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Default rebar - why is mild steel okay?


I understand vanilla concrete is permeable, otherwise we'd not need a DPC.
So why is it okay to use mild steel reinforcement? Corroding steel breaks
up concrete a treat.

Puzzled...

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Default rebar - why is mild steel okay?

I worked on bridges as a youngster - ISTR that if there would be water
pressure on the concrete it was coated with bitumen prior to
backfilling. There was a mimimum coverage on the steel (50mm?) and of
course the mix was carefully controlled and pokered to remove voids.
Also strict rules about the steel being rust free.

(Some of the guys I worked with had built the nuclear bombproof
aircraft hangars at the US airbases. Steel went up, followed by the
peace protestors delaying work, followed by a strike. 9 months later
they then attempted to clean up the steel with wire brushes before
pouring concrete. The guys assured me they'd be lucky to just stay up,
let alone take a direct hit).

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Default rebar - why is mild steel okay?


Steel needs oxygen as well as water to rust. Reinforcing bars will rust
and the concrete spall if there isn't enough cover of concrete over
them or if the concrete isn't properly compacted around them.

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Default rebar - why is mild steel okay?

Rednadnerb wrote:
Steel needs oxygen as well as water to rust. Reinforcing bars will rust
and the concrete spall if there isn't enough cover of concrete over
them or if the concrete isn't properly compacted around them.


I agree. Also concrete starts off being highly alkaline which prevents
oxidisation. However carbon dioxide in the atmosphere penetrates the
concrete at about 1mm per annum which turns it acid. When this
'carbonation' reaches the rebar it will rust and in time spall the
concrete.

There are many ways to prevent this happening. (by the way, wire
brushing rust on MS just polishes it)

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Default rebar - why is mild steel okay?

On Thu, 07 Sep 2006 11:39:19 -0700, Bookworm wrote:

Rednadnerb wrote:
Steel needs oxygen as well as water to rust. Reinforcing bars will rust
and the concrete spall if there isn't enough cover of concrete over
them or if the concrete isn't properly compacted around them.


I agree. Also concrete starts off being highly alkaline which prevents
oxidisation. However carbon dioxide in the atmosphere penetrates the
concrete at about 1mm per annum which turns it acid. When this
'carbonation' reaches the rebar it will rust and in time spall the
concrete.

There are many ways to prevent this happening. (by the way, wire
brushing rust on MS just polishes it)


Right. Where do I get galvanised rebar from then? My BM just does the mild
steel stuff. I need about 8mts

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Default rebar - why is mild steel okay?

visionset wrote:
On Thu, 07 Sep 2006 11:39:19 -0700, Bookworm wrote:

Rednadnerb wrote:
Steel needs oxygen as well as water to rust. Reinforcing bars will
rust and the concrete spall if there isn't enough cover of concrete
over them or if the concrete isn't properly compacted around them.


I agree. Also concrete starts off being highly alkaline which
prevents oxidisation. However carbon dioxide in the atmosphere
penetrates the concrete at about 1mm per annum which turns it acid.
When this 'carbonation' reaches the rebar it will rust and in time
spall the concrete.

There are many ways to prevent this happening. (by the way, wire
brushing rust on MS just polishes it)


Right. Where do I get galvanised rebar from then? My BM just does the
mild steel stuff. I need about 8mts


You'll be lucky, it's as rare as rocking horse sh!t...why can't you just use
normal mild steel but treat it first with something? - this would
effectively double it's life.


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Default rebar - why is mild steel okay?

On Thu, 07 Sep 2006 18:57:20 +0000, Phil L wrote:


Right. Where do I get galvanised rebar from then? My BM just does the
mild steel stuff. I need about 8mts


You'll be lucky, it's as rare as rocking horse sh!t...why can't you just use
normal mild steel but treat it first with something? - this would
effectively double it's life.


I thought about that, but what? All that tamping down in a mix of sharp
sand and aggregate...

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Default rebar - why is mild steel okay?

The message . com
from " contains these words:

Also strict rules about the steel being rust free.


Rolls about on floor laughing! There may be rules, but I'm sure we've
all seen rebar gleaming brightly as it waves goodbye beneath the rising
concrete. Lots.

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Default rebar - why is mild steel okay?

I think you are all worrying about nothing. There's a recommended minimum
depth below the surface and once you are there very little happens because
the concrete will stay alkaline for a long time. Even sub-sea structures
like pier columns last a long time with mild steel rebar.


