Rusty re-bar

[stubuzz]

I have seen many concrete structures being made with re-bar that starts to rust before the concrete is poured. Is this a problem?

[carlyai]

I think the rusting stops when it is imbedded in the concrete. Probably wrong as now I remember something about concrete osmosis.

I used to live in an apartment block that was built for the US servicemen during the Vietnam war.
There was lots of exposed rusting rebar poking through the concrete, but the 6 story building still stands, even though the ground floor has dropped a lot from water extraction.

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[Kwasaki]

It's good it's called " The re-bar is cured ". 

[VocalNeal]

Rusting is a chemical process involving oxygen. Once the concrete is poured and has set there is no more oxygen, so no more rusting.

[cheeryble]

2 hours ago, Arjen said:

I once heard that rebar has to be rusty to get a good "grip" on the concrete? Not sure if this is correct, or a fairytale from the seller from re-bar?

 

ISn't that what the indentations are for?  :)

[vogie]

Light rust is better than new shiny & oily.

[cheeryble]

20 minutes ago, vogie said:

Light rust is better than new shiny & oily.

Reckon I'd agree, but I would say the indentations are a major structure-strengthening feature.

[vogie]

8 minutes ago, cheeryble said:

Reckon I'd agree, but I would say the indentations are a major structure-strengthening feature.

You would be exactly correct, but the op is questioning whether rust on the rebar would be a problem.

[cheeryble]

No problem......I've scrapped what looked like heavy corrosion off rebar exposed in a 25 year old beam and there  was still very sound steel underneath.

[Kwasaki]

16 minutes ago, vogie said:

You would be exactly correct, but the op is questioning whether rust on the rebar would be a problem.

OP l thinks as all would know re-bar being installed will show corrosion quickly in most cases even in storage before sale.

The twisted ribbing of re-bar is for cement to adhere-to the thin layer or rust dis-colour re-bar is nearly always avoidable that's why they call it "cured "  if for any reason there is too much build up of rust it should be wire-brushed before the concrete pore.

 

[eyecatcher]

a bit of rust isn't going to be detrimental to the concrete, just wire brush it off and get on with the job.

 

The main problem with rusting rebar is when its already cast within a concrete structure. 

When steel rusts it expands and this is what has happened when you see those old concrete fence posts with lumps of connie spalling off.

Water has penetrated cracks/fissures and weak spots over time and oxidation occurs.

It doesnt take much to blow the concrete apart.

 

You dont want that happening on your house columns due to poor concreting, rebar being almost at the surface or the faces not rendered or sealed afterwards.

 

thats when you need to take care

[A Lurker]

The author of Concrete Planet believes rusty rebar is a massive problem. He goes as far as to suggest that we should consider not using it all together.

 

He  touches on it in this podcast interview:

 

https://player.fm/series/science-for-the-people-2062/413-concrete

[r136dg]

For residential, if it's in-bedded well within the concrete, I wouldn't sweat it (if we're talking about a little surface rust).

I've torn out slabs where they didn't pull the rebar, or mesh up during the pour. It's almost worthless down there half in the soil & like others said, continues to rust.

In the US, along with many other countries, you'll see green rebar being used on most bridge & road construction. Powder-coated or epoxy-coated I believe. Probably spendy stuff.

[bbradsby]

Typical Concrete specs for building construction call for lightly wire-brushing off loose rust scale and flakes and its good to go. Surface rust spots aren't a problem. No need to use epoxy coated rebar unless you're pouring concrete that will receive chemical or salt water immersion, mist, spray or splash. Its expensive.

[Kwasaki]

5 hours ago, A Lurker said:

He goes as far as to suggest that we should consider not using it all together.

Bamboo is good.  

[sometimewoodworker]

On 06/08/2017 at 3:11 PM, Kwasaki said:

Bamboo is good.  

Not really, it disappears in a coupe of years.

[CGW]

Common sense, (is that a term used anymore?) tells me that concrete/cement will bond to a lightly rusted steel far better than steel that is treated with "oils' to prevent rusting?

[cheeryble]

1 hour ago, CGW said:

Common sense, (is that a term used anymore?) tells me that concrete/cement will bond to a lightly rusted steel far better than steel that is treated with "oils' to prevent rusting?

 

Hi CDW

common sense tells me the same but my rational brain says it's likely irrelevant.

The amazing strength of reinforced concrete is all in the combination of high compressive strength of concrete.....at the top of the beam..... and tensile strength of rebar at the bottom of the beam. So rebar is quite bendy....the only virtue it needs is to resist being stretched itself.....which it does well.......and to not "slide" inside the concrete.

This is where the grooves come in.

For a groove......or thousands of grooves....to slide inside set concrete it would mean the concrete filling the grooves would have to compress to let the grooved steel move inside it. But we know that concrete's strength is it's incompressibility.

Nuff said.

[CGW]

2 hours ago, cheeryble said:

 

Hi CDW

common sense tells me the same but my rational brain says it's likely irrelevant.

The amazing strength of reinforced concrete is all in the combination of high compressive strength of concrete.....at the top of the beam..... and tensile strength of rebar at the bottom of the beam. So rebar is quite bendy....the only virtue it needs is to resist being stretched itself.....which it does well.......and to not "slide" inside the concrete.

This is where the grooves come in.

For a groove......or thousands of grooves....to slide inside set concrete it would mean the concrete filling the grooves would have to compress to let the grooved steel move inside it. But we know that concrete's strength is it's incompressibility.

Nuff said.

Interesting, so in reality you don't wont the concrete to bond to the rebar?

[cheeryble]

8 minutes ago, CGW said:

Interesting, so in reality you don't wont the concrete to bond to the rebar?

Given the choice i'd prefer it attached and wouldn't use rebar with more than the slight remains of protective oiling, but wouldn't worry about that.

[bbradsby]

Its always good that folks are here to help others. However, in this case, we're talking about poured in place concrete. It's building science, not opinion, that creates proper, safe concrete structures. If you're uneducated and/or inexperienced in the subject matter, posting guesses, theories and suppositions only serves to muddy the waters for those poor souls seeking help here, and potentially puts lives in danger. Please refrain. 

 

The facts: reinforcing steel must be free of foreign coatings which would reduce bond to concrete - oil would qualify, by the way. Excerpted from a set of professional specs:

 

PART 3 - EXECUTION

3.01     PLACEMENT

            A.         Place reinforcing supported and secured against displacement.  Do not deviate from true alignment.

            B.         Before placing concrete, ensure reinforcing is clean, free of loose scale, dirt, or other foreign coatings, which would reduce bond to concrete.