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Concrete In Australia : June 2013
46 Concrete in Australia Vol 39 No 2 FEATURE: BRIDGES of excess tie wires, which were left to fall to the base of the cage. One experience during construction of the headstocks was that these excess tie wires could be caught between the steel reinforcement and the cathodic anode ribbons, effectively causing a short that would render the entire zone inoperable. As there was only minor contact between the tie and the anode, continuity readings fluctuated, indicating no open-circuit conditions as required. ese readings had to be taken seriously, and the entire cage inspected until the source of the short was identified and removed. Care had to be taken that all stray tie wires were removed from the form and cage prior to casting. Monitoring during and immediately after casting showed relatively low resistance values between the various circuits, due to the higher conductivity of fresh concrete, and it was not possible to confirm electrical isolation at this time. However, after a few hours of curing, the resistances between the various circuits shifted back into an acceptable Mega-Ohm range. 7.3 Cable runs and junction boxes One of the greatest challenges was the wiring of the system after the headstocks had been completed and the girders lifted in place. Available space for cabling was sparse and operational requirements by the client prevented installation of junction boxes on upstands along the bridge walkway. As a result, junction boxes had to be located in cast in voids within the headstocks. Cables were run in the gaps between headstock and girders. Cable and junction box protection was achieved by provision of conduits and stainless steel covers. A total of 117 cables from each span had to be terminated at the 10 headstock junction boxes along the bridge. is comprised of 88 cables coming from the girders of each span, 12 cables from each headstock and 17 outgoing cables to the PSU. Each abutment had an additional small junction box. Multicore cables were then utilised to connect the terminals in the junction boxes with the terminals at the monitoring panels at the power supply unit and the DC output controllers. It is needless to say that the termination of the cables was nothing less than testing for the contractor. External cabling, junction boxes and PSU are expected to require replacement over the design life of the bridge and thus have been designed such that replacement is possible. 8.0 COMMISSIONING Construction of the bridge commenced in 2009 and was completed in January 2010. Commissioning of the cathodic prevention system was performed in February 2011 due to lack of AC power to the site. e system has now been operational for two years and is providing full protection to the bridge headstocks and girders. e reinforcement is receiving between 2 mA/m2 and 6 mA/m2 of protective current which results in a total system output of 20 A and a power consumption of 170 W. 9.0 MONITORING e client has incorporated a number of practices into its asset management strategy to ensure the long term success of this system. ese include monthly inspection of the PSU, and recording of output values and settings. is inspection is performed by the client s maintenance staff during Figure 13. Installation of the anode ribbon system. Figure 14. Tendons stressed and crossbonded to reinforcement using tie wires Figure 15. Anode junction box.