How bacteria-laced concrete heals cracks with limestone

HostMost things we build, from the sidewalk in front of your house to the giant dams holding back lakes, start to fall apart the moment we finish them. It's just the way the world works. Water gets into a tiny crack, the winter cold makes it freeze and expand, and slowly the whole thing turns to dust. But there's a group of people trying to change how we think about the stuff we build. They want to make it act more like a living thing that can heal its own wounds. How do you actually get a slab of gray rock to fix itself when it breaks?

GuestIt sounds like something out of a book, but the secret is actually alive. We mix tiny living things called bacteria directly into the wet concrete before we pour it. These aren't the kind of bugs that make you sick. They're more like very hardy workers that can stay asleep for decades. We tuck them away inside the concrete, and they stay there doing absolutely nothing until a crack forms. When that crack opens up, it lets in air and water. That's the wake up call. Once they wake up, they start a process that grows new stone to fill the gap. They basically build a patch from the inside out.

HostThat sounds like a bit of a stretch. Concrete is very harsh, right? I have heard it's like lye or bleach before it dries. It's hard to imagine anything living in that kind of environment without being fried instantly.

GuestYou're right to be skeptical. If you put a normal bug in there, it would be gone in seconds. To make this work, we have to use very specific types of bacteria. We look for the ones that naturally live in really tough spots, like dry lakes that are full of salt or places near volcanoes. These guys are used to the high levels of minerals and the harsh conditions. But even then, we don't just toss them in and hope for the best. We pack them into tiny little balls made of clay or a type of plastic that breaks down over time. Inside those balls, we also put their food. It's like a little survival kit. The bacteria stay inside those pods, safe and sound, while the concrete is being mixed and poured. They only come out when the concrete cracks and breaks their little pods open.

HostOkay, so the crack is the alarm clock. But even if they wake up, they're still just tiny cells. How do they actually create enough solid rock to seal a crack? They can't just pull stone out of thin air.

GuestThey need a source of energy and the right building blocks. That's why we put a specific type of food in there with them. It's a mix of a certain sugar and a mineral called calcium. When the bacteria wake up and eat that food, they process it and let out a waste product. That waste reacts with the other chemicals already in the concrete. The result of that reaction is limestone. It's the same stuff that makes up sea shells or the white cliffs you see by the ocean. It's very hard and it sticks to the old concrete perfectly. As the bacteria eat and multiply, they keep building up layers of this new stone until the crack is completely stuffed full. It's not fast, though. It might take about three weeks to fill a crack that's about as wide as a fingernail.

HostThree weeks? That feels like a long time if we're talking about a bridge that people are driving over every day. And if the crack is bigger than a fingernail, are we just out of luck? It feels like this only fixes the small stuff.

GuestIt's definitely for the small stuff, but the small stuff is what causes the big problems. Think about a big highway bridge. It has steel bars inside the concrete to give it strength. If a tiny crack opens up, water and road salt get inside and reach that steel. Once the steel starts to rust, it expands and pushes the concrete apart from the inside. That's when you get the big chunks falling off and the bridge becomes dangerous. This system stops that from happening. It seals the tiny doors before the water can get to the steel. We're not trying to fix a bridge that has been hit by a truck. We're trying to make a bridge that never gets old in the first place.

HostI assume there's a catch here. If this is so great, why is every sidewalk not doing this? It sounds like something we should be using everywhere if it saves us from fixing repair crews all the time.

GuestThe biggest hurdle is the price tag. Making these tiny pods of bacteria and food is a lot more work than just crushing up rocks and mixing them with cement. Right now, it can make the cost of the concrete about twice as high as the normal stuff. Most builders or city planners just look at that number and say it's too much. They're thinking about the budget for this year, not the budget for twenty years from now. But when you look at how much we spend on repairs, the math starts to change. We're seeing it used now in places where it's almost impossible to send a repair crew, like tunnels deep underground or huge water tanks. In those cases, paying more today to avoid a disaster tomorrow makes sense.

HostIt's a different way of looking at the world. We usually think of buildings as things that are still and unchanging, just sitting there until they rot.

GuestWe're moving toward a world where our roads and buildings are more like a forest than a parking lot, where they can sense when they're hurt and use their own energy to grow and heal.

HostConcrete doesn't have to be a dead block of rock if we can just find the right tiny partners to keep the cracks closed.