Short Wave - Rome wasn't built in a day, but they sure had strong concrete

Episode Date: March 6, 2023

The Roman Colosseum is a giant, oval amphitheater built almost two thousand years ago. Despite its age and a 14th century earthquake that knocked down the south side of the colosseum, most of the 150-some foot building is still standing. Like many ancient Roman structures, parts of it were constructed using a specific type of concrete. Scientists and engineers have long suspected a key to these buildings' durability is their use of this Roman concrete. But exactly how this sturdy concrete has contributed to the architecture's strength has been a mystery to researchers across the globe.A team of interdisciplinary researchers have recently discovered one answer to why these ancient Roman buildings have weathered the test of time — while many modern, concrete structures seem to crumble after a few decades. The answer: self-healing concrete. Curious about other new discoveries or potential climate solutions scientists are researching? Email us at [e-mail] pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hi, shortwavers, producer Burley McCoy here sitting in his host. So I recently took a trip to Italy. And yes, the food and the wine were great. But what I really remember most is this ancient feeling from the art and the architecture. It's hard to conceptualize how long this has been here. Now, my travel plans didn't originally include Rome, but then I heard that Emily Kwong was planning on doing an episode on Roman concrete

and how scientists had determined what made it so resilient, and I was intrigued. Like, how did they get these blocks on top of each other? I'm standing at the entrance, just beyond the security gates, of the Roman Coliseum. The Coliseum is this giant oval amphitheater built almost 2,000 years ago, and I was a little bit awestruck. You can see pictures of this thing, and... It just doesn't do it justice.

Imagining how they did that. I mean, this thing is huge. How did they get up there? I spoke to people who came from all around the world to see the Coliseum. It's so amazing. Anshu was seeing the Coliseum for the first time with her husband, Niraj,

who had visited once before from India. They told me it was part of a goal they had to see the seven wonders of the world. Meanwhile, Amani and Sora from Japan were amazed at the same. size and beauty. In Japan, where we are from, earthquakes, and the old buildings were, like, once broken. And so it's kind of unbelievable that how, like, the old building from the 2000 years ago is still, like, on site.

And even though the south side of this otherwise 150-some-foot building collapsed in a 14th century earthquake, the majority of the building is still standing. So today on the show, Emily Kwong digs into the science of ancient Roman cement, what it is and why it's lasted so long. I'm Burley McCoy, and you're listening to Shortwave, the Daily Science podcast from NPR. All right, Emily Kong, Rome completely blew my mind. Yeah, these structures you would never know they're that old just by looking at them, especially when you think about modern concrete structures that tend to crumble after a few decades. Right. It's like staggering what the...

Romans did. And one of the huge reasons why it's withstood the test of time is because the concrete is self-healing. Wait, wait. How can concrete be self-healing? It's not alive. Well, in this case, it has the ability to heal and strengthen itself internally, like a hydra. Hydra, like the Greek snake head monster that grows the snakes back once they're cut off? I'm telling you, I did not make this up. This self-healing property was one of the world's oldest architectural mysteries until this interdisciplinary team came along. My name is Linda Seymour. I am a project consultant with Simpson, Gumpurts and Hager here in Boston, Massachusetts.

My name is Admir Mashich. I'm an associate professor at the Department of Civil and Environmental Engineering at MIT. Edmier and Linda, along with scientists from MIT, Harvard, and laboratories in Italy and Switzerland, have figured out the mystery. And they published their findings in science advances in January. Okay. And when kind of viral, and as much as a material science paper can captivate the internet. Okay. All right, Emily. Which is how I heard about it. As a former biomaterial scientists, going viral in any sense, we're going to count this one as a win. Fair, fair, fair. Yeah, the paper's got a really sexy title, okay? Okay. Hot mixing.

Ooh. Colon. Mechanistic insights into the durability of ancient Roman concrete. You got to have that colon. Okay, I'm ready to rock. Give me the concrete facts. This is why you're my favorite. Okay, Burley. So concrete, let's just talk about. modern concrete. It is a combination of water, cement, and other materials that react together. Like, you should think of concrete as a chemical reaction. And some things strengthen the mix and other things weaken it. Okay. So, like, what kind of things weaken it? Well, a lot of modern buildings tend to use concrete that's reinforced with steel. And when that steel rusts and corrods over time, it expands, pulling on the concrete, and that can force it to crack and pop and spall, setting off this reaction that's bad for the structure. Okay, but the Romans didn't

build with steel. I mean, not that I saw at least. Not at all. But they saw what was structurally possible with concrete and ran with it. Over the centuries, they created a concrete recipe called Opus commentatium. That could stand the test of time. Is that Aladdin, Emily? Wow, you fancy. I think it's because you're a scientist. Yeah, this recipe was used to make these giant bricks. And in this way, Romans could build anywhere. They could make stone on site. They were able then to build harbors, build monoliths, you know, in Rome and every single city where they went.

