Flame cutting rebar 3

[FSS]

Can someone point me to a source that includes procedures on cutting rebar and why you should not flame-cut? I am trying to cite a reference in addition to common sense when explaining to a contractor that he should not use flame-cutting.

 

[Qshake]

I don't know of a particular reference other than those found in the structural steel industry. Its the same no matter what the shape of the steel looks like. Steel heated beyond a certain point begins to change properties. For example,

From 300 to 700F (not red yet) molecular structure changing
1100 to 1200F (red) Properties begin to change drastically (structure, hardening, elongation)
2400F and greater steel becomes mallable - rehardening but becomes very brittle. Greater than 2400F and the alloys are released (aluminum (1100), copper (2000), and Nickel/Manganese (2600))

Now, compare this to the workers tool, the acetylene torch. This torch with oxygen burns at 5200F. And an ordinary match will light at about 600F.

As always the contractor or worker will argue that griding or cutting with a blade introduces heat as well. True enough but not at the level a torch does.

 

[rebar]

I don't know of a source prohibiting cutting rebar to length using a torch. I don't object to it because any influence the cutting has on the properties of the rebar is limited to a small region at the ends.

Two things that should be absolutely prohibited are:
1. Heating the rebar to bend it. It must be bent cold.
2. Tack welding bars in place of using tie wire.

 

[Ron]

CRSI standard practice requires shearing, not flame cutting rebar.

There are a couple of reasons for this.

Flame cutting changes the metallurgy of the rebar, which is rather a "Heinz 57" anyway, so the predictability of its properties is not good. That's also why you shouldn't heat it to bend it. Also changes the corrosion response of the rebar.

Another reason is the rough end cut often leave a few pieces of slag and stuff that doesn't bond well, so you tend to get initially a good bite, but abrupt failure of the bite under stress. This can lead to end cracks and other problems.


Ron

 

[dik]

Aware of the CRSI requirements, and although I prefer to see them sheared, I usually don't have a problem with torch cutting of small bars such as 10M and 15M (#3, #4, and #5) because the heating effect is generally local and confined to the end of the bar. The smaller bars can generally be cut quickly by torch (still wouldn't want to hold onto it <G>). The force in the steel in the cut end is limited by the bond... at the cut end, bond is nearly non-existant and by the time the full strength is required, you are far enough away from the burned end that the effect of heating has diminished.

Problem with rebar and a good reason to be mindful of heating is that steel used is often 'stuff they couldn't use for something else' and the specification for the composition is not as tight as for other types of steel.

You can also spec weldable rebar; use these for embedded assemblies to attach other things to. The metallurgy of these bars is such that they can accommodate heating through transition, although the effected area of stick or gas shield welding is considerably less than acetylene. Again, only use small diameter bars.

Seems that cutting bars by torch is less common today; likely due to improvements in abrasive cutting disks and ease of using grinders in lieu of lugging tanks... also small portable shears are more common.

 

[evelrod]

Boys, ya'll need to get out in the field a bit more often.
Sure , a pair of bolt cutters can be used on #3 and #4 and cut off wheels can be used on just about anything---BUT flame cutting in the field is just as common today as it was 40 years ago when I was an apprentice Ironworker. A little common sense and a LOT of training can go a long way in preventing slag buildup, metalurical changes in vital areas, etc.
Yes, I have seen jobs where flame cutting was not approved, but not for the reasons you have given.
Don't mean to step on anybody's toes, just the facts as they reallly are.

Rod

 

[dik]

Rod:
toes are OK... wear Terra Lites nearly all the time... never know when I'll be on a jobsite; I spend a lot of time there...

Bolt cutters may be OK with #3... but our standard 'small' rebar is 10M and generally Grade 400 (60Ksi), sorta halfway between a #3 and #4 and unless you have a really big bolt cutter, you'll have some difficulty; it's much easier to use a grinder.

Regarding larger jobs... at one time a cutting torch for rebar was standard equipment. Now only about half the sites use these. In earlier days... almost never saw a cutting wheel and even less often encountered a small portable hydraulic shear (I don't recall having seen them earlier than about 10 years ago). Occasionally have the odd small plant or warehouse where the bars are fabricated right on site... even a small truck mounted bar bender...

And with respect to the little 'nub' on the end, I've just assumed that these improve the bond <G>.

 

[Qshake]

Rod, I agree that the practice continues as it has in the past but only because the inspector is over worked or under asserting himself. I apply the old addage to your response: two wrongs don't make a right. And just because it has been done and continues to be done doesn't make it more acceptable either.

