Concrete is the most widely used building material in the world. This durable concrete can be seen in urban sidewalks, bridges spanning large rivers, and the world’s tallest skyscrapers. However, this resilient concrete has a flaw: it develops catastrophic cracks that cost tens of billions of dollars in repair annually. But what if we could solve this problem and create self-healing concrete that performs self-repair?
What is Self-Healing Concrete and How Does it Work?
This idea is not as far-fetched as it may seem. It is possible by understanding how advanced concretes are formed and using this process to our advantage. Concrete is a mixture of coarse rocks and sand particles, called aggregates, combined with concrete additives and cement, a powdered mixture of clay and limestone. When water is added to this mixture, the cement becomes a paste that coats the aggregates and rapidly hardens through a chemical reaction called hydration. Eventually, this material becomes strong enough to support concrete structures hundreds of meters high.
Role of Microbes and Calcium Carbonate in Concrete Crack Repair
Fortunately, we are experimenting with methods that can induce advanced concretes to self-repair. Some of these solutions are inspired by the natural mechanisms of self-healing concrete. When water enters small cracks, calcium oxide hydrates the concrete. The produced calcium carbonate reacts with carbon dioxide from the air, initiating a process called self-healing, in which microscopic calcium carbonate crystals form and gradually fill the cracks. Unfortunately, these crystals can only repair cracks narrower than 0.3 millimeters.
Advantages of Using Additives in Advanced Concretes
Material scientists have succeeded in repairing cracks twice this size by adding concrete additives and hidden adhesives to the concrete mix. If we add fibers and glue-filled tubes to the mix, they break when a crack forms, releasing their sticky contents and sealing the gap. However, adhesive chemicals often behave differently from concrete and may lead to more cracks over time.
Green Technology in the Concrete Industry and Its Impact on Structural Durability
Perhaps the best way to repair large cracks is for the concrete to have the necessary tools to assist itself. Scientists have discovered that certain bacteria and fungi can produce minerals like calcium carbonate, which are also present in self-repair. Experimental concrete mixes incorporate these bacterial spores or fungi along with nutrients into the concrete mix and can remain dormant for hundreds of years. When cracks appear and water enters the concrete, the spores germinate and grow in their nutrient-rich environment, creating favorable conditions for the formation of calcium carbonate crystals. These crystals gradually fill the gaps, and after about three weeks, the active microbes can fully repair concrete cracks up to a width of approximately 1 millimeter.
Although this technique has been extensively studied, there is still a long way to go before it is globally used in concrete production. However, these repairing microbes have immense potential to make concrete stronger and more durable – which can significantly reduce the financial and environmental costs of concrete production. Ultimately, these microorganisms may lead us to have a new perspective on concrete cities and imagine these living eco-friendly concrete forests.
