CONCRETE PLACEMENT - Printable Version

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CONCRETE PLACEMENT - C M Dordi - 06-09-2022

CONCRETE PLACEMENT

The method of placing concrete is very important and needs proper planning and careful study if the structure to be concreted is to be durable, strong and having a good appearance. Placing of concrete when not done properly can lead to serious defects which may ultimately result in costly and time consuming repairs.

Placing method will depend on rate of concreting planned, type of structure, quantity of concrete required to be poured in one go, the equipment of manpower available for placing and compacting concrete, formwork design, time required for initial setting of concrete and even the ambient temperature.

Supervision during concreting at the placing and compacting point can play an important part in the quality of the concrete structure.

RECOMMENDATIONS FOR CONCRETE PLACEMENT

The Site Engineer is required to follow the recommendations given below to successfully place the concrete.

Concrete must be deposited at or near its final position and as quickly as possible without loss of workability or segregation. It is generally  recommended that concrete must be placed within 30 minutes of mixing. However, as long as the concrete is workable, placing can be done after any lapse of time so long as additional water is not added to the concrete mix.

Moving heaped concrete manually or using a poker vibrator must be avoided.

The concrete must be placed in proper layers and without occurrence of cold joints.

Whenever concrete is dumped or dropped, the direction of fall should be vertical. If not, then invariably, there are good chances of segregation occurring. 

The most satisfactory way to ensure a vertical drop is by passing the concrete through a short section of drop chute. Baffle plates are not always adequate enough because they merely change the direction of concrete flow causing segregation instead of preventing it.

It is possible to place concrete of low workability if buckets are used. However, bucket should have side slopes not less than 60 degrees with wide, free working and tight closing discharge gate.

The gate should be such that it can easily open and close depending on the discharge required.

Bucket should not be swung over the freshly finished concrete. There are chances that concrete may spill over or leak through the gates and fall on the
freshly finished surfaces.

After each use it is essential to clean the bucket making sure that hardened concrete is removed. The bucket's gate control mechanism should be cleaned and oiled lightly so that it can function smoothly.

Chutes, if used, should be of rounded cross section to avoid the accumulation of concrete in sharp corners. The slope should be sufficiently steep to permit flow of concrete without requiring a workability greater than that specified. The slope usually required is 1 vertical to 2 or 2.50 horizontal.

If belt conveyors are used, segregation at the point of discharge is likely to occur. This is prevented by providing a suitable drop chute at the discharge
end of the belt conveyor.

If climate is hot, dry and/or windy the container in which concrete mix is carried for a long distance must be kept covered to prevent drying and slump loss. Similarly long conveyor belts or chutes would also require proper cover.

Chutes and conveyor belts must be mobile enough to allow easy movement at the placing end so that concrete can be easily placed as close as possible to the desired location as concreting progresses.

Chutes and conveyor belts must be cleaned and flushed with water after the concreting is over. The water with washed concrete materials must not be
allowed to fall into the formwork or on freshly placed and finished concrete.

It is preferable not to drop the concrete from more than 1 m. height. However, cohesive concrete with an unobstructed fall can be dropped from any height and is unlikely to segregate if the drop is through a funnel and tremie (pipe). Even cohesive concrete dropped through obstruction like formwork,
reinforcement, inserts, cutouts etc. can segregate very easily even if the drop is small. As the concrete hits any obstruction the mortar from the mix will
stick to the obstructions and the coarse aggregate will roll down causing honeycombs.

Concrete should be placed in uniform layers. Concrete should not be placed in large heaps or in sloping layers. In mass concrete the layers having thickness more than 500 mm are not recommended as the weight of concrete of the top layer makes it almost impossible to vibrate and get the entrapped air from the lower level to surface out. Generally in reinforced concrete the layer thickness of 150 mm to 300 mm is most recommended. The sloping layers can cause segregation , if the mix is not cohesive.

Concrete placing and compacting operations should be compatible with each other for smooth concreting operations.

When good finish is required it is recommended to place concrete at a rate greater than 2m height per hour.

Delay and interruptions cause cold joints, honeycombing, porosity , discolouration and therefore must be avoided at all costs.

The fresh layer placed should be well 'knit' with the previous layer. Each layer must be placed only after the lower layer is properly compacted and before lower layer is still responsive to compaction. This will prevent cold joints.

Massive pours requiring continuous pouring of concrete for hours together have to be properly planned and co-ordinated. This is necessary to avoid any
stoppage which may result in stiffening of concrete, cold joints or segregation.

Concrete should be deposited near its final location and not allowed to flow laterally for more than a short distance unless it is clearly evident that the
whole mix is moving without segregation.

To attain good durabil ity and finish , concrete must be placed directly in corners and ends of walls. The flow of concrete, if any, should be from
the corners and ends rather than towards them.

In slab construction fresh concrete should be placed against the hardened concrete of the slab portion cast earlier.

Under no circumstances should the first layer placed on hardened concrete on rock surface be more than 500mm. In walls, deep beams and columns
first layer should be preferably rich in mortar.

The top layer when placed should be more or less kept in level.

Near the top of a deep lift the concrete tends to become wetter, since water, in placed concrete , rises and migrates upwards. To avoid excessive water
surfacing at the top as the level of concrete rises a drier consistency concrete should be used. This normally causes no problems for placing and compaction because the concrete in the higher levels can be easily placed and compacted than the concrete at lower level.

If bleed water rises to the surface it is probably due to deficiency of finer fines in sand or due to high slump. Such bleeding, either of clear water or water and fines, results in the weak top surface of concrete which is unsuitable either as a construction joint surface or for exposure to weather or traffic.

Continuity of concrete placement is extremely important, however, due to many unforeseen reasons the concrete placing activity may get interrupted.
Whenever such interruption occurs, provisions should be made to protect the concrete face which will later receive fresh concrete when placing  resumes.

The concrete face should be immediately covered with wet (not dripping) burlap. If the delay is long enough to require construction joint, then proper
joint must be prepared by chipping and cleaning before placing resumes.

All areas where concrete is to be placed and compacted must be easily visible. Deep sections should be provided with adequately sized inspection windows on the form face and with proper lighting. 

On wide slab work working platforms that span the entire width of the slab should always be provided to prevent any disturbance on freshly placed
concrete.

Muddy boots or feet should at all times be kept out of fresh concrete. 

It is a must to continuously check forms, bracing, props and support during placing. Any adjustment or additional support required must be immediately
provided to avoid damage to freshly placed concrete and at times to prevent a serious accident.

The temperature of concrete while placing should be generally less than 30 degrees C. For massive pours concrete placing temperatures as low as 1 0
to 20 degrees C are recommended depending on the shape and size of the structure required to be constructed.

In warm weather all mixing and conveying equipment for concrete should be painted with reflective or light coloured paints.

Subgrade, forms and areas where concrete is to be placed should be kept cool with fog sprays before concreting.

Placement of concrete should preferably be done during early morning and late night hours. 

All flat and exposed concrete surfaces should be covered with protective covering immediately after finishing. It is preferable to cover the concrete 
surface with plastic (White) sheets. As soon as concrete surface has hardened sufficiently, wet curing can commence by allowing water between the concrete surface and the plastic sheet.

for more details visit or download

 concrete transportation and placement fd.pdf

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