Cathodic Protection for Life Extension of Existing Reinforced Concrete Bridge Elements (2009)

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6There are two distinct types of chemical reactions that occur on the surface of metals embedded in concrete, anodic and cathodic. These reactions are electrochemical in nature and, as the name suggests, electrical energy is associated with these chemical reactions. These reactions occur at the metal/ concrete interface. The loss of metal (i.e., corrosion) occurs as a result of the anodic reaction. The rate of these reactions is controlled by the magnitude and the direction of the local electric field and other factors. A cathodic protection system applies an electric field such that it favors the cathodic and deters the anodic reactions. When the magnitude of the applied electric field exceeds the threshold for the local environment, the anodic reactions stop; that is, corrosion stops. The material that imposes the electric energy on the metal to be protected is called an anode. It is the primary compo- nent and, generally, a cathodic protection system is described by the anode material it uses. The strength of the electric field and the resistance of the system control the magnitude of the electric current that flows in the system. The principal requirement of the anode material is that it have the capacity to transfer the electrical charge from its surface to the elec- trolyte. The electrolyte is a conductive solution, such as pore water in concrete, through which the cathodic current flows to the surface of the metal to be protected. In the process of transferring the current, the anode will corrode (i.e., will be consumed). Therefore, the slower the consumption rate of the anode per unit of cathodic current, the longer it will last. In addition, the anode must be durable in the environment it is to be used in and be able to withstand the loading it may be subjected to. For example, the anode material placed on the deck of a bridge must be capable of withstanding the weather and the traffic. The lower the electrical resistivity of the anode material, the larger the surface area of the concrete element it can uniformly distribute the current to. There are two different ways of imposing an electrical field on the metal to be protected. The one termed impressed current uses an external electrical power source to drive a current through the anode toward the metal to be protected. In the other method, galvanic cathodic protection, another metal (anode), which is more electronegative than the metal to be protected is placed in its vicinity and electrically connected to it. The dif- ference in the natural electrical potential between the two mate- rials in the given environment generates an electrical field to drive the protective current that flows from the anode to the sur- face of the metal to be protected. IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM Impressed current cathodic protection is achieved by driving a low voltage direct current from a relatively inert anode material through the concrete to the reinforcing steel. Direct current of sufficient magnitude and direction is applied to shut down the anodic reaction and support the cathodic reaction on the steel surface. The direct current is supplied by an external power source, most often a rectifier that converts alternat- ing current to direct current. Recently, solar power and spe- cially designed batteries have been successfully used as an external power source (12). The direct current is distributed to the reinforcing steel by the anode. Figure 1 shows the basic layout required for impressed current cathodic protec- tion systems. There are various different materials and configurations that can be used as anodes in impressed current cathodic pro- tection systems. In some systems only one anode material is used and in others more than one is used. When more than one anode material is used, the material that receives power from the external power source is called the primary anode. It is important that the primary anode have as low an electri- cal resistivity as possible so that current can be distributed to longer distances with minimal loss. The secondary anode, which has a much larger surface area, receives current from the primary anode and distributes it uniformly over the area to be protected. There has been an evolution in anode materials for use on reinforced concrete structures. During the developmental process, some of the anodes did not perform as expected and were eventually eliminated. Unfortunately, agencies initially experimenting with the cathodic protection systems used some of these underperforming anodes and were left with an unfavorable impression of the cathodic protection technol- ogy. Listed here and defined are anodes that have been used in impressed current cathodic protection systems. Platinum niobium wire: This anode is comprised of a cop- per core, a niobium substrate, and a platinum cladding. Plat- inum forms the surface of the wire and is an excellent anode material with a very low corrosion rate and does not form an insulating layer in most electrolytes. The niobium sub- strate is used to provide dimensional stability and the cop- per core is used for its high conductivity and lower price. As CHAPTER TWO CATHODIC PROTECTION TECHNOLOGY

7the dimensions of this anode are very small (see Figure 2), it requires either a very conductive electrolyte or a secondary anode to distribute current over larger areas. Carbon fiber: A fiber comprised of graphite, which is very similar in application to the platinum niobium wire, except its conductivity is not as good (see Figure 3). Zinc: It is one of the most widely used anode materials and can be used as either an impressed current or a galvanic anode. Although zinc is available in many different configurations, in impressed current systems it has generally been applied to the concrete surface as a thin metallic coating using the arc spray technique. Aluminum–zinc–indium alloy: This alloy can be used as an impressed current anode. It is applied to the entire surface of the concrete element using arc spray technique. Mixed metal oxide: This uses titanium as a dimensionally stable base material, which is protected by thin, self-healing, tightly adherent oxide films. It is acid resistant and resists the passage of current in the anodic direction. The mixed metal oxide coating functions as the anode. The mixed metal oxides, formed on the surface of titanium through a process RECTIFIER (NEG) (POS) Embedded Impressed Current Anode Original Concrete System Negative (Structure Ground) Repair Area & Overlay Continuity Bond Reinforcing bars (Cathode) FIGURE 1 Basic layout of an impressed current cathodic protection system. FIGURE 2 Platinum niobium wire anode. FIGURE 3 Carbon fiber anode.

