Evaluating the Effectiveness of Treatment Options / New Zealand Transport Agency
The Auckland Harbour Bridge corrosion protective coating system has been undergoing regular maintenance since the bridge opening in 1959. Historic practice was to spot abrasive blast corroded surfaces followed by spot painting and applying a full overcoat. While this process was effective in maintaining the protective coating, it also resulted in significant amount of contaminates being discharged into the Waitematā Harbour despite of the precautions being taken.
In an effort to reduce the discharge, various options have been considered taking into account the protective coating performance and longevity, with the aim to achieve the lowest practicable environmental discharge and whole of life costs.
One option involved collecting the abrasive blasting removal of the coating via the use of full scale containment to capture contaminants. However, it was found that this option would require strengthening of the bridge to safely carry the containment under wind loading at a cost of NZ$65M over a 10-year period.
As such, by undertaking a comprehensive review of the coating maintenance, a 40 years Coatings Maintenance Plan was developed. The identified lowest whole of life solution involved:
- On the land spans, use of full containment (where it could be supported from the ground), allowing for the full removal of the coating system via abrasive blasting, and its full reinstatement. These spans are to be left as long as possible before reinstating the protective coating, while ensuring minimal, if any section loss, to the steel superstructure.
- Spot repair and overcoating of other spans to maintain the existing coating for as long as practicably possible. A more proactive intervention approach is also adopted while using abseil techniques to minimise access costs.
- An outcomes based approach for consenting purposes that involved the establishment of low level discharge limits for contaminants deemed to be environmentally safe. This enables small areas of abrasive blasting without full containment for spans other than above land.
Thus, allowing for the continued corrosion protection of the bridge 125,000m2 external surface areas in a marine environment, while providing a cost effective and environmentally responsible solution.
Read more in the chapter: 6.4.3 Using Work History Information to Improve Models