اسپيسرها
در این پست قصد دارم یکی از تکنولوژی های نوین ساختمانی که به تازگی استفاده از آن در ساختمان سازی و قالب بندی های حجیم رونق گرفته است را برای دوستان معرفی کنم. امیدوارم مورد استفاده دوستان قرار بگیرد.
در ضمن دوستانی که اطلاعاتی بیشتری در این زمینه دارند و صاحبان صنایع نوین ساختمانی می توانند مطالب خود برای معرفی و استفاده علاقمندان،در وبلاگ انجمن عمران گرگان قرار دهند.
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برای کسب اطلاعات بيشتر در مورد اهمیت پوشش بتن و نقش اسپيسر در رعايت آن ، مقاله دانشکده عمران دانشگاه هنگ کنگ راببينيد .
قالبهاي پلاستيکي بتن :
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واتراستاپ پی وی سی :
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STEEL FIXING
Steel Positioning
Steel must be placed correctly or the strength of a structure may be greatly weakened.
 After putting the semi-precast slab into position, reinforcement fixing of the in-situ slab is commenced for the in-situ slab construction above the precast slab. |
 The lintel beam and wall reinforcement details. The lintel beam is placed above the wall opening to provide structural rigidity. |
Cover
Cover is the distance from the outside face of the concrete to the nearest surface of reinforcing steel.
If this distance is insufficient, the steel will rust. As the steel changes to rust, the diameter of the actual steel decreases and the strength provided by the steel decreases. This can seriously shorten the life of a building and has been a major problem for the Hong Kong Housing Authority.
Furthermore, when steel rusts, it expands to 2.2 times its original volume. This expansion bursts the concrete open thus exposing the steel to attack by the weather which increases the corrosion.
Cover in Hong Kong varies from 20 to 60 mm. The actual value depends on the concrete strength and on the location of the concrete such as below the ground or inside a building. Other countries use higher covers.
Cover is also very important in maintaining the strength of reinforced concrete during a fire. Depending on the length of time the reinforced concrete must withstand the fire, the type of concrete, and the location of the concrete (floor, column etc.) the minimum cover varies from 20 to 70 mm.
The designers must allow for the effect of deformations on bars. If they increase the diameter of bars and if not allowed for, reinforcing steel often cannot be placed properly. Usually it is the cover that suffers.
 Natural concrete curing of the reinforced concrete wall. One can see the cover in these walls. |
Spacers
Spacers are used to maintain cover. They keep the steel the required distance away from the edge of the concrete.
Spacers
Fixing
Fixing is fastening the steel reinforcing bars so that they stay in the correct place between the spacers and relative to each other.
The steel must stay in place:
- while workers walk on it,
- while concrete is being placed,
- and during compaction.
Tie-wire is used at the junctions of bars to hold them together.
- It must be tied tightly.
- Free ends must not protrude into the cover space.
- All pieces of tie-wire must be removed from the formwork before concreting otherwise rust-stains soon occur on the finished product.
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 Tie wire used to join steel reinforcing bars. Steel chair used to separate the top and bottom layer. |
Bending
Too small a diameter weakens the steel.
Too large a diameter may cause problems such as lack of anchorage (a form of bond failure) or create difficulties in keeping other steel bars in the correct place.
| Standard bend radii (r): | |||
| Grade 250 bars-all diameters | 2 X diameter | ||
| Grade 460 bars-up to and including 20mm | 3 X diameter | ||
| Grade 460 bars-over 20mm | 4 X diameter | ||
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FORMWORK
Introduction
Formwork is the mould to shape the concrete structure. Good concrete cannot be made from a poor mould; thus formwork is very important in producing good concrete. Besides that, it has to be designed to take the concrete load and the working load generated from workers and plant. An excessive deflection or leaky and not well-fitted mould which loses grout and weakens the concrete can destroy the appearance of the concrete product. Moreover a leaky joint can cause honeycombing and joint marks on the face of concrete.
 The timber formwork is prepared on site for the construction of columns and walls. |
 Steel formwork, shown here being struck, is usually prefabricated off site. |
Materials
Timber and plywood are generally used in Hong Kong for in-situ concrete work, as shown in the AEB project illustrated in this website. Steel formwork is also used, particularly in Harmony and Concord public housing work.
Glass-reinforced plastic formwork may be used for complex shapes and special surface features.
 Timber Formwork |
 Steel Formwork |
Checklist
This section contains checklists of activities which need to be carried out before concrete is placed in formwork.
Formwork
- Correct position
- Line and level
- Accuracy within permitted tolerances
- Shape of members
- Verticality (plumb)
- Correct angles for batters etc.
- Finishing heights (mark off)
Joint
- Formwork joints flush and tight
- No nails protruding into the concrete
- Formwork joints sealed as required
- Construction joints strutted and supported
Waterstops
- Correctly positioned
- Fixed and suitably supported
Ties and Fixings
- Correct number
- Correct lengths
- Tight enough and not too tight
- Fitted with the proper washers
Finish
- All forms perfectly cleaned out
- Release agents correctly applied
Striking
The specification will normally give guidance on when forms can be struck and these times may be governed by the size and shape of the member, the concrete mix, and the weather.
To strike the wallform, ties and clamps should be loosened gradually, a little at a time.
To remove the props, release the props evenly in small stages starting at the middle of the span and working out towards the supports. This can avoid overloading of the props at the centre of the span due to large deflection at the centre.
Always withdraw or hammer down projecting nails as the formwork is struck from the concrete.
Make sure that other trades are kept away from areas below those where striking is being done.
Some Formwork Examples
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Cleaning
As soon as the formwork has been struck, it should be cleaned, not left until it is wanted again.
Timber and plywood forms should be cleaned with a stiff brush to remove any grout; a timber scraper should be used for stubborn bits of grout.
With glass reinforced plastics, a brush and wet cloth are all that should be needed.
When steel forms are to be put in store or are not going to be used for some time, they should be lightly oiled to prevent rusting.
Timber and untreated plywood should also have a coat of release agent applied for protection if they are not going to be re-used immediately.
Any depressions, splits and nail holes should be repaired with plastic wood or similar material, followed by a light rubbing down.
Before concreting, the insides of the forms should be cleaned. Where the forms are deep, temporary openings should be provided for inspection.
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 |
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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. |
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
- 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.
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.
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.