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RE: Twisting of rebar[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Twisting of rebar
- From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
- Date: Thu, 6 Nov 2008 18:24:06 +0000
Ah yes, the real world as opposed to the pristine academic or design office world where a 90 degree bend is easy to put on paper, but hard to get in the field. I visit the real world often. Basically the ACI 318 and 301 dump it to the engineer with only limited guidance.
If the twist occurs over a long enough length, it is not a problem. The other part of the unknown is the rebar chemistry itself. The ASTM A 615 spec allows a lot of latitude that effects ductility. Some bars you can bend like a pretzel, and some will snap if you sneeze on them. And they both will meet the A 615 spec.
If you have a problem, it is easy to see. The bar will crack or fracture. It sounds like heating has been a problem, but it shouldn't be. Require the used of Temple sticks or "crayons" to determine the heat of the bar and the use of a rosebud on the torch used to heat the bar. The more length you heat, the better.
If you allow cold bending, check for cracks. The maximum stress will be at the perimeter where cracks are easy to spot. You can use a dye penetrant if you want to show them that you are serious about looking for cracks. If you are really anal, have them bring out an ultrasound. Better yet, leave that up to the special inspector to look for the cracks and have him specifically report on the field bent rebar. Try to avoid heating at the interface between the concrete and rebar for 2 reasons. You want to avoid heating the concrete, and you want the bending to occur over some distance away from the concrete so that if there are cracks, you can see them easier.
R7.3.2— Construction conditions may make it necessary to
bend bars that have been embedded in concrete. Such field
bending should not be done without authorization of the
licensed design professional. Contract documents should
specify whether the bars will be permitted to be bent cold or
if heating should be used. Bends should be gradual and
should be straightened as required.
Tests7.2,7.3 have shown that A615 Grade 40 and Grade 60
reinforcing bars can be cold bent and straightened up to
90 degrees at or near the minimum diameter specified in
7.2. If cracking or breakage is encountered, heating to a
maximum temperature of 1500 °F may avoid this condition
for the remainder of the bars. Bars that fracture during bending
or straightening can be spliced outside the bend region.
Heating should be performed in a manner that will avoid
damage to the concrete. If the bend area is within approximately
6 in. of the concrete, some protective insulation may
need to be applied. Heating of the bar should be controlled
by temperature-indicating crayons or other suitable means.
The heated bars should not be artificially cooled (with water
or forced air) until after cooling to at least 600 °F.
188.8.131.52Field bending or straightening—When permitted,
bend or straighten reinforcement partially embedded in
concrete in accordance with procedures 184.108.40.206.a through
220.127.116.11.c. Reinforcing bar sizes No. 3 through 5 may be bent
cold the first time, provided reinforcing bar temperature
is above 32 °F. For other bar sizes, preheat reinforcing bars
18.104.22.168.aPreheating—Apply heat by any method that
does not harm the reinforcing bar material or cause damage to
the concrete. Preheat a length of reinforcing bar equal to at
least five bar diameters in each direction from the center of the
bend but do not extend preheating below the surface of the
concrete. Do not allow the temperature of the reinforcing bar at
the concrete interface to exceed 500 °F. The preheat temperature
of the reinforcing bar shall be between 1100 and 1200 °F.
Maintain the preheat temperature until bending or straightening
is complete. Measure the preheat temperature by
temperature measurement crayons, contact pyrometer, or
other acceptable methods. Do not artificially cool heated reinforcing
bars until the temperature of the bar is less than 600 °F.
22.214.171.124.bBend diameters—Minimum inside bend
diameters shall conform to the requirements of Table 126.96.36.199.
In addition, beginning of the bend shall not be closer to the
concrete surface than the minimum diameter of bend.
To avoid this problem in the future, terminate your bars for multiple placements with a straight bar and lap on an L bar that can accommodate variations in placement or mislocation.
Is that worth one beer or two? Oops, you are from Kentucky. I feel the fee of Woodford Reserve might be in order.
Regards, Harold Sprague
> Subject: Twisting of rebar
> Date: Thu, 6 Nov 2008 10:25:39 -0500
> From: David.Dickey(--nospam--at)masonandhanger.com
> To: seaint(--nospam--at)seaint.org
> Is there an ACI document that specifies the amount that a bar can be
> Hooked bars extend out of the top of a load-bearing 12" concrete wall.
> The concrete roof slab has not been placed yet. The hooks are intended
> to be perpendicular to the wall, but have been placed parallel. I
> recommended that the bars be heated to 1350-1400F, straightened, and
> then bent in the correct direction. The contractor would prefer to
> grasp the bar at the point where it protrudes from the concrete and
> twist the bar 90 degrees. The bar extends approx. 12" above the level
> of the concrete, so 90 degrees of twisting in a #5 bar would be
> occurring over a 12" length.
> I am entertaining this proposal because another case of heating and
> bending bars on this job (without my approval) turned out to be a
> David Dickey, P.E.
> Lexington, KY
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