Waitaki bridges meant to have weathered look

Kurow residents are being reassured two new multimillion-dollar bridges being built over the Waitaki River are not being made using ''rusty'' steel - that is the way they are supposed to look.

Work to replace the existing 132-year-old twin bridges between Kurow and Hakataramea with two modern steel bridges began in April as part of a $20.1 million project funded by the New Zealand Transport Agency (NZTA).

The agency's Waitaki bridges project manager, Steve Proud, said the structures would be the first state highway bridges in the South Island to be built with girders made from weathering steel.

Weathering, or weather-resistant, steel, a high-strength, low-alloy steel that developed a weathered (rusty) look, was not only a more environmentally friendly option, but also had economic, health and safety benefits throughout the life of the structure, Mr Proud said.

''The bridges will not need to be painted at the time of construction and there is no associated ongoing painting maintenance cost.''

The rusting process was started in the same way as with conventional steel but the specific alloying elements in weathering steel produced a stable rust layer that adhered to the base metal, he said.

Over time - two to five years - the orange-brown appearance would change to a characteristic dark-brown.

''This rust patina develops under alternating conditions of applying water and drying the steel to produce a protective barrier that prevents further oxygen or moisture reacting with the steel.

''The choice of weathering steel also reflects the heritage of the existing bridges and harmonises with the environment.''


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