The two commonest 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. 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. Over the course of many years, the cracks eventually appear on the surface and concrete starts to flake off or spall.
Fred Andrews-Phaedonos, Principal Engineer – Concrete Technology at VicRoads, said the government road authority has a range of assets throughout the State that face degradation from a range of sources.
“The iconic Westgate Bridge carries massive loads in addition to being subject to high winds and salt spray,” he said.
Inspection of the many concrete culverts and low road bridges along the hundreds of kilometres of country highway has shown that their effective operational life is being reduced as the size of interstate road trucks increases. Many structures were designed for vehicles half the size and weight of modern trucks.
“Current estimates suggest that a proportion of Australia’s bridges are structurally or functionally deficient and require major rehabilitation, strengthening, improvement or replacement to bring them to current design standards,”
Queensland Sugar Limited (QSL) operates and maintains a range of assets, the major ones being its storage sheds and wharves from where raw sugar is loaded onto ships. According to David Edelman, Project Engineer at QSL, the company’s massive storage sheds—some of which are 45 metres wide and 400 metres long—also face a slow acting but pervasive threat.
“Sugar makes a mildly acidic solution that can slowly eat away at the concrete floors and walls of the sheds,” Edelman said. “This damage leads to a pot-holed, uneven surface and breaking of the concrete at joints, which adds to the difficulty of washing the floors in addition to presenting hazards to workers. The sugar forms a sticky, unsafe coating that builds up over time and makes work inside the sheds difficult meaning the floors have to be washed periodically.”
To minimise the damage caused by the sugar-attack, the walls and floors of the company’s storage sheds are coated with a sealer. Deeper holes and cracks are filled with epoxy, and joints are kept maintained to prevent sugar attacking deeper into the slabs. In addition to this chemical attack, the continual operation of large, front-end loaders moving hundreds of thousands of tonnes of raw sugar through the sheds and onto conveyor belts and ship loaders damages the concrete surfaces.