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Comment by ortusdux | Hacker News Reader

Stainless steel rebar is an often overlooked option. In theory, solid SS rebar should outlast the concrete, but it is a difficult thing to accurately study. In favorable conditions, regular rebar reinforced concrete starts to need major repairs after ~40 years due to corrosion.

The Progresso Pier in Mexico was build over 80 years ago with SS rebar, and reportedly has not needed any renovations. A pier built 20 years later using mild steel rebar has been almost completely destroyed by the ocean.

I wish more large infrastructure projects would use it. The up-front costs can be 2x higher, but the lifetime savings win out in many situations.

https://www.amusingplanet.com/2015/10/progreso-pier-worlds-l...

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Stainless steel needs oxygen[1], otherwise it will eventually corrode with pitting and "crevice corrosion"[2]. I wonder what the ingredients are in stainless rebar, and what the oxygen environment is like.

[1] https://www.thermofisher.com/blog/metals/is-stainless-steel-...

[2] https://www.cruisingworld.com/how/beware-stainless-steel-cor...

Hey this is a little late, but now they are looking at non oxidizing basalt based rebar and mesh's as an alternative to steel. It's work a look. it's got a lot going for it.

I came here to mention that. SS rebar is, indeed, a thing and would be an interesting combination with long lasting (fly ash) concrete, etc.

How long lasting is fly ash concrete?

"How long lasting is fly ash concrete?"

We don't know. Existing structures that use "Roman Concrete[1]" are (roughly) 2000 years old and counting ...

[1] https://en.wikipedia.org/wiki/Roman_concrete

How does the Pantheon in Rome keep its structure? Is it just concrete?

Since it’s 2000 years old now.

Roman concrete is not like modern concrete. The process in modern concrete does not stop and eventually makes it too brittle and falls apart, Roman concrete does not do this and can last a very long time.

https://science.howstuffworks.com/why-ancient-roman-concrete...

Here's a good video on the use and tradeoffs of Roman concrete engineering: https://www.youtube.com/watch?v=qL0BB2PRY7k

TL;DW: whereas modern construction uses rebar as a way to keep concrete from fracturing under tensile stress, the Romans made their constructions enormous so that the weight of the structure itself would compress the material and keep it from failing from tensile stress. Their monuments weren't built huge just because it cool, but also because it was practical. But large concrete constructions are both expensive and take years and years to cure, and depending on your concrete chemistry the strongest mixtures can also be much more difficult to work with.

Thanks, so very cool how they were able to make such structures!

Compressive shapes and good concrete.

You start talking about rebar and other complexities when you want a shape that puts concrete in tension (where it's very weak), instead of compression.

Basically - lots of domes and arches.

Wish we could re-discover how to produce Roman Concrete, which has already proven its long-lasting efficacy. https://en.wikipedia.org/wiki/Roman_concrete.

My understanding is that modern concrete would last as long as Roman concrete would, if we built the same types of designs that Romans used concrete for. The big difference is that we want to span gaps without using large unwieldy arches, so we need tensile strength, so we need to use steel reinforcing bar in our concrete, which is the eventual pathway to failure. (Well, that and heavy machinery like semi-tractor trailers, which the Romans also didn't have to design around)

The Romans did not have the quantity of cheap steel necessary for this, so they ensured only compressive loads on their concrete, so it lasted about as long as you'd expect a random rock subject to only compressive loads in a field to last.

> so it lasted about as long as you'd expect a random rock subject to only compressive loads in a field to last.

Roman Concrete is a bit more special than some 'random rock' thanks to the presence of "aluminous tobermorite" which makes the concrete much stronger and more chemically stable

https://interestingengineering.com/scientists-discover-why-r...

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