The concrete placing method will depend on the rate of concreting planned, type of structure, the quantity of concrete required to be poured in one go, the equipment or manpower available for placing and compacting concrete, formwork design, the time required for the initial setting of concrete, and even the ambient temperature under supervision.
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 after mixing.
- Moving heaped concrete manually or using a poker vibrator must be avoided.
- The concrete must be placed in proper layers and without the occurrence of cold joints.
- Whenever concrete is dumped or dropped, the direction of fall should be vertical. If not there are good chances of segregation occurring.
- 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 the rounded cross-section to avoid the accumulation of concrete in sharp comers. The slope should be sufficiently steep to permit the flow of concrete without requiring 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 discharge is likely to occur. This is prevented by providing a suitable drop chute at the discharge end of the belt conveyor.
- If the 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.
- Chutes and conveyor belts must be mobile enough to allow movement.
- Chutes and conveyor belts must be cleaned and flushed with water after the concreting is over.
- It is preferable not to drop the concrete from more than 1m in 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).
- 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 500mm 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 the surface out. Generally, in reinforced concrete, the layer thickness of 150mm to 300mm is most recommended.
- Concrete placing and compacting operations should be compatible with each other for smooth concreting operations.
- When a 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, and discoloration 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 the 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 coordinated. This is necessary to avoid any stoppage.
- 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 durability and finish concrete must be placed directly in corners and ends of walls.
- In slab construction, fresh concrete should be placed against the hardened concrete of the slab portion cast earlier.
- Under no circumstances the first layer placed on hardened concrete on the rock surface should be more than 500mm. in walls, deep beams, and columns the 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 the top as the level of concrete, 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 a 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 result in the weak top surface of the concrete which is unsuitable either as a construction joint surface or for exposure to weather or traffic.
- Continuity of concrete placement is extremely important, whenever an 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 a construction joint, then a 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 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 place concrete and at times to prevent a serious accident.
- The temperature of concrete while should be generally less than 30oC. For massive pours concrete placing temperatures as low as 10o to 20oC are recommended depending on the shape and size of the structure required to be constructed.
- 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 sheets. As soon as the concrete surface has hardened sufficiently wet curing can commence by allowing water between the concrete surface and the plastic sheet.
Concrete Placement for Massive Structures
For large equipment and structural foundations, it is recommended to place concrete in layers not exceeding 500mm. for mass concrete dams, it is required that the concrete is placed in 2500mm lifts in about 5 layers at a time.
Concrete must be placed without the occurrence of the cold joint. This requires the previously placed layer to be compactable and in unset condition, even after placing a fresh layer over it. Step placing method in mass concrete is generally recommended for proper concrete placement.
If concrete is required to be placed in deep sections and through layers of reinforcement, it is recommended that at predetermined locations, preferably at 1m to 2m spacing, adequate gaps be provided in the reinforcement so that drop chutes or pipe chutes or flexible hose of concrete placing booms can enter and concrete can be placed without obstruction. This will prevent segregation. If concrete is placed in conjected reinforcement areas, it will either segregate or it will not flow into the coverage areas. This is especially observed in beam-column junctions in both slender and massive structures. The reinforcement details and placement must be properly planned both at the design stage as well as at the construction stage to avoid problems during or after the concrete placement.
Plain and massive concrete can be placed easily using skips or buckets which is discharge in one go, from a height usually between 1m to 2m. This helps the concrete to slump and spread under its own momentum and avoids heaping up.
Concrete Placing for Slender Structures
Concrete Placing in Slender Columns
The following precautions are necessary:
- Concrete should be placed within formwork in one continuous operation, preferably at a constant rate between 1m to 2m vertically per hour. The rate of concreting however should match with the compaction method adopted.
- As far as possible, the column should be placed in one lift with the minimum number of joints.
- There is no limit to the height through which a cohesive concrete mix can be allowed to fall freely in a tremie or a pipe chute. However, tremie may be absolutely necessary, if the structure to be cast is very deep and/or densely reinforced.
- Care should be taken to ensure that concrete mix is not dumped in large quantities, but is gradually placed in small quantities. If mechanical modes of concrete transportation and placement are used, the concrete will have to deposit on a platform very close to the placing point and then gradually unloaded such that layers of 300mm are compacted at a time. At times similar method will have to be adopted for mechanically transported mixes as the slender section may not permit entry of concrete hose pipe.
- If concrete is placed through dense reinforcement or hits the formwork when it falls, it will not only damage the formwork and displace the reinforcement but will also segregate; resulting in honeycomb patches especially in the lower half of the structure. It is therefore preferable to start the concrete with a richer mix (more cement mortar) and then revert back to the normal mix for subsequent layers. The first layer above the already cast starter (kicker) is the most critical as generally the first batch of concrete while placing is most prone to segregation. If concrete is placed without segregation and compacted properly even densely reinforced columns can have a good appearance and surface finish.
- If concrete is to be pumped into the formwork, then the flexible hose must be lowered up to the bottom and then gradually raised as concreting proceeds. However, the rate of pumping has to be very gradual and care should be taken that the concrete is not rapidly built up thereby causing air entrapment in the layers below the freshly placed concrete.
- It is essential that concrete being placed and compacted is visible at the spot at which it is placed. Proper lighting if necessary should be provided to allow proper inspection.
- In columns of significant height, if the concrete has to be done in one go, it is very essential to have inspection windows at every 2m height interval, so that concrete can be inspected and compacted through these windows. Tremie pipe chute or lowering the flexible hose of concrete placing boom up to the bottom is a must to prevent segregation.
Concrete Placing in Deep Beams, Wall and Shells
Following precautions are necessary
- The concrete placing has to be planned right from the stage the RCC drawing is prepared in the design office. The reinforcement detailing, including the clear cover, should be such that it is possible to lower the concrete at the placing point without segregation.
- The concrete equipment operators must be explained the sequence of pour and the locations through which the concrete has to be placed so that concrete does not segregate.
- The concrete must be placed as uniformly as possible avoiding heaps and inclined layers. If skips are used, the discharge from the skip is required to be controlled so that it is moved horizontally along the length of the wall and the concrete is spread in uniform horizontal layers.
- At stop-end (vertical construction joint) good compaction is essential. The stop-ends also must be rigidly fixed. Concrete placed and compacted without stop-ends is very inferior in quality. This is because there is a tendency to avoid compaction near the proposed construction joint so that the concrete does not flow further away.
- If cut-outs, openings, void-formers are introduced within the formwork and the concrete is required to be placed around it, the following precautions must be taken.
- Concrete must be placed and compacted from one side
- Continue placing and compacting the concrete from one side till the concrete emerges below the cut out on the other side.
- Place concrete on the other side now and compact the same.
- If concrete is not placed with care then voids will be formed.
- If the concrete wall is rectangular in shape or circular and is to be concreted without a vertical joint, then it is preferable to place concrete on either side of the starting point and move circumferentially away from the starting point on both sides till such time the entire layer is poured and compacted.
- The second layer has to be started once again from the same starting point as the previous layer.