Published on 12/4/2018
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Lets talk about vibrating concrete
The University of Hong Kong gives us an insight
Source for this article : http://www.cityu.edu.hk
These are mobile items of mechanical plant used to vibrate (shake) air out of fresh concrete.
There are 2 major types of vibrators:
- External vibrators (Form vibrators)
- Internal vibrators (Poker/Immersion vibrators)
All types of vibrators have motors, which can be driven by:
- Compressed air
- Main supply electricity (230 or 400V & 50Hz) motors
- High frequency electricity motors
- Petrol or diesel (liquid fuel)
Concreting to the precast facade and then compact the concrete by Poker Vibrator to force out the air-bubbles trapped inside.
Internal Vibrator Usage
Internal vibrators are often used inefficiently. They often run wastefully, or at a reduced efficiency, for about 70% of their operating time, this being made up as follows:
|out of the concrete and left running||15%|
|wrongly positioned in the concrete||35%|
|vibrating already compacted concrete||20%|
This means that the poker is doing useful work for only 30% of the time, which is why it is so necessary to plan the compaction in advance, along with the placing method and technique, so that both operations are done as economically and as quickly as possible.
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20 Rules for Using Internal Vibrators
- Make sure you can see the concrete surface. Lights may be required in thin deep sections.
- Put the head in quickly. When inserting the poker, allow it to penetrate to the bottom of the layer as quickly as possible under its own weight. If done slowly, the top part of the layer will be compacted first, making it more difficult for the entrapped air in the lower part of the layer to escape to the surface.
- Insert the head vertically. This minimises the voids created by inserting the head, and allows air bubbles to rise up unimpeded by a slopping vibrator.
- Do not stir. This only increases the voids.
- Leave the poker in the concrete for about 10 seconds.
- Withdraw the poker slowly. The main thing is to see that the hole made by the poker is closed up; otherwise you will be left with a hole in the finished concrete. If this does happen-and it is often difficult to prevent if the concrete is very stiff-put the poker back in near enough to the hold for the next spell of vibration to close it up. For the final insertion, withdraw the poker even more slowly and wiggle it about to ensure that the hole closes up properly.
- Repeat insertion. Ensure that in plan (as seen on the surface) all areas covered.
- Avoid touching the form face with the poker. Not only will the form face be damaged but a mark will be left on the finished concrete surface. To be on the safe side, keep the vibrator about 75-100 mm from the formwork.
- Avoid touching the reinforcement with the poker. Provided that all the concrete is still fresh, vibrating the reinforcement should not do any harm and could improve the bond. The danger lies in the vibrations in the reinforcement being transmitted into parts of the section where the concrete may have stiffened, in which case the bond may be affected.
- For shallow flat slabs lie the head in the concrete and drag the head slowly through the concrete. Alternately an external vibrator may be able to be used. If say ¹ of the head can be immersed, the vibrator will not be very effective, and compaction will be slow.
- Avoid using the vibrator to make the concrete flow. You may get some areas of segregation and other areas of fines only concrete. The latter may be weak and cause discoloration.
- Avoid sticking the poker into the top of a heap. Although heaps should be avoided in placing they are sometimes unavoidable or caused by mistake. To flatten a heap, insert the poker around the perimeter. Do this carefully to avoid segregation.
- Make sure that the poker extends about 100mm into any previous layer. This will knit the layers together, and any laitance on top of the previous layer will be mixed with the bottom of the new one. The new layer should not yet of course be rigid.
- The maximum depth of the new layer equals vibrator head length minus 100 mm. If the depth of concrete is greater, then the new part will not be fully compacted.
- Put the whole length of the poker into the concrete. This is essential to keep the bearings cool.
- Avoid leaving the poker running when it is not in the concrete. Otherwise there is a risk of bearings overheating.
- Avoid sharp bends in flexible drives. Otherwise the shaft can be broken.
- Remember that, where finish is important, a little bit of extra vibration can reduce the number of blowholes.
- Make sure the drive motor will not vibrate itself off the staging. Do not remove it by pulling the flexible drive.
- Clean it afterwards.
Cover Photo courtesy of Getty Images
Information extracted from IPTC Photo Metadata
Length of Time Required for Full Compaction
- Initial consolidation is rapid and the level of the concrete drops, but the entrapped air has still to be removed.
- As the concrete is vibrated, air bubbles come to the surface. When the bubbles stop it can be taken as a sign that not much more useful work can be cone on the concrete. The distance of the bubbles from the poker is also a useful guide to its radius of action.
- Sometimes the sound of the poker can be a helpful guide. When the poker is inserted there is usually a dropping off in frequency, and when the pitch (whine) becomes constant the concrete is free of entrapped air.
- The surface appearance also gives an indication of whether or not compaction is complete. A thin film of glistening mortar on the surface is a sign that the concrete is compacted, as is cement paste showing at the junction of the concrete and formwork.
On columns and walls where surface finish is of importance, there is sometimes a tendency for blowholes to occur in the top 600 mm of a lift; the reason is that, unlike the lower layers, this top layer does not have the advantage of the weight of additional concrete on top to increase the compaction. It can often help to revibrate the top 500 mm within the first 15 minute after the initial compaction.
In thick sections of slabs and beams, and particularly with mixes are prone to bleeding, there is a danger of plastic cracks appearing within 1-2 hours after compaction. If they are noticed within this time, and provided the concrete is till workable, revibration of the top 75-100 mm can close them up again.
Source for this article : http://www.cityu.edu.hk
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