Bob's Bunker Page 34

The fellow on the top of the wall is holding the hose of a concrete vibrator. An electric motor which he is holding in his left hand drives a flexible shaft inside the hose. At the end of the hose is a cylindrical tube which has a weight that is driven by the flexible shaft. The weight is off balance and so as it rotates, it vibrates. By vibrating the concrete, air bubbles that are entraped within the concrete mix are forced to bubble up to the surface creating a wall that is less likely to have air pockets.
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