do not use a mix so stiff that it requires too much labor to place it. The vibrators available in engineer
construction battalions are called internal vibrators because the vibrating element is inserted into the concrete.
An external vibrator is applied to the form and is powered by an electric motor, a gasoline engine, or compressed
air. When using an internal vibrator, insert it approximately in 18-inch intervals into air-entrained concrete for 5
to 10 seconds and into non-air-entrained concrete for 10 to 15 seconds. The exact period of time that you should
leave a vibrator in the concrete depends on its slump. Overlap the vibrated areas somewhat at each insertion.
When possible, lower the vibrator into the concrete vertically and allow it to descend by gravity. The vibrator
should not only pass through the layer just placed but penetrate several inches into the layer underneath to ensure
a good bond between the layers. Vibration does not normally damage the lower layers, as long as the concrete
disturbed in these lower layers becomes plastic under vibratory action. You will know that you have consolidated
the concrete enough when a thin line of mortar appears along the form near the vibrator, the coarse-aggregate
disappears into the concrete, or the paste appears near the vibrator head. Withdraw the vibrator vertically at about
the same rate that it descended. Some hand spading or puddling should accompany all vibration. Do not vibrate
mixes that you can consolidate easily by spading because segregation may occur; you should not vibrate concrete
that has a slump of 5 or 6 inches. Also, do not use vibrators to move concrete any distance in the form.
Figure 4-7. Using a vibrator to consolidate concrete.
Manual consolidation methods require spades, puddling sticks, or various types of tampers. To consolidate
concrete by spading, insert the spade downward along the inside surface of the forms, as shown in Figure 4-8,
through the layer just placed, on into the layer underneath several inches. Continue spading or puddling until the
coarse aggregate disappears into the concrete.