That's evident. Right. You saw a lot of it for your own eyes. Now, to figure out what made this concrete self-healing, Admir and his colleagues focused their research. on the ancient city of Pravernaum. They grabbed 2,000-year-old mortar samples from the city wall, and using imaging and mapping techniques developed in Admir's lab, they figured out what was inside. We love a good mapping technique. What did they see? So Roman concrete has three components that are pretty unique, limestone, volcanic material, and water. You take the limestone rock, which is going to be the white rock, you take the volcanic ash, which is the remnants of volcanic explosions,

of long ago. You mix it together with water, and that's how you're going to essentially get the baseline Roman mortar concrete. I mean, it seems so simple. Yeah. For years, architects and historians speculated that it was the volcanic material that made the concrete so strong. And it does, but it doesn't explain the self-healing. What's more, when chemical images of those samples came back to the lab, those big white chunks of lime were really no. They weren't really mixed in. Here's Admir again. Yeah, I could imagine chocolate or, you know, Torone, you know, this Italian white chocolate-like bars that have inclusions of maybe, I don't know, nuts.

Okay, I don't know what those chocolate bars are, but I'm imagining, you know, like chunks of things and another thing. So the lime were the chunks of things, but like what were they doing in there? That was the question. The team then took on the enormous task of characterizing these lime chunks and figuring out what role they played in the concrete over time. And they figured out two things. Here's Janil Mirage, who was part of the study. Janiel is now an associate at Exponent, an engineering and scientific consulting firm. The first thing was that we saw that these lime clasts were different in composition at their cores and along the edges,

which suggested that maybe they weren't totally inert over time. And the second thing that we saw was this material extending from the relics lime clasps into the surrounding cement matrix. And it kind of felt like this, wow, there's evidence. I love the science mystery. Okay, evidence of what? Evidence of what? Of self-healing, of lime filling in those cracks, reaching out into the chocolate bar to be like, I'll seal you. I'll seal you too.

Linda processed a lot of this data to determine the chemistry of what they were at. actually seeing. Amazing. And the lime gave up its secret. When the Romans were making their mortar, they were heating up their lime to turn it into something called quicklime. Oh, yeah, yeah. I have heard of quick lime. I might need a refresher. Yeah, it's a chemical sibling to limestone and very reactive. But because the Romans were introducing this quick lime to water as they were mixing, the heat it produced set up a chemical foundation that could strengthen the concrete later. Yeah, here's how. When tiny cracks started to form, the quick lime stopped them from becoming a big crack. And that's because when it rained, the lime reacted with the water to recrystallize as various forms of calcium carbonate.

Oh, yeah. Quickly filling the crack or reacting with the volcanic ash to strengthen the material. And calcium carbonate being the major component of limestone. Mm-hmm, mm-hmm. Okay, so basically, you are saying the lime was waiting to save the coliseum of tiny monster cracks that formed over time. Yeah, it was just doing its chemical job. with the help of water. And it was observed back then. Pliny the Elder wrote in the first century AD that as soon as this material comes into contact with, quote, the waves of the sea and is submerged,

it becomes a single stone mass impregnable to the waves and every day stronger. But, okay, so did the Romans know this? Did they like know this concrete was going to be self-healing? Yeah, they knew the interaction with water made it stronger, but it's hard to say. So to test out this hypothesis, the team went so far as to hot mix their own concrete. Using ancient and modern recipes, deliberately cracking the concrete running water through the cracks. And within two weeks, the concrete which contained quicklime healed. Oh my gosh. The cracks were sealed and water could no longer flow through.

Two weeks, that is amazing. And it makes so much sense like looking at that 2,000-year-old building that it's still standing. So this seems really useful, like, for people today. Are these scientists going to use this research going forward? Yeah, so Admir and Linda actually do have a patent for self-reparing mortar concrete that is inspired by the Romans, by this research. And while there is self-healing concrete on the market, this is special because we know that Roman concrete has stood the test of time. Right. And that matters for the health of our planet.

modern concrete gives off carbon dioxide when formed. And for material scientist Anisa Ramirez, the possibility of concrete requiring less repairs because it's self-healing is a welcome discovery. This is one way that the material can be greener. So I find that that's fascinating, that it's sort of like a message in a bottle. The Romans made the material.

We had to kind of figure out how they did it so that we can make better materials and then, you know, in turn, be better, stewards of our environment. Oh, that's really beautiful. It's like a telegraph from the past, helping us solve a science mystery to take care of our future. Yeah, exactly. It's a monumental piece of research from scientists who have been studying building materials for a long time. This paper was made possible through a collection of disciplines and ideas and minds. And this approach is what Admir really believes helped solve this mystery.

I think that's the space I really enjoy to navigate in where you bring different perspectives and put it on the same table and then make conclusions that probably we would never be able to do if we were just focusing on one specific thing from one specific perspective. Yeah, Emily, this really resonates. Before I went into science, I don't think I would have imagined how important it is for scientists from different disciplines to come together to solve problems. Like, we all bring our expertise, and he's right, you don't solve these big problems without having different expertise. And that's why diversity in science is so essential. So I think that's a really beautiful thought. And I really appreciate you bringing us this story, Emily.

Burley, hit me up any time on your next vacation. And I will explain to you how it's architecturally possible. Done. If you have a science question, send us an email at Shortwave at NPR.org. This episode was produced by Thomas Liu, edited by Rebecca Ramirez, and fact-checked by Annoe Oza. The audio engineer for this episode was Valentina Rodriguez. Brendan Crump is our podcast coordinator, our senior director of programming is Beth Donovan, and the senior vice president of programming is Anya Grundaman. I'm Emily Kwong.

I'm Beryl. I'm Beryl. I'm Beryn from NPR.