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Comment by steve918 | Hacker News Reader

The problem with modern concrete falling apart prematurely is usually the use of rebar as a reinforcement.

And we use rebar because we like long, flat bridging spans that can hold hundreds of tons at a time — something Roman concrete has no hope of ever doing, unreinforced.

Not necessarily countering your point, but perhaps giving some perspective.

The City I grew in (Cordoba) has a roman bridge[1], originally built 1st century BC and still standing. It supported a 2-way street until 2004, when it was pedestrianised.

Caveats:

* It has gone over a great deal of renovations/partial rebuilds on its history, so its durability can not really be attributed to roman concrete alone. Only 2 of the original 16 arcades remain. One could argue that "it's not the same bridge" any more (Theseus' Bridge).

* I ignore wether they used concrete to build the original one. I'm not an architect. For all I know, the initial construction could have been mortarless stone, and cement (as well as reinforcement) could have been added in some of the later renovations.

That said, all bridges require periodical servicing/renovations anyway, in order to remain functional. And this one has stand for 2000 years.

[1] https://en.wikipedia.org/wiki/Roman_bridge_of_C%C3%B3rdoba

This doesn’t really give perspective, it’s basically what they’re saying. Roman bridges are gravity structures, the entire thing is in compression and heavy, the arches have really limited spans.

Compare to the Millau Viaduct for instance.

Well in order to compare things properly we would have to check if the Viaduct is still there 2000 years from now.

That's a completely different axis is the point, you can not solve the issues the viaduc solved, let alone build anything even remotely similar, using roman materials (to say nothing of techniques).

does fiberglass rebar work? I remember reading that it works well for horizontal layouts but was not sure about use in pillars.

Yes, alternate forms of rebar work. You can also coat rebar with epoxy, or use stainless steel.

The two core issues are cost, and limited understanding of the long-term properties of the material (as well as experience e.g. epoxy-coated rebar needs to be handled carefully lest you scratch off the epoxy).

FWIW the Practical Engineering channel has an entire series on concrete which covers (the basics of) this and more: https://www.youtube.com/playlist?list=PLTZM4MrZKfW90PdaBFt70...

>... e.g. epoxy-coated rebar needs to be handled carefully lest you scratch off the epoxy).

That sounds impractical at scale. Whenever I pass by construction I see the materials haphazardly strewn about. How could you possibly ensure that all of the rebar used is never scratched when it has to pass through dozens/hundreds of hands before it is finally set?

I've always noticed it carefully packed and stacked on site. The epoxy coated rebar is usually formed and connected into structures that are placed/suspended in the forms or roadway. The epoxy is also very durable.

Isn't basalt the best non-steel option all around?

I don't think anyone's ever claimed that it was better for all modern applications.

Rebar reinforced concrete fails, I assume, because the rebar rusts? If so, isn’t the issue greatly reduced (potentially) if we were to use the quicklime process of the Romans? If the self-healing prevents water penetration, does that stop rebar from rusting?

Steel is really strong vs tension, not so much vs water / rust / chemical reactions.

It's also relatively cheap. Maybe carbon nanotube ropes could replace that after we have cheap mass production?

The point, is that the steel needs water to rust. The cracks allow the water, but supposedly the Roman concrete self heals and water does not continue to penetrate. One assumes, therefore, limited rust (perhaps). In practice this may not be the case, but surely someone has tested? Steel has the right properties + price; if you can keep the water out.

Concrete isn't waterproof, it might resist the flow of water, but as even most rocks are, there is at least a slow migration of water through the material.

There's already other ways to reinforce concrete, one I know of is embedding kind of fiber-glass mesh in it instead of rebar. I quick google suggests that this might be called Fiber Reinforced Polymer (FRP) reinforcing, or possibly that's a different thing I'm not really an expert.

I assume you could use a whole bunch of different materials really, including other metals that don't rust in the same way as steel does.

Remember that roman concrete often used iron for reinforcement. The Colosseum is a famous example of this. The only reason it's not still there today is because metal, particularly iron, was extremely valuable so after the fall of the Roman empire, people tore whatever iron they could out of the structure (akin to people today stealing wiring and copper piping from the walls of houses).

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