It is 1984. "Terminator" movie is for the first time aired in the cinemas. This is when the concept of self-healing materials was probably first acknowledged by the general public. For the industry the idea of materials that can repair their own damage is like a "holy grail" - it would solve a lot of expensive problems, but few believe that it can actually work. When we say "self-healing", we think: smart, high-tech, sophisticated, futuristic. Would you describe simple, primitive, grey concrete as "smart"? Probably not. But what if concrete could heal itself, just like a human body...?
Concrete is after water the most used substance on Earth. It is a fantastic building material with high compressive strength and good durability. Unfortunately, due to its brittle nature, it is prone to cracking. Cracks create an open path for acidic ions to reach the steel reinforcement. This leads to corrosion and significantly weakens the concrete structure. To prevent material deterioration and deal with unwanted cracks, concrete structures require expensive repairs. In Europe, according to data from 2009, 50% of the annual construction budget is used for structure rehabilitation. In Sweden, only for horizontally cast structures, the crack repair costs reach up to 3 million Euro per year. To make matters worse, repairing concrete with, well... more concrete, is responsible for increased global pollution. In fact, if we considered cement industry as a country, then it would be standing on the podium together with China and USA when it comes to global carbon dioxide emissions. What could help in this situation? Perhaps a material that can heal itself without much effort - a smart self-healing concrete.
Concrete has self-healing imprinted in its "genes". Have you ever wondered why ancient Roman structures managed to survive for so long? Concrete's chemical composition allows for a partial crack closure and the regain of mechanical properties. For the process to be activated the crack has to be subjected to certain environmental conditions, for instance water exposure. Different ions travel from the cement binder towards the crack and react there with water and carbon dioxide forming calcium carbonate crystals and other types of chemical compounds, filling the crack. The chemical and physical principles of healing are still not fully understood. The effectiveness of the process is being improved since right now it has several shortcomings, e.g. only limited crack width can be fully closed. Self-healing abilities of concrete can be manipulated in two ways. It can be done by changing its mix composition - altering the concrete's "genotype". Another way is to apply different curing regimes on the crack surface, depending on the environment and type of structure. It is like prescribing a medication that matches the illness...
How to approach this problem? The bottom line is to understand the inner workings of cement hydration and the mechanism of the self-healing. Researchers from Luleå University of Technology, among several other universities, are working to "boost" the self-healing process. Recent results have shown that compounds similar to the ones found in bones can be formed inside the crack if a specific water based exposure is used. Fibers can also facilitate crack closure by acting like stitches and bridging the crack.
And what if you added to concrete something that does not belong there but can heal the crack? The fascinating story of self-healing concrete is filled with calcium carbonate producing bacteria, innovative microcapsules and even fungi. But this is a topic for another day.