[sporkland]

For someone totally outside this space I had assumed concrete is rigid and brittle and rebar reinforcement allows for it to bend and flex without breaking. But your comment makes it feel like it keeps the concrete in tension with compression forces to make it sturdier. Can you ELI5 what the rebar does to help?

[walty]

You're going to love this playlist from Practical Engineering [1]. Grady explains all of this much better than I could. The second video explains reinforcement and the fifth explains pre-stressing.

[1] https://www.youtube.com/watch?v=UOHURuAf5iY&list=PLTZM4MrZKf...

[LegitShady]

you have it backwards. The reinforcement handles tensile stress, keeping more of the concrete section in compression, where its strong. In concrete where the reinforcing isn't tensioned, the concrete below the reinforcing will crack as it is very weak in tension. In concrete that is pretensioned, the tension in the rebar is used to apply compression on the concrete so that it might never get into a tensile load and crack, which also reduces deflection under load.

The rebar handles tensile loads, the concrete handles compressive loads. You put the rebar where tension will exist so that it can handle that. Some amount of cracking on non-pretensioned concrete is normal and unavoidable.