Major disruption and cost were threatened when it was found that salt from winter de-icing had permeated the bridge’s concrete deck, triggering severe corrosion of its steel reinforcement. Corrosion had to be halted and fast.
Replacing the entire deck was logistically and economically a non-starter, and requiring 1000m³ of concrete with a carbon footprint of 300t it was environmentally unappealing too.
Instead, we proposed cathodic protection (CP), which involved passing a low voltage electrical current from an inert anode to the reinforcing steel to stop corrosion.
CP was to be installed on the underside of the deck. Traditional methods involve applying either a mesh anode coated in sprayed concrete, or drilling holes into which discrete anodes are installed. But difficult access and the construction of the bridge itself made both unworkable.
We resolved the dilemma with a first-of-its-kind solution. We sat anodes in a foam-filled glass reinforced polymer tray or ‘cassette’. These were bolted to the underside of the deck. Electrical connectivity was provided by surrounding the anodes with gel that sucks moisture from the air: the wet, swollen gel provides contact with the deck and enables current to flow.
After 12 months of operation, physical inspection of the deck supported data from remote monitoring, showing that corrosion had been fully halted. The system has a 25-year design life but there’s no reason why it should not continue to provide protection indefinitely.