huangyhg

can rebar be bent twice?
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?
check out our whitepaper library.
ofthesea,
per aci 318-05 section 7.3, bars can be bent once and only "cold bent".
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.
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
ccb1,
i have attached aci 318-05 section 7.3. for my learning, where it is written that rebars can be bent only once?
sorry here is 7.3
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?
jrisebo...that's what it sounds like to me.
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."
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.
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.

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]