Re “… unreinforced concrete…”
They were brilliant engineers.
It still is somewhat of a mystery, but the current belief is that due to the nature of the way they created their concrete was the root of its durability. They didn't mix it all together and homogenize it in a tumbler the way we do today, they laid it down manually, throwing the ingredients like lime randomly via shovelfuls.
These structural inconsistencies created pathways for a low temperature self-healing process following the irregular pathways created during the curing process that baked in the inconsistency of the mixture which were leveraged by natural stresses and processes to actually repair cracks on its own.
(I hope I didn't botch that explanation) I couldn't find the original one I looked at a few years back, but this link describes it:
Thanks!
Concrete is strong in compression but has almost zero strength in tension. The rebars that we see in modern concrete are to add strength in tension.
Buildings such as the Pantheon are designed such that the concrete is always in compression. The dome is a perfect example. Thus reinforcement would have been redundant.
But they were still brilliant engineers.
I’ll add that the concrete is porous so that moisture can penetrate. When the moisture gets to the rebar it rusts. When steel rusts it expands. This has the effect of popping off big chips of concrete in a process called “spalling”. The failure is not in the concrete but in the rebar.