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Comment by silicon2401 | Hacker News Reader

Why don't we just use concrete by itself? Is steel necessary for using concrete the way we do in modern times?

Basically concrete is good only for compressive loads. Under tension it easily cracks. The steel reinforcement adds tensile strength to concrete.

In fact, the concrete MUST crack in the tension zone in order for the steel to take effect and provide tensile capacity to the member.

Only if you don’t prestress the reinforcement.

https://en.m.wikipedia.org/wiki/Prestressed_concrete

How often are skyscrapers built using prestressed concrete?

Great video on prestressed concrete members - https://www.youtube.com/watch?v=P13Mau2VUWw

(Really good channel overall on civil engineering)

For the way we use it, yes. Concrete works well in compression but is horribly weak in tension; a supervisor of mine compared it unfavourably to a ginger nut biscuit. Probably hyperbolic but you get the idea.

Big, arch-y structures like the Roman pantheon work, because everything is in compression. But that's a lot of not-economically-useful space.

In order to use concrete by itself, you have to limit yourself to specific architectural structures which are very much unlike modern buildings - essentially, you can build quite high ancient-looking structures with everything held up on arches (which take up space and material), but you can't build a simple box-shaped building from concrete without steel rebar.

It is now possible to use wooden structural components to build modern skyscrapers. Maybe not 100 storeys buildings but 30 storeys is feasible[1].

[1] https://www.bbc.com/future/article/20171026-the-rise-of-skys...

Yes, steel is required. In the design of concrete structure, concrete is assumed to have zero (0) tensile strength.

Explaining the asnwers in another way, yes, steel is necessary for both large horizontal openings without pillars and large vertical heights.

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

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.

Epoxy coated rebar was a trap, and is now being banned around the world. Paradoxically, the epoxy coat can make corrosion worse than the bare steel, let alone galvanised.

Not sure why you're being downvoted, because you're right, atleast on the west coast of north america. My firm hasn't use epoxied rebar since like 1990, and we removed all references to it on our drawings long ago.

The coatings were super fragile, and often damaged while being placed in the field. Then someone had to come along with magic paint and touch up all the cracks. But of coarse they would never find 100% of them...

He's getting downvoted because more modern versions of epoxy coated rebar are still widely used in new construction. It wasn't the silver bullet for salted roadways it was sold as but it has its place.

which regions and industry do you see them used? In west coast buildings, its uncommon.

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.

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