Q.: We hear recommendations both for and against using internal vibrators in constructing floors and other flatwork. What's best?

A.: One of the reasons for cautioning against the use of internal vibrators is concern over segregation and the possibility that this may happen if workers use internal vibrators to move piles of concrete into position. It can also happen if they use them to vibrate high slump concrete that doesn't require vibration for placement. However, if there are rebars in the floor, internal vibrators are useful in helping consolidate the concrete against the steel. Also, in any slab that is 8 inches thick or more, internal vibrators should always be used so that the full thickness becomes uniformly consolidated.