I have recently made my first Wikipedia edit.
The article on the Pantheon in Rome made the following claim:
“The exact composition of the Roman concrete used in the dome remains a mystery. An unreinforced dome in these proportions made of modern concrete would hardly stand the load of its own weight, since concrete has very low tensile strength, yet the Pantheon has stood for centuries. It is known from Roman sources that their concrete is made up of a pasty Calcium hydroxide|hydrate of lime, with pozzolanic ash (Latin ”pulvis puteolanum”) and lightweight pumice from a nearby volcano, and fist-sized pieces of rock. In this, it is very similar to modern concrete. The high tensile strength appears to come from the way the concrete was applied in very small amounts and then was tamped down after every application to remove excess water and trapped air bubbles. This appears to have increased its strength enormously.”
Some quick research on the Internet found no evidence for the Roman concrete having particularly high strength, and this article: The Pantheon by David Moore quoted research showing that the maximum flexural tensile stresses were very low.
My edit was as follows:
It is known from Roman sources that their concrete is made up of a pasty hydrate of lime, with pozzolanic ash (Latin pulvis puteolanum) and lightweight pumice from a nearby volcano, and fist-sized pieces of rock. In this, it is very similar to modern concrete. No tensile test results are available on the concrete used in the Pantheon; however Cowan discussed tests on ancient concrete from Roman ruins in Libya which gave a compressive strength of 2.8 ksi (20 MPa). An empirical relationship gives a tensile strength of 213 psi (1.5 MPa) for this specimen. Finite element analysis of the structure by Mark and Hutchison found a maximum tensile stress of only 18.5 psi (0.13 MPa) at the point where the dome joins the raised outer wall. The stresses in the dome were found to be substantially reduced by the use of successively less dense concrete in higher layers of the dome. Mark and Hutchison estimated that if normal weight concrete had been used throughout the stresses in the arch would have been some 80% higher.
4. H. W. Cowan, The Master Builders. John Wiley and Son, New York, 1977, p. 56
5. R. Mark and P. Hutchinson, “On the Structure of the Pantheon”, Art Bulletin. March 1986
6. Moore, David, “The Pantheon”, http://www.romanconcrete.com/docs/chapt01/chapt01.htm, 1999