Brian Salazar and his UC Berkeley team have developed a new way to reinforce concrete with a polymer lattice, an advance that could rival other polymer-based enhancements and improve concrete’s ductility while reducing the material’s carbon emissions.

Tech Briefs: How did this project start?

Brian Salazar: The lattice we used is called the octet structure — Buckminster Fuller used it for his kites, and it’s been used for many other structures as well. Before we worked on this, our team was working on a project using cellular cementitious materials. We were directly shaping concrete into a lattice structure and we found that provided good performance. But concrete is a very brittle material that always has to be reinforced. However, people haven’t thought about the best way of structuring that reinforcement. At the same time, we had a facility at UC Berkeley called the Jacobs Institute for Design Innovation, which had come up with some wonderful 3D printing technologies. So, we considered the possibility of merging these two frames of thought: using a lattice structure in concrete and using the advanced additive manufacturing techniques to change the way we reinforce concrete.

Tech Briefs: Why did you use a polymer?

Salazar: [Polymers] have been used to reinforce concrete in the past, typically in fiber form though, little pieces of fiber mixed in with concrete. But there are lots of disadvantages with that: those fibers are rather expensive, it costs money to produce them, and the amount of fiber you could put into concrete is very small, around 2%, whereas, we are able to do 33%. But it does have benefits: the polymers are much lighter in weight than the concrete; they are insulating, while steel is a heat conductor; and they are corrosion resistant, as opposed to steel, which will corrode over time.

Tech Briefs: How does the strength of the polymer compare to steel?

Salazar: It’s much lower strength and we see that in our experiments. But we are able to find applications where you don’t necessarily want such high strength, where you might care more about some of the other aspects. For example, you might care more about making a structure very ductile. At the moment, we’re looking at applications where we might want to insulate something in a cold environment. Concrete on its own is a very good insulator and the addition of the polymer might increase the toughness while still allowing it to keep those insulating properties.

Tech Briefs: What makes a lattice an octet?

Salazar: An octet is a specific geometry. For one thing it’s isotropic, which means if you look at it from the front face or the top face, you’ll see the same image. The other thing about it is, it’s triangulated — all of the connections are triangles.

Tech Briefs: How do you determine the size of the lattice work, the size of the triangles, the specific geometry of the lattice?

Salazar: There are a lot of considerations. One is, we want the openings to be large enough so we can pour concrete in there. If they were shrunk to a quarter of the size, for example, you might not be able to fill it with concrete. The other consideration is, we need to consider the manufacturability of the portions of the lattice that get printed. When you 3D print you have to consider the size of the nozzle. A bigger nozzle gets you faster speed, but you’re not able to make as fine a structure, so it’s a bit of a compromise. You have to be able to fill it with concrete and also print it in a reasonable amount of time. And of course, you have to be able to get enough polymer in there so it will actually reinforce the structure. It took a lot of thought.

Tech Briefs: Will you be considering different kinds of lattices in the future?

Salazar: Yes, I’d love to do another study about this. For example, we chose the octet structure primarily because it’s very well-known. But there’s nothing to say it’s the best one for this particular application. Being able to determine the optimal shape for a particular application, is very much in my field of interest.

The 3D printer creates polymer lattice reinforced beams. Special camera equipment shows that, when tested under bending, the beams are highly flexible and most of the cracks are blunted by the lattice.

Tech Briefs: How would you approach that?

Salazar: It’s a computational problem, we use computational modelling techniques.

Tech Briefs: What is the difference between “compressive strength” and “peak load”?

Salazar: Here, peak load refers to the maximum load the structure is able to withstand under bending. There are two types of tests we perform. One, to determine the compressive strength, the sample is squeezed between two plates. The other is a bending test, which is a little more complicated, but more interesting as well. When you bend the structure, it will also experience tensile stresses, and concrete is very weak in tension. So, we wanted to increase the performance of concrete in the tension region. We found that increasing the amount of polymer decreased the compressive strength but in the bending tests the opposite happened — the tensile strength increased.

Tech Briefs: You used ultra-high performance concrete for your tests. Is that required? How will that affect the cost of producing this?

Salazar: We have also done experiments where we used ordinary run-of-the-mill concrete and found similar results. What’s special about ultra-high-performance concrete is that it’s even more brittle than conventional concrete. So, when you have something that’s more brittle, being able to turn that into something that is very ductile, is actually a bigger accomplishment. It’s amazing that this technique works on both types of concrete.

Tech Briefs: If it’s more brittle, what makes it ultra-high performance?

Salazar: UHP refers to the fact that it can reach higher loads. It can stand more weight, but that’s different from how much it can bend.

Tech Briefs: How does your process reduce the environmental impact?

Salazar: The relatively high environmental impact of concrete includes the total life cycle: mining, producing, and transporting. With less concrete in a given structure, the environmental impact is reduced across the board.

Tech Briefs: Aren’t polymers petroleum-based products?

Salazar: They are. Per volume of material, a polymer will actually emit more CO2 than concrete. However, we envision a way for us to eventually develop this in a way that would be more environmentally friendly. That might mean inventing new polymeric materials that are more CO2 – friendly. Or there might already be some polymers that incorporate recycled materials that would have lower environmental emissions. So you’re right, you can’t just take a polymer that you’d find at the store and use that to lower your environmental emissions — there’d have to be some research to develop polymeric reinforcements that would allow the overall structure to have lower environmental emissions.

Tech Briefs: When I read about this, I was picturing using poured concrete for buildings, but would you have to make these beams in advance?

Salazar: Both are done — if they’re made off-site, we call them precast. I think, in particular, lattice reinforced concrete could be really beneficial for the precast industry because the structures would be much lighter to transport. I think there’s some potential there.

Tech Briefs: How difficult do you think it would be to scale this process up for commercialization?

Salazar: You mean manufacturing an entire structure? I think the route is there. We know a lot about the concrete itself. So, the question is, how do we make these lattice structures sized for larger structures? There are currently manufacturing techniques out there that can make a lattice structure like this. I don’t see the need for any major developments for that to happen.

Tech Briefs: Do you have a rough idea of a timeframe for when this might be commercialized?

Salazar: I would not be surprised if in 10 years there’s a structure out there that uses this system.

An edited version of this interview appeared in the January 2021 issue of Tech Briefs.