Ensuring high construction quality at beam-column joints demands careful oversight from construction management. Poor control in this area can directly compromise the overall structural integrity. Compared to other components, what key issues should be addressed when managing quality at beam-column joints?
Disclosure and Preparation Work
(1) Conduct technical briefings ahead of time, emphasizing critical steps such as pouring, prefabrication, assembly, and compaction within the core zone of beam-column joints. This proactive approach helps eliminate uncertainties related to pouring in these key areas.
(2) Develop specialized and effective methods for concrete pouring and vibration tailored to the project’s specific conditions. Avoid blindly copying plans from other projects, although some fundamental principles may be referenced appropriately.
(3) Strictly control relevant parameters to ensure structural performance post-pouring. Use building materials scientifically, and rigorously monitor the mix ratio parameters of ready-mixed concrete. Since high-strength concrete involves high cement content, water-cement ratio, sand content, and slump, select vibration techniques carefully to guarantee concrete interface compaction.
Main Reinforcement and Hoop Reinforcement Installation
(4) Before pouring, securely fix both the main reinforcement and hoop reinforcement, applying spot welding if necessary. If hoop reinforcement is not firmly tied to the main reinforcement, collisions during pouring, or loose fixation to formwork, can compromise reinforcement stability and the structure’s integrity. During concrete pouring and subsequent vibration, ensure that construction equipment does not contact the main reinforcement. After pouring, reinforce the stability of the main reinforcement and formwork, and consider welding support reinforcements.
(5) Prior to installing beam reinforcement, insert the entire skeleton into the column’s longitudinal reinforcement and rest it on floor formwork using wooden pads. Then thread and tie the beam longitudinal reinforcement. This method ensures that when the beam settles, the node area skeleton moves with it without deformation or separation. It also maintains proper stirrup spacing and quantity in the node area, meeting specification requirements effectively.
Reinforcement Anchoring
(6) Follow technical instructions strictly before anchoring steel bars, paying particular attention to the steel bar anchorage length specified in the design. Verify that anchorage lengths meet these requirements. Avoid reducing anchorage length to save costs, which could compromise joint quality. Due to dense steel bars and limited working space in node areas, some sites may be tempted to shorten anchorage lengths for convenience, which is inadvisable.
(7) When construction conditions are challenging, coordinate to adjust processes and create sufficient working space. For insufficient anchorage sections, add short bars or weld anchor plates, ensuring the hoop reinforcement’s bending angle matches the straight section length.
(8) Use double U-shaped hoop stacking or weld the hoops into closed loops. If stirrup quality or quantity is inadequate, the column’s longitudinal bars may shift, causing stirrup displacement and deformation. To facilitate construction, some sites reduce stirrup counts in node areas; however, it is crucial to maintain stirrup quantity, adjusting spacing or increasing diameter as needed based on construction conditions.
Steel Bar Cutting
(9) When cutting and preparing steel bars, consider adding several short bars at the same level as the stirrups. Their length should correspond to the stirrup height in the node area. First weld the stirrup openings, then weld the column stirrups with short bars at the designed spacing. These welds can be on one or both sides of the stirrups, forming a closed skeleton around the upper and lower openings.
Concrete Pouring
(10) Inadequate compaction during vibration can cause the node’s structural strength to fall below standards. Concrete strength at beam-column joints usually matches that of the column but should be 1-2 grades higher than the beam slab. If construction gaps exist at the beam bottom and beam slab and node concrete are poured simultaneously, the concrete strength may significantly decrease despite matching grades.
(11) Improper pouring can also cause cracks in the node area, often due to large strength differences between column and beam concrete, making joints prone to cracking. When strength differences are small, pour both at once based on beam strength. If the difference exceeds 10MPa, pour separately, completing within the initial setting time. During vibration, avoid touching steel bars; manual vibration can be used if necessary.
(12) To ensure quality in densely reinforced areas, take special measures during pouring:
- Adjust aggregate particle size, pour fine concrete matching the assembled structure’s strength grade, manually shovel the material, use a small vibrating rod, and assign a dedicated person to vibrate until the surface is smooth and free of bubbles.
- Loosen upper steel bars in advance, pull out some bars to create feeding gaps for the vibrator. Secure steel bars firmly after vibrator insertion and continue pouring.
- When pouring concrete with different strength grades, pour the higher-strength concrete first (usually a smaller volume), supplemented by tower crane buckets. Before initial setting, pour lower-strength concrete for beams and slabs promptly to meet design requirements.
Concrete Vibration
(13) When beam and column concrete strengths differ, pour and compact them separately. For high-rise buildings using premixed concrete pumps, pour and compact the core area of beam-column nodes layer by layer, leaving a 45° slope at the floor beam and slab. Before concrete sets, pump concrete for floor beams and slabs together.
(14) This method requires careful control of interfaces between high- and low-strength concrete to avoid cold joints. It is advisable to leave construction joints at the top and bottom of columns and beams to reduce pouring and compacting time for high-strength concrete in the core node area and prevent cold joint formation.
(15) For core areas with dense steel bars, use small insertion vibrators to enhance vibration, eliminate blind spots, and ensure concrete compaction and design strength.
Article source: Architectural Technology Magazine
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