Rebar Threaded Coupler Testing

Rebar threaded connection sleeve inspection mainly conducts scientific testing and analysis on the dimension and tolerance testing, surface quality and appearance testing, material property testing, hardness testing, connection joint uniaxial tensile property testing, and connection joint high-stress repeated tension-compression and large deformation repeated tension-compression property testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Dimension and tolerance testing: This is the basic item of the sleeve testing, mainly testing the key dimensions such as the internal thread pitch diameters, major diameters, minor diameters, pitches, thread profile angles and lengths of the sleeves. According to the standards such as JG/T 163-2013, the dedicated thread plug gauges are used for the comprehensive inspection, combined with the high-precision callipers and micrometers for the individual dimension measurement, ensuring the matching precision with the rolled or upset threads at the rebar ends, meeting the connection strength requirements.

Surface quality and appearance testing: Whether the sleeve outer surfaces have the defects such as the cracks, laps and rust affecting the use is checked. The internal thread surfaces should be smooth and complete, without the missing teeth, damaged teeth, rust and oil stains. For the sleeves with the anti-corrosion treatments (such as the galvanizing and dacromet), the coating uniformity, adhesion and thicknesses also need to be checked, preventing the long-term durability of the connections from being affected by the corrosion.

Material property testing: Through the sampling of the sleeve raw materials, their mechanical properties and chemical compositions are tested. The mechanical properties mainly include the tensile strengths, yield strengths and elongations after fracture, ensuring that the sleeve body strengths are not lower than the strengths of the connected rebars. The chemical composition analysis aims to confirm the material designations, usually requiring the high-quality carbon structural steels and alloy structural steels, and controlling the contents of the harmful elements such as the sulfur and phosphorus.

Hardness testing: The Rockwell hardness testers or Brinell hardness testers are used to carry out the hardness testing on the sleeves. The reasonable hardness ranges can not only ensure that the sleeves have the sufficient strengths and deformation resistance to withstand the tightening torques and service loads during the rebar connections, but also avoid the brittleness caused by the excessively high hardness or the damages when biting with the rebar threads.

Connection joint uniaxial tensile property testing: This is the core property testing of evaluating the connection reliability of the sleeves. The sleeves and the standard-conforming rebars are tightened according to the specified torques to make the connection joint specimens, and the tensile testing is carried out on the universal testing machines. The testing items include the tensile strengths, residual deformations and total elongations at the maximum forces. It is required that the joint tensile strengths are not less than the standard values of the tensile strengths of the connected rebars, and have the high ductility.

Connection joint high-stress repeated tension-compression and large deformation repeated tension-compression property testing: Used to evaluate the performance of the sleeve connections under the dynamic loads such as the simulated earthquakes and wind loads or the large deformation conditions. Through the dedicated equipment, the repeated tension-compression loads of the specified times and amplitudes are applied to the connection joints. After the testing, the joints still need to meet the specified tensile strength requirements, and the brittle failure should not occur, to ensure the safety of the structures under the rare disasters.

Testing Scope

Standard connection sleeves: Suitable for the connections with the straight threads (such as the rib-removing rolling and direct rolling) or upset straight threads at the rebar ends, mainly used for the conventional position connections with the fully centered and non-eccentric rebars. The testing needs to cover all their specifications (such as Φ12-Φ50), and verify their adaptability with the rebars of the different strength grades (HRB400, HRB500, etc.).

Positive and reverse thread (adjustable) connection sleeves: Used for the occasions where the two rebars cannot be rotated or the connection lengths need to be adjusted, and the thread rotation directions at the two ends of the sleeves are opposite. During the testing, in addition to the routine items, the coaxiality of the threads at the two ends, the engagement lengths and the locking reliability in the actual adjustment and use need to be particularly paid attention to.

Reducing connection sleeves: Used to connect the rebars of the different diameters. The testing emphasis is to verify the structural design rationality at the diameter-changing parts, the stress concentration conditions of the transition areas, as well as the machining precision of the threads of the different specifications at the two ends, ensuring that the large and small diameter rebars can both achieve the equal-strength connection effects.

Flared connection sleeves: Mainly used for the scenarios such as the segmented hoisting and docking of the rebar cages. One end has the guiding flare, which is convenient for the fast centering and introduction of the rebars. The testing needs to evaluate whether the flare angles, guiding properties have the adverse effects on the connection strengths and fatigue resistance.

Lengthened, lock-nut and other special sleeves: The sleeve variants designed for the special engineering needs (such as the ultra-long connections and anti-loosening occasions). The testing scope needs to add the corresponding special tests according to their special design purposes, such as the torsional stiffness testing of the lengthened sleeves and the anti-loosening property testing of the lock-nut sleeves.

Raw material and finished product factory delivery inspection: The testing scope runs through the whole production process of the sleeves. Including the sampling inspection of the incoming raw materials, first article inspection and patrol inspection during the production process, as well as the batch sampling inspection before the finished product delivery. The factory delivery inspection usually includes the sampling testing of the dimensions, appearances, thread precision and tightening connection properties.

