Cold Climate Organisms Offer Clues To Crackless Concrete
Concrete is a seemingly simple mix of wet cement, but it’s been the foundation of many civilizations. Ancient Mayans and Romans used concrete in their structures, and it is the basic building block of the sky-scraping concrete jungles we inhabit today. But it turns out, it’s still possible to improve.
In an effort to create crack-free concrete that can resist the stresses of freezing temperatures, one group of researchers looked to organisms that live in sub-zero environments. Their results were published this week in the journal Cell Reports Physical Science. Engineer Wil Srubar, who is an author on that study, talks about how nature can serve as inspiration in the quest to create more sustainable concrete, wood, and other building materials.
Step inside Srubar’s Living Materials Laboratory and see regenerating bacteria cement, transparent wood, and more in a SciFri article!
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Wil Srubar is an assistant professor of Civil, Environmental, and Architectural Engineering at the University of Colorado, Boulder in Boulder, Colorado.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. If you’ve listened to this program for a while, you know that I am a concrete nerd. I am just fascinated with how a seemingly simple mixture of wet cement can dry and cure into durable, hard building material that has lasted for thousands of years. Think of the concrete in ancient Mayan and Roman ruins. And of course, it’s a basic building block of the sky-scraping concrete jungles we inhabit.
So I’m always wondering, is it possible to improve on concrete, to make it better? Well, we can always look to nature, which has a pretty good construction track record. For example, if we wanted to create crack-free concrete that resists the stresses of freezing temperatures, why not check out what organisms that live in subzero environments do to stay alive?
That’s what one group of researchers did, and published their results this week in the journal Cell Reports Physical Science. My next guest is an author on that study, and also experimenting not only with modifying concrete, but creating totally new, different building materials, like transparent wood. Yeah, we’ll talk about it. Wil Srubar is an assistant professor of civil, environmental and architectural engineering and principal investigator of the Living Materials Lab at the University of Colorado in Boulder. Welcome to Science Friday.
WIL SRUBAR: Thank you very much for having me.
IRA FLATOW: You’re a civil engineer. How did you get interested in mixing biology with engineering?
WIL SRUBAR: So I grew up on a farm outside of Houston. And for anyone who’s been on a farm, things are very much alive. It’s really when I started to live in cities that I came face to face with concrete every day, and started to question why are buildings and cities so static and why is nature very much alive. Take Central Park in New York City, where there’s a definitive line as to what is natural and what is human. And I really started to question that paradigm.
So I was a structural engineer by training. And so I was always asking questions of how can structural engineers and other building professionals contribute to sustainability. And we were using a lot of concrete, a lot of steel, a lot of glass, and materials that aren’t necessarily born of the earth. I know that nature has figured out a couple of different really cool ways to make different materials, even to survive really extreme conditions and extreme environments, so I thought maybe we could learn a few things from natural organisms and infuse that behavior into the materials with which we choose to build.
IRA FLATOW: As someone who observes a lot of concrete because I just can’t help myself, one of the issues with concrete, I’m sure as you know, and as people can tell when they walk on it, is it’s subject to cracking under stress from the elements. And as I said before, you looked at this in your latest study. And how did you approach this problem?
WIL SRUBAR: So concrete, as you know, Ira, is a porous material. It’s permeable, and it’s really prone to what’s called freeze/thaw damage. So water can permeate the surface of concrete. Water, as we know, expands when it freezes. And so that pressure, that pressure that builds up inside, if it can’t be resisted by the concrete, or if that pressure has no place to go, it will pop off the surface of concrete.
This particular study really challenged the conventional way by which we mitigate freeze/thaw damage in the concrete industry today. So the primary way is by putting in tiny little air bubbles into concrete mixtures. And as you can imagine, putting in tiny little air bubbles to help alleviate that pressure has some drawbacks.
There are three primary drawbacks, one being that those tiny little air bubbles actually lowers the strength of the concrete. So air doesn’t carry load, so it will reduce the compressive strength of concrete. Number two, tiny little air bubbles exacerbate other durability issues. So just like kind of superhighways for water and other harmful ions, like chloride, to get into the concrete and cause chloride-induced corrosion, for example.
