Is this bent piece of rebar going to be used as reinforcing steel or for some other purpose?
If as reinforcing steel usually cold working is used with rebar bending machines to achieve specific minimum bending radii. I'm sure you know all this, but the purpose is to ensure no change is made to the metallurgical properties of the steel. Will the final shape comply with your local codes?
Heating a #7 or #8 bar enough to make a hook will damage the bar. My position has always been that reinforcement shall not be heated for the purpose of field corrections. The following is from advice by ARC, an Australian reinforcing steel provider.
"Heating and Bending
Hot bending is not a normal factory operation. It is more likely to be done on-site, generally with poor supervision and inadequate quality control. Since the strength of the steel will be reduced, uncontrolled hot bending is a dangerous practice.
As a general rule, heating Grade D500N bars of any type must be avoided at all times. AS3600-2009 Clause 17.2.3.1 (b) gives a maximum temperature for reinforcement as 600° C. At 600° C the bar is only just starting to change colour. If any colour change is observed whilst heating, the reinforcement should be discarded. If the bar is heated over 450º C, the steel is softened due to changes in the crystalline structure of the metal. Once heated over 450º C the yield strength of the bar is reduced to 250 MPa. The only practical method of monitoring heat in the bar is the use of heat crayons.
Galvanised bars should not be hot bent.
Using Bars after Heating
Overheating beyond 600°C will alter the structure of the steel. 450°C has been found to be a realistic limit because above this temperature the yield stress, while under load, reduces to 250 MPa."
A starter question would be where are you and what codes are in effect?
ACI 301 would be a reference if it is applicable. You are limited in the heat that may be applied to the bar and the heat that may be applied near existing concrete should the bar already be cast in place.
Is the desire to hot bend because the proper reinforcing bar was not sent to the field?
Onsite fabrication of bars of this size with hand tools using heat as a substitue for proper benders is not a good idea.
msquared, I'm sure there isn't a "fabrication shop" involved.... most likely just Bubba, a torch and a cheater pipe.
joe, the short answer is no. Why not just insist on properly sourced material from a reputable fabricator/supplier. If Bubba refuses, then tell him it's on him.
You might be able to accomplish a field-expedient bend with a conduit bender, at a muffler shop, or maybe even with a cheater pipe and the vise on the back bumper of the fitters truck. Cold bending is really the proper way.
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
Rebar is normally bent cold, using equipment intended for that purpose, either in a fab shop or out in the field, and to min. radii and configurations spec’ed by ACI. Heating and welding can be done, but both operations require tightly controlled temps., processes and methods, because the heat input can change the steel metallurgically and will also change Fy and Fult of the steel. There should be procedures written and followed for these operations. Heating, to bend anything in the field, should certainly not be done by some know-it-all who thinks he knows how to run a cutting torch, which is all too often the case in the field.
gte447f,
I agree, Saw first hand what a torch and a pipe and/or sledge hammer will do to a rebar. Rebar broke like glass. Made them replace the are effected and splice new bars. they were not happy since it involved removing formwork.
Here is a reference that is not a specification or guideline but a bit of research into the subject matter. Surprisingly this report recommends heating well above 700 degrees F and yet below 3300 degrees F. And that the most effective range is 1400 to 1500 degrees F. So too little heat is bad and too much heat is bad. Moreover, the report goes on to note that even in the recommended range there are reductions in performance. One can conclude that heating the rebar is crtical and is not likely achieved properly in the field or shop and thus should not be done as general rule.
Because rebar is not heat treated, it is unlikely that heating and cooling it will cause any loss of property. likely up past 700 or so degrees, anyway.
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