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.
