Durable and climate-friendly
A construction industry without concrete is almost unthinkable—but the manufacture of concrete is extremely harmful to the climate. In a project that has recently been awarded funding from the Werner Siemens Foundation, materials scientist Ueli Angst from ETH Zurich is seeking ways to improve the carbon footprint in the construction industry. In particular, he’s challenging the long-held conviction that only highly alkaline concrete can protect rebar from corrosion.
Across the globe, 600 gigatonnes of concrete—enough to build ten Matterhorns—have been used so far in construction projects. And there’s no end in sight. Indeed, concrete is the most common human-made material in the world, with several reasons accounting for its popularity: it’s relatively cheap, for example, and the substances needed for its manufacture occur naturally almost everywhere in the earth’s crust. What’s more, when reinforced with steel, concrete is incredibly strong and durable, making it ideal not only for constructing residential and office buildings, but also major infrastructure projects like bridges and tunnels.
On the downside, producing these massive amounts of concrete is bad for the climate: concrete manufacturing is responsible for up to eight percent of all human-related CO2 emissions, and it releases three times more carbon dioxide into the atmosphere than global air traffic.
This is now common knowledge in the concrete and construction industry, where various approaches to realise climate-friendlier concrete production are being tested. However, while on the whole very promising, all these endeavours have fundamental problems and have yet to find practical application.
Reinforced concrete has a weakness
Civil engineer and materials scientist Ueli Angst, professor at the Department of Civil, Environmental and Geomatic Engineering at ETH Zurich, is familiar with the technical and economic obstacles to making concrete a sustainable building material while also retaining its durability. Now, thanks to funding from the Werner Siemens Foundation, he can focus his attention on understanding the underlying problems and processes. His investigations are centred on corrosion of the steel reinforcements in concrete, as reinforcement steel (also called rebar, short for reinforcing bar) in the concrete is susceptible to rust, which, over time, can cause considerable damage. Indeed, the phenomenon of rebar corrosion is the Achilles heel of reinforced concrete. Angst therefore sees preventing corrosion, and hence the deterioration of reinforced concrete structures, as the key goal. Otherwise, the consequences are immense costs, safety hazards and environmental pollution. “According to current doctrine, however, only highly alkaline reinforced concrete protects against corrosion,” says Ueli Angst.
Is high-alkaline concrete the only solution?
To manufacture concrete, the binder material cement is mixed with water and aggregate materials such as sand and gravel. Cement is made using limestone and raw materials like clay; after being ground, this mineral mixture is heated in a furnace at high temperatures to create cement clinker. In a process called calcination, the heat breaks the limestone down into calcium oxide and the greenhouse gas CO2. The calcium oxide formed during calcination is a component of the cement clinker and later, when the cement is admixed with water to manufacture concrete, it generates a highly alkaline (basic) environment in the concrete—precisely the high alkalinity that protects the rebar in the concrete from corrosion. However, calcination is also responsible for sixty percent of the CO2 emitted during concrete manufacturing. The remaining forty percent are caused by heating the furnace, transporting raw materials, and grinding the minerals and cement clinker.
Do ecological cements hold up?
In the past twenty years, various methods to improve the environmental impact of concrete have been tested. One approach is using more sustainable energy sources to heat furnaces, but the overall gains are rather limited. To date, the most effective approach has been to change the composition of the raw materials. “The only way to avoid geogenic CO2 emissions is by replacing limestone with another raw material, or by not burning it,” Ueli Angst says, while also emphasising that scientific and industrial research is pursuing numerous interesting procedures in this area. “These ecological cements may well present similar or even better mechanical properties than those of conventional types,” he explains, “but still today there’s a persistent belief that cement manufactured using less CO2 is also less durable.”
Concrete and steel are viewed as a builder’s dream team: conventional concrete is highly alkaline, with pH values around thirteen, and the high alkalinity causes a so-called passive coating to form on the steel’s surface, inhibiting iron dissolution in the steel and thus protecting the rebar from corrosion.
Alkalinity as an article of faith
Angst explains that this “high-alkaline corrosion protection is attained only at the price of major CO2 emissions during concrete manufacturing”. If calcination is carried out without the CO2-emitting “bad guy” limestone, or even if less is used, the inevitable result is less alkalinity. “This is the dilemma we’re facing in research and industry.”
