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CONCRETING

Placing Concrete

The main objective in placing is to deposit the concrete as close as possible to its final position as quickly and efficiently as you can, so that segregation is avoided and it can be fully compacted.

Concrete can be transported by a variety of different methods ranging from wheelbarrows, dumpers and ready-mix trucks to skips and pumps, and though it is obviously desirable to place the concrete directly into position this is not always possible: for example, it will seldom be practical to discharge from a dumper or ready-mix truck directly into the top of a column or wall.

Concreting using skip and crane
Concreting using skip
and crane
 

Concrete pump for placing concrete
Concrete pump for placing concrete

 

Concreting

Someone experienced in the construction of formwork, preferably a tradesman, should always be standing by when the concrete is being placed.

He should have a supply of suitable materials such as props, bolts etc. to handle dangerous situations.

Grout loss is an indication that joints were not tight or some movements has occurred during placing.

The vibrations transmitted to the formwork can loosen wedges and fixings so a close watch on all fastenings is necessary to avoid loosening. Similarly, wedges should be regularly checked and tightened.

All split concrete or grout leakage should be cleaned or diluted with spray water immediately after concreting to make striking and cleaning easier especially with steel formwork.

Remove timber spreaders which were used to hold formwork apart as concreting proceeds.

Check cracking, excessive deflection, level and plumb, and any movement.

Concrete should be deposited at, or as near as possible to, its final position.

The concrete should be placed in uniform layers. Avoid placing it in large heaps or sloping layers because there is always a danger of segregation, especially with mixes tending to be uncohesive.

In walls and columns no layer should be more than about 450 mm thick. With layers thicker than 450 mm, the weight of concrete on top makes it almost impossible-even with vibration-to get the air out from the bottom of the layer.

In thin slabs compacted by a vibrating beam, restrict the layers to 150-200 mm. With greater thickness, vibrators have to be used.

Place the concrete as quickly as possible. But not faster than the compacting method and equipment can cope with.

Where a good finish is required on columns and walls, fill the forms at a rate greater than 2 metres height per hour. Also avoid delays and interruptions because these will cause colour variations on the surface.

Make sure that each layer of concrete has been fully compacted before placing the next one, and that each new layer is placed while the underlying layer is still responsive to vibration. This will make the layers "knits" together.

Avoid the formation of cold joints. Good planning is necessary, particularly with large pours.

In columns and walls, the placing must be done in such a way that the concrete does not strike the face of the formwork; similarly, avoid heavy impact against reinforcement, as the force could displace it.

Always make sure that the concrete can be seen as it is being deposited.

 

Compacting Concrete: Theory

After concrete has been mixed, transported and placed, it contains entrapped air in the form or voids. The object of compaction is to get rid of as much as possible for this unwanted entrapped air; down to less than 1% is usually the aim.

The amount of entrapped air is related to the workability: concrete with a 75 mm slump contains about 5 % air, while concrete of 25 mm slump contains about 20 %. This is why a low-slump concrete requires more compactive effort-either a longer time or more vibrators compared with a concrete with a higher slump.

 

Reasons for Removing Air

Voids reduce the strength of the concrete. For every 1 % of entrapped air, the strength falls by about 5 to 6 %. So a concrete with, say, 3 % voids will be about 15-20 % weaker than it should be.

Voids increase the permeability, which in turn reduces the durability. If the concrete is not dense and impermeable, it will not be watertight, it will be less able to withstand mildly aggressive liquids, and any exposed surfaces will weather badly; in addition, moisture and air are more likely to get to reinforcement and cause it to rust.

Voids reduce the contact between the concrete and the reinforcement and other embedded metals; the required bond will then not be achieved and the reinforced member will not be as strong as it should be.

Voids produce visual blemishes such as blowholes and honeycombing on stuck surfaces.

Fully compacted concrete will be dense, strong, durable and impermeable. Badly compacted concrete will be weak, non-durable, honeycombed and porous. The air must be removed.

 

Vibration

Rodding, spading are all ways of removing air from concrete to compact it, but the best and quickest method is vibration.

When a concrete mix is vibrated it is "fluidised", which reduces the internal friction between the aggregate particles. The fluidisation of concrete allows entrapped air to rise to the surface, and the concrete becomes denser.

With a properly designed cohesive mix, segregation and bleeding will be minimised. With an over-wet mix, the larger aggregate pieces may settle during compaction, with the result that a weak layer of laitance will finish up on the surface; if this does happen, the laitance must be removed. It therefore pay to see that mix is right in the first place!

 

Vibrators

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
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%
Total: 70%

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.

 

20 Rules for Using Internal Vibrators

  1. Make sure you can see the concrete surface. Lights may be required in thin deep sections.
  2. 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.
  3. 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.
  4. Do not stir. This only increases the voids.
  5. Leave the poker in the concrete for about 10 seconds.
  6. 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.
  7. Repeat insertion. Ensure that in plan (as seen on the surface) all areas covered.
  8. 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.
  9. 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.
  10. 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 of the head can be immersed, the vibrator will not be very effective, and compaction will be slow.
  11. 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.
  12. 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.
  13. 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.
  14. 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.
  15. Put the whole length of the poker into the concrete. This is essential to keep the bearings cool.
  16. Avoid leaving the poker running when it is not in the concrete. Otherwise there is a risk of bearings overheating.
  17. Avoid sharp bends in flexible drives. Otherwise the shaft can be broken.
  18. Remember that, where finish is important, a little bit of extra vibration can reduce the number of blowholes.
  19. Make sure the drive motor will not vibrate itself off the staging. Do not remove it by pulling the flexible drive.
  20. Clean it afterwards.

 

Length of Time Required for Full Compaction

  1. Initial consolidation is rapid and the level of the concrete drops, but the entrapped air has still to be removed.
  2. 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.
  3. 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.
  4. 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.

 

Revibration

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.