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Osbios t1_j9tszdy wrote

Reply to comment by OathOfFeanor in It seems like we havent heard anything about carbon nanotubes for almost a decade. is there anything exciting involving those these days? by mintysmellshowntell

> Carbon nanutubes work the same way on the nano scale.

Could this replace the steel bars in reinforced concrete? Or will it "just" improve concrete in other properties?

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OathOfFeanor t1_j9tuvrd wrote

No, unfortunately the tensile strength gains are not that good.

But as you said it improves the concrete in other ways. Another significant benefit is that the carbon nanotubes reduce autogenous shrinkage of the concrete (basically shrinkage = cracking)

In some slabs people are already doing them without rebar depending on circumstances (subgrade, climate and frost line, expected usage and load, etc.), so this could extend that range just a bit as a middle ground. Someone who wants a concrete slab to hold a picnic table can get away with that, someone who wants a foundation for a building cannot.

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Kangermu t1_j9u04ni wrote

I'm confused now... You had said They provide significant increases in tensile strength which is hard to achieve in concrete. but it's also not as good as rebar? Is the idea to use both?

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darkfred t1_j9u967l wrote

Rebar is simultaneously the thing that makes concrete capable of long unsupported spans, and the main reason concrete is not permanent and needs to be maintained to not fall apart.

The difference between nanotubes and rebar is a matter of scale. We will probably always need both. In bridges the rebar or cables would keep the entire block under compressive tension so the concrete can be used in long spans. The nanotubes keep it from flaking and cracking locally from temperature variations. If we can reduce the internal steel grid to a more directional set of tensioned cables then there are less places where water intrusion and could cause the steel reinforcement itself to rust, expand and crack the concrete. And even when it does the carbon or fiberglass micro structures will prevent the cracks from starting and reduce their spread.

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toalv t1_j9v5miy wrote

Right now we just use bulk fiberglass to accomplish the exact same thing at a extremely small fraction of the cost for basically identical performance. It's a nonstarter.

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darkfred t1_j9v9fa8 wrote

Absolutely, i mentioned fiberglass because right now it's essentially the same thing, with the same strength. Even if the carbon fibers get as long as fiberglass it's not a game changer from current practice, cause it will still be an order of magnitude more expensive.

The real game changer would be the technology for weaving the nanotubes into macro scale carbon fiber mesh load struts that were rigid enough to be cast in concrete in place of steel reinforcement or structural beams and don't need to be resin impregnated but mechanically bond directly to the concrete.

This would be huge because the negative thermal expansion coefficient of woven nanotube fiber could be manipulated by casting temperature to prestress the truss and even an entire slab from the inside out and might, despite carbon fibers being much more elastic than steel, completely replace it in some applications.

edit: OTOH, if someone doesn't figure out some elegant solution like this to the problems of carbon fiber's elasticity and thermal expansion coefficient, it's just never going to work with concrete. Steel and concrete are a magical material properties match on every front except for oxidation.

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OathOfFeanor t1_j9wh6ac wrote

You are confusing carbon fiber with carbon nanotubes.

Carbon fiber is fiberglass with carbon instead of glass. It's the same thing, but stronger. But it's still encased in a plastic binder, still micro or macro in size (not nano-scale).

Carbon nanotubes are pure carbon (sometimes with some oxides in there at low concentrations).

Their behavior and impact on the chemistry of the concrete are entirely different.

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darkfred t1_j9xc3ix wrote

Not really. From a macro engineering perspective all of the current research in carbon nanotubes for structural use is aimed at producing longer tubes, often referred to as carbon nanotube fibers. Nanotube fibers that are long enough to be used in the same way as carbon fiber, or glass fiber, woven together and bound in a matrix to produce strong macro scale materials.

ALL current carbon fiber use in concrete is mechanical and exactly equivalent to how glass fibers are used. Except that the carbon nanotubes are far smaller so although they provide stronger local support the scale of the cracking they prevent is reduced vs other fiber mix-ins.

Like I said, real innovation will come when carbon nanotubes are long enough to be used at a structural scale rather than microscopic. It's neat to talk about futuristic meta-materials and they can slightly change the thermal conductivity and microscopic crack formation of concrete but from a large scale engineering perspective most of the small scale nanotubes that can be purchased in bulk today would be the same as mixing a bag of soot into the concrete.

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OathOfFeanor t1_j9xe82n wrote

> Not really. From a macro engineering perspective all of the current research in carbon nanotubes for structural use is aimed at producing longer tubes, often referred to as carbon nanotube fibers. Nanotube fibers that are long enough to be used in the same way as carbon fiber, or glass fiber, woven together and bound in a matrix to produce strong macro scale materials. > > ALL current carbon fiber use in concrete is mechanical and exactly equivalent to how glass fibers are used. Except that the carbon nanotubes are far smaller so although they provide stronger local support the scale of the cracking they prevent is reduced vs other fiber mix-ins.

This is absolutely 100% false and if you just browse the ACI YouTube channel you will find a whole ton of research of nano-scale carbon. The goal is NOT to make it micro or macro scale because the nano scale provides unique benefits.

It is incredibly different from how glass fibers are used and I recommend you educate yourself on the subject or else just stop talking down about a new technology you are unfamiliar with.

Again, carbon fiber and carbon nanotubes are very different materials.

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OathOfFeanor t1_j9wgxes wrote

> we just use bulk fiberglass to accomplish the exact same thing...basically identical performance

Absolutely false, but these sorts of attitudes are exactly why things take so long to propagate in construction.

Adding fiberglass fibers of any size to concrete will make only a fraction of the difference to tensile strength compared to the carbon nano tubes. With CNTs we're talking increases of something like 40% to the Young's Modulus which is significant.

The nano scale also improves the concrete's ductility. Fiberglass fibers have no effect on shrinkage, either. Fiberglass does not serve as a nucleation point for cement hydration products. The list goes on.

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[deleted] t1_j9u6hti wrote

Perhaps you could use less rebar. It has to be simultaneously the best and worst thing to happen to concrete.

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OathOfFeanor t1_j9u7m2h wrote

Yes, the idea is to use both.

It allows for smaller amounts of concrete and reinforcement to be used to achieve the same effect.

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BBAstronot t1_j9u1ola wrote

Is production still prohibitively expensive? Those properties seem like a good fit for roads in places with extreme temperature swings.

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oldcrustybutz t1_j9wak5y wrote

Basalt rebar is taking up some of that role, higher tensile strength, less corrosion. Cost is still somewhat high though.

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