And the third, these air bubbles, getting it exactly right is really, really finicky. And if you don’t get it exactly right and the concrete hardens, you really have to go in and replace it, which is really, really expensive and time-consuming. Recent research has also shown that those tiny little air bubbles, even if those are fully filled with water and the concrete undergoes freeze/thaw damage, it doesn’t really help.
So we really scratched our heads and thought, I wonder how nature deals with freezing temperatures. And so we discovered the field of antifreeze proteins. And antifreeze proteins are produced by a myriad of fish, plants, insects, and bacteria. And these small molecules bind to the surface of a nucleated ice crystal inside the organism and prevent it from growing and coalescing into a much larger and destructive ice crystal.
The next logical step is, well, why don’t we just get a bunch of this protein and put it into concrete? And we tried that, actually. For anyone who studies proteins knows that proteins can get really unhappy in a non-native environment. If you look at the pH of concrete, if you squeezed concrete and a drop of water came out, the pH is really high. It’s about 12 and 1/2 to 13. And proteins really don’t like that proteins. Proteins have this nice folded and chiral structure, and they like to unfold and even disintegrate at really high pH.
So we thought, well, let’s just find a polymer that mimics this behavior. That’s exactly what we did. We found a biomimetic polymer that was a little bit more robust, a little bit more stable at high pH that also displayed this ice recrystallization inhibition activity. And we found that it did, in fact, resist freeze/thaw damage in a solid ceramic, just like concrete.
IRA FLATOW: Where would we find a polymer like this in nature?
WIL SRUBAR: Yeah, so the molecule that we used was a synthetic molecule that’s been used a lot in the biomedical field for making different types of plastics. Similar molecules are actually used in foods. So for example, gelatin is an example of a molecule that has been used in some food products, like ice cream, to keep ice cream really, really smooth, to keep those ice crystals really, really small.
IRA FLATOW: Wow. So you put jello in the concrete.
WIL SRUBAR: (LAUGHING) We didn’t quite use gelatin in this study, although my lab has an affinity for structural proteins.
IRA FLATOW: Now, I understand that you’ve also made concrete with bacteria, cyanobacteria. And living concrete?
WIL SRUBAR: Yeah. So in a study we published a few months ago, we created really a hybrid living/building material that displayed both biological and structural function. So in contrast to– folks may be familiar with the concept of self-healing concrete, where researchers are putting bacteria inside of concrete that can help seal and heal cracks. In contrast to that approach, we basically used bacteria to help us biomineralize and toughen a sand anhydrous gel scaffold. So there’s really no cement.
And what we found is that we can keep our material alive. The bacteria actually enable the material to be produced, and also to– what was really exciting about that study, we showed that we can keep the bacteria alive and enable the material itself to regenerate. We grew three generations of materials from one parent generation. And we’re really just only scratching the surface with that type of concept and that type of approach to creating engineered living building materials that display biological functions.
IRA FLATOW: Wait, wait, I have to explore this a little bit more. So you start out with a wet cement, and you put bacteria in it, and as it dries, the bacteria stays alive?
WIL SRUBAR: In our approach, we used no cement. So we combined photosynthetic cyanobacteria, so really green cyanobacteria, with sand and a little bit of a hydrogel. And what the bacteria did is it grew, it flourished, and under the right conditions, it biomineralizes, meaning it makes little tiny minerals. And those biominerals really serve as that kind of cement component. And so it really toughened and helped bind the sand particles together.
But that hydrogel kind of retained a little bit of moisture, which enabled the bacteria to thrive and survive the initial manufacturing. And we showed that the viability can be one to two times, in terms of orders of magnitude, higher than the traditional self-healing concrete approaches, where you add bacteria directly to a concrete mixture. In those cases, really less than 1% of the initial inoculum will survive.
IRA FLATOW: So you come in in the morning, and where you had one brick concrete block before, you have two?
WIL SRUBAR: (LAUGHING) We are often asked, will my house grow into a skyscraper if we don’t control it? But what’s really great about the material system we’ve engineered is that, by just tailoring the temperature and the humidity and really the moisture, you can control that bacterial growth. It’s a lot like putting yogurt and other food in the refrigerator. That really kind of shuts down bacterial metabolisms, so you can kind of control it and reawaken it when you need to, but keep it at bay if you need to, as well.
