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Concrete In Australia : June 2013
Concrete in Australia Vol 39 No 2 45 • Reference electrodes had to be installed in between tendons. • Pouring and compaction of concrete which cause: ° Changes to the concrete cover over the anode ribbons (shorts to casting form). ° Shorts between anodes and reinforcement. ° Damage to cabling, anodes and connections, if not properly secured. Some of these risks were also applicable to the onsite casting of the headstocks and abutments, which also had their own set of risks, including: • Construction of formwork around anodes which are external to the main reinforcement cage, potentially damaging or moving the anodes. • Shorts between anode and reinforcement caused by impact to the anodes or stray tie wires. • Lack of reinforcement continuity. • Ability to achieve required cover and separation between anode, surface and reinforcement, and lack of ability to confirm this. To address these risks, a suitable construction methodology and a stringent QC process with checklists for each stage of construction was developed in liaison with designer, cathodic prevention contractor, the casting contractor and site construction managers. e construction process and QA procedures were managed by the CP contractor and inspected periodically throughout the construction works by the designer. Some key QA issues the process included are summarised below. 7.1 Electrical continuity Particular attention was paid to the connections between: • anode ribbons • anode ribbons and conductor bars • positive cable connections and conductor bars. All connections were made by multiple spot welds, which were inspected. A critical component and source of system failure are cables connections, in particular positive cable to anode connections. If these connections or the cable cores are at any time during their life exposed, eg to the concrete, current output at the damaged location will occur leading to very rapid failure of the connection/cable. erefore, all cable to conductor bar connections were cast into epoxy inside a PVC tube. ree negative connections and three positive connections were provided per girder/zone respectively in order to provide redundancy. In order to achieve full protection and to prevent damage due to stray currents, all reinforcement bars, ligatures and tendons must be connected into the negative circuit. Tack and lap- welding of each ligature and reinforcement bar several times to neighbouring or crossing bars provided excellent electrical continuity throughout the entire reinforcement cage. Welding of the high tensile steel tendons was not possible and continuity was achieved by connecting the prestressed tendons to the cage using heavy gauge tie wires at multiple locations along the girders. Main challenges were the potential for tendons to snap during the process of tensioning, the very limited space inside the cage after installation of the tendons and the time to perform this work had to be kept to a minimum to avoid casting delays. Closed circuit resistance below 0.5 Ω was confirmed after curing of the girders prior to grouting up the exposed ends of the cut tendons. is is acceptable in accordance to AS2832.5, which recommends a criterion for continuity of 2 Ω. Electrical continuity of the negative connections and within the positive zones was confirmed by testing the resistance among the three negative cables and the three positive connection cables of each zone. is testing was conducted prior to casting and after curing. 7.2 Electrical isolation In order to provide a functioning cathodic prevention system, the anode ribbons and the reinforcement must remain electrically isolated. e same applies for the anode ribbons of the different zones. erefore, the resistance between the different anode zones, and between the positive anode zones and the reinforcement connections was tested multiple times during the construction process. Open circuit was the acceptance criterion prior to casting. In addition to a visual inspection, the tests were repeated after the formwork was closed to confirm that no steel or ribbon was in contact with the steel formwork. Typical construction methodology included the cutting off Figure 12. Cast girders in various stages of construction.