To provide a little more detail than could fit into the graphical article about consolidation in the July issue, Fred Oswald of Oztec offers a few additional tips:

When vibrating a wall the important consideration is to make sure that all of the concrete is exposed to the vibration. For thin walls, where an insertion at the center of the wall would be sufficient to reach both walls, a good rule of thumb would be to choose a vibrator with a radius of influence equal to 3/4 the total thickness of the wall. For example, if you have an 8-inch wall you would want a vibrator with a radius of influence of 6 inches, which would be a 1- 1/2-inch vibrator head (this can change depending on the slump of the concrete).

This determination cannot always be simplified to a single calculation based on the thickness of the wall versus the vibrator's radius of influence. For example, when pouring thick walls, more than one insertion may be required at each length of wall (that is, insert on one side of the wall, and then at the other side before moving laterally to the next insertion points). For these cases, we recommend using the largest head size that can safely be used, given the design of the rebar within the wall.

These are, of course, general guidelines; the actual mix design will have a significant influence on which size vibrator head to choose. For dryer, stiffer mixes a larger, more powerful head becomes more appropriate. For looser mixes, smaller heads can be well suited for the job and can avoid problems with over-vibration.

Another major consideration, especially nowadays, is the placement and congestion of the rebar. High rebar congestion causes two main problems when it comes to consolidating concrete. First, it makes it difficult to actually insert a vibrator head in between congested rebar, forcing the contractor to use a smaller than ideal vibrator head in order to fit between the rebar. Second, tightly congested rebar has a tendency to act like a wall, preventing the vibrations from a vibrator inserted in the interior of a wall from reaching the form face. The opposite is true when using external vibrators - the vibrations from the external vibrator ONLY reach the concrete at the form face and are blocked from the interior of the wall by the rebar.

Oztec's patented RebarShaker was designed to address this problem. By attaching the RebarShaker to the top of the rebar, you can use the rebar itself to transmit the vibration into the mix. By using the RebarShaker, you achieve 360o consolidation, leaving you with excellent consolidation from the center of the wall right up to the form face. This tool also eliminates the difficulty of working an internal vibrator head in areas of tight rebar congestion.

Fred Oswald is president of Oztec Industries.