Kumar IronUpdated
In this guide
Why heat is a problem for TMT
TMT stands for thermo mechanically treated. The bar is cooled quickly after rolling, which gives a hard outer layer and a softer, ductile core. That is where its strength comes from.
If you heat the bar with a torch or a gas flame to bend it, the hard outer layer can soften or change. Makers commonly warn against heating above about 600 degrees Celsius. A bar heated red hot loses part of its strength, and you cannot see the damage afterwards.
Bend bars cold
IS 456 and IS 2502 require reinforcement to be bent cold, using a proper bending tool, around a pin of the right size. Hot bending is permitted only with specific approval of the engineer, and generally not for high strength bars.
Use a bar bending bench with a pin or a mandrel of correct diameter, and bend slowly in a single smooth motion. Do not hammer.
Bend radius basics
IS 456 asks for an internal bend radius of at least 4 times the bar diameter for high strength deformed bars (and 2 times for plain mild steel). In plain terms, the pin you bend around should be at least 8 times the bar diameter.
Standard hooks have their own rules. A 180 degree hook is commonly finished with a straight tail of at least 4 times the bar diameter, and stirrups in earthquake zones use a 135 degree hook with a tail of 6 times the diameter (minimum 65 mm), as per IS 13920. The structural drawing shows the exact detail.
| Bar diameter | Pin diameter (8d) |
|---|---|
| 8 mm | 64 mm |
| 10 mm | 80 mm |
| 12 mm | 96 mm |
| 16 mm | 128 mm |
| 20 mm | 160 mm |
Never re-bend a bar that has already been bent and straightened. The bend point is already stressed, and a second bend can crack it.
When welding is allowed
Welding TMT bars is not banned, but it needs control. The heat of the weld softens the tempered layer around the joint and can leave a brittle zone. It also depends on the steel chemistry, especially the carbon equivalent (CE). Higher CE means the bar is harder to weld without cracking.
Welding is accepted for special cases, such as joining bars larger than the lap limit, or connections designed by the engineer. It should be done by a qualified welder, with the correct electrode, and with the weld method specified by the engineer. IS 2751 and IS 9417 cover welded joints in reinforcement. Check the mill test certificate for the CE, and ask the maker if the grade suits welding.
- Do not tack weld cross bars to main bars in beams and columns to hold the cage. Use binding wire.
- Do not weld bars together in place of a lap, unless the engineer asked for it.
- Do not weld close to bends.
Lap vs weld
For normal houses, the right way to join bars is a lap, with the bars overlapping by the length on the drawing and tied with binding wire. The load passes through the concrete. A lap does not heat or harm the steel.
Weld only when the engineer specifies it. Our lap length guide explains typical overlaps.
Frequently asked questions
- Can I heat a TMT bar to bend it?
- No. Bend cold around a pin. Heating can destroy the tempered layer and you cannot see the damage.
- Can I weld two TMT bars end to end?
- Only if the engineer specifies it, with the right method and welder. Normally use a lap.
- What happens if I bend a bar back after bending?
- The bend point can crack. Avoid re-bending.
- Is it ok to straighten a bent bar?
- A slight bend is fine. Do not straighten sharp bends, and use another bar for the main reinforcement.