concrete is vibrated, the internal friction between the aggregate
particles is temporarily
destroyed and the concrete behaves like a liquid; it settles in the
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.
compressive strength and bond between concrete and rebar and decrease
cold joints, honeycombing, excessive entrapped air, and
Causes concrete within a circular field of action to act like a liquid.
How to Vibrate:
the concrete to determine the vibrator's field of action. High-powered
vibrators and high-slump concrete have large fields of action. The rule
thumb is that the field of action is 8 times the vibrator's head
a vibrator so the fields of action overlap.
the vibrator vertically, allowing it to penetrate rapidly to the bottom
of the lift and at least 6 inches into the previous lift.
at the bottom of lift for 5-15 seconds.
vibrator up at a rate of 15 seconds for a 4-foot lift ( about 3 inches
concrete surface takes on a sheen.
Large air bubbles no longer escape.
hear the vibrator change pitch or tone.
You feel a change in vibrator action.
vibrator run outside of the concrete-it will overheat
use a vibrator to move concrete horizontally-it may cause seggegation
force or push a vibrator into concrete- it won't remain vertical and
caught in the reinforcement.
start a job without a spare vibrator.
vibrate a lift greater than about 0.5 m in one go-vibrator may not be
(due to weight/friction of overlying/surrounding concrete) in expelling
from the lower part of the layer.