Corrosion impact on bridge infrastructure
Corrosion will affect all types of metals to varying degrees of severity and speed. Unless comprehensive management plans are developed and implemented, steel and other metals will ‘rust’ and reinforced concrete will spall and crack. Corrosion can be prevented or minimised by either ‘isolating’ the material from its environment with some sort of coating or implementing an active intervention system such as cathodic protection. Bridges carry massive loads from moving vehicles which impose vibrational and other stresses onto structures. Approximately 200,000 cars and trucks cross Melbourne’s Westgate Bridge each day. The Auckland Harbour Bridge carries a similar volume of road traffic, although it is estimated that half the people crossing the bridge in the morning peak hour are on buses. The owners and managers of these assets must ensure that bridges are safe, while maintaining acceptable levels of service for the duration of the expected life of the asset. If appropriate asset management strategies are implemented, it is possible to restore an asset to near its original condition and maintain its functionality for the remaining service life and, possibly, even beyond. The two most common causes of concrete corrosion are carbonation and chloride or ‘salt attack’. The alkaline (high pH) conditions in concrete forms a passive film on the surface of the steel reinforcing bars, thus preventing or minimising corrosion. Reduction of the pH caused by “carbonation” or ingress of chloride (salt) causes the passive film to degrade, allowing the reinforcement to corrode in the presence of oxygen and moisture. Leaching of the alkalinity from concrete also lowers pH to cause corrosion of steel reinforcement. Stray electrical currents, most commonly from electrified traction systems, can also breakdown the passive film and cause corrosion of steel reinforced concrete and prestressed concrete elements. As reinforcing bars rust, the volume of the rust […]