MISSINGHAM BRIDGE, BALLINA, NSW
Missingham Bridge is a nine-span road bridge across the 150m wide tidal estuary of North Creek in Ballina. In 2006, the Roads and Traffic Authority implemented a preventive maintenance strategy designed to protect the reinforced concrete pile caps, piers and superstructure from the effects of salt water.
A contract was awarded to Marine & Civil Maintenance to supply and install a cathodic protection ("CP") system to the pile caps and piers, and apply protective coatings to the piers, headstocks and superstructure. The work was to be carried out in partnership with the client; the RTA provided access to the piers via boats, scaffolding and underbridge inspection units; traffic management; site facilities; and assistance with the installation.
The CP design called for the pile caps to be protected by discrete anodes that would be grouted into holes drilled vertically into the top of the caps. In the piers, which featured vertical grooves on the wide faces, the anode specified was a mesh ribbon which was to be grouted into vertical slots sawn in the concrete surface. The soffits of the headstocks also required ribbon anodes.
A trial CP installation was carried out on one pile cap to determine the optimum spacing for the discrete anodes. All other elements remained as designed.
The piers were coated with a breathable methacrylate coating that is compatible with the CP underneath. The superstructure, consisting of box girders, deck slab soffit and the sides of the headstocks, were treated with a silane crème designed to protect the concrete against the ingress of chloride ions from the sea.
The CP cables were collected via junction boxes into conduits and a cable tray to a transformer-rectifier at one end of the bridge. The system was commissioned in April 2007.