Rebar makes earthquakes worse, as seen in California's freeway collapse. The rebar depends on the concrete to prevent corrosion, but that is only good for roughly 50 years. Rust causes expansion, which cases cracking. The outer concrete falls away, leaving rebar to get crushed under the load. The obvious fixes, like stainless steel rebar, have thermal expansion coefficients that don't match concrete, so instead you get cracking even without corrosion.

Lots of Roman stuff is still standing in areas that get earthquakes. The solution is to use a conservative design, with arches and thick walls. Domes are good. We can improve on this with 3D printing, using a structure like mammalian bone: solid near the surface, and spongy in the middle.

"Primitive" structures and other structures engineered to avoid tension in the concrete. Concrete doesn't have zero tensile strength. Apparently it's a few MPa, so a fifth or a tenth of the compressive strength. http://www.engineeringtoolbox.com/amp/concrete-properties-d_...

The large dome of the (Roman) Pantheon is unreinforced concrete. Most compressive structures built before the modern period (arches, domes, buttresses) were not reinforced with tension elements, just plain masonry.