Introduction of Rebar Mechanical Connection Technology

Reinforcement mechanical connection technology is a new type of rebar connection process, known as the "third-generation rebar joint" after binding and electric welding, with joint strength higher than the base metal of rebar, speed 5 times faster than electric welding, no pollution, and saving steel 20% and other advantages.
At present, the commonly used types of steel mechanical connection joints on the market are as follows:
1. Sleeve extrusion connection joint: the joint formed by the plastic deformation of the steel sleeve of the connecting piece and the tight bite of the ribbed steel bar through extrusion force. There are two forms, radial extrusion connection and axial extrusion connection. Due to the inconvenient on-site construction of the axial extrusion connection and the insufficient stability of the joint quality, it has not been promoted; while the radial extrusion connection technology, the connection joint has been widely used. The sleeve extrusion connection joints used in engineering are all radial extrusion connections. Due to its excellent quality, sleeve extrusion connection joints have been widely used in construction projects in my country since the early 1990s.
2. Tapered thread connection joint: a joint formed by the special tapered thread at the end of the steel bar and the tapered thread of the connecting piece. The birth of the taper thread connection technology overcomes the shortcomings of the sleeve extrusion connection technology. The taper thread head is completely prefabricated in advance, and the on-site connection takes up a short period of time. It only needs to be operated with a torque wrench on site, and there is no need to move equipment and pull wires. It is well received by various construction units. However, the quality of the tapered thread connection joint is not stable enough. Because the small diameter of the processed thread weakens the cross-sectional area of the base material, thereby reducing the joint strength, generally only 85-95% of the actual tensile strength of the base material can be achieved. There is still a certain gap between my country's tapered thread connection technology and foreign countries. The most prominent problem is that the pitch is single. The pitch of steel bars with a diameter of 16 to 40 mm is 2.5 mm, and the pitch of 2.5 mm is most suitable for the connection of steel bars with a diameter of 22 mm. The strength of the connection of too thick or too thin steel bars is not ideal, especially the tapered thread connection of steel bars with a diameter of 36mm and 40mm, it is difficult to reach 0.9 times the actual tensile strength of the base material. Many production units claim to have reached the standard strength of the steel bar base material, which is to use the super strong performance of the steel bar base material, that is, the actual tensile strength of the steel bar is greater than the standard value of the steel bar tensile strength. Because the taper thread connection technology has the characteristics of fast construction speed and low joint cost, it has also been widely used since the early 1990s. However, due to the large defects, it is gradually favored by straight thread connection joints. replace.
3. Straight thread connector
Equal-strength straight thread connection joints are the latest international trend of steel bar connection in the 1990s. The quality of the joints is stable and reliable, and the connection strength is high, which can be compared with the sleeve extrusion connection joints. Moreover, the taper thread joints are convenient and fast in construction. Therefore, the emergence of straight thread connection technology has brought a qualitative leap to steel bar connection technology. At present, my country's straight thread connection technology presents a scene of a hundred flowers blooming, and a variety of straight thread connection forms have emerged.
Straight threaded joints mainly include upsetting straight threaded joints and rolled straight threaded joints. These two processes adopt different processing methods to enhance the bearing capacity of the thread at the end of the steel bar, so as to achieve the purpose of equal strength between the joint and the base material of the steel bar.
1. Upsetting straight thread connection joint: the joint formed by the straight thread produced after upsetting the end of the steel bar and the thread of the connecting piece. Its craft is:
First, the end of the steel bar is upset by the upsetting equipment, and then the thread is processed. The small diameter of the thread is not smaller than the diameter of the steel bar base material, so that the joint and the base material can reach the same strength. Foreign upsetting straight thread connection joints have hot upsetting and cold upsetting at the end of the steel bar. Hot upsetting is mainly to eliminate the internal stress generated in the upsetting process, but the investment cost of heating equipment is high. For the upsetting straight thread connection joints in my country, the steel bar ends are mainly cold upsetting, which requires high ductility of the steel bar. For the steel bar with low ductility, the upsetting quality is difficult to control, and brittle fracture is easy to occur.
