[theoh]

"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.

[crwalker]

The concrete in the Pantheon is exposed to max ~20 psi tensile stress, and the concrete can handle about ~200 psi tensile stress.

http://www.romanconcrete.com/docs/chapt01/chapt01.htm