The Pantheon dome in Rome is the oldest unreinforced concrete dome in the world, built between 118-128 AD. It has a depth and width of 142.4 feet, configured as a perfect sphere. To support the massive weight of the dome, the Romans built 20 foot thick concrete walls and used innovative techniques to reduce the dome's weight, such as using pozzolans and amphra in the concrete mix and adding coffering. The oculus opening distributes stresses at the dome's weakest point. The Pantheon's concrete design has enabled the 142 foot tall dome to remain intact for nearly 2,000 years.

1. PANTHEON DOME
Oldest Unsupported/Unreinforced Dome in the World

2. ORIGIN
 Commissioned by Emperor Hadrian
 The word “Pantheon” loosely translates to
“all of the gods”… the Pantheon serves as a
temple to the Roman gods, the dome being
the divine feature
 Built between 118-128 A.D.
 1,887 years old (still in use today)

4. DOME
 Depth from dome’s apex to floor is exactly equal
to the width of the dome (142.4 ft)
 Meaning: the rotunda (round floor beneath
dome) and dome are configured in accordance
with a perfect sphere
 Because of no reinforcement to keep the dome
from trying to “push” or “flatten” out, Romans
built 20 ft thick walls to serve as buttresses for
the dome
 Based on the “arch” concept

6. WEIGHT OF DOME
 The most critical issue Roman engineers
faced was the dome’s own dead weight
 Roman engineers needed a material with a
larger strength to weight ratio than traditional
stone. Answer: concrete
 Strength was not a central issue in the
design, but rather the weight and stresses
associated with the material used

7. ROMAN CONCRETE
 Very strong by modern standards
 For the Roman engineers, the concrete was
still too heavy, so they needed to develop
ways to reduce the amount used
 Roman engineers conducted a stress
analysis of the dome and determined that the
compression stresses were smaller towards
the top. Solution: pour a progressively
thinner layer of concrete

8. ROMAN CONCRETE (CONTINUED)
 Romans relied on the chemical reactions that
took place between the lime and pozzolans they
added into the mix to strengthen their concrete
 Pozzolans – volcanic ash found in and around
Puzzuoli Bay near Naples (varies in dome)
 Roman engineers used small hollow jars called
amphra as aggregate… used to displace
concrete to make mix lighter

9. ROMAN CONCRETE (CONTINUED)
 Coffers – ingenious Roman innovation which
removed an array of concrete sections…
made the dome lighter and more appealing,
all while maintaining thick sections to serve
as reinforcement
 Oculus – used to distribute the stresses
acting on the dome’s weakest point along a
rounded perimeter… also serves as an
aesthetic feature

11. REFERENCES
 Rome: Engineering an Empire
 https://www.youtube.com/watch?v=C5obOUDy
Q5s
 The Secrets of Ancient Roman Concrete
 http://www.history.com/news/the-secrets-of-
ancient-roman-concrete
 The Pantheon
 http://www.romanconcrete.com/docs/chapt01/ch
apt01.htm