Anchoring steel for monoslab footing.


Residencial code calles for one #5 bar in the center of a mono slab footing or 2 #4 bars. Anyone know if it's ok to drive a verticle piece of rebar into the ground to tie these horizontal bars to, or are they so anal retentive that they think the steel will rust into the concrete?
Thanks, Bob
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Robert Olin
Bob\'s Water & Septic LLC
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Anal. (although since it is not a bridge, a high rise, or an overpass, I do not see why not!) Most of the times I just use a dobe - concrete block that ties to steel. Or, make an angle out of a smaller rebar........right angle......say 2' x2' and rest it on the concrete 3" x 3" dobe and tie the steel to it.
I cannot imagine the rust traveling from the ground........all the way up to possibly compromise the steel horizontal piece. john

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Rebar should NOT touch the ground or be driven into the ground as a stake to support other bars. There is a reason that most specs. call for a minimum clearance of 2-3 in. from the surface of the concrete or earth contact. As John stated earlier, use some concrete (NOT RED) bricks for chairs to suspend the horiz. bars from the ground surface. The rust will travel to the bars in the concrete over time and cause destruction of the footing or slab. Surface rust or oxidation on reinforcing steel is harmless if contained, however when the bars are subject to severe rusting they expand and will ultimately cause the concrete to fail.

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Hi Tom. I have always wondered about this, and we always do put steel on concrete blocking. not red brick......either. there is a maintained 3" from earth contact in most plans. Is the steel like a electrical wire that would transmit rust? I still have trouble visualizing rust following steel. Anyway, I do use concrete dobies..... john

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wrote:

Absolutely, rust will travel along steel (such as rebar) under paint and protective coatings. Because of the mechanics of how this rust spreads, it is very difficult to prevent, hence we limit the ability for water to contact steel.

Think of it this way. You get rust on the edge. Rust is very porous. Water travels in the rust, to new virgin steel, and rusts it. It is a continuous process, with the rust able to travel great distances by water being drawn in by capillary action.
About the only thing that limits it (rust) is to use galvanized coatings such as hot-coat zinc. (Electroplate zinc is usually not thick enough to withstand long periods of time...) Not much rebar is zinc coated however.
What makes things worse is some soils/sands are loaded with stuff that promotes rust, such as salt! (even inland, you can have corrosive sand, as the sand is often the result of ocean action over a long period of pre-history...)

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Well that helps explain it better. I know I have worked on State Jobs....that required epoxy coated rebar and tie wire.....any cut was coated as well as wire..... This rebar was in the large size family also....1" and even larger...... john

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I have had to deal with several fairly major repairs where the steel had rusted and the rust expanded enough to move the concrete, break the concrete, or cause other damage. One involved heaving a door mull enough to prevent the panic bars from latching, unbond the terrazzo, and pressure the transom glass overhead. Several others have involved shattering stair noses on exterior concrete steps.
None of these involved rebar driven in the dirt, but rather moisture through the concrete. I don't have any stories about failed footing steel and have often wondered about the requirement. Corps work is adamant about steel cover with no ground contact.
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John,
I think Peter and Dan have answered your question, so I won't go into the tech. end of the whys and wherefores unless you want that info. If you do, you know how to contact me. Hope the weather is nice in No. CA this time of the year.
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Weather is beautiful. I am going to stick to my usual method of placing rebar on dobies. and keeping an earth contact away.....at least 3" john

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Thanks guys. I don't want to do it twice so I'll will not be driving rebar into the ground as stakes. I will need one #5 bar in the middle of the footing. Thats about 10" above the ground. So, would you guys suspend it? Thanks, RO
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Robert Olin
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Yes, it is quite normal to hang grade beam cages, etc. Another possibility would be to pour a portion, place the steel, and then complete the pour. It will mean tying laps and corner bars in location, and an inspector may not accept your word to install.
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It's not ok....steel is not supposed to touch soil.... 3" minimum cover
Use concrete dobies, thats what they're for
cheers Bob
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