How a seemingly sound high-rise concrete building can just collapse. From Charles Hugh Smith at oftwominds.com:
There is a downside to steel reinforcing bars: they rust.
The second most remarkable thing about the sudden collapse of the Florida condo building was the rush to assure everyone that this was a one-off catastrophe: all the factors fingered as causes were unique to this building, the implication being all other high-rise reinforced concrete condos without the exact same mix of causal factors were not in danger.
Before we accept this conveniently feel-good conclusion, there are a few things we should consider about reinforced concrete high-rise condos.
1. This may seem too obvious to be important, but concrete is a heavy material. Fill a 5-gallon bucket with wet concrete, let it cure (harden) and then pick the bucket up–if you can.
2. Conventional concrete is not water-proof; it absorbs moisture. Construct a concrete wall against an excavated cliff of damp earth saturated with underground moisture and the concrete wall will be damp unless it is sealed essentially perfectly–no easy task.
3. Steel reinforcing bars add specific kinds of strength to concrete, which is rather brittle in its conventional unreinforced state: tilt a slab of unreinforced concrete on a large, sharp rock and hit the elevated half of the slab with a sledge hammer, and the slab will crack on the (fulcrum) rock.