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Designers should consider possible congestions in reinforced concrete members while designing concrete structures. They should follow specifications of applicable codes to prevent congestions. Sometimes, one of the options to avoid congestion is by increasing the width of the concrete members to create adequate space.
Lap splicing of steel bars would double the reinforcement ratio at the splicing location. Therefore, use a staggered arrangement of bars to ensure that the increase in the reinforcement ratio does not occur at the same place. Alternatively, mechanical splicing or welded splicing are recommended for splicing positions of concrete elements.
Additionally, embedment has to be positioned in a manner that it does not hinder the concrete placement. Finally, the formwork designer should take placement of boxout, embedment, and formwork height into consideration.
Tips for Designers to Avoid Reinforcement and Embedment Congestion in Concrete Members
Arrangement of Reinforcement Steel Bars
The designer can consider the following tips to avoid congestions in reinforced concrete elements:
- Provide enough space between steel bars for pouring concrete into the forms according to the building code requirements.
- If viable, increase the size of concrete members to create adequate space for concrete placement.
- In heavily reinforced concrete members, it is possible to create access points through placed steel bars to permit concrete pouring.
- The adequate concrete cover should be maintained; otherwise, the aggregate in the concrete mixture would not be able to pass between formwork and steel bars.
- It should be remembered that a line on a drawing is not to scale. For instance, a nominal 25 mm bar on a drawing is nearly 30 mm in diameter in practice due to deformations and wires used for tying.
- Bear in mind that bars that are vertically aligned may deflect downward. As a result, the distance between two rows of steel bars would be less than that determined in the design.
- If steel congestion is expected in beam-beam and beam-column joints, draw the joint to scale and put steel dimensions provided by the manufacturer. Then, check whether 75 mm or 50 mm diameter poker vibrator can pass through reinforcements.
Splicing of Reinforcement Steel Bars
- Use mechanical splicing to avoid steel congestions if bar splicing is expected to cause congestions in concrete members.
- Welded connection is another means for splicing steel bars that can reduce steel congestion.
- Both mechanical and welded splicing techniques lead to a localized increase of the bars' diameter, which should be considered in the detailing clearances and spacings.
- Staggering of reinforcement at splicing positions can sometimes be an excellent solution to prevent congestions.
Formwork can cause congestions if they are not appropriately designed. The formwork designer should take form height, location of boxouts and embedment, concrete hose location, and number, location, and size of tie rods into consideration. The following tips can ease congestions in concrete members:
- Use external tie rods for narrow walls.
- Increase spacing in load-bearing members and use higher capacity ties and sheathing.
Embedment and Boxout Arrangement
It is required to consider concrete mix proportion, nominal maximum aggregate size, and detailing of reinforcement steel bars while embedment and boxout are arranged:
- Use void forms to prevent penetration of forms. If forms are 0.6 m or more in each direction, install concrete placement and vibration pipes. These pipes go through a formed blockout to allow proper concrete placement and vibration.
- Install concrete placement and vibration tubes through boxouts intended to be removed later and those greater than 0.6 m x 0.6 m.
- Stay-in-place boxouts like metal window frames cannot be cut to place concrete pouring and vibration tubes. Alternatively, use boxouts that have tolerances that permit them to be shifted.
- Create access holes at the bottom of boxouts that span from one form face to another one. Seal the hole when the concrete reaches the bottom of the boxout.
Congestion of reinforcement occurs if a high density of steel reinforcement is put in concrete members. This commonly occurs when steel bars are closely spaced, reinforcement of different concrete elements meet at a junction, and embedment are closely spaced in concrete members, etc.
Designers should follow building code requirements for structural concrete in terms of the placement and spacing of reinforcements, embedment, boxout, etc. Sometimes, designers may have to increase concrete member size to create space and prevent congestions.
Boxout may cause honeycomb formation in concrete since fresh concrete cannot go under the boxout. So, place concrete placement and vibration pipes through formed blockout to make way for concrete placement.
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