Sundsvall bridge

Keeping the Sundsvall Bridge from falling apart

Bacteria is causing the Sundsvall Bridge to rust faster than expected and it may not last the planned 120 years. Swerea has investigated possible methods for correcting the problem.

The bridge was inaugurated at the end of 2014 and was built to facilitate travel between the northern and southern parts of Sundsvall. Instead of driving directly through the city centre, traffic now goes via the bridge over the fjord.

Higher corrosion rate than expected

Just months after the inauguration, the Swedish Transport Administration discovered that the rate of corrosion was significantly higher than expected for the bridge foundation’s supporting construction. It was concluded that even if there was no immediate danger, the bridge would not last the intended 120 years.

The Swedish Transport Administration engaged Swerea to find a solution. Swerea’s study was underway throughout 2015, beginning with an inventory of all conceivable methods for protecting the pilings and then continuing with testing of the most promising methods.

“With Swerea’s help we’ve come up with a technically and financially optimal solution to the problem,” says Magnus Borgström, manager for the bridge construction project.

Cathodic protection stops corrosion

The preferred method calls for cathodic protection, which involves applying a voltage between anodes placed in the water and the rusting bridge supports. A current is then connected that immunises the metal and stops rust.

“This is a proven method, primarily used for ships and offshore platforms. The challenge has been to attain a wellfunctioning system in the brackish water of the fjord,” says Bertil Sandberg at Swerea.