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Unreinforced concrete is largely useless for modern construction, because it basically has negligible tensile strength. You can't make beams, columns, or even pre-fabbed wall panels of any useful strength without reinforcement.

One of the reasons unreinforced concrete may last a lot longer is because its only going to exist in places that don't subject it to tensile stresses. That being said, changes like differential settling can create these stresses after construction.

'Deterioration' can mean many things in terms of concrete.