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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-it
may cause seggegation
· 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.
· Don’t
vibrate a lift greater than about 0.5 m in one
go-vibrator may not be effective (due to weight/friction of
overlying/surrounding concrete) in expelling air from the lower part of
the
layer.
(Acknowledgements: www. prairie.com)
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