Another problem is that even reinforced concrete structures can rust if the conditions are right—or wrong, as it were. Environmental factors that promote rusting include road salt or seawater: the porous nature of concrete allows the salt to infiltrate and reach the rebar, where it can trigger corrosion—the beginning of the end for a reinforced concrete structure unless cost-intensive corrective action is taken. As an example: in Switzerland, over half a billion Swiss francs are spent every year on corrosion prevention measures and repairing damaged traffic infrastructure—which translates to roughly one thousand francs a minute.
Angst stresses that while it’s correct that high alkalinity in conventional concrete prevents corrosion, “the general conviction that concrete has to be highly alkaline to protect against corrosion poses an obstacle in the search for less carbon-intensive alternatives”.
Indeed, high-alkaline concrete as a requirement was set down in textbooks and building standards some fifty years ago. Angst says there was good reason for establishing this doctrine, especially in the nineteen eighties and nineties, and it was instrumental in achieving greater durability in reinforced concrete structures. But today, in light of global warming, he believes the practice should be reviewed—and is convinced that he can overturn the old, ingrained belief in alkalinity and replace it with a new, climate-compatible approach.
Hundreds of buildings examined
Angst’s theory that corrosion prevention is possible without high alkalinity is based on numerous observations. “Three years ago, we conducted a detailed analysis of studies made on nearly four hundred buildings in Switzerland, Finland and Japan, all of which were older structures whose concrete was no longer alkaline due to exposure to carbon dioxide in the atmosphere. Nevertheless, only five to ten percent of these buildings presented significant corrosion problems,” Angst explains. “We interpret this as clear empirical evidence that protection from corrosion can be guaranteed in concrete that isn’t highly alkaline. The critical question is how to avoid corrosion in the problematic five to ten percent.”
Prevention first
But what is it that really protects reinforced concrete and other porous building materials from corrosion? This is what Angst and his interdisciplinary team plan to find out in the next ten years—with funding from the Werner Siemens Foundation. Although the problem has been known for decades, a fundamental and detailed understanding of the progression of corrosion in reinforced concrete is still lacking. Angst and his team are adopting a comprehensive, interdisciplinary approach to investigate the various complex and interdependent processes, including chemical reactions, electrochemical processes, changes in pore structure, and the transport of various substances and gases dissolved in the concrete pore water. Concrete wetness is another key factor in preventing corrosion.
“We want to achieve a paradigm shift: rather than striving to stabilise alkalinity levels in concrete for a hundred years, we want to control corrosion,” Angst says. To realise this shift, the research focus must be placed on corrosion, and the related processes thoroughly described and quantified. Only then will it be possible to develop viable forecasting models and new testing methods to ensure both the durability of concrete and climate-friendly manufacturing.
Basic research for practical application
How exactly does rust eat its way through reinforced concrete structures? What conditions facilitate corrosion? How can deterioration be curbed? And how can the health of a concrete structure be monitored? Angst hopes that the answers to his central research questions will find application in industry and education as quickly as possible, and he’s convinced that the information on corrosion prevention currently found in textbooks, technical manuals and industry standards will have to be revised.
The ETH professor would also like to incorporate the new knowledge about corrosion in concrete into study programmes “so that the next generations can build on the latest findings”. One way of doing this is using what are known as demonstrators—climate-friendly reinforced concrete models equipped with sensors that provide tangible information on how to prevent deterioration in sustainably manufactured concrete. Angst also has plans to add short educational videos on corrosion and the durability of reinforced concrete to his recently launched YouTube channel, which he envisions as a platform to transfer knowledge to a large, international audience. After all, CO2 knows no borders, and the paradigm shift that Angst wants to bring about must take place on a global scale.
To Angst, it’s clear that reducing greenhouse gas emissions as quickly as possible is important—and that climate-friendlier concrete can make a significant contribution. “However, we can’t forget that durability is critical,” he says. “Otherwise, we’re passing a time bomb on to future generations by building structures that need major repairs after just a few decades—repairs that will once again release unnecessary emissions and use up natural resources.”
Concrete, the carbon sink
Once the desired paradigm shift has taken place, the conditions for climate-friendly and sustainable concrete manufacturing will be set. Yet Angst’s vision goes even farther than toppling an old dogma. He hopes that, in the long run, we’ll be able to store the greenhouse gas CO2 in concrete, with no corrosion. Scientists have already discovered numerous possible ways to capture carbon dioxide in concrete, and Angst is now conducting a preliminary study on an idea he finds particularly fascinating: using microorganisms to accelerate the uptake of CO2 in concrete. This means that, in future, some of the vast amounts of concrete already used in existing structures could serve as a carbon sink.
