently, zinc anodes encap- sulated in proprietary material designed to keep the zinc anode active (activated anodes) have also been used inside fiberglass jackets backfilled with cementitious material in non-tidal zones. The activated anodes are available in various shapes and sizes and have zinc at its core surrounded by the proprietary material. In Figure 12, one configuration of activated zinc anodes, known as point anodes or hockey pucks used in repairs is shown. These anodes look very much like hockey pucks and have a tie wire attached to fasten them to the reinforcing in repair areas. The purpose of using these anodes is to avoid acceleration of corrosion around the perimeter of the repair FIGURE 11 Expanded zinc mesh in jacket. FIGURE 12 Hockey puck anode installation in repairs.