Oh and yes rebar is fundamentally required in many cases, but the Romans didn’t have it.
The rebar doesn’t make cement stronger. It just keeps it in one piece after it cracks.
*Tell the 3D printing guys that sugar makes the concrete flow better...s/
No rebar; they used volcanic ash. We can use fly ash.
Interesting wiki article!
https://en.wikipedia.org/wiki/Roman_concrete
My understanding is that Rebar makes concrete more resilient and tougher, while limiting its lifespan.