22. Gas pressure welding of Rebar

1) Definition of gas pressure weldingIt is welding method directly of raw material itself to press under the condition which is

before heat melting(1,200°C~1,300°C) for junction part with oxy-acetylene gas to fix on

both side rebars to connect in carrying out of rebar concrete structure building. Also, it is

economic splice method than folded tying (lap splice), mechanical spiral tying(mechanical

splice) and also the strength of junction part is stronger than raw material.

※ Rebar splice is classified to folded tying (lap splice) , mechanical spiral tying(mechanical

splice), compressed tying(welded splice). Folded tying (lap splice) is commonly have used as

important method for rebars of small diameter before. However, Folded tying (lap splice) has

irrationality such as complexity of rebar arrangement and bigger material section and also the

increase of mold cost and so it is prohibited to folded tying (lap splice) for over D29 deformed

rebar on Korean concrete standard specification and mechanical spiral tying(mechanical splice),

compressed tying(welded splice) will be applied frequently. Therefore, compressed

tying(welded splice) is stronger and economical splice method than Folded tying (lap splice) if

rebar arrangement with density or rebar diameter is bigger and it is using for the rebar over D29.

2) Composition of automatic pressure welding equipment

<Welding units>① Oxygen pressure controller

② Acetylene pressure controller

③ Acetylene outlet hose

④ Oxygen outlet hose

⑤ Welding torch

⑥ Welding tip(4 edges)

<Compressor units>① Pump

② Pedal pump

③ Hydraulic pressure hose

④ RAM cylinder

⑤ External cylinder

⑥ Fixed clamp

⑦ Flexible clamp

⑧ Rebar

3) Working sequence to tie gas pressure welding The method of gas compressed tying(welded splice) is classified to manual gas

compression and automatic gas compression method and the principle is same but

difference is by manual or automatically for splicing.

① Grinding the section of rebar with flat by using grinder

② Install rebar for splicing to pressure welding equipment

③ Heat and pressure

- Heat rebar with gas mixed oxygen acetylene

- Heat machine will be used multi-hole burner

- Heat and pressure at the same time

- Don't heat till the temperature of rebar melting but heating to 1,300°C as rebar

exterior temperature (it is different from welding splice)

- Pressure to 300~400kg/cm2 per rebar 1cm

- Use hydraulic regularly for pressure machine

- It shows time-passage of standard heat, pressure power as above graph.

- If starting heat, pressure from A point, the section space will be decreased on B

point and finally adhere completely to D point.

- It makes inflation by increasing pressure between F-G to keep this temperature

< Time passage of heat, pressure power >

1st heat pressure

pressurepower

pressurepower

temperature temperature

time(second)

2nd heat pressure 3rd heat pressure

4) Required time to heat for pressure welding per diameter

5) Inspection of gas pressure weldingInstruction of welded part includes visual inspection, ultrasonic detecting test and

tensile test, etc.

Selection of inspection test should be determined in accordance with suggestions on

the drawings.

① Visual inspection

welding distortion, deflection, shape and dimension and any other defect can be

inspected with eyes.

② Tensile test

- In case of manual welding equipment, tensile test is conducted for sampled

specimen

- In case of automated welding machine, test piece is manufactured for tensile test

③ Ultrasonic detecting test

In inspection of welded part, visual inspection and destructive or nondestructive

test are run parallel in general.

Among these, destructive test not only takes times and costs but also had some

errors because only a few test pieces are sampled from the entire welding spliced

bars.

As a solution, reliable nondestructive test is conducted to find out defects from all

welded parts

Required heat time

40 ~ 50sec

50 ~ 60sec

60 ~ 1min 20sec

Rebar diameter

HD16

HD19

HD22

Required heat time

1min 15sec ~ 1min 45sec

1min 45sec ~ 2min 15sec

2min 15sec ~ 2min 55sec

Rebar diameter

HD25

HD29

HD32

6) Caution if pressure welding works

① The builder will get the admission of person in charge to select gas pressure

welding worker.

② You must check and maintenance inevitably before working for oxygen , acetylene

instrument and protect it from direct rays.

③ The compression section should be decreased by cutting of plane after deleting oil,

paint, cement piece, etc but to be cut the neighbor surface by grinder.

④ The final pressure of rebar discharge direction should be over 300kgf/cm2 per

dimension.

The diameter of compression protrusion part should be over 1.4times of rebar

diameter and diameter of compression protrusion part should over 1.2times and

the form should be smooth.

But , it is followed by in case it is fixed by shop drawing separately.