Is green concrete a myth?

Concrete is a source of controversy and various research, since it is the second-most widely used substance on the planet after water. Much of concrete’s environmental impact comes from cement, its key ingredient whose production accounts for around 8% of world CO₂ emissions. Gravel is another important ingredient of concrete, produced by heavily extractive processes and that often results from rock crushing.

A valuable “liquid stone,” concrete is used extensively in our walls, floors, foundations, and infrastructure. But, is it possible to build concrete structures that are less damaging to the environment?

Improving existing processes

This can be done by reducing the impact of cement, for instance by using clean energy or adapting its composition. Structures can also be designed to be lighter-weight and streamlined, thus requiring less concrete. This is also a potential benefit of 3D printing in construction.

Today, concrete is one of the biggest sources of waste in Switzerland. Crushed to make reclaimed gravel, it can be used under roads or to produce more concrete by replacing at best 50% of the natural gravel. However, concrete made from reclaimed gravel – known as “recycled concrete” – requires as much cement as regular concrete to fabricate, meaning its carbon footprint is almost the same. Yet it is currently branded as sustainable.

Leveraging existing concrete

While concrete rubble can be crushed into gravel, another option – and one that’s less common – involves reusing concrete as a substitute for stone in pavement, masonry walls, retaining walls and gabion walls. The challenge is that large concrete rubble pieces are often heavy and oddly shaped, hindering their reuse in construction. As part of our research, we scanned concrete rubble of irregular shapes and planned their assembly to build slender walls. Through full-scale prototypes and testing, we demonstrated the load-bearing potential of these walls and their environmental benefits.

We should consider the greenest concrete to be concrete that’s well maintained and, most importantly, that’s never poured in the first place.

Obsolete concrete structures can also be sawn into blocks of specific shapes for use in new structures. That’s the idea behind the Re:Crete footbridge built by EPFL researchers out of blocks from an old building, and the rebuiLT student project to construct a community pavilion in Vaud Canton that advocates low-tech systems.

These solutions aim to reduce the environmental footprint of concrete. But we should consider the greenest concrete to be concrete that’s well maintained to avoid obsolescence, that’s strengthened to extend its lifespan, and, most importantly, that’s never poured in the first place.

Maxence Grangeot, EPFL researcher at the Structural Xploration Lab (SXL, part of the Smart Living Lab) and at the Laboratory for Creative Computation (CRCL)

• This article was published in October 2024 in three local dailies – La Côte (Vaud Canton), Le Nouvelliste (Valais Canton) and Arcinfo (Neuchâtel Canton) – under a joint initiative between EPFL and ESH Médias to showcase the R&D being carried out at EPFL on advanced construction techniques.