In the construction of modern construction projects, the welding of steel bars is widely used. The commonly used welding methods for steel bars include flash butt welding, arc welding, and electroslag pressure welding. The welding quality of steel bars is related to the weldability and welding process of steel. A brief discussion on the key points of the construction technology of steel reinforcement welding is now made.

Welding process

The welding process includes arc ignition, arc stabilization, electroslag and top pressure, etc. Before welding, the rust within about 120mm of the end of the steel bar should be removed, clamp the clamp on the lower steel bar, and the upper steel bar upright clamp Secure in the movable electrode and fill the flux box with flux.

When manual electroslag electric pressure welding is used, the direct arc ignition method can be used. The upper and lower steel bars are first contacted, and after the welding power is turned on, the upper steel bars are immediately lifted by 2 to 4 mm to ignite the arc; then, continue to lift slowly The steel bar is several millimeters. After the arc is stably burned, it is gradually sent down as the steel bar melts, and is transferred to the electroslag process. After the steel bar melts to a certain level, the welding power supply is cut off and the top pressure is quickly carried out, and the cooling is 1~ After 3 minutes, you can open the flux box, retract the flux, remove the fixture, and knock off the slag.

Problem and cause analysis

1. Quality problems and phenomena

The length of the welding seam is not enough, the surface of the welding seam is not smooth, there are large dents and welding bead. There is an undercut phenomenon in the weld, the welding rod does not meet the requirements, the weld skin is not knocked off, and the axes of the two joining steel bars are inconsistent.

2. Reason analysis

(1) The welder is unskilled and has not obtained the welder examination certificate.

(2) The weld length of the inflatable mandrel was not measured after the welding was completed.

(3) The welding rod is unqualified, or the specifications of the selected welding rod are incorrect.

(4) After the welding is completed, the welding skin of the inflatable mandrel is not knocked off.

(5) Two welded steel bars have no pre-bending at their overlapping ends.

Precautions

1. Before operation, check whether all tools, electric welders, power switches and circuits are in good condition, whether the metal shell is safely and reliably grounded, whether the incoming and outgoing wires have a complete protective cover, and whether the incoming and outgoing wires are welded firmly with copper joint welding heads.

2. Each welding machine should have a dedicated power control switch and a secondary protection device. It is strictly forbidden to replace the fuse with other metals, and cut off the power supply after completion.

3. The steel bars are all machined and bent according to the design size and shape, and the steel bar processing site is hardened, and a rain shed is installed.

4. The steel bar stocking card must be approved by the technical supervisor before the material is opened. The steel bars formed by the material are listed in the order of the drawing number and stacked neatly. The storage site of the steel bar should take anti-corrosion measures.

5. Remove oil stains, paint, scales and rust on the surface before processing the steel bars to keep the surface clean. When the steel bar is straightened, the wire rod is straightened with a winch, and the straightening of the cold-drawn steel bar requires that the cold-drawn rate of the first-level steel bar is not greater than 2‰.

6. Rebar blanking strictly implements the batching list system. The items are filled in completely and accurately. The blanking must be technically explained in accordance with the construction drawings, and the blanking shall be carried out according to the required specifications and dimensions after the verification is correct.

7. After the steel bars are cut, the first steel bars of the same batch, the same type and the same model should be inspected and accepted. Only after the inspection is qualified, the cutting can be continued.

8. Flash butt welding is the first choice for longitudinal joints of steel bars. When arc welding is used, double-sided welding should be used as much as possible. The weld length is more than 5d for double-sided welding and 10d for single-sided welding. The steel bar joint complies with the regulations of the relevant steel bar welding and acceptance regulations.

9. Operators must be certified to work.

How to choose lashing, welding, machinery?

Whether the selection of the steel bar connection method is correct is directly related to the quality of the project. The three common steel bar connection methods are analyzed and compared in combination with specifications and standards: ① lashing and lap joint, ② welding connection, ③ mechanical connection to facilitate engineering design And the construction staff correctly choose it.

One, lashing and lap

The steel bar lashing connection shall meet the corresponding requirements of the national standard GB50010 "Specification for Design of Concrete Structures".

