"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.