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Concrete In Australia : June 2013
44 Concrete in Australia Vol 39 No 2 FEATURE: BRIDGES A designer has to select from a range of features and capacities to ensure that the system will allow adequate control and monitoring without being excessively complicated or expensive. For this project, a 21-controller PSU was used with controllers of varying capacity. e unit is capable of providing constant current or constant voltage operational modes, and has inbuilt meters and a switch panel for monitoring the 113 embedded reference electrodes. Figure 11 shows the PSU monitoring and control panel with cabinet doors open. 7.0 CONSTRUCTION EXPERIENCE In order to meet the construction methodology, a stepwise installation process had to be developed in particular for girder CP system and incorporated into the production of the girders. ese steps included: • Assembly and tack welding of the reinforcement cage. • Installation of the negative connections, anode ribbons with conductor strips, positive connections and conduit. • Transfer of the cage into the casting form. • Installations and prestressing of the high tensile steel tendons. • Installation of the reference electrodes, where required. • Closing of the casting form. • Casting and curing. • Cutting of tendons and removal from the casting form. Simpler procedures were later adapted to suit the headstocks which were constructed insitu. High slump concrete had been selected by the casting contractor and approved by the client to accommodate for the very high steel density. is was in turn beneficial for the cathodic prevention system installation, since the load imposed by the concrete upon the anode ribbons during the casting was reduced, as well as the need for compaction. However, several risks remained which had to be addressed by the QC procedure. Critical risks for the girders were: Figure 10. (l) Headstock anode layout and (r) the installation. Figure 11. The 21-controller PSU. • Lifting into the casting form causing swaying and twisting. • Steel density was very high and access for inspections restricted as soon the cage had been placed in the casting form. • Damage could be caused by installation and stressing of the tendons after installation of the majority of CP components. • Crossbonding of the tendons had to be performed with limited access.