Thorough compaction and thus the removal of air is paramount for concrete to cure to full strength and therefore durability. Currently the most effective way of achieving this is the induction of vibration into the concrete. Vibration can be induced by a traditional poker and drive unit.
Concrete should be placed in layers no more than 450mm deep. Each layer should be vibrated before the next layer is placed. This ensures the air bubbles can escape to the surface and don’t get trapped. The next layer has to be vibrated before the previous layer has hardened. The vibration should penetrate the first layer by about 100mm to ensure a good bond between the two layers.When using a poker only a small area around the poker will be compacted to an appropriate degree. The size of this area depends on a number of factors; size of the poker head, frequency and amplitude of the vibration, slump of the concrete, aggregate and admixtures, the amount and size of reinforcement bars and distance from the formwork. Watch the surface around the poker, the size of the compaction circle can be determined by the ‘fat’ that rises to the surface. This ‘fat’ aids in providing a final finish to the concrete. For complete compaction the poker should be inserted at intervals so these circles overlap.
How to Vibrate
- Insert vibrator vertically, allowing it to penetrate rapidly to the bottom of the lift and at least 6 inches into the previous lift.
- Hold it at the bottom of the lift for 5 to 8 seconds.
- Pull vibrator up at a rate of 15 seconds for a 4-foot lift, or about 3 inches per second.
Spacing Tips
- Space out the insertion of the vibrator so the fields of action overlap.
- Watch the concrete to determine the vibrator’s field of action.
- High-powered vibrators and high slump concrete have larger fields of action.
- Rule of thumb: the radius of the field of action is four times the vibrator’s head diameter.
Stop Vibrating when:
- The concrete surface takes on a sheen.
- Large air bubbles no longer escape.
- You hear the vibrator change pitch or tone.
- You feel a change in vibrator actions
Vibrating Don’ts
- Don’t let a vibrator run very long outside of the concrete – it will overheat and fail.
- Don’t force or push a vibrator into concrete; it won’t remain vertical and may get caught in the reinforcing steel.
- Do not over-bend the vibrator – it will fails
CAUTION!
Do not drag the poker through the concrete as pockets of air are likely to be trapped.
Do not over vibrate or hold the poker in place for too long as it will cause the aggregate, sand and cement to separate out and reduce the strength of the concrete.
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