14 DAY TRIAL // Researchers at the University of Michigan have announced that they have developed a new type of concrete, which is able to heal itself, even when under extreme stress. The team say that the innovative material does not split in large cracks, like average concrete does, but in tiny splinters, which can then fix themselves. In addition, when, for instance, a bridge pillar made of the new material cracks, even extreme stress is not enough to bring the whole structure down.
“It's like if you get a small cut on your hand, your body can heal itself. But if you have a large wound, your body needs help. You might need stitches. We've created a material with such tiny crack widths, that it takes care of the healing by itself. Even if you overload it, the cracks stay small,” the UM E. Benjamin Wylie Collegiate Professor of Civil Engineering Victor Li, who is also a professor of Materials Science and Engineering at the university, explains.
During the experiments, Li's team applied a three-percent tensile strain on the new material. This means that its original size was mechanically increased, a phenomenon that rarely occurs in real life. This type of force is equivalent to stretching a 100-foot-long piece of metal or concrete another three feet. In reality, such a deformation would destroy metal and crack regular concrete wide open. The exact procedure has been recently published online in the journal Cement and Concrete Research.
“We found, to our happy surprise, that when we load it again after it heals, it behaves just like new, with practically the same stiffness and strength. Self-healing of crack damage recovers any stiffness lost when the material was damaged and returns it to its pristine state. The material can be damaged and still remain safe to load,” Li says of the results.
The key behind the success was the use of ECC, a bendable engineered cement composite on which LI and his team have been working for 15 years. It allows the new concrete to last under tensile strengths of up to five percent, whereas the regular variety breaks into a million pieces at just a 0.01-percent force. The team also learned that they had to keep the tiny fractures inside the new material at a level lower than 150 nanometers (50nm, preferably), in order to avoid any accidents.
The material is laden with extra concrete, which interacts with water and carbon dioxide to form a tiny crust of calcium carbonate that is a material that can regularly be found in seashells. “Our hope is that when we rebuild our roads and bridges, we do it right, so that this transportation infrastructure does not have to undergo the expensive repair and rebuilding process again in another five to 10 years,” the expert shares.
“Also, rebuilding with self-healing bendable concrete would allow a more harmonious relationship between the built and natural environments by reducing the energy and carbon footprints of these infrastructures. As civil and environmental engineers, we are stewards of these mega-systems. Advanced materials technology is one means to keep them healthy,” Li concludes.