I need to suspend some rebar 3″ from the bottom of a footing hole. What’s the best method for doing that?
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Replies
EricGunnerson,
They make saddles to do that. I have seen rebar wired in place but that often exposes the wire to a direct path to moisture which will cause rust to penitrate to the rebar..
I've seen rebar wired all the time by a number of steel reinforcing companies. Now I'm not saying that simply because that is what I've seen or done means it's right. I'm also fully aware that there should be no ground contact with the rebar. But I find it hard to believe that pieces of small gauge wire will create enough of a path for moisture to rust out a footing full of rebar.
I would like to know is this theory or have you ever encountered ( or heard of) a footing problem due to the rebar rusting out from moisture penetration from tie wire.
If you use tie wire, do not tie it to the rebar. Don't wrap it twice around. Don't twist it in any way.
Just make a simple saddle for the rebar to sit in.
After the mud is in place, cut the wire and pull it out.
How would something like thin nylon or plastic string work so you wouldn't have to mess with removing it?
we kind of do it like sam t
however, our inspectors do want to see it tied in place to keep it x inches fromthe form boards ( must be this years "topic to see")
so we do loop it around, and hang ift from the spreader cleats, when were pumpong mud, we make sure the rebar is not near any form boards etc, then when we are screeding, we cut it loose an tuck it in to the mud
however, that said, we do have one local city, that we have to stick it on dobbies and when we do , we use thos elittle oncrete 2x2 blocks
some inspectors are just tooooooooo anal
Set it on concrete (or regular) bricks.
Jerry
Is there some reason you can't just set the rebar on dobie blocks? They make 'em three inches thick.
Pour 3", let it set up a little, set the rebar(s) on top of it, pour the rest. --------------------------------------------------------
For a good time, visit MyToolbox.net See some of my work at TedsCarpentry.com
Get a bunch of 3" 'dobies with the twist wire on top. Cinch them to the rebar with a twister (a cheap tool and speeds up tying wire a bunch).
I hate the metal chairs...hard to keep them upright and in place unless the bottom of your footing and your rebar are perfect.
Maybe it's a regional thing, but they won't let us prop rebar up on brick or block here in central Maryland. They want us to tie it to short rebar stakes driven into bottom of footer dig. Not really sure there's a risk of rust chasing the rebar into the footer in an underground situation. I've seen it on out ot ground concrete, especially in coastal applications.
Kimball
never heard of that before. I've always learned not to have any ground contact to avoid the rust issue. But I'v never heard of 16 gauge wire being a problem as far as being able to transfer enough rust to rust out rebar in a footing.
anyone ever hear of that happening?
I don't think rust is much of a factor either in footings.
According to Frank, he learned in his physics class that rust needs oxygen down there to actually oxidize.
I'm more worried about getting hit by an astroid.
blue
lol...them dang asteroids. Some say that's what killed the dinos.
well common sense would tell me that it would take more than 100 years for water to migrate to the bar and rust out, so why would I care.
Roman Colosseum, still standing after nearly 2000 yearsSamT
There are small blocks of concrete with a length of tie wire formed in that are intended to be used in such cases. These blocks, the ones I saw, were something like 2' by 4" and perhaps 1-1/2" thick with two 6" lengths of iron tie wire sticking out the flat to tie them to the rebar and keep them in place during the pour. These are purpose built for the job and you should have no problems with inspectors. These are ideal as they have enough cross section to keep the rebar up off the soil without sinking in even if the surface is soft. Pretty common on commercial jobs.
2 foot long dobies, or typo?
Typo. 2 by 4 by 1-1/2 inches is what I am familiar with. Two-foot long would be OK in some cases but something that long and thin in concrete might tend to fall apart in handling.They were also used along the sides of beams and columns to keep the rebar away from the forms. later we would pull the forms and you could often see where the blocks were poured in.
brownbagg,
Why should you care?
You may have a point regarding the length of time before it becomes a problem , however if your inspector wants you to seperate the steel from the ground that's what you should do unless you like tearing things up.
The suggestion regarding nylon string obviously hasn't spent a lot of time pouring concrete. The idea of not wrapping the wire around the rebar hasn't seen concrete thundering into the form either.
I do like the simple idea of, pour a few inches of concrete, let it set a little bit and then set the rebar in place really has merit, again if your inspector will let you, here they want everything in place and the trench bone dry before they will let you pour..
The typical footing form has braces across the top to prevent it from spreading. It's a real hassle to set rebar in there after the braces are set, so a lot of guys form one or both sides of the footing, set the rebar, and then brace it. If you are strictly bracing to the outside, then it would be possible, but we brace across and what you are suggesting would be highly impractical here. Not to mention the need to have all of the bar pre-bent and sitting there on the ground right in the way of foot traffic.
