Concreting using skip
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.
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.
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.
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!
These are mobile items of mechanical plant used to vibrate (shake) air out of fresh concrete.
There are 2 major types of vibrators:
All types of vibrators have motors, which can be driven by:
Concreting to the precast facade and then compact the concrete by Poker Vibrator to force out the air-bubbles trapped inside.
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.
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.