did anyone made tensile tests on a roman concrete specimen?
Rebar is wildly used because reinforced concrete is not only used in compression but in beams, slabs ... and could be avoided in arched structures. Arches and domes can last a lost longuer but limit choices in what can be built. Roman concrete should be compared to modern concrete without rebar or this is comparing apples to oranges.
It's just a different tradeoff. We could absolutely make "Roman" concrete that lasts millennia, but it has a much lower strength than modern concrete so you can't make something like a harbor quay out of it. That said, "lasting a thousand years" is not as useful as it sounds, 99+++% of buildings will need to be demolished before that time anyway.
> you can't make something like a harbor quay out of it
Are you sure about that? The Romans built a lot of harbours. We don't really have any data on the compressive or tensile strength of Roman concrete so it's hard to say for sure.
You can't make a _modern_ harbor quay out of it, one that is strong enough for the super large container cranes. The main problem is of survivorship bias: all the flimsy Roman harbors have disappeared so yes, only the most sturdily constructed ones remain. They would still not last under the "modern" load of 40+ ton vehicles.
Where the similar recipe/technique has been applied is in the construction of some large dams, where they call it roller compacted concrete.
see romanconcrete.com