Building materials and the race to net zero

Corrosion of steel in concrete

A good example here is the concrete industry. Concrete is the most widely used man-​made material and a massive CO₂ emitter – generating around three times as much carbon dioxide as aviation. The concrete industry has set itself the goal of reducing CO₂ emissions to net zero by 2050,² and an intensive search is underway to find climate-​friendly alternatives, such as low-​emission cements or reusing materials after a building is demolished.³ But which of these new building materials are really the most sustainable in the long term?

The Achilles heel of reinforced concrete structures is corrosion of the embedded steel. This is often caused by de-​icing salts that penetrate the porous concrete and attack the reinforcement. The steel then rusts and the concrete crumbles. Corrosion is the most prevalent form of deterioration in civil engineering, and incurs huge costs – in Switzerland, some 1,000 Swiss francs per minute in road infrastructure alone.⁴ Predictions of the condition and safety of civil engineering structures are therefore crucial. Unfortunately, however, the prediction models in use today have a limited ability to reliably forecast the corrosion of steel in concrete.

Wrong focusing hampers progress 

In a perspective,⁵ which I recently published together with researchers from North America and Europe, we reviewed the historical development of scientific prediction of corrosion damage. We found that all existing prediction models are based on a single theoretical concept that simplifies the service life of reinforced concrete as a two-​stage process: in the first stage, corrosion-​inducing agents penetrate the concrete cover; in the second stage, the reinforcing steel corrodes. This concept has dominated research for decades without making predictions much more accurate. Our analysis shows that the dominance of this one concept creates a bottleneck, severely hindering the emergence of new approaches.

Current prediction models focus on the first phase, which considers processes that occur before corrosion of the reinforcing steel sets in. In addition, these models draw largely on empirical experience with earlier building materials and so have very limited validity when it comes to new materials. What’s more, current regulations and testing methods are also based on the traditional paradigm and again only apply to new building materials to a limited extent. Worse still, and most regrettably, these regulations often put modern, low-​emission building materials at a disadvantage for practical use.

Rethinking research and practice

For these reasons, I am convinced that a paradigm shift is urgently needed for forecasting corrosion. Science and engineering practice must break free of the two-​phase concept and focus instead on the corrosion itself. We need scientifically sound models that reliably predict the actual corrosion damage and its effects on the load-​bearing behaviour of a structure – for both old and new building materials.

Tools for strategic foresight

Deploying better predictive models would yield great benefits. They would enable more proactive maintenance of our increasingly ageing infrastructure, namely by determining the right time for repair – neither too late nor too early. Maintaining infrastructure is one of the grand challenges facing all industrialised countries. Unless significant advances are made in diagnostics and prognostics, this huge task will be almost impossible to accomplish without sacrificing the availability or safety of structures.⁶

Precise corrosion predictions are also the key to increasing sustainability in civil engineering. After all, what counts is the ecological footprint not just at the birth of a structure, but during its entire lifecycle. And life cycle analyses are difficult if you don’t know the service life.

Endeavours to promptly decarbonise the construction industry are to be welcomed. But for all our enthusiasm for net zero, we shouldn’t lose sight of the long-​term outlook beyond 2050. What we build today must also be safe and durable for generations to come. Only with sound forecasting models can we make the right decisions today for tomorrow’s world and select truly sustainable building materials and processes from the range of promising ones.