If ancient Romans already know how to make concrete in the 3rd BC, why aren't there more concrete structures recorded in history?

Answer 1

The concrete in question required, pozzolana, a volcanic ash from Pozzuoli Italy. Charlemagne's historian had access to the recipe, for example, in the 8th century. It was "rediscovered" in the 13th, but that's irrelevant to the question at hand. Rome falls, Constantinople rises. Constantinople doesn't have access to the lime and volcanic ash necessary for Roman concrete.