What If Concrete Fixes Itself without robots?

Concrete is the most widely used construction material in the world. 

It is found in swathes of city pavements, bridges that have vast rivers, and the tallest skyscrapers on earth. But this substance does have a weakness, it is prone to fatal cracking that costs tens of billions of dollars to repair each year. But what if there is a solution to this problem by creating concrete that heals itself? This idea doesn't seem simple. It goes down to an understanding of how concrete forms, and how to utilize that process to our benefit. Concrete is a combination of coarse stone and sand particles called aggregates, that mix with cement. When water gets added to this mix, the cement forms a paste and coats the cluster, quickly hardening through a chemical reaction called hydration. Eventually, the material grows strong enough to build up buildings that reach hundreds of meters into the sky. While people have been using a variety of methods to produce cement for over 4,000 years, concrete has a short lifespan. After 20 to 30 years, natural processes like concrete shrinking, excessive freezing, and heavy loads can lead to cracking. Concrete is often used as secondary support around steel reinforcements. In this concrete, even small cracks can channel water, oxygen, and carbon dioxide that corrodes the steel and lead to a disastrous collapse. On structures like bridges and highways that are constantly in use, detecting these problems before they lead to catastrophe becomes a huge and costly challenge. But not doing so would also endanger thousands of lives.  When water enters these tiny cracks, it hydrates the concrete’s calcium oxide. The resulting calcium hydroxide reacts with carbon dioxide in the air, starting a process called autogenous healing, where microscopic calcium carbonate crystals form and gradually fill the gap. Unfortunately, these crystals can only do so much, healing cracks that are less than 0.3mm wide. Material scientists have figured out how to heal cracks up to twice that size by adding hidden glue into the concrete mix. If we put adhesive-filled fibers and tubes into the mixture, they’ll snap open when a crack forms, releasing their sticky contents and sealing the gap. But adhesive chemicals often behave very differently from concrete, and over time, these adhesives can lead to even worse cracks. So perhaps the best way to heal large cracks is to give concrete the tools to help itself. Scientists have discovered that some bacteria and fungi can produce minerals, including the calcium carbonate found in autogenous healing. Experimental blends of concrete include these bacterial or fungal spores alongside nutrients in their concrete mix, where they could lie passive for hundreds of years. When cracks finally appear and water trickles into the concrete, the spores germinate, grow, and consume the nutrient soup that surrounds them, modifying the local environment to create the perfect conditions for calcium carbonate to grow. These crystals gradually fill the gaps, and after roughly three weeks, the hard-working microbes can completely repair cracks up to almost 1mm wide. When the cracks seal, the bacteria or fungi will make spores and go passive once more— ready to start a new cycle of self-healing when cracks form again. Although this technique has been studied extensively, still there are ways to go before incorporating it into the global production of concrete. But, these spores have huge potential to make concrete more resilient and long-lasting— which could drastically reduce the financial and environmental cost of concrete production. Eventually, these microorganisms may force us to reconsider the way we think about our cities, bringing our inanimate concrete to life.