It’s no exaggeration to say that modern civilisation is built on concrete. It is the foundation upon which our bridges, buildings, roads and structures are laid and the primary material they are constructed from. Concrete is used precisely because it is so solid, unyielding and can withstand the onslaught of sun, wind and rain.
However concrete is not immortal. It has its flaws; vulnerabilities that enable the intrusion of substances such as water, carbon dioxide and chlorine that can reach into the steel-reinforced heart of a piece of concrete and cause corrosion from the inside out.
Unfortunately exposure to the substances and circumstances that encourage this corrosion is likely to increase as climate change brings warmer, wetter conditions to many parts of Australia.
A report from Dr Xiaoming Wang and colleagues at CSIRO’s Climate Adaptation Flagship and the University of Newcastle examines the possible impacts of a changing climate on the deterioration of our concrete infrastructure. They conclude that these impacts are likely to be sizeable, but can be addressed in design and maintenance standards.
What causes concrete corrosion?
Concrete itself is not necessarily the problem in concrete structure deterioration. All concrete structures are reinforced, for example with steel bars to cope with the tensile forces imposed on concrete structures, and this steel is at the same time concrete’s greatest strength and its greatest weakness.
Concrete deterioration is most commonly caused by one of several mechanisms. The first is straightforward physical deterioration caused by the pressure of a substance such as water flowing through a structure and slowly eroding it.
The second major mechanism of concrete deterioration is a chemical interaction between one of a number of compounds. One of these is chloride, found in seawater.
“The chloride sitting in the seawater firstly deposits on the surface of the structure by wind or by direct contact, such as the splash of the sea water onto the structure. Combined with water and oxygen, it penetrates into the surface of steel rebars to cause corrosion,” says Dr Xiaoming Wang, Senior Principal Research Scientist with the CSIRO National Climate Adaptation Flagship.
This kind of concrete corrosion is a particular issue around coastal areas, while concrete deterioration caused by carbon dioxide is a problem in both coastal and inland areas.
In this situation – called carbonation – carbon dioxide seeps into the concrete, changing the pH of the concrete to become more alkaline. This causes depassivation, whereby the ‘passive’ layer of oxide that protects the steel reinforcement is destroyed by the lower pH, allowing corrosion to take hold.
While uncertainties may exist around the finer details of climate change, we can be certain enough that the next 50-200 years will bring increasing temperatures to Australia. This in itself presents a problem for concrete structures.
“With most chemical reactions and physical processes, if you increase the temperature you will mostly see them getting quicker so the corrosion rate will starts to be much more severe than the situation in low temperature,” Dr Wang says.
In addition, increased concentrations of atmospheric carbon dioxide – particularly in urban areas – mean greater penetration of carbon dioxide into concrete. Another important catalyst is humidity, which is likely to change depending on location, providing either more or less moisture to the corrosion process.
Strengthening concrete against deterioration
Corroded steel reinforcement in concrete
Given the likely increase in conditions conducive to concrete deterioration, a high priority is adapting concrete structures to prevent this from happening, or at least slowing it down.
This is relatively cheap and easy to do for new structures, says Dr Wang.
“The first thing is you just extend the penetration time for carbon dioxide or chloride to reach the steel reinforcement, so what you can do is to increase the thickness of the concrete cover.”
Thicker concrete means far greater concentrations and pressures are required to force the chemicals through the concrete to the steel reinforcement surface.
“If we do that during the construction period, that’s the cheapest way because if you’re just adding a 5 cm or so thickness of concrete, that’s very cheap as long as it allows you to add it in,” he says.
Dr Wang and colleagues have provided a guidance to show what increased thickness is required to adapt in different environmental exposures.
A second option is to increase the quality of the concrete itself by making it less porous, so the carbon dioxide or chloride are difficult to penetrate.
These are the simplest and cheapest existing options. More complex, and costly, alternatives include using stainless steel reinforcement, coating the steel with a thicker layer of protection such as epoxy resin.
However adapting existing structures is far more challenging, and really the only advice that can be given is to increase the amount of monitoring and maintenance that is performed.
Engineering climate adaptation
Building inspection of carbonation-induced
Increased concrete corrosion is just one of many engineering challenges that will be faced in a changing climate. At a recent Climate Adaptation for Engineering Symposium in Melbourne, speakers from all around Australia addressed the need for engineering solutions to adapt building and infrastructure to extreme weather events and climate change.
CSIRO Climate Adaptation Flagship Director Dr Paul Hardisty emphasised the importance of engineering for a climate-uncertain future.
“Engineering is in many ways on the front line of climate adaptation,” Dr Hardisty says. “It is here that practical measures to improve resilience can be first and best applied.”
Dr Wang says the symposium demonstrated that there was a wealth of knowledge and research on adapting to climate change, but the challenge was moving this knowledge into engineering practices.
“When talking to the infrastructure sector, they often have the perception that current practice is already good enough to accommodation potential future risks, so this is the problem I can see, how can we mainstream climate adaptation into current engineering practice, and we still have a long way to go.”
However he hoped that providing simple options with simple messages, such as a chart detailed the increased thickness of concrete required to weather climate change, would help this to happen.
“You have to give them the information which directly links to their practice – you don’t tell people how much the temperature increase would be in the future, you want to tell people how much thickness you want to increase to reduce those risks.”