of thermal decomposition, have good electrical conductiv- ity and anodic properties. The wear rate of these anodes is extremely low, uniform, and constant over all current densities. These anodes are supplied as expanded mesh (see Figure 4) and ribbon. A solid ribbon is also available. Cast iron anodes: High silicon cast iron in various geometric shapes has been used as an anode material. These anodes were only used in the asphalt coke breeze overlay systems and are not used anymore. Conductive rubber: The conductive rubber anode is manu- factured from ethylene–propylene–diene monomer containing 25% by volume acetylene black conductive carbon and is produced as sheets with corrugation on one face. Ceramics: These are supplied as tubular anodes and are manufactured from ceramic/titanium composite (see Figure 5). These anodes are designed to provide protection in a local area and do have good characteristics as an anode material. The following have been used as secondary anodes on reinforced concrete structures: Conductive polymer grout: This material was developed by the FHWA and is manufactured with vinyl ester resin with 8 appropriate additives and coke breeze as the filler material. It has excellent freeze–thaw durability, bonds to concrete, and has electrical resistivity in the 10 ohm-cm range. Conductive coatings: These coatings are essentially paints with graphite added to improve conductivity. The available anode materials can be used in various com- binations and configurations to meet the requirements of the structure. Combinations of anodes and configurations that have been used to date are discussed here. On bridge decks, the configurations used to date can be categorized as follows: 1. Conductive overlay systems. 2. Non-overlay slotted. 3. Non-conductive overlay. The only type of conductive overlay that has been pro- moted to date is the coke breeze overlay. Conductive coke breeze overlay systems use silicon–cast iron plate anodes placed on the deck surface or in recesses on the concrete deck surface. A conductive asphalt overlay is then placed. This is followed by placement of a conventional bituminous mixture, which serves as the wearing course. The slotted non-overlay system requires sawing slots into the concrete, which form a uniform grid over the entire sur- face (see Figure 6). Anodes are then placed in the slots, which are backfilled with a conductive material. Several different anodes can be used in the slots and they include platinum nio- bium wire, carbon fiber, and mixed metal oxide ribbon. The slots are backfilled with an FHWA conductive polymer ma- terial when the primary anode is the platinum niobium wire or the carbon fiber. A cementitious backfill material can be used with the mixed metal oxide ribbon anodes. The older systems FIGURE 4 Mixed metal oxide mesh anode. FIGURE 5 Ceramic tubular anodes.

9in Missouri were overlaid with a wearing surface of asphalt and the newer ones are overlaid with concrete. Several different combinations of anodes and configura- tions have been used with a non-conductive overlay. One of the earlier designs used a grid of anodes on the concrete sur- face, usually platinum niobium wire or carbon fiber encapsu- lated with a mound of conductive polymer to increase the surface area of discharge. A cementitious overlay was then placed to restore the wearing surface. This configuration is not used any more. The other alternative involves placing a mixed metal oxide mesh anode on the surface of the deck as shown in Figure 7 and overlaying it with either a portland cement or latex modified concrete. On the underside of the deck and other superstructure elements such as beams, girders, diaphragms, hammer heads, and caps the surface-applied systems are generally used. The surface-applied systems involve application of an anode material over the entire surface of the concrete. The most common surface-applied anodes are conductive paint and thermally sprayed zinc. The conductive paint anode is applied by spray or roller and a decorative overcoat is then applied, if desired, for aesthetic purposes (see Figure 8). A thin layer of zinc is applied to the concrete surface, often using the arc spray technique. On hammerheads and caps, mixed metal oxide anode encapsulated in shotcrete has been attempted. In one application, the mixed metal oxide anode was placed on the underside of a roadway of a tunnel and encapsulated with shotcrete. In another, mixed metal oxide mesh was installed on the top and bottom surfaces of historic arches and encapsulated with shotcrete. The aluminum– zinc–indium alloy has also been installed using the arc spray method. On bridge substructure elements the configurations used can be categorized into: 1. Surface applied, 2. Encapsulated, and 3. Non-encapsulated. FIGURE 6 Slotted non-overlay system (Platinum niobium wire anode and FHWA conductive polymer). FIGURE 7 Placement of mixed metal oxide mesh on a deck surface. A cementitious overlay is normally placed on the mesh anode. FIGURE 8 Conductive paint systems with decorative overcoat on hammerheads and columns.

