[Retric]

We have some ideas. For one thing a reasonable amount of documentation still exists that speaks how things where run.

Rome was not building all that much stuff, they simply didn’t have nearly the population or wealth we do. Think the population of Tanzania and the GDP of Libya spread across as much land as India.

Meanwhile we can see structures like Portus Adurni that are mostly intact. A meaningful percentage of their larger structures are still around.

[Gibbon1]

And after the fall of Rome the economy was smaller and more fragmented.

Last time I looked into this I came away with the observation that preindustrial concrete was very labor intensive. We use it for it's physical properties and because it's 'cheap'. Roman concrete wasn't cheap at all.

The central Roman government was able to marshal the effort necessary to produce it on a ongoing basis for particular uses. But I understand Romans mostly used brick and mortar for ordinary construction.

After the Roman Empire fell apart concrete probably didn't make sense.