Concrete is quite literally the foundation of the modern world, especially the city. Skyscrapers, roads, and bridges are made of the stuff. About half of what we humans make is concrete. The first wave of consistent usage was in the Greco-Roman era, but it fell out of vogue for some reason. We are currently nearing the end of a second wave. It’s foundational, but considered an ugly necessity by modern sensibilities. We cover it over with paint and more aesthetic materials. But who knows? Perhaps new technologies in concrete will catalyze a third wave of excitement over the material.

One of the most recent and exciting innovations in concrete is self-healing concrete. What catalyzed the investigation was the discovery of bacteria in extremely alkaline hot springs. These pools had a pH ranging from 9 to 11, and were long considered too hostile to support life. One species in particular caught the attention of scientists: B. pasteurii. It’s perfectly comfortable living in rocks and excretes the mineral calcite.

Biologists and engineers have collaborated to develop a concrete that can repair itself by adding B. pasteurii into concrete mixes along with a starchy compound that serves as a food source. These bacteria can remain dormant in hostile conditions (e.g., trapped inside cement), but in the event of a crack in the sidewalk, exposure to moisture will bring them out of their hibernation. As they eat the starches, they produce calcite (a component in concrete) which fills in the cracks. Calcite production increases geometrically as the unleashed bacteria multiply. This process of biological cementation restores concrete to 90 percent of its original strength.