1 Scope of Application
Rebar coupler connection is applicable for the connection of main reinforcement (threaded steel) in bridge pile foundations, piers (including thin-walled hollow piers), etc. Compared with rebar welding, rebar coupler connection can accelerate the construction progress, reduce the construction difficulty for operators, and thereby ensure the qualification rate of the spacing between main rebars and the rebar cover.

2 Process Flow Chart

Note: This flow chart reflects a qualified construction process for straight thread rebar coupler connection. If a certain process is unqualified after self-inspection, the next process shall not be carried out.

3 Main Technical Parameters
3.1 Rebar connection shall preferably adopt welded joints or mechanical connection joints.

3.2 The connection joints of tensioned reinforcement shall be arranged at positions with smaller internal forces and shall be staggered. For welded joints and mechanical connection joints, no two joints shall be allowed on the same rebar within the joint length section. The percentage of the cross-sectional area of joints in the total cross-sectional area of tensioned reinforcement arranged within the joint length section shall comply with the provisions in Table 1.

Notes: 1. The welded joint length section refers to the range of 35d (d is the rebar diameter) but shall not be less than 500mm. 2. Fewer joints shall be arranged on the same rebar. 3. The welded joints of tensioned reinforcement at the connections of prefabricated components shall not be restricted by this requirement.

3.3 Couplers shall meet the following requirements:

3.3.1 The material of couplers and lock nuts shall preferably be high-quality carbon structural steel or alloy structural steel, with no cracks or other defects on the surface.

3.3.2 Possess product quality certificates. Finished couplers shall be packaged and stored by category without confusion or rust.

4 Process Requirements
4.1 Support Arrangement: The center line of the threading machine's main shaft shall be aligned with the center line of the rebar to be processed placed on the support. Meanwhile, the erection of the support shall ensure that the rebar is placed horizontally.

4.2 Rebar Cutting: Rebar cutting can be performed using rebar cutting machines, abrasive wheel cutting machines, etc., and gas cutting shall not be allowed. During rebar cutting, the end face of the cut shall be perpendicular to the rebar axis, without horseshoe shape or deflection. If the end is not straight, it must be straightened before cutting.

4.3 Rebar Threading: For qualified thread heads, a plastic protective cap shall be put on one end in a timely manner, and a coupler of the same specification shall be screwed onto the other end and tightened. They shall be stacked neatly by specification for use.

4.4 Rebar Connection4.4.1 Before rebar connection, first recover the plastic protective caps on the thread heads and the plastic sealing covers at the ends of the couplers, and check whether the thread is intact and clean. If debris or rust is found, clean it with an iron brush.4.4.2 After connecting each joint, mark it with paint immediately to prevent missing tightening.

4.5 Direct Rolling Thread Processing: Directly roll threads using rebar rolling machines (models: GZL-32, GYZL-40, GSJ-40, HGS40, etc.). This method features simple thread processing and low equipment investment; however, the thread accuracy is poor, and the thread diameter varies due to uneven rebar thickness, which affects construction.

4.6 Rib Extrusion and Rolling Thread Processing: First pre-press and flatten the transverse and longitudinal ribs of the rebar using special extrusion equipment rollers, then roll the threads. The purpose is to reduce the impact of rebar ribs on the formed threads. This method improves thread accuracy to a certain extent but still cannot fundamentally solve the impact of rebar diameter variation on thread accuracy, and two sets of equipment are required for thread processing.

4.7 Rib Stripping and Rolling Thread Processing: Use rebar rib stripping and rolling machines (models: GHG40, GHG50) to first strip the transverse and longitudinal ribs of the rebar, so that the cylindrical diameter of the rebar before thread rolling reaches the same size, then perform thread rolling forming. This method has high thread accuracy, stable joint quality, fast construction speed, and moderate price, with broad development prospects. The processing dimensions of the rib stripping and rolling heads shall comply with the provisions in Table 1. 