1 Applicable and non-applicable scope

In the “Reinforcement Connection” in the national standard GB50010 "Specifications for Concrete Structure Design", “the longitudinally stressed steel bars of axial tension and small eccentric tension members shall not be lashed and lapped; when the steel bars in other components are lashed and lapped, the The diameter of the tensioned steel bar should not be greater than 25mm, and the diameter of the compressed steel bar should not be greater than 28mm".

The non-applicable scope of steel bar lashing lap joints is: "For components that require fatigue calculation, the longitudinal tension steel bars shall not use lashing lap joints"; the longitudinally stressed steel bars of axial tension and small eccentric tension members; the diameter exceeds Tension steel bars of 25mm and compression steel bars with a diameter of more than 28mm should not be used.

2 advantages

General steel workers can operate under any environmental conditions, without additional processing, installation and testing equipment, construction speed is faster, and the quality is fully guaranteed.

3 shortcomings and shortcomings

1) There are twice as many joint steel bars in the overlap area, too much steel bars occupy the cross-sectional area, the steel bars at the joints of the members cannot be released, and the concrete is not compacted by pouring and vibrating;

2) Most steel is used;

3) The power transmission performance is the worst.

Two, welding connection

There are five types of steel welding connection: flash butt welding, arc welding, electroslag pressure welding, gas pressure welding and embedded steel submerged arc pressure welding, which should meet the national standard GB50010 "Specification for Concrete Structure Design" and the national standard JGJ18 "Rebar Welding and Acceptance" The corresponding requirements of the Regulations.

1 Applicable and non-applicable scope

According to the "reinforcing bars connection" in GB50010 "Specifications for Design of Concrete Structures", the longitudinal tension bars of the "reinforcing steel bars" in the "reinforcing steel bar connection" shall not use lashing lap joints or welded joints. They shall not be attached to the steel bar except for end anchoring. "Include accessories" shows that: it is applicable except that longitudinal tensile steel bars of components that require fatigue calculations are not suitable; but for fine-grained hot-rolled ribbed steel bars and ribbed steel bars with a diameter greater than 28mm, the welding should be determined by tests, and the residual heat It is not advisable to handle steel bars by welding.

2 advantages

Some welded joints are cheaper and can be welded at any position within the allowable range of joints (if welding conditions are available).

3 shortcomings and shortcomings

1) Special construction equipment and materials and electricity are required, and the energy consumption is the largest;

2) Strict requirements for personnel and certain requirements for the construction environment, which cannot be used anytime and anywhere: For example, electroslag pressure welding can only be used for vertical steel bar connection, but flash butt welding and air pressure welding can only be welded in the processing field and the connection length is limited. Steel bar

3) The quality cannot be fully guaranteed, unqualified products are prone to be produced, and there is a lot of waste.

Three, mechanical connection

There are three types of mechanical connection of steel bars: straight thread, taper thread and sleeve extrusion, which should meet the corresponding requirements of the national standard GB50010 "Specification for Design of Concrete Structures" and the national standard JGJ107 "Rebar Welding and Acceptance Regulations". Because the sleeve backlog, the tapered thread connection quality is unstable and the joint force cannot reach, it is rarely used now, and the most commonly used straight thread joint is used as the analysis and comparison object.

1 Applicable and non-applicable scope

All apply, no unavailable range is specified.

2 advantages

General reinforcement workers can operate under any environmental conditions, with the fastest construction speed, continuous and unlimited extension, the best force transmission performance, super strong connection, and stable and best quality.

3 shortcomings and shortcomings

The cost is more expensive.

Cost analysis

1) The cost of lashing connection is the cheapest, but it is actually only used in steel bars with a diameter of no more than 14;

2) The construction cost of welding connection is in the middle;

3) The construction cost of straight threaded joints is the most expensive, because the price of the finished sleeve is higher.

Conclusion suggestion

1) Except for mechanical connection joints, various joints have their own scope of application, and attention should be paid not to exceed the scope of use;

2) Although the steel bar straight thread connection is the most expensive, it is the most widely used because of the best quality and easy operation;

3) Although the price of welding connection is moderate and recommended, the market share is gradually decreasing due to high operating requirements, high energy consumption, and quality that cannot be fully controlled.

Source: Laixuewang, Welding Technology

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