rebar - why is mild steel okay?

[visionset]

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...

[dom]

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).

[Rednadnerb]

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.

[Bookworm]

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)

[visionset]

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

[Guy King]

The message from " [e-mail]" contains these words:

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.

[Newshound]

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.

[Phil L]

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.

[Weatherlawyer]

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.

[The Natural Philosopher]

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

There have been concrete ships built before now..

[dom]

Free of detaching, scabby rust.

[Ian White]

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.

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.

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.

[Phil]

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).

[Bookworm]

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.