1. Entrapped air can be removed by hand rodding—jamming a rebar or tamping rod into the concrete or using a flat-nosed spade.

This is effective for small placements (like cylinders) or with higher slump concrete, but it’s not possible with a wall or column. Mechanical consolidation relies on vibration.

2. Concrete in a form is usually consolidated using vibration. Vibration can be applied to the outside of the form (external vibration) or in the concrete.

External is commonly used for precast concrete. Vibration eliminates the air pockets, spaces out the coarse aggregate, and produces better surface finishes.

3. For most cast-in-place concrete placed in forms, consolidation is accomplished with an internal vibrator—a vibrating head on the end of a shaft—usually a flexible shaft.

The operator controls the power unit and a spinning cable rotates an eccentric weight in the head to create the vibration.

4. The important thing to know about a vibrator is its radius of influence—the distance to where it can no longer effectively consolidate concrete.

This is a characteristic of the vibrator but is also influenced by the concrete mix. Select a vibrator with a radius of influence that will consolidate all of the concrete.

5. The two things the operator needs to know are how far apart to insert the vibrator and how deep to go. The center-to-center insertion spacing should be 1.5 times the radius of influence. To minimize lift lines, the total insertion depth is the depth of the lift being vibrated plus 6 inches into the previous lift.

Credit for all: Patrick Welsh

6. The vibrator should be allowed to sink under its own weight to the proper depth then be moved up and down for five to 15 seconds. It is then pulled out slowly, about 15 seconds for every 2 feet. When in doubt, vibrate more—with well-proportioned concrete mixes you are not likely to segregate the aggregate.

Thanks to Oztec Industries. See their vibrator tips here.