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Concrete In Australia : June 2013
40 Concrete in Australia Vol 39 No 2 FEATURE: BRIDGES construction and energised at the time of commissioning of the structure. is would ensure that the structure remains passive even when chloride threshold levels capable of initiating reinforcment corrosion are reached at reinforcement depth. If a cathodic protection system is planned for a later stage then there are at least two main options: • e CP system can be installed with all anodes, reference electrodes and cables ready for wiring up at the time of construction. Completion of wiring and installation of a TRU can be performed when need for the CP system is identified. • A more basic approach is to provide electrical continuity of the reinforcement at the time of construction, and retrofit anodes and reference electrodes at the time of installation of the CP system. is would involve cutting or drilling to install anodes, reference electrodes and cabling. ese options have certain advantages and disadvantages as summarised in Table 3. 5.0 CASE STUDY -- RAIL BRIDGE OVER TIDAL RIVER e example selected for discussion in this paper is a 250 m long rail bridge crossing a tidal river constructed in 2010. Table 1. CP sacrificial anode types (passive) for steel in concrete. Table 2. CP impressed current anode types (active) for steel in concrete. Anode type Comments Zinc anodes in activating backfill • Cathodic protection criteria can often not be satisfied. • Limited life. • Increase of current if needed, requires installation of additional anodes. • Would not be used for cathodic prevention. • Significant number of anodes and breakouts required. • Sometimes utilised with DC power source during initial time of operation to boost protection levels. Zinc metal spray, connected to reinforcement using conductor strips or applied directly • Demonstrated track record of performance and provision of adequate protection to reinforcement. • Applicable to all surface configurations. • Coating can be readily re-applied if consumed. • High initial current output and consumption if current is not controlled. • Unsuitable for immersed or low concrete resistivity conditions. • Performance may vary with seasonal weather (moisture) conditions. • Atmospheric corrosion of zinc to form zinc oxide or carbonate products on external surface. • Would not be used for cathodic prevention. Anode type Comments Mixed metal oxide (MMO) coated titanium mesh • Usually applied over large areas as repair and embedded into a supplementary cover concrete (mortar). • Good bonding between parent concrete and mortar is essential. MMO coated titanium ribbon • Installed in cut slots into the concrete surface (retrofit) or using insulating spacer to mount onto the reinforcement prior to casting (at time of construction). • Backfilled with acid buffering, low shrinkage, low resistivity grout. • Not recommended for use in tidal zones if retrofitted. Discrete MMO coated titanium or ceramic based anodes • Primarily used for retrofit in splash and tidal zones. • Backfilled with acid buffering, low shrinkage, low resistivity grout. Platinised titanium or niobium • Installation as for MMO coated anodes. • MMO coated anodes are now used preferentially. Platinised Ribbon and discrete anodes • Installed in cut slots or drilled holes in coke backfill. • Only suited for application in horizontal surfaces, not commonly used anymore due to development in alternative grout materials. Conductive polymer coatings and overlays • Would only be applied at the time of energisation. • Demonstrated effectiveness for bridge decks in US. • Limited life between 10-15 years. • Less suited for environment with severe exposure such as UV or splash. • Would not be used for cathodic prevention. Thermal spray coatings • Typically Zn or Zn alloys; Ti also used. • Widely used for CP of bridges in US. • Humectant can be used to improve performance.