Like Dik many of our jobs involve significant bars (up to #14s) and flame or sawcutting takes considerable time. I have and continue to enforce saw cutting and repair of the coating (should it exist). Do I pride myself in catching all these field fixes? No. But I like to think that by working with the field worker and explaining why it is important to do it the right way there will be mutual respect. Afterall, it matters if I were a roofer coming to a fieldworker's house and doing shoddy work and so he should come to my client's and expect to do the right thing also. Most often than not an agreement is reached and we're all happy at the end of the day.

Incidentally, same goes for ironworkers as goes for rod-benders (yes! there is a difference). I have worked on a number of steel truss bridges and will always require a little effort on the part of the ironworker (connector or bolter) to extract/construct pins or bolts at a panel. When extracting I always go to the &quot;monday&quot;, &quot;hell-dog&quot;, porta-power and finally to the torch if necessary.

 

[evelrod]

To all:
My last job where rebar was involved was the Navy base in San Diego a LONG time ago (if I never have to see another rebar it will be too soon), but I still have friends who are rodbusters and they tell me all is just about as I remember it. As for the structural part, without a torch and a good man (or woman these days) to use it, most jobs just wouldn't get done. I have NEVER allowed my men to slot a hole or burn anything when there was ANY alternative available.
It is unfortunate that there isn't a certification necessary to use a cutting torch, because I've run afoul of some TERRIBLE burners. Lots of new stuff around now like plasma cutters, chop saws, hydraulic sheers and the like that were not available to me, but we will still need the torch in the forseeable future.
As for the &quot;hell dog&quot; , BOY---I am &quot;two days older than dirt&quot;, and I just barely remember those monsters.
The inspectors around here do a GREAT job as I see it( most are retired tradesmen).
Structural engineers have a tough time keeping up with the everchanging rules here in California, my hats off to them.

Yes dik, I have a big toe that only has one joint because I DIDN'T wear the right boots!

Very interesting---Rod

 

[Ron]

Hey Q...congrats!

We often have requirements that at one point in time were proper and necessary. Having a good bit of experience in both field and analytical work, I'm not convinced we have progressed a great deal in workmanship. I know I'll get an argument on this, but I see more and more compromise on the part of designers because they are not familiar with the &quot;why&quot; just the &quot;how&quot; of construction. As Qshake said, two wrongs don't make it right.

I often get the addage that &quot;we've been doing it that way for 20 years with no problem&quot;. That is a comment made by someone with 1 year of experience, 20 times...not the comment of one with 20 years of experience. We all get lulled into thinking that because a structure hasn't failed in its lifetime it was properly constructed. VERY FEW structures ever get tested to their design loads. That's why they are still standing, not necessarily because they were built so well.

Remember, most builders only see a building for the warranty period. Owners, Architects, and Engineers see them for their performance lives. There is often a big difference!

Anyway, my point through this diatribe (yes, I think I have a point)is that we should not construct to the lowest common denominator and a torch on the jobsite usually does this.

Ron

 

[Qshake]

Thanks Ron, I agree with you point as well!

 

[evelrod]

Gee guys, it's just a tool!!!

Rod

 

[Hariharan]

It would appear that the objection is to flame cutting epoxy
coated rebars. Fully justified and endorsed. What is
the problem with flame cutting uncoated bars? Rebars are
not subject to major stresses at the ends. Any degradation
at the ends would not affect performance?

Hariharan

 

[evelrod]

Thank you Hariharan for the spark of common sense!

To Ron

In 43 years I have had the priviledge of working in both the engineering and construction fields and seldom have I been involved in a project that did not require compromise.
I have worked with some brilliant engineers and remarkably competent field personnel on some very intersting jobs, none of which were completed without field revisions.
There are seldom perfect solutions to all problems, and I have had to do more than one set of &quot;As Builts&quot;, but it has never stopped me from trying to do the job correctly.

Sorry about this diatribe, but the concept of working to the lowest common denominator is offensive to me. I don't associate with stupid people. Ignorance can be cured, but stupid is just plain stupid.


Rod

 

[JRM]

If the bar is flame cut on the end, and only the end area is affected by loss of strength from the heat; does flame cutting really matter? Is this concern on the flame due to uncontrolled use (intentionally or accidently) on other areas of bars (not at the end)?

 

[v2]

I agree with &quot;rebar&quot; above.

I would like to add that the only concern I have is that a torch can be a fire hazard. the concrete form, plywood, can get cought on fire. it happened a couple times. Now I require a fire extinguisher to be always nearby handy to he iron workers using the torch.

I have allowed torch cutting to be used on #18 bars.

V2

 

[v2]

I am a structural engineer in the high-rise business.