The advantages of upsetting straight thread connection joints are high strength, fast on-site construction speed, low labor intensity of workers, all straight thread heads of steel bars are prefabricated in advance, and on-site connection is an assembly operation. Its shortcoming is that the upsetting phenomenon is easy to appear during the upsetting process. Once the upsetting is off, the heavy upsetting must be cut off; the internal stress is generated during the upsetting process, and the ductility of the upsetting part of the steel bar is reduced, which is prone to brittle fracture. Thread processing requires two steps. Two sets of equipment are completed in the process.
2. Rolled straight thread connection joint: the joint formed by the direct rolling of the end of the steel bar or the extrusion (rolling) of the rib rolling or the stripping of the rib and the rolling of the straight thread and the thread of the connecting piece.
The basic principle is to use the characteristics of metal materials that are plastically deformed and then cold hardened to enhance the strength of metal materials, and only plastic deformation and cold hardening occur on the surface of the metal, while the performance of the original metal is still maintained inside the metal, so that the steel bar joint and the base metal reach equal strength.
At present, there are three types of common rolled straight thread connection joints in China: direct rolled thread, extruded (rolled) rib rolled thread, and stripped rib rolled thread. The thread accuracy and size obtained by these three types of connection joints are different, and the quality of the joints also has certain differences.
(1) Direct rolling straight thread connection joints:
Its advantages are: simple thread processing, less investment in equipment, but the disadvantage is poor thread accuracy and false thread phenomenon. Due to the uneven thickness and large tolerance of the steel bars, the processed thread diameters are inconsistent, which causes difficulties in on-site construction, makes the sleeve and the wire head inconsistent in tightness, and some joints appear to be pulled off. Due to the change of the steel bar diameter and the influence of the transverse and longitudinal ribs, the life of the thread rolling wheel is reduced, the additional cost of the joint is increased, and the on-site construction wearing parts are frequently replaced.
(2) Extruded (rolled) ribbed rolling straight thread connection joint:
This connection joint uses special extrusion equipment to pre-flatten the transverse rib and longitudinal rib of the steel bar, and then roll the thread, in order to reduce the influence of the steel rib on the accuracy of the forming thread.
Its characteristics are: compared with direct rolling, the accuracy of thread forming is improved to a certain extent, but it still cannot fundamentally solve the influence of the inconsistency of steel bar diameter on the accuracy of thread forming, and thread processing requires two processes and two sets of equipment to complete.
(3) Stripped rib rolling straight thread connection joint:
The process is to strip and cut the transverse rib and longitudinal rib at the end of the steel bar first, so that the diameter of the cylinder before the steel bar thread rolling reaches the same size, and then perform thread rolling forming.
The stripped rib rolling straight thread connection technology is a new type of steel bar equal strength straight thread connection developed by the Building Mechanization Research Branch of the China Academy of Building Research, which is the first at home and abroad. Through the type test, fatigue test, low temperature resistance test and a large number of engineering applications of the existing HRB335 and HRB400 steel bars, it is proved that the performance of the joint not only meets the performance requirements of the first-level joint in JGJ107-2003 of the "General Technical Regulations for Mechanical Connection of Reinforced Bars", but also realizes The same strength connection is achieved, and the joint also has excellent fatigue resistance and low temperature resistance. The joints have passed 2 million fatigue strength tests, and there is no damage at the joints. When tested at a low temperature of -40ºC, the joints can still achieve the same strong connection with the base metal. Rib stripping and rolling straight thread connection technology is not only suitable for the same and different diameter connections of HRB335 and HRB400 grade steel bars with a diameter of 16-40mm (recently expanded to a diameter of 12-50mm) in any direction and position, but also can be applied to fully required In concrete structures with high requirements for steel strength and butt joint ductility and concrete structures with high fatigue performance requirements, such as airports, bridges, tunnels, TV towers, nuclear power plants, hydropower plants, etc.
Compared with other rolled straight thread connection joints, stripped rib rolling straight thread connection joints have the following characteristics:
① Good thread profile, high precision, smooth tooth surface;
②The thread diameter is consistent, easy to assemble, and the connection quality is stable and reliable;
③The rolling wheel has a long service life and the additional cost of the joint is low. The rolling wheel can process 5000-8000 wire ends, which is 3-5 times longer than that of direct rolling;
④The joint has passed the fatigue strength test of 2 million times, and there is no damage at the joint;
⑤ Tested at -40C low temperature, the joint can still reach the same strength as the base metal, and has good low temperature resistance

Reprinted by Shanghai Hufeng Machinery Co., Ltd.