Testing Methods

Thread precision comprehensive inspection method: The calibrated go thread plug gauges and no-go thread plug gauges are used for the inspection. The go gauges should be able to be smoothly screwed into the full lengths of the sleeve threads, and the screwing amount of the no-go gauges should not exceed 3P (P is the pitch). This method efficiently and comprehensively judges whether the manufacturing tolerances of the internal threads are qualified, and is the most commonly used quality control method at the production sites.

Individual dimension precision measurement method: The high-precision instruments such as the thread micrometers, three-wire method, tool microscopes or profile scanners are used for the independent and precise measurement of the parameters such as the pitch diameters, pitches and thread profile half-angles of the internal threads. This method is mainly used for the process analysis, arbitration inspection or re-inspection when there is a dispute over the gauge inspection results, and can provide the specific dimension deviation data.

Mechanical property testing machine method: According to JGJ 107-2016 (Technical specification for mechanical splicing of steel reinforcing bars), the uniaxial tensile, high-stress repeated tension-compression and large deformation repeated tension-compression testing are carried out on the rebar connection joints on the microcomputer-controlled electro-hydraulic servo universal testing machines. Through the real-time acquisition of the load, displacement and deformation data by the transducers and software, the curves are drawn, and the strength, deformation and ductility indicators of the joints are precisely evaluated.

Torque-rotation angle method (construction tightening control): In the actual construction or simulated installation, the digital torque wrenches are used to tighten the sleeves. The rotation angles when reaching the specified tightening torque values are recorded, or the torque-rotation angle curves are monitored. This method is not only used to verify whether the construction meets the requirements, but also the curve patterns can indirectly reflect the smoothness and consistency of the thread fits.

Non-destructive testing method: For the important projects or the sleeves suspected of the internal defects, the methods such as the ultrasonic flaw detection (UT) or magnetic particle flaw detection (MT) can be adopted. The ultrasonic flaw detection is mainly used to detect the volume defects such as the inclusions and cracks inside the sleeves; the magnetic particle flaw detection is very sensitive to the linear defects such as the surface and near-surface cracks and laps.

Physical and chemical analysis method: The spectroscopic analysers are used for the fast quantitative analysis of the chemical compositions; the metallographic microscopes are used to observe the micro-structures of the material (such as the grain sizes and inclusion grades); the salt spray test chambers are used for the accelerated corrosion resistance testing of the anti-corrosion coatings. These methods guarantee the sleeve qualities from the material essence.

Testing Standards

Domestic Standards:

1. JG/T 163-2013, Coupler for rebar mechanical splicing

2. JGJ 107-2016, Technical specification for mechanical splicing of steel reinforcing bars

3. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

4. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

International Standards:

1. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Thread gauges (plug gauges and ring gauges): The dedicated standard measuring tools of testing the precision of the sleeve internal threads and rebar external threads. The go/no-go gauges are used for the fast comprehensive judgment, and the calibration gauges are used to calibrate the working ring gauges. The precision grades of the gauges must be higher than the precision grades of the threads to be tested, and need to be regularly sent to the metrology institutions for the verification, ensuring the reliability of their own precision.

Universal material testing machine: The core equipment of carrying out the mechanical property testing of the connection joints. The modern testing machines are mostly the electro-hydraulic servo or electronic type, equipped with the high-precision force transducers and extensometers, controlled by the computers and capable of the automatic data acquisition and processing. They need to meet the requirements of the JGJ 107 standard for the loading rates, control precision and data acquisition frequencies.

Repeated tension-compression testing device: The dedicated equipment used for the high-stress repeated tension-compression and large deformation repeated tension-compression testing of the connection joints. It can apply the tension-compression alternating loads of the specified frequencies, amplitudes and cycles according to the standard provisions, and is equipped with the high-precision force and displacement measurement systems, used to evaluate the seismic performance of the joints.

Hardness tester: The Rockwell hardness testers (HRC and HRB scales) or Brinell hardness testers (HBW) are commonly used. The appropriate scales need to be selected according to the estimated hardnesses of the sleeve materials and standard requirements. Before the testing, the instruments should be calibrated. During the testing, the tested surfaces of the sleeves need to be flat and clean, and the multi-point measurements are taken at the different parts to obtain the representative results.

High-precision dimension measuring instruments: Including the digital vernier callipers, outer diameter micrometers, inside diameter micrometers, thread micrometers, three-wire measuring sets, as well as the more advanced coordinate measuring machines (CMM) and optical profilometers. These equipment are used for the precise digitalized measurement of the sleeve inner and outer diameters, lengths and individual thread parameters, providing the data support for the quality analysis and process improvements.

Torque wrench and calibration instrument: The key tools of applying and controlling the tightening torques in the construction and quality inspections. Divided into the preset type, digital type and dial type. All the torque wrenches must be regularly calibrated with the torque calibration instruments, ensuring that their indication errors are within the allowable ranges (usually ±5%), to guarantee the tightening force consistency and reliability of the connection construction.

Spectroscopic analyser and metallographic microscope: The laboratory equipment used for the material composition and structure analysis. The portable spectrometers can quickly screen the material designations at the sites; the benchtop spectrometers and metallographic microscopes are used for the more precise quantitative analysis and structure rating, and are the important means of controlling the raw material qualities and heat treatment process effects.