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Guy King wrote:
The message . com
from " contains these words:

Also strict rules about the steel being rust free.


Rolls about on floor laughing! There may be rules, but I'm sure we've
all seen rebar gleaming brightly as it waves goodbye beneath the
rising concrete. Lots.


I like the way it's lovingly cared for at the BM too, polished every day and
treated with oil and grease so that it's not bright orange when it's
delivered.




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Default rebar - why is mild steel okay?


Newshound wrote:
I think you are all worrying about nothing. There's a recommended minimum
depth below the surface and once you are there very little happens because
the concrete will stay alkaline for a long time. Even sub-sea structures
like pier columns last a long time with mild steel rebar.


This thread is the first time I have ever heard of the steel needing to
be rust free. Iron left in sea water aggregates a lime scale concretion
that preserves it.

I would imagine that if rust was a problem the various agencies in
charge of the safety of building would put a stop to it and that the
concrete mufacturers would have invented an additive to safeguard
against the problem.

I imagine the problem with expanded steel is that the plaster or
whatever it is used with, does not completely immerse the metal.

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Default rebar - why is mild steel okay?

Newshound wrote:
I think you are all worrying about nothing. There's a recommended minimum
depth below the surface and once you are there very little happens because
the concrete will stay alkaline for a long time. Even sub-sea structures
like pier columns last a long time with mild steel rebar.


Indeed. Because concrete is air and water proof if done correctly.

There have been concrete ships built before now..
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Free of detaching, scabby rust.

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Bookworm wrote:
Rednadnerb wrote:
Steel needs oxygen as well as water to rust. Reinforcing bars will rust
and the concrete spall if there isn't enough cover of concrete over
them or if the concrete isn't properly compacted around them.


I agree.


A very thin coating of 'adherent' rust helps the concrete to bond to the
steel. That is why rebar (or mesh) is normally bare steel, and isn't
stored with any particular effort at weather protection. But it does
need to be used while fairly new, before the rust becomes thicker and
more flaky - at which point the rebar no longer meets the relevant BS
and is not fit for sale.

At first sight it all seems a bit hit-and-miss, being so dependent on
the turnover through the supplier's yard, and also being so dependent on
somebody's judgement about the state of the rust; but in practice it
works out all right for non-critical applications.


Also concrete starts off being highly alkaline which prevents
oxidisation. However carbon dioxide in the atmosphere penetrates the
concrete at about 1mm per annum which turns it acid. When this
'carbonation' reaches the rebar it will rust and in time spall the
concrete.

Once the rebar does start to rust again, the swelling will crack and
spall the concrete, which lets air and water in, and then the whole
thing goes very quickly.

The other killer is chloride ions (salt) in the concrete mix, which is a
big problem with sea walls.

At that point they start to think about protecting the rebar, although
that brings new problems of its own. A painted surface will not bond
well to the concrete, so the rebar has to be keyed into structure some
other way. I don't know specifically about galvanizing, but again the
soft zinc coating could weaken the surface bond. IOW there's a lot to be
said for plain bare rebar - it isn't just about cheapness.


There are many ways to prevent this happening. (by the way, wire
brushing rust on MS just polishes it)

The main way to prevent it happening is to keep to the standards for
minimum cover thickness. The aim is not to protect the rebar forever,
only to prevent corrosion becoming a problem within the design lifetime
of the structure.

Critical structures such as bridges and sea walls obviously have higher
standards of protection than yer average small building slab.



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Default rebar - why is mild steel okay?


visionset wrote:
I understand vanilla concrete is permeable, otherwise we'd not need a DPC.
So why is it okay to use mild steel reinforcement? Corroding steel breaks
up concrete a treat.

Puzzled...

--
Mike W


Aliied Steel & Wire in Cardiff used to have a division called Allied
Bar Coaters that pickled & epoxy coated mild steel rebar....someone
must have used it (or perhaps no-one did, IIRC they closed some years
back).



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Ian White wrote:




The main way to prevent it happening is to keep to the standards for
minimum cover thickness. The aim is not to protect the rebar forever,
only to prevent corrosion becoming a problem within the design lifetime
of the structure.


Another way of protecting rebar in critical situations is to de-rust it
completely with an abrasive (not wire brush) and coat it with epoxy
resin.

I am pretty sure you can still buy epoxy coated rebar.

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