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 products can increase up to six times that of the original steel, thus increasing pressure on the surrounding material which slowly cracks the concrete. The most exposed elements usually deteriorate first, and it may take 5 to 15 years for the effects of reinforcing steel corrosion to become visibly noticeable. Cracks eventually appear on the surface and concrete starts to flake off or spall.
There are remedial options to stop corrosion of reinforcement. These include cathodic protection, electrochemical chloride extraction and electrochemical re-alkalisation.
In addition to the range of repair and protection approaches, the latest concrete structures incorporate new materials and production methods which improve longevity and performance. Because of the research into concrete additives, construction companies and engineering consultancies have access to all the latest technologies that yield a suite of proactive and reactive processes and procedures to maximise the durability of reinforced and pre-stressed concrete.
The physical aspects of applying a coating or repairing a section of steel or concrete present their own challenges for owners and operators of bridges. The towers and stays of suspension-type bridges often require staff to have advanced abseiling skills so they can access them. Metal structures usually need specialised equipment and scaffolding to allow workers to safely perform maintenance work.
New Zealand has approximately 2300 bridges of varying size associated with the country’s highways. A large proportion of the bridges are concrete decks on steel frames and supports or pre-stressed concrete structures, in addition to bridges made of conventional reinforced concrete and timber.
The iconic Auckland Harbour Bridge is a steel truss and box girder design. For many years, the maintenance of this bridge involved a continuing program of painting, where applicators started at one end and when they got to the other end, went back to the beginning again. According to Mandeno, this has changed. “Old oil-based paints became very brittle and could crack then delaminate,” he said. “In the late 1990s they changed to a moisture cured urethane which gives approximately a 20-year lifespan before the bridge needs to be repainted.”
Many roads throughout the region are being upgraded to allow for longer and heavier trucks. All road authorities face similar challenges when managing the risks of ageing infrastructure designed to a much lower standard, whilst still providing access for modern heavy vehicles.
Short span structures like culverts are only exposed to one axle group at any one time whereas longer span structures built during the past century are now required to carry substantially more load than they were originally designed for.
In New Zealand, many of the older timber rail bridges nearing the end of their useful life are being replaced by ‘weathering steel’ girder bridges which should provide a longer operational lifespan.
Officially known as “structural steel with improved atmospheric corrosion resistance,” weathering steel is a high strength, low alloy steel that, in suitable environments—those not exposed to high levels of salinity and pollutants—may be left unpainted allowing a protective rust “patina” to form and minimise further corrosion. Alloy components such as copper, chromium, silicon and phosphorus form less than two per cent of the steel but it retains appropriate strength, ductility, toughness and weldability so that it can be used for bridge construction.
All structural steel rusts at a rate determined by the amount of moisture and oxygen to which the metallic iron is exposed. As this process continues, the oxide (rust) layer becomes a barrier restricting further ingress of moisture and oxygen to the metal, and the rate of corrosion slows down.
The rust layer that forms on most conventional carbon-manganese structural steels is relatively porous and flakes off the surface allowing a fresh corrosion cycle to occur. However, due to the alloying elements in weathering steel, a stable rust layer is produced that adheres to the base metal and is much less porous. This layer develops under conditions of alternate wetting and drying to produce a protective barrier which impedes further access of oxygen and moisture. It is possible that if the rust layer remains sufficiently impervious and tightly adhering, the corrosion rate may reduce to an extremely low one.
It can be relatively simple to calculate loads and stresses on bridges when weights are distributed evenly across the structure, but road authorities also must deal with heavy and over-dimension loads. Movement of such vehicles requires special planning as there are some roads and bridges that are physically unable to support massive weight concentrated into a small area.
Modern technology can assist in managing some structures sensitive to vibration from heavy vehicles. Electronic sensors can be set up to monitor vibrations and other stresses on structures so that many data points are logged that can be downloaded for analysis. Sensors can also be connected to remote cameras that are triggered whenever a threshold vibration level is exceeded to identify which vehicles are producing these effects.
It is strongly recommended that a durability plan be developed which then becomes a critical tool in supporting an overarching asset management strategy. The plan should clearly outline likely corrosion-related risks and agreed mitigation approaches as early as possible in an asset’s lifecycle, ideally during the planning and design stage.