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OCR for page 12
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 silversilver 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). FIGURE 13 Expanded zinc mesh system without encapsulation with a zinc bulk anode. Current probes and corrosion null probes can also be used to monitor the amount of cathodic current reaching the mon- owing to differences in pH and chloride ion concentration itored area from the cathodic protection system. By measur- between existing concrete and the patch repair. A cylindrical ing the current picked up by these probes, one can determine configuration of this anode can be installed in excavations in if sufficient current is distributed to monitored areas. concrete to provide local cathodic protection. SELECTION AND DESIGN OF CATHODIC Perforated sheets, expanded mesh, and bulk anodes have PROTECTION FOR REINFORCED CONCRETE been used in the marine environment without any encapsula- BRIDGE STRUCTURES tion. In some instances, the perforated zinc or the expanded mesh anode were sandwiched between fiberglass or a recy- The cathodic protection system must be matched to the struc- cled material panel and the concrete of the pile and held in ture material, its corrosivity (presence of chloride ions or place by compression bands (see Figure 13). The bulk anode carbonation), geometry, and the environment of use. The is generally installed under water to supplement the protec- application of cathodic protection current results in the gener- tion provided by any of the other zinc anode configurations ation of alkalinity at the steel/concrete interface and is directly used on marine piles. The current requirement in the tidal proportional to the current applied. This may accelerate the zone can be very high and perforated sheets and expanded alkalisilica reaction if the aggregate is susceptible to it. Com- mesh zinc anodes, encapsulated or not, have a very high con- pared with an impressed system, a galvanic system is less likely sumption rate in this region and need the bulk anode to keep to affect the alkalisilica reaction. When applying cathodic pro- their consumption rate to acceptable levels. 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 INSTRUMENTATIONS quantity, results in hydrogen embrittlement and subsequent failure of the steel. In impressed current systems, potential The impressed current systems require a rectifier to provide controlled rectifier systems are required when high strength power to the system. The rectifiers convert alternating cur- steel is cathodically protected to avoid hydrogen embrittle- rent to direct current and supply it to the system. They also need circuitry to control the output, which can be regulated ment. Certain galvanic anodes such as zinc, which do not either by controlling the output voltage or the current. The polarize the steel to hydrogen evolution potential, can be safely current controlled systems are preferred. Some rectifiers also used on high strength steel. The reinforcement and all embed- have potential control, whereby the potential of the reinforce- ded metals to be protected must be electrically continuous or ment is measured and the output of the rectifier adjusted to made so during installation as discontinuous metals can cor- maintain the potential or keep it under a given limit. rode owing to the discharge of current from their surface. Most modern rectifiers come equipped with remote mon- Chloride contamination of sound concrete is an important itoring and control systems. These are used to remotely mon- factor in selecting cathodic protection as an alternative. If the itor the operation and the health of the system and control the concentration and distribution of chloride ions in sound con- output. Only remote monitoring systems are used in galvanic crete is likely to result in corrosion initiation in the future, then cathodic protection systems as these systems inherently can- none of the barrier systems are effective. Stopping additional not be controlled. The modern remote monitoring and con- ingress of chloride ions does not necessarily delay the initiation trol units can be accessed by means of telephone connection, of corrosion as sufficient ions are already present. Electro- cell phone, or the Internet and are capable of sending alarms chemical chloride extraction and cathodic protection are the when any of the system parameters stray out of the normal only techniques that will not require removal of chloride con- operating values. taminated concrete, which can result in significant cost savings.
OCR for page 13
13 Therefore, it is imperative that a concrete and corrosion condi- tem. It also permits the control of the system to be more respon- tion survey be conducted to obtain the necessary information to sive to actual corrosion conditions in various sections of the match the appropriate cathodic protection to the structure. reinforced concrete elements and makes it easier to locate any problems that may occur. Standards limit a zone on a bridge As important as ascertaining the compatibility of cathodic deck to a maximum of 604 square meters (6,500 square feet) protection is the selection of the appropriate type of system. of concrete surface (9). In the case of bridge substructure sys- The type and geometrical configuration of the anode is one of tems, a single beam, piling, etc., may comprise a complete the most critical components in the success of a system. A zone regardless of the concrete surface area. Each zone rep- particular application may preclude the use of some of the resents an independent cathodic protection system consisting available anode materials. It is important that the selection of anode material, wiring, an external power source (if an of an anode material and configuration not impact the over- impressed current cathodic protection system is used), con- all durability or the operating capacity of the structure; for nections to the reinforcing steel, and appropriate monitoring example, it should not cause acid attack of the concrete or devices. When a rectifier is used in an impressed current sys- aggravate freezethaw damage, nor should it add additional tem, it may be equipped with a control card so that the cur- dead load, which could result in reduction of its overall live rent, voltage, or potential in each zone can be controlled load capacity or operating clearances. independently. The pre-design survey of the structure might include obtaining information that may affect the development Typically, a cathodic protection system is subdivided into of cathodic protection zones, installation and location of sys- smaller sections called zones to simplify control of the sys- tem components.