IRA FLATOW: That’s amazing. So this is sort of, as you say, self-healing concrete.
WIL SRUBAR: Correct. It’s certainly a different approach to a concept of self-healing concrete. What we really challenged in the paper that we published was the concept of linear manufacturing of building materials, and really harnessing the exponential growth of bacteria. So by taking one parent generation, splitting it into two, and having those halves grow into two full bricks, and we did that two subsequent times, we really challenged that linear manufacturing approach, where you’re making one widget at a time, to show that you could, theoretically, manufacture building materials at an exponential scale.
IRA FLATOW: You’ve also used E. coli bacteria to produce materials. Why are bacteria so good for creating construction materials, and what would you do with those?
WIL SRUBAR: What we’re particularly excited about is blurring the boundaries between the field of synthetic biology and material manufacturing. So if you look at some organisms, they’re really good at making materials. And in recent years, the synthetic biology or genetic programmability of certain microbes has just enabled us to toy, again, with ideas of genetically programming bacteria to make hierarchically ordered and architecturally patterned minerals and materials with really tailored properties.
So that particular study was looking at infusing an ability into E. coli bacteria to produce tiny limestone nanoparticles that had different shapes and different properties. And it was really among the first studies to be able to show that we can perhaps, one day, use bacteria, genetically program them, and really genetically program blueprints of buildings and materials right into their DNA, which is really exciting to think about.
IRA FLATOW: Now, once you make the concrete, you lay the foundation, you need something to stick everything together. And I understand you also work in bioadhesives.
WIL SRUBAR: Correct, yeah. So again, going back to those structural proteins, just like gelatin or even other types of polysaccharides and polypeptides, and some of them are really sticky, and really is an untapped field with understanding how even wasps build their nests and how insects build their colonies by sticking together dirt and sand particles with saliva and structural proteins. So we’ve certainly played in the lab with some bioadhesives from structural proteins, and have shown that they rival the best engineered wood adhesives on the market today.
IRA FLATOW: I also understand that you make transparent wood. Transparent wood? Really?
WIL SRUBAR: (LAUGHING) We can make wood transparent, absolutely. So the process is actually quite simple. If you take balsa wood that you get at your local hardware store, or any kind of wood veneer, what you can do through a simple chemical process is remove all of the brown lignin from the wood. It’s a lot like paper-making, where you can remove the brown lignin through an oxidation process.
And what you end up with is a really nice white scaffold that is certainly not transparent, but it does look like a piece of 3D paper almost. And if you infill the porous structure of that wood with a polymer resin that has a refractive index that matches that of cellulose, which is about 1.5, what you get is a material that has the ability for light to pass through.
So we’re really excited about that because you can start to imagine applications such as light fixtures inside of buildings or even parts of windows, sun shades, things of that nature that would lend themselves well to something that’s a little bit more bio-based, biorenewable, that has the great properties of glass and other translucent to transparent materials, but has a much lower manufacturing energy than making glass from melting sand.
IRA FLATOW: I’m Ira Flatow, and this is Science Friday from WNYC Studios. Of course, the $64 question is how do you get the building industry to adopt any or all of these new materials.
WIL SRUBAR: As we know, the building industry is extremely conservative. And rightfully so, right? We have to hold paramount the safety, health and well-being of the public. Your lives every day to architects and engineers that have done the right calculations to keep people safe. So anytime a new product is introduced into the market, there certainly is some criticism and some skepticism. And I applaud that.
What we really need is a little bit of help from policymakers to help put, for example, caps on embodied carbon on our building materials to help infuse more biological and bio-based materials into the built environment. And we also need really motivated and dedicated clients and owners, building owners and developers to stand up and say these are certainly materials that I want to explore, and I want to be a testbed for different materials that I’d like to see in the building industry.
IRA FLATOW: Wil, we’ve unfortunately run out of time. I want to thank you for taking time to be with us today.
WIL SRUBAR: Thank you so much. It’s really been a pleasure.
IRA FLATOW: Wil Srubar is an assistant professor of civil, environmental and architectural engineering, principal investigator of the Living Materials Lab at the University of Colorado in Boulder.