Weathered steel gives bridges rustic look


When construction of the girders for the decks of the new $20.1 million Waitaki Bridges begins this month, locals could be forgiven for thinking the bridges are being built from recycled "rusty" steel.

The two new bridges will be the first state highway bridges in the South Island to be constructed using weathering steel, a high-strength, low-alloy steel that develops a weathered (rusty) look. 

The NZ Transport Agency’s Waitaki Bridges Project manager Steve Proud says weathering or weather-resistant steel is not only a more environmentally friendly option but also has economic, health and safety benefits throughout the life of the structure.

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

Mr Proud says the rusting process is started in the same way as conventional steel but the specific alloying elements in weathering steel produce a stable rust layer that adheres to the base metal. Over time - two to five years - the orange-brown appearance changes 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.”

While not suitable for all locations, he says the Waitaki Bridges are in the ideal environment for this form of construction.

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

The Transport Agency’s first bridge to be built from weathering steel was on the State Highway 1 Mercer to Longswamp off-ramp. The bridge was built in 2006 and the protective layer has been slowly forming, the uncoated steel changing from rusty red-brown to dark brown.

Editor's note: The two new two-lane bridges being built on State Highway 82, between Kurow and Hakataramea, replace two ageing single-lane wooden bridges over the Waitaki River and are part of the Transport Agency's $1 billion programme of investment in Canterbury's transport network from 2012 to 2015.