Can Rebar be bent twice?

[ofthesea]

I have a contractor who bent a #5 bar 90 degrees and now needs to bend it back up. Does the bar undergo some diminished tensile capacity when bending it multiple times?

 

[nuche1973]

ofthesea,
Per ACI 318-05 section 7.3, bars can be bent once and only "cold bent".

 

[dgkhan]

When the rebar was bend first time. The inside corner area went under compression and become hardened, and hence less ductile. Now when you straighten it up, you impose compression on outside face of corner, which now became hardened and less ductile. So you created two zones with less ductility and hence less tensile capacity. Alternatively, rebar can be heated to avoid this but cost factor!! ! May be new rebar will be cheaper.

 

[concretemasonry]

Rebar is cheap. Have it bent properly. This may require a delay in the project that the contractor will not like. - You can avoid the delay if you sign and approve the procedure. - Is it in a critical application or only prescriptive?

Dick

 

[dgkhan]

CCB1,
I have attached ACI 318-05 section 7.3. For my learning, where it is written that rebars can be bent only once?

 

[dgkhan]

sorry here is 7.3

 

[JStructsteel]

It says that "Grade 60 can be cold bent and straightend to 90 deg" Does that mean bent once, then straigtened (2nd bend) is allowed?

 

[ofthesea]

Jrisebo...THat's what it sounds like to me.

 

[enginerding]

Will multiple bending reduce tensile strength? Technically not because of strain hardening. The problem is that the material can undergo only so much inelastic deformation before it breaks - think of bending a paperclip back and forth.

With reinforced concrete, ductility is is key. We need the bars to be able to yield without breaking. Bending the bar back and forth has already "used" some of this yielding. One bar, one time? Probably OK. But be careful how you give your answer. It is not uncommon for someone to interpret "it will be OK" as "it will be OK no matter how many times I do it, or in what circumstance."

 

[IFRs]

I interpret "It says that "Grade 60 can be cold bent and straightend to 90 deg" to mean that during fabrication, if you are trying to get a 90 degree bend but over or undershoot you can fix it by re-bending without the need to start over.

 

[archeng59]

In my opinion, bending rebar more than once in the same location of the bar is similar to bending tie-wire or baling wire multiple times in the same location of the wire. Each bend reduces strength and eventually it breaks. The rebar is a larger diameter and requires several more "bends" to cause the bar to break, but the results are eventually the same.

 

[boffintech]

Don't think I've seen rebar partially embedded in concrete bent "at or or near the minimum diameter specified in 7.2." or bent gradually. Usually they just beat it down with the backhoe bucket.

 

[hokie66]

ofthesea,

You have taken the trouble to post this same question in the AS/NZ forum. If you are asking about Austalian conditions, read the anwers you have been given in that forum. The reinforcing material is different.

 

[Ron]

Rebar can be straightened, cold or heated; however, it should be checked for cracks. If you bend it with heat, make sure the temp. is controlled and with either, use magnetic particle inspection to check for cracks.

 

[JStructsteel]

I guess we have got off track, the code says you can bend and then straighten, so to answer the original question, I would interperet that there is no loss of strength...after 2 bends, yes you have issues.

 

[DRC1]

Note that rebar is commonly bent in the field to provide access during construction and then bent back. This is especially true at backwalls. I have vever heard of any problems due to this.

 

[msquared48]

Note the implication in 7.3.2 though where it is the Engineer's responsibility to OK a second bend, depending on the situation, the implication being that there could be a problem - strain hardening and cracking leading to a possible critical failure.

Regardless of what the code says about the grade 60 steel tests, I would be still be very leary of any cold bending without some preheating. Grade 40, no worries. Just my early training coming to light I guess.

Mike McCann
MMC Engineering

 

[BAretired]

I think temperature may have something to do with it too. I remember hearing a story about bars being offloaded from a truck at about -40C (same as -40F). The bars were apparently just dropped on the frozen ground and they snapped as a result of the impact.

I found this story hard to believe and I'm not sure I do, but I wouldn't want to cold bend any bar at very low temperatures.

Best regards,

BA

 

[cloa]

That would below the ductile-brittle transition temperature so the steel had no ductility. The dislocations (imperfections in the structure) need a certain amount of heat to be mobile. Moving dislocations is how plasticity works.

 

[msucog]

from my construction experience, bars are bent all the time and you probably don't even know it. anything bigger than #5 will likely require substantial force to bend. i personally would not allow my people to bend any bar bigger than #5...but yes, we would bend partially embedded footing bars up out of the way for equipment and then back down. we would attemp to not bend past 90 degrees and would make the bend somewhat "gradual". we also would avoid bending column, beam, and wall rebar. i would say that my comments are not typically for the industry...it is usually bent whichever way (and as many times) is easiest for the laborer doing the work. at least they bent it unstead of cutting it completely off and leaving it out all together at the time of concrete placement but after inspection (seen this with my own eyes before). i suggest you provide your response with a specific location and criteria noted so that the contractor doesn't use your comments as a blanket "okay" for everything across the project. and maybe tell them to avoid this situation in the future and/or have mechanical couplers on hand if it's something that you don't feel comfortable letting them bend back and forth.