Concrete Was, Is, and Will Continue to Be a Critical Structural Material – How Can We Bypass its Shortcomings?

Concrete Limitations

Concrete has become the central material in construction in the past few decades, especially in dense urban environments. It has progressively replaced bricks, stones, and wood, thanks to its low cost, easy use, and scalability.

But it is not without issues.

Sustainability

First, it is far from a sustainable product when it comes to resources consumed. It consumes tremendous amounts of sand, to the point it has been described that the world is “running out of sand”.

The production of cement is also a very energy-intensive activity. It is also almost exclusively powered by fossil fuels, resulting in cement production being responsible for 8% of the world’s CO2 emissions.

This can be compared to the CO2 emissions of cars and vans, which are responsible for 10% of the world’s global emissions. So, making concrete more sustainable would be as impactful as turning all the world’s cars into EVs and powering them only with green energy.

Durability

If concrete buildings lasted centuries, at least we could consider that the sand consumption and CO2 emissions were something done once and for all, with the construction standing for a long time after it.

But this is far from being the case. Reinforced concrete buildings, the bulk of what is built today, have an average lifespan of only 50-100 years. They then need to be completely destroyed, the rubble taken away and buried somewhere, and rebuilt from scratch.

New Concrete Technologies

This combined high environmental costs and lack of durability means that concrete needs to be improved. Of course, we could theoretically revert entirely to using only stone, clay, and wood as basic materials for construction. However, the practicality and low cost of concrete makes it very hard to dislodge from its position as the preferred material by architects.

Removing Steel

The central culprit in the low durability of modern concrete is the steel rebar (reinforcing bar) inserted into it. Steel rebar provides important structural advantages, by radically improving the tensile strength of concrete (the ability to resist being pulled or stretched).

The problem is that steel, which is mostly made of iron, ultimately rusts. When rusting, it expands, causing the concrete around it to crack. This is often called concrete cancer, and it is why reinforced concrete lasts only so long.

Source: USNW Sydney

Graphene Concrete

As we are inventing new materials, new solutions can be applied to concrete. By adding graphene to concrete, an exceptional 2D material, researchers at the University of Virginia have managed to boost the performance of concrete, while making it more durable and reducing carbon emissions.

It was published in the Journal of Building Engineering under the title “Rheological, mechanical, and environmental performance of printable graphene-enhanced cementitious composites with limestone and calcined clay.”

They focused on concrete used in 3D printing methods, a topic we covered in further detail in “Home Ownership is More Prohibitive than Ever Before in North America – Can 3D Printing Change This?”.

The graphene-enhanced LC2 concrete could reduce greenhouse gas emissions by approximately 31% compared to traditional printable concrete mixtures. This is because instead of using steel, itself a large carbon emitter when produced, they used graphene, which is made of pure carbon. So at least for the reinforcing part, this new concrete captures carbon instead of emitting it.

Still, the cement part is the same carbon-intensive process, but this is a good first start.

And because graphene is much more stable and does not oxidize (rust), this sort of concrete should be safe from concrete cancer. If the resulting building lasts a lot longer, it will mean a lot less emissions over the long term.

Hemp Rebar

Advanced materials like graphene may not be the only option. This is good as graphene is, for now, rather expensive, which might hinder the commercialization of these high-performance, low-emissions concrete.

Hemp (the non-psychoactive species of cannabis plants) is increasingly used in construction materials. When properly processed, it can be turned into insulation material, hemp blocks for small buildings, and even hemcrete (hemp+concrete).

Source: 2050 Materials

It could also be shaped into rebar for concrete in the form of Natural Fiber-Reinforced Thermoplastic (NFRT) composite rebar, or hemp rebar, mixing hemp-based rope with a thermoplastic sleeve.

Source: CASE

As hemp is a plant fiber, it is carbon-negative, locking in atmospheric carbon. And hemp is also very resistant and non-flammable.

Self-Healing Concrete

Not all concrete lasts only a few decades. Some forms of concrete have been actually sturdy enough to last millennia, back from the Roman Empire. For example, the famous Pantheon is the world’s largest un-reinforced concrete dome and was built in 128 AD.

The first key is that such concrete structures did not use any rebar. They were also made with special materials, like volcanic rocks and ashes, that made the concrete somewhat self-repairing.

So while it may not be perfectly scalable, with volcanic rocks not as abundant as sand, it could be an option for many constructions that do not truly need rebar and are unlikely to need rebuilding.

Heating Concrete

Besides corrosion, another factor that can damage concrete buildings is cycles of freezing-melting in winter. Each time, the water expands when freezing into ice, chipping a little bit of concrete or causing cracks. Over time, this can extend into increasingly severe damages.

In “Self-Heating Concrete Could Help our Roads, Aquifers, and Wallets”, we explored how low-temperature phase changes material (PCM) can be incorporated in micro-capsules of paraffin and directly mixed into concrete.

This could help keep concrete frost-free in temperatures around 0°C and reduce winter road maintenance needs.

Carbon-Free Cement

With cement’s carbon emissions almost as high as cars’, an important improvement is needed to reduce or fully remove the associated carbon emissions, which is easier said than done.

Green Energy

As cement production requires a lot of heat, we need to see the development of electric or thermal-based alternatives to fossil-based kilns and calciners.

This is not necessarily a very complex technical task, but seeing the entire industry change their equipment and adopt this new technology will take time.

In addition, electric alternatives to fossil fuel-based industrial processes are only as green as the electricity they use. So a steady supply of carbon-free electricity is also required.

Limestone-Related Emissions

A big issue with removing carbon emissions from cement production is that not all the emissions are from the energy consumed. If this was the case, just using green electricity to heat the limestone into cement would be enough.

But the limestone itself, the base raw material for cement, is an emitter of CO2, following this formula (with Δ=heat): CaCO3 + Δ → CaO + CO2 in a process called calcination.

But some alternatives exist:

• Replacing limestone: by using calcium silicate, instead of calcium carbonate (limestone), the company Brimstone proposes to create carbon-free cement.
  • As calcium silicate is a very abundant mineral, this is mostly a question of developing the technology and making it commercially competitive.
• Carbon capture: as for now, most cement-producing facilities are using fossil fuel-based calciners and limestone, it could be easier to capture the emissions than to avoid them.