The idea of not wrapping the wire has been on more pours than you'ld understand.SamT
"The suggestion regarding nylon string obviously hasn't spent a lot of time pouring concrete."Ah, Frency, looks like you have taken a trick from your GOP friends - if you can't win the argument then slander the person stating the idea. OK, having worked two years as a concrete laborer on heavy construction (and a stint as a concrete lab tech and then another stint as a concrete rebar draftsman) may not have been a "lot of time" to you, but sure seemed like it to me at the time...The mild steel tie-wire commonly used by Ironworkers has the advantage of being cheap, easy, and quick. It is also relatively tough, but monofilament nylon of the same diameter is actually slightly stronger than the same size mild steel. Nylon fishing line is readily available with in 500 lb test. There are even plastic clips that are now used to hold rebar in place and the plastic is probably not as strong as nylon. The advantage of mild steel is that an Ironworker can give a couple of wraps of steel wire and twist the ends in about ten seconds where as with nylon, it would require tying some type of knot, which would take far longer. I also know that if you are dumping concrete into a form with enough force to break a couple of wraps of 500 lb test nylon line, you are going to have some fairly severe separation problems among other things. So, what is your problem with nylon. And how how many thousand yards of concrete does one have to pour before they know something?
CaseyR
Sure it will work as you say there is a whole lot of strength in nylon string.. However the time it takes to tie it is what concerns me.. a simple slip knot doesn't work when you are dealling with slippery nylon.. that plus most concrete guys I know have hands like, well like claws.. large strong big and well not exactly dainty..
But I yield to your experiance.. While I've never seen it done in my 15 plus years in construction it doesn't mean that it can't be done..
What about using zip ties? They would probably be just as quick and very strong.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
I thought that is what they were talking about at first. seemed like a good idea to me.
Edited 4/19/2006 5:35 am ET by alrightythen
I don't think rust is much of a factor either in footings.
According to Frank, he learned in his physics class that rust needs oxygen down there to actually oxidize.
....will get it from H2O.
here, they frown on concrete brick and some will fail you, but ok with others. rebar driven in ground definanite you cannot do. and clay bricks are not allowed. most are going to plastic chairs.
Oh, well. Codes here won't allow rebar to make ground contact.....Adobes have always been good with every inspector on every foundation I've done. But then, our soils are pretty acid.
One more word on the wire chairs: IMHO, about all they're good for is for girls with big hands and a tennis ball to play "Jacks" with. :-)
Hanging the stuff from the forms with tie wire or punched strap shouldn't be a problem....better than driving rebar dowels in the ground, I'd think.
Might I ask why 3" is the magic number? Although a poly vapour barrier beneath a concrete floor is now accepted, why not one beneath your footings so that moisture can't get in or out while the liquid stone is curing. That way you could use 2" & satisfy the requirement for rebar cover without allowing the concrete to "degrade due to contact with soil", the reason for a spec of 3" I have been given to understand.
Best
on some jobs they make us line the footing with a plastic vapor barrier and on one a thick tar like paper. about a 70 lb paper
Do they allow you to reduce your rebar cover when the vapour barrier is installed?
I'm always interested in the thinking behind the code requirements.
nope, not code but crazy architect
Your gonna need alot of J in the B orange juice, and M,Donalds milk cartons.
Fill them up , after you've drank the contents, with concrete 3" deep. Let harden. Remove outer wax restrictor covering. Then place each block gingerly below your rebar to maintain your 3 " minimum soil from steel clearance.
TRIGGER
Practicality wins!If you guys have ever worked to seismic specs, you would not dream of
placing rebar until the bottom of the excavation was "blinded" by ####two inch, unreinforced, weak mix skim. The rebar would then be supported by 'chairs', saddles, or dimensioned blocks. The bottom cover is not just to prevent oxidation (read rust), but to allow the rebar designed
strength to fully develop. Side cover is a different issue.This is a good thread.Lapun.
Lapun,
So, what's the purpose of "blinding?" The only time I've seen it was when the bottom of the dig was too soupy to walk on and we couldn't build the forms. This was in SoCal above the San Andreas. The Brawley Seismic Zone in the Salton Trough.
SamT
Good question, Sam.The wet puddled excavation is the obvious instance. The totally dry earth trench can be disadvantage to the retention of homogenious concrete essential properties, and thereby reducing designed structural
strength. Blinding can reduce honeycombing in the lowest earth contact
zone, which will permit migration of noisture through the bottom cover
of rebar - which is what thwe original thread was all about.
Then there are other benefits too. During concrete placement, the rolling wet-edge may advance more readily with low slump concrete, without picking up loose earth,etc.
Because of its regidity, blinding is superior to a plastic vapour barrier, and is not at risk of penetration or tearing.
In domestic sized operations, plastic may benefit the concrete, but it
is not an adequate platform for the maintenance of an accurate level of
rebar cover. In multi-storey construction plastic membranes have a function on post-poured below grade backfil, and slab curing, but that
is all.
The BIs who permit tieing to rebar stakes are,IMHO,subverting better industry standards. They need retraining. Trigger should be one of their lecturers.Lapun.