As the vibrator is withdrawn the concrete surrounding it flows back into the hole left behind by the withdrawing vibrator. If the rate of withdrawal is too rapid the air will get trapped inside the hole.

Gradual withdrawal of vibrator is recommended so as the hole left behind by the vibrator closes fully without any air being trapped.

A withdrawal rate of 3 inch/second (i.e approx.80 mm/sec) is generally considered satisfactory.