In a comprehensive overview of concrete*, Dr. Carolyn M. Hansson pointed out that concrete is in itself an inherently durable material. The example of the Roman Colosseum, still standing after nearly 2000 years, was cited. A further relevant example is the Roman Pantheon.

The Roman Colosseum
(see www.greatbuildings.com for more details)

However, in this overview it was stated that modern reinforced concrete structures were increasingly failing to survive much shorter specified service lives.

Corrosion damage in a North American elevated highway structure exposed to de-icing salts - notice spalling of the concrete cover, leaving the rebar exposed.

* C.M. Hansson: "Concrete: The Advanced Material for the 21st Century", Metallurgical and Materials Transactions A, 1995, pp.1321-1341. (The 1993 Edward DeMille Campbell Memorial Lecture - ASM International)

Links:
Roman Pantheon
More information on reinforced concrete and monitoring at www.corrosion-club.com.

The Portland Cement Association (with information on concrete basics):
www.portcement.org

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

Infrastructure deterioration - collapse of Roman Empire