The processing length of the thread head is 1/2 of the length of the standard coupler, with a tolerance of +2P (P is the pitch). Operators shall inspect the thread head processing quality in accordance with the requirements of Table 1. For every 10 thread heads processed, inspect once with go-no-go ring gauges (Figure 1). Qualified thread heads after self-inspection shall be randomly sampled and inspected by quality inspectors. The thread heads produced in one working shift shall be taken as one acceptance batch, with a random sampling rate of 10% and no less than 10 pieces. If the qualification rate is less than 95%, double sampling shall be conducted. If the qualification rate in the re-inspection is still less than 95%, all rebar thread heads shall be inspected one by one, unqualified thread heads shall be cut off, the cause shall be identified, and threads shall be reprocessed.

4.8 Rolled Straight Thread Couplers: The connecting couplers for rolled straight thread joints are made of high-quality carbon structural steel. The types of connecting couplers include: standard type, positive and negative thread type, reducing type, adjustable type, etc., which are the same as the types of upsetting straight thread couplers. The specifications and dimensions of the connecting couplers for rolled straight thread joints shall comply with the provisions in Table 2.

5 Inspection, Maintenance and Upkeep
After the completion of rebar connection, the tightening torque shall comply with the requirements of Table 3.

6 Joint Quality Inspection
6.1 When applying rolled straight thread joints in projects, the technical providing unit shall submit a valid type test report.

6.2 Before the start of rebar connection operations and during construction, joint connection process inspection shall be conducted for each batch of incoming rebars. The process inspection shall meet the following requirements:

 (1) The number of joint test pieces for each specification of rebar shall not be less than 3; 

 (2) Tensile strength test shall be conducted on the base metal of the rebars for joint test pieces;

 (3) The tensile strength of each of the 3 joint test pieces shall not be less than the standard value of the tensile strength of the corresponding grade of rebar, and shall also not be less than 0.9 times the actual tensile strength of the rebar base metal.

6.3 On-site inspection shall include tightening torque inspection and uniaxial tensile strength test. For structures with special requirements for joints, the corresponding inspection items shall be separately specified in the design drawings.

6.4 Use a torque wrench to sample and inspect the construction quality of joints according to the joint tightening torque values specified in Table 9-74. The sampling quantity is: 15% of the number of joints for beams and column components, and the number of sampled joints for each component shall not be less than one; for foundation, wall, and slab components, every 100 joints shall be taken as one acceptance batch, and less than 100 joints shall also be taken as one acceptance batch, with 3 joints sampled per batch. All sampled joints shall be qualified; if one joint is unqualified, all joints in the acceptance batch shall be inspected and tightened one by one.

6.5 The uniaxial tensile strength test of rolled straight thread joints shall be conducted by acceptance batch. Under the same construction conditions, joints of the same grade, type, and specification using the same batch of materials shall be inspected with 500 joints as one acceptance batch. If all uniaxial tensile tests of ten consecutive acceptance batches pass the first sampling inspection on-site, the number of joints in the acceptance batch can be expanded to 1000.6.6 For each acceptance batch, 3 test pieces shall be randomly sampled from the engineering structure for uniaxial tensile test. When the tensile strength of all 3 test pieces is not less than the strength requirement of Class A joints, the acceptance batch shall be judged as qualified. If the tensile strength of one test piece does not meet the requirement, double sampling shall be conducted for re-inspection. There are three failure modes for the uniaxial tensile test of rolled straight thread joints: rebar base metal fracture, coupler fracture, and rebar slipping from the coupler. Any failure mode shall be judged as reasonable as long as the strength requirement is met.
7 Notes
To ensure the quality of straight thread rebar coupler connection, the wrench used for tightening the coupler must be a (digital display) torque wrench, and ordinary wrenches shall not be used. When tightening with a torque wrench, if the coupler does not move and makes a "click" sound, it indicates that the connection is qualified; if there is no sound and the coupler moves, it indicates that the tightening is loose and the connection is unqualified.