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Concrete In Australia : June 2013
42 Concrete in Australia Vol 39 No 2 FEATURE: BRIDGES However, the zones between the girders are likely to become chloride contaminated over the life due to spray, with the problem being increased since no washing by rainwater would occur in these areas. It further would require ongoing concrete testing and monitoring of chloride ingress, which would have been challenging. Since this bridge is a critical asset for the rail authority, the decision was made to install a cathodic prevention system to the prestressed girders at the time of construction of the bridge, in order to provide the required durability and to include the headstocks. 6.0 CATHODIC PREVENTION SYSTEM e greatest challenge was to provide a cathodic prevention system that could be installed using simple procedures, would provide sufficient current output while addressing areas of varying current demand and is able to match the design life of the structure. Another paper by the authors 2 outlines the design details of the girder cathodic prevention system, including experiences in Figure 2. View along the new bridge. Figure 3. View from old bridge (now demolished) of new bridge. Figure 4. Cross section through pier with girders. the casting yard. However, details relating to the durability of the system are discussed in the following section. 6.1 Girder anode system design In order to ensure the effectiveness of the system, it was important to provide an anode layout that ensures even distribution of current over the entire steel surface and thus equal protection levels. is had to address several challenges: • e steel density in the lower 1/3 of the girder was significantly higher than in the top 2/3. • Current output along each anode must be even to avoid excessive output in some sections leading to premature failure. • Overprotection must be avoided to prevent disbondment between reinforcement and concrete. • Overprotection has to be avoided, in particular for the high tensile steel tendons, since these can suffer from hydrogen embrittlement. • To obtain better control, the girders were divided into two horizontal zones with dedicated anode ribbon circuits. is was identified as necessary to accommodate for a significantly higher reinforcement density in Zone 2, which is further characterised by more severe exposure conditions since it includes the soffit area, as shown in Figure 7. With the design being based on a conservative current density of 5 mA/m2 of steel surface, it was calculated that the required current per linear metre of girder for Zone 1 is 6 mA and 5 mA per linear metre of girder for Zone 2. Due to the elongated shape of the girders, titanium based, mixed metal oxide (MMO) coated saw-tooth type ribbon anodes were selected, as shown in Figure 5. To secure the anodes to the reinforcement, anode support had to be installed at every crossing of an anode ribbon with a reinforcement bar and had been provided, as shown in Figure 6. e selected 6.35 mm wide saw-tooth type ribbon has a maximum current output capacity of 3.15 mA per linear metre of ribbon as per manufacturer s data sheet. e number of