The concrete that heals its own cracks

After water, what is the second most consumed material on earth? The answer might surprise you.

It’s concrete, the primary material used in construction and the backbone of our infrastructure. And while recent breakthroughs and innovations in material science have resulted in significant advances in sectors as diverse as pharmaceuticals and electronics; concrete has historically received relatively little attention and investment.

It has suffered from being perceived fundamentally as a cheap and straightforward commodity, where the application of often expensive cutting-edge material technologies is simply not justified. I believe we have now reached the point where this view can no longer be sustained for two main reasons.

The first is that for every person on the planet, approximately three tonnes of concrete are manufactured and used by the construction industry each year. The production of the cement most commonly used to make concrete is responsible for up to 8% of all anthropogenic CO2 emissions.

Once you factor in the effects of the extraction and production of the various necessary aggregates and other components which are added to the cement, the environmental burden created by concrete production becomes an issue that we cannot safely continue to ignore.

The second is that concrete will also play a more fundamental, if perhaps changed role, in our future infrastructure. Concrete has traditionally been designed to meet a prescribed specification, including around what is considered to be inevitable material degradation over time. Mitigating this heretofore unavoidable degradation necessitates expensive maintenance regimes, and in many cases complete renewal.

Recent figures from the UK and US on the state of their concrete civil assets speak for themselves: the UK spends £40 billion per year on the repair and maintenance of its ageing infrastructure, while the US is reported to need ~$3 trillion over a five year period to raise the overall quality of its infrastructure from poor to acceptable. The spiralling costs of repair and maintenance mean that an alternative to the traditional design must be found.

To address these concerns, we need to design concrete structures to be both far more sustainable and resilient, so that their whole life carbon emissions, whole life costs and whole life societal impacts are significantly reduced. Solutions have been and are being found in part through incremental advances and improvements in areas such as resource and energy efficiency during the manufacturing process, carbon sequestration and reuse, product efficiency and sustainable construction.

But the most radical game changer will come from the impact of advanced material innovations which have the potential to transform the whole nature of concrete as a material in the way it responds and adapts to its changing environment.

Examples of such material innovations, usually referred to as ‘smart materials’, whose impact and applications could revolutionise the concrete construction sector, include nanomaterials, multi-functional materials and biomimetic materials, to name a few.

One particularly promising area of research looks to develop materials that take inspiration from the human body in its defence and self-healing processes by incorporating self-immunity and self-repair mechanisms. Self-healing mechanisms that have recently shown promise in concrete include microcapsules that contain a repair agent, bacteria that precipitate calcite (natural cement), shape memory polymers that shrink when triggered (similar to stitching cracks) and vascular flow networks (similar to the blood vein system) that circulate a healing agent.

The ultimate goal is to develop concrete materials that continually monitor, regulate, adapt and repair themselves without external intervention. This will significantly extend the life of concrete structures and minimise disruptions, repairs and replacements, saving both lives and resources, as well as leading to significant reductions in whole life carbon emissions. It also offers some quite specific and valuable safety advantages in relation to those applications where the concrete is inaccessible or is in an extremely aggressive environment such as underground nuclear waste encapsulation facilities, as well as in oil and gas well-cementing applications.

It is particularly pleasing to see that research in the development of self-healing concrete has so far been successful. Initial EU research funding in the area of self-healing materials focussed on materials such as polymers, polymer composites and metals, and concrete was not seen as a contender. However, the recent research advances made in self-healing concrete far exceeded expectations, fuelling increased international research funding in this area.

Another important category of smart materials are nanomaterials, which manipulate the inherent material structure at the nano-scale, manifesting itself into more resilient properties and functions at the macro-scale.

Advances have been made in the incorporation of nano-scale mineral and chemical additives, which as well as providing enhanced mechanical and durability performance, have also delivered self-cleaning concrete. More recently, some promise has been seen in the research on the applications of carbon nanotubes, which turn concrete into a self-sensing material.

Perhaps the greatest excitement surrounds the potential for the ‘new wonder material’, graphene, which could deliver all the above credentials put together and on top of this eliminate the problem of steel corrosion, which is the biggest and most expensive deterioration problem in concrete.

I see increasing interest from industry asking about potential applications for graphene in concrete, so there is evidence that the construction sector is beginning to take the potential for innovation in this space more seriously.

We need to build on this momentum.

Yes materials will need to be relatively cheap still, but when they are only needed in small quantities, the overall cost will be significantly reduced when concrete has been turned into a ‘smart’ material. We need to move away from narrow considerations of price to a broader discussion of value for money when it comes to construction materials.

When the whole life cost of infrastructure is considered - both in terms of carbon emissions and cost of repair, maintenance and renewal – the application of material innovations to concrete has a great deal to offer.