MC Magazine

Consolidation Using Internal Vibrators

Proper use of the internal vibrator (often called a stinger) is crucial to obtaining a high-quality product. The ideal stinger would allow you to vary its amplitude and frequency, although most do not. However, all vibrators are characterized by their amplitude and frequency. The amplitude refers to the strength of the vibration or how far away from the vibrator each vibration cycle can be felt. Frequency is how many times the equipment vibrates each minute. There is a good chance that someone else has decided on these settings, but you should know what they mean.

Proper vibration makes the concrete stronger and more durable, because it helps remove pockets of air that are trapped within the form. If you do not vibrate the concrete properly, the product may become less watertight and may begin to deteriorate rapidly once in service. If concrete begins to deteriorate, inspectors can often trace the type of deterioration to poor vibration.

You should never use the stinger to move the concrete around. For instance, with a conventional mix, you should not place all of the concrete in one side of the form and use the vibrator to move it around to the other sides. This will cause the mix to segregate or separate – aggregates will pull away from the cement paste – and this can only hurt the product. When concrete is placed into formwork, it should always be placed neatly, in layers, right where it will stay – and you should vibrate immediately after it is placed.

Vibration layers should be 12 to 24 inches. It is important for the stinger to go straight in and straight out. Do not insert or remove the stinger at an angle. Allow the vibrator to sink into the concrete under its own weight at about one second per foot. Allow it to vibrate for approximately 5 to 15 seconds for a wet mix and longer for stiff mixes, and withdraw it slightly faster than you inserted it – about 3 feet per second – because as you come back up, you are bringing air back up to the surface. As you vibrate, you will notice these bubbles of air breaking at the surface.

If you are placing concrete in more than one layer (lift), you should allow the stinger tip to penetrate the layer of concrete directly below it. You have already consolidated this lower layer, but if the stinger tip does not penetrate at least 2 to 4 inches into layer below, the two layers are not properly consolidated together and can create a weakness in the structure.

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