The famous Roman Pantheon, originally a temple to the Gods, has also been cited as an example of the durability of concrete. This building, still standing and a tourist attraction, is some 18 centuries old and was built without reinforcing steel. 

Comments by David Moore P.E. in his book "The Roman Pantheon: The Triumph of Concrete" on first experiencing the Roman Pantheon:

"But surely this could not be the right building...... It was too big, too new...... I asked the guard at the door for some assurance...... The building was indeed built by the Romans some 1800 years ago, and it had not been rebuilt".

References:

David Moore: "The Roman Pantheon: The Triumph of Concrete", 1995.

Links:

Information on Roman concrete at:
www.romanconcrete.com

The Pantheon and other famous buildings at:
www.greatbuildings.com

History of Concrete and other introductory material in a module of MAST, Materials Science and Engineering Department University of Illinois, Urbana/Champaign at:
http://matse1.mse.uiuc.edu/~tw/concrete/concrete.html

More information on reinforced concrete and monitoring on www.corrosion-club.com.