Key Construction Control Points for Reinforcement Cages in Bored Cast-in-Place Piles

In bored cast-in-place pile construction, the reinforcement cage is a core load-bearing component. Its fabrication quality, storage, lifting, positioning, and installation directly affect pile integrity, bearing capacity, and durability.

Based on extensive site experience and engineering practice, this article systematically summarizes the key technical points for reinforcement cage fabrication, storage, lifting, positioning, and anti-floating control, with a special focus on the application of mechanical rebar coupler connections, as promoted by GLUS.

1. Reinforcement Cage Fabrication

1.1 Dimensional Tolerances (Reference Standard)

According to the Specifications for Inspection and Evaluation of Highway Engineering Quality, the allowable fabrication tolerances for reinforcement cages are as follows:

1.2 Fabrication Method & Structural Reinforcement

• Reinforcement cages shall be fabricated using the reinforcing ring forming method, with reinforcing rings arranged at intervals of approximately 2 m

• Reinforcing rings shall be placed inside the main bars

• Triangular internal braces are recommended to enhance rigidity

• Main bars shall be spot-welded to the outer side of the reinforcing rings

• Main bars must be perpendicular to reinforcing rings before tying stirrups

1.3 Welding Requirements for Main Bars

• Main bar lap welding length: 10D (double-sided weld)

• Due to incomplete weld formation at start and end points, it is recommended to:

  • Increase weld length by 10 mm

• Welding joints shall be staggered, with spacing meeting code requirements

• Reinforcing rings welded by butt welding show good performance and are recommended

Common quality risks:

• Incomplete weld penetration

• Burn damage to main bars due to excessive current or poor welding skills

GLUS Recommendation
Project teams should:

• Organize welding skill assessments

• Avoid high-current welding aimed only at speed

• Assign clear responsibility to team leaders

2. Segmented Fabrication of Reinforcement Cages

Due to:

• Large cage length and weight

• Limited drilling rig mast height (typically ≤ 10 m)

Reinforcement cages are generally fabricated in segments.

Key requirements:

• Segment length preferably matches standard rebar length

• Segment length not less than 6 m

• 50% of joints shall be staggered

• Rebar axes must be aligned on the same centerline

• Bottom bars may be slightly flared (bell-shaped) to:

  • Prevent tremie pipe hooking

  • Reduce cage floating

3. Application of Mechanical Rebar Coupler Connections

3.1 Why Use Coupler Connections?

Traditional welding methods result in:

• High rebar waste (10D lap length)

• High labor and electricity costs

• Quality variability

Mechanical coupler connections, such as parallel thread couplers, offer:

• Stable and controllable connection quality

• Reduced steel consumption

• Faster installation during cage lowering

 Field data shows that mechanical connections can reduce overall cost, depending on project conditions and skill level.

3.2 Key Quality Control Points for Coupler Connections

To ensure structural performance, attention must be paid to:

1. Threaded end processing

   • Rebar ends must be cut flat

   • Thread length must meet design requirements

2. Thread accuracy

   • Threads must not be too tight or too loose

   • Avoid over-cutting

3. Alignment control

   • Prevent one-sided engagement (one end more, one end less)

   • Mark rebars before assembly for consistency

4. Pre-assembly

   • Ensure sufficient site space for axial alignment

   • Number main bars to avoid mismatch

  Improper alignment may lead to:

• Reduced tensile strength

• Failure to meet design requirements

4. Sonic Logging Tube (CSL Tube) Installation

4.1 Tube Specifications

• Standard tube lengths: 9 m / 6 m / 3 m

• Typical specification: Φ50 × 1.2 mm

4.2 Connection Methods

Recommended: Bolt-and-nut mechanical connection

Advantages:

• Fast installation (over 5× faster than welding)

• No special equipment required

• No hot work on site

• Reduced safety risks

• Lower cost and material loss

• Not affected by weather conditions

4.3 Installation Requirements

• Tube length shall match cage length

• Bottom tube shall extend 200–300 mm below cage bottom

• Tubes must be filled with clean water

• Seal with dedicated caps

• Prevent blockage during concreting

5. Concrete Cover Control

Recommended Method: Concrete Roller Spacers

• Spacer thickness = design concrete cover thickness

• Arrangement:

  • 4 spacers every 2 m, evenly distributed

• Welded to main bars

Functions:

1. Ensure cage centering

2. Control concrete cover thickness

3. Prevent scratching of borehole wall

 During lowering, install spacers progressively to avoid damage.

6. Storage of Reinforcement Cages

• Store cages on level, dry ground

• Place wooden blocks of equal height under reinforcing rings

• Prevent contact with soil

• Arrange segments in order

• Attach labels indicating:

  • Segment number

  • Length

• Protect from rain and corrosion

7. Lifting, Lowering & Positioning Control

7.1 Anti-Deformation Measures

• Use temporary longitudinal stiffeners (e.g. wooden poles)

• For long cages:

  • Bind two straight wooden poles inside the cage

7.2 Lifting Method (Two-Point Lifting)

• First lifting point: lower part of the cage

• Second lifting point: between midpoint and upper third

• Lift sequence:

  1. Lift lower point slightly

  2. Lift both points together

  3. Stop lower point once cage leaves ground

  4. Continue lifting upper point until vertical

 Do not drag the cage on the ground.

7.3 Cage Positioning

• Slowly lower cage vertically

• Avoid collision with borehole wall

• Temporarily support cage at reinforcement rings during segment welding

• Align upper and lower segments on the same axis

Final positioning:

• Use surveyed pile top elevation to back-calculate cage position

• Verify repeatedly before final welding

8. Prevention of Reinforcement Cage Floating During Concreting

Main Causes:

1. Tremie pipe hooking the cage during lifting

2. Upward concrete impact force during rapid pouring

Control Measures:

• Control tremie pipe position:

  • Bottom ≥ 3 m below cage bottom

  • Not higher than 1 m above cage bottom

• Slow down pouring rate when concrete reaches cage zone

• Maximum recommended rise rate:

  • ≤ 0.5 m/min (depending on pile diameter)

• Adjust tremie embedment to increase concrete confinement

• Control concrete slump and ensure continuous pouring

If cage floating is detected:

• Immediately reduce pouring speed

• Use “slow lift – quick drop” tremie operation to settle cage

9. Common Problems & Preventive Measures

Common issues include:

• Cage floating

• Cage falling during lowering

• Misalignment

• Deformation

• Blocked CSL tubes

Key preventive actions:

• Strengthen lifting points

• Improve weld quality

• Ensure proper lifting method

• Use equal-length lifting slings

• Never use folded two-rope lifting

Safety during lifting is critical. Accidents have occurred—safety first, always.

GLUS Engineering Value

At GLUS, we believe that reinforcement cages are not just steel assemblies—they are structural safety systems.

By combining:

• Mechanical rebar coupler technology

• Standardized fabrication control

• On-site construction guidance

GLUS helps contractors:

• Improve connection reliability

• Reduce welding risks

• Enhance construction efficiency

• Achieve stable, inspectable quality in bored pile projects

Contact GLUS for rebar couplers, threading equipment, and integrated reinforcement connection solutions for foundation engineering.