Rebar Mechanical Splicing Connector Testing

Reinforcing bar connection joint inspection mainly conducts scientific testing and analysis on the tensile properties, bending properties, fatigue resistance, macro metallographic examination, micro-structures and hardness, appearance and dimensions, 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

Tensile properties: This is the key mechanical indicator of evaluating the quality of the reinforcing bar connection joints. During the testing, the specimens with the joints are placed on the universal testing machines, and their tensile strengths, yield strengths and elongation after fracture are determined. The tensile strength of the joint specimens should not be lower than the specified standard tensile strength values of the connected reinforcing bars, and the fracture positions should be outside the joint length ranges, or the ductile fractures occur at the joints, to verify whether their force transfer properties are reliable.

Bending properties: Used to evaluate the ductility and integrity of the ribbed bar mechanical connection joints or welded joints under the bending deformations. Through the forward bending and reverse bending tests, whether there are the cracks, fractures or excessive deformations in the joint areas is observed. For the qualified joints, after reaching the specified bending angles and mandrel diameters, no transverse cracks should appear on their tensile surfaces, ensuring that they can work normally in the structural bending parts.

Fatigue resistance: Targeting the reinforcing bar joints in the structures bearing the repeated or alternating loads (such as the bridges and crane beams). This testing applies millions of cyclic loads, simulating the property degradation conditions of the joints under the long-term dynamic loads, evaluating their fatigue strength limits and judging whether the joints will have the fatigue fractures at the levels far lower than the static strengths. This is crucial for guaranteeing the structural durability.

Macro metallographic examination: Mainly used for the welded joints (such as the flash butt welding and electroslag pressure welding) and some mechanical connection joints (such as the sleeve extrusion joints). Through the preparation of the joint cross-section specimens, after the grinding, polishing and etching, the weld fusion conditions, heat-affected zone structures and whether there are the macro defects such as the incomplete penetration, slag inclusions and cracks are observed under the magnifying glasses, intuitively rating the internal quality of the joints.

Micro-structures and hardness: For the welded joints, especially the micro-structures of the heat-affected zones (such as the martensite and widmanstatten structures), the analysis is carried out to evaluate whether the welding thermal cycles cause the embrittlement. At the same time, the Vickers or Rockwell hardness testing is carried out, drawing the hardness distribution curves of the joint areas, ensuring that the hardness gradients are smooth without the abnormally high hardness zones, preventing the stress concentrations and delayed cracks.

Appearance and dimensions: This is the most basic on-site testing item. It is checked whether the appearances of the joint areas (including the welds, sleeves and threads) are complete and free of the defects visible to the naked eyes such as the cracks, overburning and serious deformations. At the same time, the dedicated gauges (such as the thread go/no-go gauges and callipers) are used to measure the key dimensions such as the sleeve lengths, outer diameters, thread precision and reinforcing bar insertion depths, ensuring that they meet the design and process specification requirements.

Sealing (only for the special connections): For the reinforcing bar connection joints applied to the prestressed duct grouting or with the special anti-corrosion requirements (such as the mechanical connections with the sealing rings), the air tightness or water tightness testing needs to be carried out. By applying the specified air pressures or water pressures to the closed cavities and maintaining for a certain time, the pressure drops or leakages are observed, to verify whether the sealing protection properties of the joints meet the standards.

Testing Scope

Mechanical connection joints: Mainly including the straight thread joints (peeling and rolling, direct rolling), taper thread joints and sleeve extrusion joints. The testing scope covers the sleeve materials, machining precision, thread profiles and precision, extrusion passes and indentation depths, joint tightening torques, as well as the final mechanical properties such as the tensile and bending, ensuring that they meet the requirements of the "equal-strength connections" or "high-ductility connections".

Welded connection joints: Covering the flash butt welded joints, arc welded joints (lapping plate welding, lap welding and molten trough lapping plate welding, etc.), electroslag pressure welded joints and gas pressure welded joints. The testing scope not only includes the tensile and bending properties of the joints, but also needs to comprehensively check the welding process parameters, welder operation levels, weld appearances and internal qualities (non-destructive testing) according to the welding process qualification requirements.

Lapped binding joints: Although not the physical connections, their force transfer properties depend on the bonding and anchoring of the concrete. The testing scope is mainly the on-site inspection of whether the lap lengths meet the design and specification requirements (related to the concrete strengths, reinforcing bar specifications and positions), as well as whether the densified arrangements of the stirrups in the lap areas are in place, ensuring the sufficient anchoring and splitting control abilities.

Different reinforcing bar specifications and grades: The testing covers the connection joints of the hot-rolled ribbed bars (such as HRB400 and HRB500) and hot-rolled plain bars of the various diameters and strength grades. The testing requirements of the different specifications and grades, such as the tensile strength ratios and elongation requirements, are adjusted according to the corresponding standards, ensuring that the connection properties of the joints match the performance requirements of the base reinforcing bars.

