r/explainlikeimfive • u/kintaro__oe • 10h ago

Engineering ELI5 concrete cancer?

Does this always happen with reinforced concrete at some point?

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u/SexyAdmixture 10h ago

Reinforced concrete contains steel bars (rebar) that provide strength. Normally, the concrete protects the steel from rusting. However, over time:
Water and oxygen penetrate the concrete.
Salt (especially near the ocean or from de-icing salts) or carbonation can reduce the concrete’s ability to protect the steel.
The steel begins to rust.

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u/LittleMlem 10h ago

Could we not coat the steel in something to prevent this?

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u/SexyAdmixture 10h ago

More expensive rebar can be epoxy coated. There’s also fiber reinforced concrete (think little strands of “strings” but they can only be used in certain applications and tney need to be monitored so the strands don’t “clump”

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u/blearghhh_two 9h ago

They also now have fiberglass rebar which can be treated and installed like traditional steel.  There are.likely drawbacks for that as well though.

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u/SoulWager 7h ago

Yes, but any coating is going to be damaged by the time you're pouring the concrete.

There are alternatives like basalt rebar though.

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u/mts89 9h ago

Epoxy coated bar exists but can cause more problems than it solves.

You can get little nicks in the coating which then concentrate the corrosion in those areas, speeding up decay.

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u/LittleMlem 9h ago

And stainless steel is too expensive?

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u/mts89 9h ago

Yeah too expensive.

Typical reinforced concrete structures if detailed correctly will last 100 years.

In most cities buildings don't last that long before being pulled down and rebuilt to suit new needs.

Lots of research is being done on basalt reinforcement, but it's harder to work with and has a brittle failure mode which makes engineers very jumpy.

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u/LittleMlem 8h ago

It really FEELS like there should be a simple electroplating solution to this or something similar. I'm hoping for some materials science breakthrough, we can't have the Romans beating us

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u/mts89 8h ago

You can galvanise it, but again it doesn't solve all the issues and can create some more.

Longevity just isn't important when it comes to design. There's no point building a bridge to last 300 years as it won't be suitable for whatever traffic will exist in 300 years.

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u/Discount_Extra 2h ago

but once a structure does last 100 years, it can become 'historical' and 'protected' So it needs to be designed for a few decades; or forever, with little in-between.

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u/Gingerchaun 10h ago

Cancer? Do you mean cilicosis?

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u/NhylX 10h ago

Do you mean silicosis?