Revealing the secrets to longer-living concrete
After water, concrete is the most widely used substance on Earth. It’s the foundation to major structures all over the world and has been used for centuries – since the Roman Empire. But now, our oldest concrete-based infrastructure is approaching the end of its expected lifespan.
Understanding the longevity of concrete is imperative for the safety of concrete buildings. We need to know when a concrete asset must be replaced, or removed, but also should avoid doing this too soon. This is not only financially wasteful, but also has a significant impact on the environment. In fact, concrete accounts for around 8-10% of global CO2 emissions with a staggering 5.2 billion tonnes per annum created from predominately cement. A better understanding of the timescales for using concrete safely could have an enormous impact on our overarching carbon footprint.
In an effort to improve our understanding of the ageing of concrete, a team of scientists at Institut Laue-Langevin (ILL) in Grenoble set out to explore how moisture affects cracking and degradation, measuring behaviours that were yet to be explored before by the scientific community. The study was conducted with LafargeHolcim, one of the biggest cement producers in the world.
Cracking and drying can become a vicious circle in cement-based materials. Heat, as well as time the environmental humidity conditions, can contribute to the drying of a material and therefore contribute to its breaking down. In reinforced concrete (strengthened with steel rods), small gaps, or fissures, that form can even start to expose the metal, causing rusting and corrosion and that compromises the safety of the structure.
It is crucial to understand this behaviour. As our cities grow, our use of cement does too, and testing conditions, such as frequent flooding or droughts as a result of our changing climate, will continue to contribute to this breakdown.
NeXT-Grenoble (Neutron and X-ray Tomography in Grenoble) is a world-leading collaboration between ILL and Universite Grenoble Alpes to develop state-of-the-art methods for investigating materials. NeXT was used in this research to provide a 3D map of the internal structure of a cement paste sample. The cement was heated to induce drying, and simultaneously X-ray and neutron imaging was conducted in multiple snapshots over time. This allowed the scientists to capture the network of cracks and the movement of water around the material.
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Advancement of the drying front in concrete heated to fire-like conditions. Top right: comparison of different concrete mixtures (blue lines) showing significantly different behaviours despite identical temperature profiles (red lines). Bottom right: neutron imaging even allows to capture the accumulation of moisture in the colder regions ahead of the drying front, believed to be the source of the explosive spalling of concrete at high temperatures.
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Example of the high complementarity of the x-ray and neutron tomography information available at NeXT: X-ray (right) provide an insight in the evolution of the microstructure (opening of cracks) in heated concrete and neutron reveals its interplay with moisture migration/drying.
Neutrons are sensitive to hydrogen, the main component of water molecules, meaning that researchers could capture a precise 3D picture of water as it travels within the sample. The images were aligned with extreme precision to ensure the neutron and X-ray ‘maps’ matched up.
The study showed that as soon as a crack is formed there is accelerated drying near the fracture. This instigates changes in saturation of the cement pores, as well as shrinkage, which can contribute to premature aging and degradation. This is the first time this effect on concrete has been measured by the scientific community.
Further experiments will confirm whether this relationship is definitively causative, but it is an important step in unravelling the mystery of the full potential of future concrete. Similar approaches could be applied to investigate the potential recycling of this material to improve the sustainability of its lifecycle.
With the scale of concrete use on the planet, identifying the smallest ways we can optimise it as a material, will have a massive ripple effect on building safety, sustainability, and potentially save lives around the world.