One of the big reasons is ductility. We want structures to show signs of distress before failing catastrophically. With ductile reinforcing (steel), the deformation at which failure occurs is easily 5 times larger than the deformation where yielding occurs. Large cracks will open up, it hopefully gets noticed, and there is a chance to intervene.

With GF or CF reinforcing, failure is sudden and catastrophic.

> Why can’t we use carbon fiber or aluminum or anything that won’t rust?

We can, we do, just steel is super duper cheap, and workable.

Ironically, it's China now who leads the world in GFRC (glass fibre reinforced concrete) construction to get those miniscule cost savings on steel (despite it being world's biggest steel producer.)

There is also basalt fibre reinforced concrete that is supposed to be superior to glass fibre, and can come in workable varieties (heat it up with a torch, and bend.)

"Why can’t we use carbon fiber or aluminum or anything that won’t rust?"

You can. Stainless steel rebar is a thing and would, largely, solve the problem of spalling due to rusting steel reinforcements.

As you can imagine, stainless steel rebar adds a significant cost.

To avoid the problems of rust, epoxy coated steel is often used. It costs more, so in situations where the contract is won by the lowest bidder (unless it is expressly required), you don't get it.

So as a result, buildings that could last 200 years start to crumble after 50.

We actually can - fiberglass rebar has many advantages over steel, among the first being that it does not corrode and expand. It's also much lighter and easier to transport to and on the worksite.

It is starting to see widespread use in the construction industry, but far too slowly. A quick search on "fiberglass rebar" will return many manufacturers and articles.

Cost would be my guess.

> Why can’t we use...

You can. Nobody’s stopping you.

Perhaps you meant “they” instead of “we”.

Pet peeve.

I’ve found when you use the proper pronoun — ‘they’ in this case, unless you make concrete — it focuses your thinking.

That is, if you want to know why ‘they’ do something, you’ll naturally ask ‘them’. But, when you use ‘we’, you’re more likely to just imagine your own solution.

Nothing personal.

‘We’ is the bane of my existence...

In order for this post to be somewhat relevant, I’ll add this... I see corroding rebar as a feature and not a bug; it assists in the natural degradation of concrete and it gives me some comfort to know that the distant future will have no signs of the concrete monstrosities that litter out landscape. Thanks, rebar.