Why are concrete schools crumbling in the UK

A huge and unfolding issue in the UK where "cheaper" concrete used to build a many UK schools has far extended its useful life of 30 years and is forcing school closures due to risk of, and actual, failures. This begs the question - what do we do with massive amounts of infrastructure after it extends its useful life? Most modern reinforced concrete is rated for 50-100 years if it is properly maintained - when was the building that you are currently sitting in built?