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Rusted Rebar & Concrete?


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#1 Theduardo

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Posted 13 January 2008 - 09:21 PM

Rebar with a decent coat of rust is somewhat common down here.

Does the rust have a significant effect on the quality and strength of the concrete form being poured?
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Posted 13 January 2008 - 11:33 PM

I grew up working for my father who was a remodeler/carpenter. I've worked on quite a few construction crews here in Arkansas (humidity) and out in California for a short time. I even helped a few people that I knew when I lived in Michigan pour driveways, patios, etc.

In all of the construction sites and homes that I've seen them pour concrete in slabs, foundations and so on, even the two story house that my father built, I never once witnessed any rebar that wasn't rusted when they poured concrete over it. My parents still live in the house that we built 30 yrs. ago and there is not one single crack in the slab portions of the basement, garage, and driveways that we poured. I would make a good guess that rusted rebar, as long as the integrity has not already been compromised by being rusted at great length, would have no bearing on structural integrity.

I've had the experience of demolition of a lot of concrete over the years that I worked in construction and most of the rebar and wire mesh reinforcing that was in the concrete as far as I remember did not have any red rust evident. It was almost as if the rust was 'killed' or halted by being entombed in the concrete. I can only assume that between the chemicals in the concrete and lack of sufficient oxygen limits the rebar's capacity of continuing to rust. I'm no expert in those areas, but can only lend what I've witnessed.



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#3 Tobus

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Posted 14 January 2008 - 07:06 AM

Technically, yes, rusted rebar can cause problems down the road. It's mostly a factor on rebar that's near the surface. The rebar will continue to rust over time, due to oxygenation that occurs as concrete cures and draws in oxygen from its exposed face. The rebar expands as it rusts, and spalls the concrete off the surface. We sat through a long presentation last year on this very subject.

On commercial projects (like my company does), the rod-busters are usually required to sandblast the rebar if it's very rusty. But more often than not, I see them pouring on rusty rebar. Not super-duper rusty, just light surface rust.

As for strength, rust doesn't really affect it. Not unless it's really badly pitted, where the rebar has lost a significant portion of its cross-sectional area. I'm sure you've noticed that rebar has "ribs" or "bumps" on the outer surface, and this is what "grabs" the concrete to provide strength. So if these nubs are still present, and the rebar still has its tensile strength, it should be fine in concrete in terms of its strength. The only real issue is long-term performance near the surface.

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#4 Chris

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Posted 14 January 2008 - 08:56 AM

Good to know.
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#5 Summers

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Posted 11 December 2008 - 07:39 AM

Rebar with a decent coat of rust is somewhat common down here.

Does the rust have a significant effect on the quality and strength of the concrete form being poured?


Comments shared thus far have merit and can be contributing causes of the rust. Any time rust on re-bar begins to swell and discolor a concrete as well as expand to cause cracks, the REAL answer to what has happened is : The Steel has Lost the Passivating Layer !
Concrete when poured and surrounds the steel has a pH of 13 +or-. This high pH forms a layer on the steel that prevents it from corroding[Passivating Layer] ! With time, the pH begins to fall and this protection is lost.

As stated, salts, atmosphere and carbonation begin to play a role in the degradation of the concrete. The most deletrious affect is moisture. Concrete is a thermodynamically unstable material that absorbs moisture into the concrete matrix. [pour a small amount of water on a surface and observe] As water enters the gel pore capillary system of concrete, it carries contaminates with it, that speed the lowering of a pH significantly. When the pH goes below 11.5, the passivating layer is gone, an electrochemical reaction commences causing rust to form. As the rust progresses, it expands the size of the re-bar exerting pressure on the concrete creating cracks [Spalling]. At this point, additional cracks allow more water/moisture into the matrix and the rust really takes off! At a pH of 13 it takes 8000 ppm of Chlorides to commence corrosion on steel, at a pH of 11.5 that # is 70 ppm !! Drastic Difference

Concrete mix, Depth of cover and good practices all play a roll, however a concrete that absorbs moisture is the major culprit. Use of Epoxy coated re-bar was thought to be the answer! After monumental and catastrophic failures of coated steel was observed, it has been determined that these protective measures do not work. When a coating is chipped or a small nick on a surface is present, the lowered pH then concentrates at that site and instead of even surface rust you wind up with corrosion all the way through the bar and all tension properties are lost, resulting in a structural failure at that point!

Waterproofing of a concrete with StableCrete will prevent moisture intrusion and also prevent internal chemistry from migrating out to a surface reaction with the atmosphere. Freeze thaw does not happen since there is no moisture in the concrete to freeze. Because the concrete is dry, the Ohms resistance of the concrete is maintained at high levels, preventing the flow of electrons required for corrosion cells to form.

Hope this helps!! Glenn
"Cost Effective Answer to Concrete Corrosion"

#6 purple

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Posted 11 December 2008 - 10:27 AM

Welcome Glen! :)
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