The concrete dome of the Pantheon in Rome. Public domain image by MariamS on Pixabay.
The
concrete forming the dome of the Pantheon in Rome didn't need to be
reinforced, presumably because none of the components are under tension
and the structure is effectively a 3D arch (A dome can be thought of as
an arch rotated through 360 degrees around its keystone). Arches are
strong and can withstand large loads pushing down from above with the
stones being squeezed together and put under compressive forces,
resisting breaking. They also direct forces out to the side of the arch.
Concrete or stone is strong in compression, so it can be "squashed" and
loaded by stacking pieces one on top of another as in a stone or block
wall, or pillars. However it's relatively weak in tension. So if you put
a plain concrete beam over a wide opening and load it sufficiently, the
concrete will snap on the underside as it's stretched or put under
tension. Steel is strong in tension and reinforced concrete makes use of this property by employing embedded steel bars called rebar to form a composite material. This is strong under both compressive and tensile loads.
