Vibration

Vibrators consolidate concrete by pushing the coarse aggregate downward, away from the point of vibration. Vibrators allow placement of mixtures that are too stiff to place any other way, such as those having a 1 or 2 inch slump. Stiff mixtures are more economical because they require less cement and present fewer segregation or bleeding problems. Do not use a mix so stiff that it requires too much labor to place it.

Mechanical Vibrators

The best compacting tool is a mechanical vibrator, shown in Figure 63. The best vibrators available in engineering construction battalions are called internal vibrators because the vibrating element is inserted into the concrete.

When using an internal vibrator, insert it at approximately 18-inch intervals into airentrained concrete for 5 to 10 seconds and into nonair-entrained concrete for 10 to 15 seconds. The exact period of time to leave a vibrator in the concrete depends on its slump. Overlap the vibrated areas somewhat at each insertion. Whenever 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 the vibrating action. You know that you have consolidated the concrete properly when a thin line of mortar appears along the form near the  vibrator, the coarse aggregate disappears into the concrete, or the paste begins to appear near the vibrator head. Then, withdraw the vibrator vertically at about the same gravity rate at which it descended.

Some hand spading or puddling should accompany all vibration. To avoid the possibility of segregation, do not vibrate mixes that you can consolidate easily by spading. Also, don’t vibrate concrete that has a slump of 5 inches or more. Do not use vibrators to move concrete in the form. 

Vibrating (consolidating) the Concrete

When surface vibrating slabs up to 6 inches thick, provided they are not reinforced or contain only light mesh, use low-frequency vibrating
screeds — 3000 to 6000 vibrations per minute. This will provide adequate consolidation depth without creating an objectionable layer of fines at the surface. High frequency, low amplitude screeds are satrifatory when applied solely to accommodate the finishing operation. 6 to 8 inch slabs that are not reinforced may be consolidated by either surface or internal vibration.

For slabs more than 8 inches thick or thinner slabs containing embedments or substantial reinforcement, it is recommended to use an internal vibration. When an internal vibrator (Figure 64), is inserted into fresh concrete, the concrete near the vibrator tends to act like very thick liquid in the field of action area that is affected by the vibrator. By watching the way the concrete acts near the vibrator you can judge the size of the field of action. Big high-powered vibrators have larger fields of action than do small vibrators. A vibrator’s field of action is larger in high slump (very wet) concrete than it is in stiffer concrete. To consolidate concrete completely, you must be sure that the fields of action overlap from one insertion point to another.

The head of an internal vibrator should be completely immersed during vibration. For thick slabs it is possible to insert a vibrator vertically, for thinner slabs it should be inserted at an angle or even horizontally. Note that letting the vibrator contact the subgrade could contaminate the concrete with foreign material. The vibrator should remain in the concrete until the surface of the field takes on a sheen look. There are several precautions when consolidating concrete:

• DO NOT use a vibrator to move concrete horizontally (“run the concrete”) because the coarse aggregates will separate from the mortar.
• DO NOT leave a vibrator in the concrete too long in concrete mixes which have a slump of more than 3 inches, this will cause segregation. If in doubt about the adequacy of compaction, it is generally better to vibrate more in stiffer mixes because the danger of overvibrating stiff mixes is minimal.
• DO NOT let a vibrator run very long when it is not in concrete — it may burn out. Concrete acts as a coolant for the vibrator.
• DO NOT use an electrically powered internal vibrator without wearing good rubber gloves and rubber boots — there is a shock or burn hazard.

Hand Methods

Manual consolidation methods require spades, puddling sticks, or various types of tampers. To consolidate concrete by spading, insert the spade along the inside surface of the forms as shown in Figure 64, through the layer just placed, and several inches into the layer underneath. Continue spading or puddling until the coarse aggregate disappears into the concrete. Do not attempt on large placements.