In-situ Corrosion Protection Performance Assessment of Petrolatum-Based Pile Wrapping Systems at the Tasman Bridge

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The Tasman Bridge in Hobart was constructed in 1965 and is supported by bored piles. The bored piles consist of mild steel tubes (casings) filled with reinforced concrete. Divers have determined that local corrosion of the steel casings has occurred to expose underlying pile concrete.

The occurrence of corrosion of the steel casings is most commonly at “below low water” and “below lowest astronomical tide (LAT)” as well as local areas within the immersion zone together with local areas of steel casing loss within the tidal zone (as there is no splash zone and atmospheric zone, as the piles are protected by reinforced concrete pile skirts attached to reinforced concrete pile caps) and are not considered typical for the corrosion profile of carbon steel piles in seawater. It is therefore proposed that Accelerated Low Water Corrosion (ALWC) and microbiologically influenced corrosion (MIC) are responsible for the local losses of steel casing to the Tasman Bridge piles.

A scenario analysis of the options for protection of the piles was undertaken which included identification of the timing of the protection options together with the inspection and maintenance requirements associated with each option. Petrolatum-based wrapping and jacketing of the piles from the top of each pile to over exposed concrete areas and “bands” immediately below LAT and continued 0.5 to 1m onto sound pile casing was the protection option adopted for further consideration.

Piles have been wrapped/jacketed with three different system types to confirm the performance levels offered by each system. The performance of the systems is being assessed in-situ using electrical resistance corrosion sensors and reinforcement corrosion rate probes installed at different locations around and at different heights for the wrapped/jacketed piles. Two control piles with no jackets are also being monitored.

The paper will provide some relevant discussion of the corrosion and its mechanisms prevalent to the Tasman Bridge piles as well as the pile wrapping systems trialled and being performance monitored in-situ. Details will be provided of the corrosion sensors and probes adopted including their different locations on the piles. An assessment of the results obtained to-date for up to 21 months of exposure will be made. It is also the intent of the authors to publish the future data obtained from the corrosion sensors and corrosion rate probes as they have been designed for a 25 year life.

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