Testing Methods

Tensile testing method: In accordance with the standards such as GB/T 228.1 and the joint testing provisions of the relevant specifications, the joint specimens are loaded on the universal testing machines at the specified rates until the fracture. The tensile strengths, yield strengths, elongation after fracture and fracture positions are recorded, and whether the joints meet the strength requirements and fracture modes specified in the standards is judged.

Bending testing method: In accordance with the standards such as GB/T 232, the joint specimens are placed on the support rollers of the bending testing devices, and the pressures are applied through the bending mandrels to bend them to the specified angles. The forward bending tests the tensile surfaces of the joints, and the reverse bending tests the compressive sides, checking whether there are the cracks or fractures on the tested surfaces.

Fatigue testing method: The joint specimens are loaded with the alternating loads of the specified stress ratios and amplitudes on the fatigue testing machines (such as the electro-hydraulic servo fatigue testing machines). The loading frequencies are usually several hertz to dozens of hertz, and after the specified numbers of cycles (such as 2 million times), whether the specimens have the fatigue fractures and whether the residual static strengths meet the requirements are checked.

Metallographic examination method: The cross-section specimens are cut from the joints, and the metallographic specimens are prepared after the mounting, grinding, polishing and etching. The macro defects and micro-structures are observed under the stereomicroscopes and metallographic microscopes, and the hardness testing is carried out on the various areas (base material, heat-affected zone and weld) with the Vickers or Rockwell hardness testers, drawing the hardness distribution curves.

Non-destructive testing method: For the important structural welded joints, the ultrasonic flaw detection is used to detect the internal defects such as the cracks, lack of fusion and slag inclusions. The probes transmit and receive the high-frequency ultrasonic waves on the joint surfaces, and the positions, sizes and depths of the defects are judged according to the waveforms, amplitudes and positions of the reflected waves.

Appearance and dimension inspection method: The appearances of the joint areas are checked with the naked eyes or with the aid of the magnifying glasses, and the dedicated gauges such as the thread go/no-go gauges and torque wrenches are used to measure the thread precision and tightening torques. The sleeve lengths, outer diameters and reinforcing bar insertion depths are measured with the callipers, ensuring that they meet the process requirements.

Testing Standards

Domestic Standards:

1. GB/T 1499.2, Steel for the reinforcement of concrete — Part 2: Hot rolled ribbed bars

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

3. JGJ 18, Specification for welding and acceptance of reinforcing steel bars

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

5. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

6. GB/T 232-2010, Metallic materials — Bend test method

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

8. GB/T 4340.1-2009, Metallic materials — Vickers 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

Universal testing machine: The core equipment for the tensile and bending testing of the joints. Its measuring ranges and precision need to meet the standard requirements. Usually equipped with the hydraulic servo or electronic control systems, it can precisely control the loading rates, automatically record the load-displacement curves and calculate the key data such as the tensile strengths and yield points. It is the main basis of the mechanical property rating.

Torque wrench: Used to install and inspect the tightening torques of the mechanical connection joints (especially the threaded connections). Divided into the pointer type, preset type and digital display type. During the on-site installation and acceptance, the calibrated torque wrenches must be used, ensuring that the applied torque values are within the ranges specified in the standards. This is the key to guaranteeing the pretension forces and load-bearing capacities of the threaded connections.

Thread gauge (go/no-go gauge): Specially used to detect the machining precision of the reinforcing bar end threads and connection sleeve threads. Including the go gauges (T) and no-go gauges (Z). The go gauges should be able to smoothly screw into all the effective thread lengths, and the screw-in amounts of the no-go gauges should not exceed 3 thread pitches. This is the important tool of controlling the assembly quality and load-bearing uniformity of the threaded connections.

Ultrasonic flaw detector: Mainly used for the non-destructive testing of the welded joints of the important structures. The probes move on the joint surfaces, transmitting and receiving the high-frequency ultrasonic waves. According to the waveforms, amplitudes and positions of the reflected waves, whether there are the defects such as the cracks, lack of fusion and slag inclusions in the joints and their sizes and depths are judged. It has the characteristics of the high sensitivity and strong penetration.

Metallographic microscope: Including the stereomicroscopes (macro) and optical/electron microscopes (micro). Used to observe and analyse the prepared joint cross-sections, evaluate the weld fusion qualities, heat-affected zone widths, micro-structure morphologies and the various metallurgical defects, providing the direct micro evidence for the process improvement and failure analysis.

Hardness tester: The Vickers hardness testers or Rockwell hardness testers are commonly used. By measuring the hardness values of the different joint areas (base material, heat-affected zone and weld) and drawing the hardness distribution maps, the strength changes, hardening tendencies and possible softening zones of the material can be indirectly evaluated. It is the important auxiliary means of evaluating the rationality of the welding processes.