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.
Looking it up on archive.org shows it was generated this year.
Until an earthquake occurs, and then all of those mostly-down forces turn into side-to-side forces.
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.
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.
Tension, as you say, is indeed the term used in physics and engineering for the force on an object pulling it from either end.
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.
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".