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

11 FIGURE 10 Zinc foil and arc sprayed zinc systems. FIGURE 11 Expanded zinc mesh in jacket. FIGURE 12 Hockey puck anode installation in repairs. 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

owing to differences in pH and chloride ion concentration between existing concrete and the patch repair. A cylindrical configuration of this anode can be installed in excavations in concrete to provide local cathodic protection. Perforated sheets, expanded mesh, and bulk anodes have been used in the marine environment without any encapsula- tion. In some instances, the perforated zinc or the expanded mesh anode were sandwiched between fiberglass or a recy- cled material panel and the concrete of the pile and held in place by compression bands (see Figure 13). The bulk anode is generally installed under water to supplement the protec- tion provided by any of the other zinc anode configurations used on marine piles. The current requirement in the tidal zone can be very high and perforated sheets and expanded mesh zinc anodes, encapsulated or not, have a very high con- sumption rate in this region and need the bulk anode to keep their consumption rate to acceptable levels. INSTRUMENTATIONS The impressed current systems require a rectifier to provide power to the system. The rectifiers convert alternating cur- rent to direct current and supply it to the system. They also need circuitry to control the output, which can be regulated either by controlling the output voltage or the current. The current controlled systems are preferred. Some rectifiers also have potential control, whereby the potential of the reinforce- ment is measured and the output of the rectifier adjusted to maintain the potential or keep it under a given limit. Most modern rectifiers come equipped with remote mon- itoring and control systems. These are used to remotely mon- itor the operation and the health of the system and control the output. Only remote monitoring systems are used in galvanic cathodic protection systems as these systems inherently can- not be controlled. The modern remote monitoring and con- trol units can be accessed by means of telephone connection, cell phone, or the Internet and are capable of sending alarms when any of the system parameters stray out of the normal operating values. 12 Newer hybrid rectifiers can use an alternating current source, a battery, or solar power as input and provide con- trolled direct current output to the system. They have a built- in remote monitoring and control system and can also be used on galvanic systems for monitoring purposes. Half-cell reference electrodes, specifically silver–silver chloride, are generally embedded in the concrete to monitor the potential of the protected steel. The shift in the potential of the protected steel owing to the application of the current is an indicator of the level of protection achieved. A NACE Standard Practice requires a 100 mV shift in the potential for complete stopping of corrosion (13). Current probes and corrosion null probes can also be used to monitor the amount of cathodic current reaching the mon- itored area from the cathodic protection system. By measur- ing the current picked up by these probes, one can determine if sufficient current is distributed to monitored areas. SELECTION AND DESIGN OF CATHODIC PROTECTION FOR REINFORCED CONCRETE BRIDGE STRUCTURES The cathodic protection system must be matched to the struc- ture material, its corrosivity (presence of chloride ions or carbonation), geometry, and the environment of use. The application of cathodic protection current results in the gener- ation of alkalinity at the steel/concrete interface and is directly proportional to the current applied. This may accelerate the alkali–silica reaction if the aggregate is susceptible to it. Com- pared with an impressed system, a galvanic system is less likely to affect the alkali–silica reaction. When applying cathodic pro- tection to high strength steels, caution must be exercised. If not properly designed and controlled, hydrogen gas can be gener- ated at the metal surface which, when adsorbed in sufficient quantity, results in hydrogen embrittlement and subsequent failure of the steel. In impressed current systems, potential controlled rectifier systems are required when high strength steel is cathodically protected to avoid hydrogen embrittle- ment. Certain galvanic anodes such as zinc, which do not polarize the steel to hydrogen evolution potential, can be safely used on high strength steel. The reinforcement and all embed- ded metals to be protected must be electrically continuous or made so during installation as discontinuous metals can cor- rode owing to the discharge of current from their surface. Chloride contamination of sound concrete is an important factor in selecting cathodic protection as an alternative. If the concentration and distribution of chloride ions in sound con- crete is likely to result in corrosion initiation in the future, then none of the barrier systems are effective. Stopping additional ingress of chloride ions does not necessarily delay the initiation of corrosion as sufficient ions are already present. Electro- chemical chloride extraction and cathodic protection are the only techniques that will not require removal of chloride con- taminated concrete, which can result in significant cost savings. FIGURE 13 Expanded zinc mesh system without encapsulation with a zinc bulk anode.

13 Therefore, it is imperative that a concrete and corrosion condi- tion survey be conducted to obtain the necessary information to match the appropriate cathodic protection to the structure. As important as ascertaining the compatibility of cathodic protection is the selection of the appropriate type of system. The type and geometrical configuration of the anode is one of the most critical components in the success of a system. A particular application may preclude the use of some of the available anode materials. It is important that the selection of an anode material and configuration not impact the over- all durability or the operating capacity of the structure; for example, it should not cause acid attack of the concrete or aggravate freeze–thaw damage, nor should it add additional dead load, which could result in reduction of its overall live load capacity or operating clearances. Typically, a cathodic protection system is subdivided into smaller sections called zones to simplify control of the sys- tem. It also permits the control of the system to be more respon- sive to actual corrosion conditions in various sections of the reinforced concrete elements and makes it easier to locate any problems that may occur. Standards limit a zone on a bridge deck to a maximum of 604 square meters (6,500 square feet) of concrete surface (9). In the case of bridge substructure sys- tems, a single beam, piling, etc., may comprise a complete zone regardless of the concrete surface area. Each zone rep- resents an independent cathodic protection system consisting of anode material, wiring, an external power source (if an impressed current cathodic protection system is used), con- nections to the reinforcing steel, and appropriate monitoring devices. When a rectifier is used in an impressed current sys- tem, it may be equipped with a control card so that the cur- rent, voltage, or potential in each zone can be controlled independently. The pre-design survey of the structure might include obtaining information that may affect the development of cathodic protection zones, installation and location of sys- tem components.