Concrete Vibration Hints

Thorough compaction – removal of air – is essential for concrete to achieve full strength and therefore durability. The most effective way of achieving this for a wide variety of applications is the efficient use of a vibrating poker.

Concrete should be placed in layers no more than 450mm deep and each layer should be vibrated fully before the next layer is placed. This ensures the air bubbles can rise to the surface and don't get trapped under a layer of concrete that isn't being vibrated. The next layer has to be vibrated before the previous layer has hardened. The poker should go in to the first layer by about 100mm to ensure a good bond between the two layers.

The poker should be inserted vertically into the concrete. An area around the poker will be fully compacted. The size of this area depends on a number of factors; size of the poker, frequency and amplitude, slump of the concrete, aggregate and admixtures, amount and size of reinforcement bars, distance from the formwork.

By watching the surface of the concrete around the poker, the size of the full area of the compaction circle can be determined. For full compaction the poker should be inserted at intervals so these circles overlap.

Slowly withdraw the poker vertically from each position allowing the void left by the poker to fill.

In normal circumstances to not drag the poker through the concrete as pockets of air are likely to be trapped.

Do not over vibrate, holder the poker in place for too long will cause the aggregate, sand and cement to separate out and reduce the strength of the concrete.

Choosing a vibrator

The size of the job and the power supplies available on site will determine which type of poker to use.

• Hand held, small jobs powered by mains electric or a generator
• Pneumatic pokers, powered by a standard site compressor
• Flexible shaft pokers, used with a dedicated petrol or diesel drive unit
• Electric high frequency, 230v or 400v 3ph converted to 42v.

The selection of head size is determined by the application. Small diameters for small concrete volumes, large diameters to increase productivity on large pours. The distance between rebars and ease of handling should also be considered before selecting the larger sizes.

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