Q.: We have recently been having trouble with vibration of concrete in wall forms. Normally we try to give good, thorough vibration with an immersion vibrator, but if we vibrate the mix very long on this job we get excessive mortar rising to the surface. The concrete comes from a supplier that we have not purchased from previously. We have reduced the slump as much as we can and that helps a little bit but not enough. Many people say you can't overvibrate concrete, but we seem to be overvibrating this concrete.

A.: Perhaps you have an improper mix design, with too little aggregate. You might modify the mix design by making a series of mixes through which you gradually increase the aggregate content and possibly reduce the water-cement ratio further. You could vibrate each mix of the series and choose the mix in which the vibrator doesn't cause the fines to separate.

This is an interesting point that was brought out a generation ago by an English investigator, D. A. Stewart. He found that mortar came to the surface when he vibrated a mix that had a water-cement ratio of 0.52 and a cement-to-aggregate ratio of 1:6. He continued to add fresh concrete and vibrate the mold, letting the separating mortar spill over the top until he got a mix which would not accept any more coarse aggregate. He then analyzed what was in the mold and found that the water-cement ratio had dropped to 0.49 and the cement-to-aggregate ratio had changed to 1:7.3.

He then made a new mix of the proportions that the vibrator had thus selected for him and found that the mix compacted well, with no segregation. This mix, with its lower water-cement ratio and lower cement-to-aggregate ratio, was obviously less expensive than what he started with.

It is reported that in a mix with too much coarse aggregate the coarse aggregate will segregate at the top in the same way. Hence, the vibrator can be used to establish the proportions that it can handle most efficiently to produce the densest mix.