Structures, water, computers, languages and people (not necessarily in this order)

Bad vibrations

As you probably know, vibrating concrete consolidates the material, avoids segregation, minimises holes and protects the steel reinforcement. High resistance cement mixtures are very difficult to place and it is required a careful combination of fluidificant and vibrating. Although over-vibration of the concrete is not a good thing, generally speaking, it is worse to under-vibrate than to over-vibrate concrete.
I found a couple of interesting pics that show the rapid growing of the holes in the deck of a bridge.

If you understand a bit of Spanish, I recommend you have a look at the desperate on-site engineer's rant at the Urbanity forum.