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Engineering of connections in structures, analysis of gas pressure welding machine

Evaluation of performance, advantages, and international standards in gas pressure welding of rebar

Introduction of gas pressure welding machine (GPW)

The gas pressure welding machine (GPW), is one of the solid-state joining methods that was first developed in the 1930s in Japan and the United States. According to the report of the Japan Society of Civil Engineers, the rebar forging method, due to its “high mechanical stability and uniform behavior under cyclic loads”, quickly became standardized in Japanese infrastructure projects.

In GPW, the two ends of the rebar, without melting, are joined together only by uniform flame heating and controlled axial pressure. This process creates a homogeneous joint, free of impurities and without a melted zone; something that is referred to in Japanese articles as a Solid-State Homogeneous Joint.

In the Japanese standard JIS Z 3891, it is stated that the appropriate temperature for GPW must be in a range where the steel enters a “stable plastic state”; usually about 1150 to 1250 degrees Celsius. At this temperature, the crystal structure of the steel softens and, under hydraulic pressure, the two ends of the rebar interpenetrate.

An article from the Welding Journal (AWS) explains that in GPW, unlike arc welding, “no molten pool is formed and therefore the risk of solidification cracks, porosity, and slag inclusions does not exist.” These characteristics have made GPW used as the main method of rebar joining in critical projects such as steel bridges, railway lines, earthquake-resistant structures, and urban tunnels.

Technical advantages of gas pressure welding (GPW)

1. Higher strength than the base rebar

In tests at the Tokyo Institute of Technology, the GPW joint in tensile testing usually fails in the rebar region, not at the joint location. This means the joint is stronger than the rebar itself.

2. Excellent behavior under seismic loads

Studies by the JSCE Earthquake Engineering Committee show that GPW joints, under cyclic loading, have “uniform behavior without stress concentration.” This is vital for countries like Japan.

3. Elimination of lap splices and reduction of steel consumption

In a report by the Korea Infrastructure Safety Corp, it is stated that the use of GPW in tunnel projects has reduced steel consumption by up to 12%.

4. Stable and inspectable quality

In Japanese standards, the quality of the joint is controlled through inspection of the upset shape and non-destructive tests. Due to its high repeatability, this method is ideal for high-volume projects.

Components of the gas pressure welding machine (GPW Welding Machine)

1. Welding unit (Burner Unit)

• Special heating torch, capable of producing neutral and reducing flames
• Multi-hole burner with U-type flame pattern
• Four-edge nozzle for uniform heat distribution
• Precision oxygen and acetylene regulators
• High-pressure hoses with EN 559 standard

2. Pressure unit (Hydraulic Unit)

• Hydraulic jack with short stroke and high pressure
• Hardened jaws with self-centering design
• Pressure control system with ±5% accuracy
• Holding clamps with JIS standard

3. Cold cutting unit (Rebar Cutter)

• Reduction gearbox
• Holding clamp for 90-degree cutting of the rebar end
• Air-cooled cutting blade with tungsten carbide inserts

Steps of performing forging welding based on JIS Z 3891 standard

1. Surface preparation: cold cutting of both rebar ends and squaring them
2. Alignment: precise positioning in the jaws
3. Initial heating: bringing the joint area to the plastic state
4. Application of initial pressure: start of surface upsetting
5. Final heating and main pressure: formation of the upset and creation of the solid-state joint
6. Natural cooling: without thermal shock and without using water

This cycle usually takes between 50 and 130 seconds depending on the diameter and grade of the rebar.

Global applications of gas pressure welding

According to the Japan Railway Technical Research Institute, more than 90% of rebar joints in railway and building projects in Japan are performed using GPW. In South Korea, this method is also accepted as a standard in tunnel and high-rise building projects.

Standard for training and testing of gas pressure welding

Training of GPW operators is usually carried out based on Japanese and European standard guidelines and includes equipment familiarization, pressure and temperature settings, identification of the correct upset shape, and performing destructive and non-destructive tests on welded samples. Preparing a standardized training and skill testing procedure is essential to ensure joint quality in construction projects.

Conclusion

The gas pressure welding  (GPW) is a solid-state, standardized, and highly reliable technology that has its roots in Japanese research and is used today worldwide for heavy projects. By creating a homogeneous joint, high strength, suitable seismic behavior, and reduced steel consumption, this method is one of the best options for modern civil engineering projects.