[aaron38]

I wrote a paper on ancient construction (not just Rome) back in the day. Roman concrete, such as in the pantheon, wasn't poured into forms like modern concrete. It was hand laid and hand pounded into place. It's more masonry than concrete. The pounding eliminated voids and air bubbles, made the concrete much more dense, stronger. As they built, they could vary the aggregate, structure, chemistry as they went. Essentially roman concrete was 3D printed - by hand. In no way would it be economical to build that way today.