Except for concrete placed underwater, you must compact or consolidate all concrete after placement.
PURPOSE OF CONSOLIDATION
Consolidation eliminates rock pockets and air bubbles and brings enough fine material both to the surface and against the forms to produce the desired finish. You can use such hand tools as spades, puddling sticks, or tampers, but mechanical vibrators are best. Any compacting device must reach the bottom of the form and be small enough to pass between reinforcing bars. The process involves carefully working around all reinforcing steel with the compacting device to assure proper embedding of reinforcing steel in the concrete. Since the strength of the concrete member depends on proper reinforcement location, be careful not to displace the reinforcing steel.
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. However, do not use a mix so stiff that it requires too much labor to place it.
The best compacting tool is a mechanical vibrator (figure 7-42). 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 air-entrained concrete for 5 to 10 seconds and into nonair-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. 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.
Figure 7-42.-Using a vibrator to consolidate concrete.
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 that 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, dont vibrate concrete that has a slump of 5 inches or more. Finally, do not use vibrators to move concrete in the form.
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 (figure 7-43), through the layer just placed, and several inches into the layer underneath. Continue spading or puddling until the coarse aggregate disappears into the concrete.
Figure 7-43.-Consotidation by spading and a spading tool.