Prairie Material Consolidation

Consolidation

In some types of construction, the concrete is placed in forms, then consolidated. Consolidation compacts fresh concrete to mold it within the forms and around embedded items and reinforcement and to eliminate stone pockets, honeycomb, and entrapped air. It should not remove significant amounts of intentionally entrained air.

Vibrating Concrete
Vibration, either internal or external, is the most widely used method for consolidating concrete. When concrete is vibrated, the internal friction between the aggregate particles is temporarily destroyed and the concrete behaves like a liquid; it settles in the forms under the action of gravity and the large entrapped air voids rise more easily to the surface. Internal friction is reestablished as soon as vibration stops.

Proper vibration will:
• Increase compressive strength and bond between concrete and rebar and decrease concrete permeability.

• Decrease cold joints, honeycombing, excessive entrapped air, and segregation.

• Causes concrete within a circular field of action to act like a liquid.

How to Vibrate
Watch the concrete to determine the vibrator's field of action. High-powered vibrators and high-slump concrete have large fields of action. The rule of thumb is that the field of action is 8 times the vibrator's head diameter.

• Insert a vibrator so the fields of action overlap.

• Insert the vibrator vertically, allowing it to penetrate rapidly to the bottom of the lift and at least 6 inches into the previous lift.

• Hold at the bottom of lift for 5-15 seconds.

• Pull vibrator up at a rate of 15 seconds for a 4-foot lift ( about 3 inches per second).

Stop vibrating concrete when:

• The concrete surface takes on a sheen.

• Large air bubbles no longer escape.

• You hear the vibrator change pitch or tone.

• You feel a change in vibrator action.

Vibrating
Don'ts

• Don't let a vibrator run outside of the concrete-it will overheat.

• Don't use a vibrator to move concrete horizontally.

• Don't force or push a vibrator into concrete; it won't remain vertical and may get caught in the reinforcement.

• Don't start a job without a spare vibrator.


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