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Concrete In Australia : June 2013
Concrete in Australia Vol 39 No 2 43 parallel ribbons installed can provide approximately 25 mA per linear metre of girder in Zone 1 and approximately 19 mA in Zone 2. e required higher current demand per square metre of concrete surface is accommodated for by the number of ribbons placed in this area, and the ribbon spacing with a maximum ribbon spacing of 330 mm being provided in Zone 1 and 120 mm in Zone 2. As shown in Figure 8, the layout was selected to ensure even current distribution. e number of parallel ribbons installed can provide approximately 25 mA per linear metre of girder in Zone 1 and approximately 19 mA in Zone 2. Consequently, operating the ribbons at low output will ensure that the design life can be met. Eight silver/silver chloride reference electrodes are installed in one girder per span at selected locations as shown in Figure 9 to monitor polarisation levels, to ensure protection and to avoid overprotection of the prestressing tendons. 6.2 Headstock/abutment design e headstocks required a much simpler design in comparison to the girders. ey are standard reinforced concrete and contain no prestressed or post-stressed components. ese elements were formed and cast onsite following the installation of the cathodic prevention componentry. As a result, this system had to be simple, easy and quick to install Figure 5. Saw-tooth type ribbon anode. Figure 6. Anode support. onsite to ensure there were no significant delays to the overall construction project. Figure 10 shows the typical headstock anode layout. Anode ribbons were again selected for installation within the headstocks, however, due to the very dense reinforcement, a higher current demand was required and 19 mm wide ribbons were utilised for the soffit region of the headstocks. In this case, there were no constructability issues as with the girder design. ree silver/silver chloride reference electrodes were installed within each headstock to allow monitoring of potentials. 6.3 Power supply unit (PSU) In a concrete cathodic protection/prevention system, the power supply unit is a critical component, responsible for energising, monitoring and controlling the system. Systems for concrete CP are more complicated than regular cathodic protection systems, and usually contain multiple output controllers with specialised output control mechanisms, monitoring and user interfaces. Figure 7. Girder cross section zoning. Figure 8. Cathodic prevention system layout. Figure 9. Reference electrodes layout.