Well, old concrete, even pre-"quick set" has been found to be much more "durable" (colloquially speaking - not really an engineering term.) And in most applicat

Well, old concrete, even pre-"quick set" has been found to be much more "durable" (colloquially speaking - not really an engineering term.) And in most applications it isn't concrete's compressive strength which causes failure - for either old concrete or new concrete. So I would hesitate to defend current building practices based on the lone metric of compressive strength. But, again, I'm not a structural engineer (though I do know one who works for the state, and he isn't allowed to express his real opinions about the danger of our current infrastructure - many building failed in earthquakes which they should have survived easily.)