[litoE]

As I understand it, concrete has excellent resistance to compression but fails easily on traction, while steel bars are exactly the opposite. That is why you put rebar in concrete: the steel handles the traction loads and the concrete handles the compression. This works well because both materials have similar coefficients of thermal expansion, so as the temperature changes they both expand and contract at the same rate. I suppose you can engineer fiberglass to have the same thermal expansion coefficient and use it to replace steel (assuming it is just as strong on traction). But how would you "build without rebar even"? Wouldn't your beams start cracking at the bottom, where they are subject to traction?

[mikepurvis]

To build without tension you have to build structures that basically look like Roman structures [1]: a bunch of tightly spaces arches so that the entire thing is in compression, with no meaningful tension anywhere.

But it turns out that's pretty inconvenient; we really like doing dozens of feet of span for highway overpasses, building floors, and everything else. So we put rebar in all the concrete and just acknowledge that that means it has an absolute maximum lifespan of a century or two, and will certainly not last for millennia the way pure concrete in pure compression can.

[1]: https://www.theartnewbie.com/blog/rome/roman-arch

[Modified3019]

The “it’s built so everything is under compression” is great thing to bring up, but fyi your link is AI generated, filled with senseless repetition and “it’s not just x, it’s y”.

Looking it up on archive.org shows it was generated this year.

[mikepurvis]

Blah, so you're right. I was just looking for a link with some representative reference pictures, but I should have done more diligence. Thanks for pointing it out.

[cm2187]

They could do large arches too, see https://en.wikipedia.org/wiki/Basilica_of_Maxentius

[throw0101d]

> To build without tension you have to build structures that basically look like Roman structures [1]: a bunch of tightly spaces arches so that the entire thing is in compression, with no meaningful tension anywhere.

Until an earthquake occurs, and then all of those mostly-down forces turn into side-to-side forces.

[Zardoz84]

or go and revive Gothic architecture

[pinkmuffinere]

> Wouldn't your beams start cracking at the bottom, where they are subject to traction?

You have a few mistakes here. I’m not trying to demean you, but I’m going to number them just for clarity, as it can get confusing when there are many misunderstandings.

1. You are intending to ask about tension (which the rebar helps with), not traction (the force your tires exert against a road).

2. Tension is not only experienced at the bottom of beams, the location with the most tension will depend on the geometry. For a vertical beam, I think tension will probably be pretty even through the whole beam in most “normal” designs and loading configurations. But it will really depend on the geometry and on the loads being applied.

3. I think when you say concrete beams you’re meaning columns (apologies if I’m wrong about this). Concrete columns are remarkably good at holding up without rebar, because they experience almost exclusively compression! And indeed, ancient Roman designs did not use rebar at all :). It’s certainly possible.

[litoE]

1.- My apologies for the terminology - I took one overview class on the subject in Spanish, and haven't looked at it in the last 60 years. 2.- Agreed - that's why rebar on horizontal beams (and slabs) sometimes goes on the top edge and shifts to the bottom edge. 3.- By beams I do mean horizontal beams. This also applies to horizontal slabs. However, vertical columns also need rebar, especially in earthquake-prone areas. If the ground moves horizontally in an earthquake, the (vertical) columns have to transmit the motion to the upper parts of the structure. This create huge tension stresses on one side of the columns so they need vertical rebar rods on their perimeter.

[kergonath]

> 1. You are intending to ask about tension (which the rebar helps with), not traction (the force your tires exert against a road).

I don’t know if it is the case here, but it is a common mistake for some non-native English speakers. In some languages traction is a false friend.

[projektfu]

And in English, traction also means pulling, but it got somewhat misapplied to the friction of tires, causing confusion in the language. For example, a tractor is something that pulls, and when a broken leg is put in traction, it's suspended to pull it apart and keep the muscles from pulling it together.

Tension, as you say, is indeed the term used in physics and engineering for the force on an object pulling it from either end.

[deepsun]

> exactly the opposite

No, steel is better in both ways, ten times over. It's just more expensive, concrete is a "filler" to cheap out construction.

If you think about it, all engineering is about cheaping out things. It's pretty easy to build awesome projects having unlimited budget.

[selcuka]

As one of my professors said back in time: "Steel is one of the best materials for a number of applications. We don't use it only because it's abundant. "Abundant" and "good" are not always mutually exclusive."

[aw1621107]

Just FYI, I think you're looking for "(in) tension" instead of "(on) traction".

[cma]

There's also stainless steel rebar

[mikepurvis]

Even without the rebar rusting, I think you can still have issues with the steel and concrete having difference thermal expansion properties, particularly for outdoor strucutures where a freeze/thaw cycle is in play.

Basically the rebar works itself loose over time and creates micro-fractures in the concrete that then get moisture in them and can cause expansion damage— all without the rebar itself ever rusting or "failing".

[cma]

Maybe so, OP mentioned they were very similar and that still seems to hold for stainless. How about the aggregates within? The composite makeup can result in different CoTE than the individual aggregates, I think that's one part of concrete sidewalks cracking though, sometimes near shaded/unshaded boundaries. Roman concrete supposedly had some self healing properties before the cracks grew, from the lime inclusions.

[21asdffdsa12]

https://en.wikipedia.org/wiki/Galvanic_anode