‘Paradigm change’ needed for forecasting corrosion damage of bridges

A group of researchers from Switzerland, the US, Canada and Norway have said the most common cause of degradation and failures of reinforced concrete structures is chloride-induced corrosion of embedded steel elements.

They warned that the issue is a “pervasive, urgent problem” that requires more attention and public awareness.

An underlying concept of a chloride threshold is widely used, and all existing models to forecast corrosion performance of reinforced concrete structures exposed to chloride environments are based on this one common theoretical concept.

But the researchers believe change is needed to address the growing challenges of ageing structures losing functionality and potentially collapsing, as well as their associated greenhouse gas emissions.

“Corrosion of steel within concrete is a complex phenomenon,” said Ueli Angst, from ETH Zürich in Switzerland.

“In the generally very high alkaline environment of concrete, where the pH may be higher than 13, steel is considered passive, which means it is covered by a thin layer of protective oxides and its corrosion rate is negligibly low.”

But concrete is porous, and when exposed to salts, such as seawater or road salts, chloride ions can eventually penetrate the concrete and reach the steel. At some point, the protective passive layer will be destroyed and corrosion may start. Depending on actual exposure conditions, corrosion may occur at a faster or slower pace.

In reality, steel corrosion within concrete is a continuous process rarely separable into uncoupled, sequential phases.

The researchers believe more focus should be placed on the quantification of the corrosion rate from the moment steel is placed within concrete until it reaches the end of its service life.

To achieve this, a multiscale, multidisciplinary approach combining scientific and practical contributions from materials science, corrosion science, cement/concrete research, and structural engineering is needed, they said.

“Despite huge amounts of research, no clear chloride threshold could be found, and the influencing factors are complex,” said Burkan Isgor, from Oregon State University. “Unfortunately, mainstream research is still in search of this threshold, which presents a major barrier to developing reliable corrosion forecast models.”