Concrete mix for footings...

Ivor Windybottom · Saturday at 2:36 PM

I was told it was accepted and desirable for rebar to be rusty, by a civil engineering lecturer way back in the 20th century! The pitting helps the concrete grip, and it all gets completely neutralised/preserved by the cement anyway. It seems logical, there's some evidence supporting it online but I really don't know enough to get into a keyboard war about it.

It can be placed in the middle or top or bottom, depending on the needs. For a beam it's normally near the bottom as that's where the tension is. For a slab it's often in two layers, one is near the bottom and the other near the top, as you don't know in advance where the compression and tension will be. In the middle arguably helps a little either way. Whatever though, it needs to be covered, i.e. at least a reasonable depth below any exposed surface. As yours will be inside a building it doesn't matter at all. Just as long as it's not sticking out so you'll trip over it on the way to the fridge you have nothing to worry about!

But what you have is way beyond what many builders use so don't worry.

Cats are a hazard! I had a few marks from our little angels in the past. They learn though, after I'd done a load of concreting while sorting the garden I saw ours walk up to the edge of a new bit and gently press it with one paw. Then only after checking it was solid she walked on it. Bless.

James Davis · Saturday at 2:48 PM

These ones didn't check. Plodded right through whilst it was still soft on the surface and put deep prints in it! Perhaps they like the smell of manure, which is the odour this slab is giving off?

Thanks for the other information. My builder said it used to be laid at the bottom of the slab without these feet things on. He's been doing extensions for decades. So I suppose even if it didn't end up as well placed as we intended, there are still steel bars in a 150mm C25 slab so I'll be unlucky if I have any problems. Beyond the cat prints!!

Ivor Windybottom · Saturday at 2:54 PM

Lying on the bottom was definitely very wrong. It needs to be encased or it will rust. Some builders go for years doing stupid things as it's the way they've always done it.

I put mine in the middle resting on concrete half-bricks, as they happen to be about half of the 150mm slab depth. Worked for me, stayed supported despite me and my barrow and it's all concrete now, without holes where the plastic cup things would have been if I'd done it properly.

Dereekoo · Saturday at 3:02 PM

only reason to put a single layer of mesh in the top face of a slab would be to control drying shrinkage to reduce size of cracks which should not be a problem anyway if the water cement ratio is around 0.45 and the mix is not to rich in cement hence C20 and below is preferable.

James Davis · Saturday at 3:10 PM

Ivor Windybottom said:
Lying on the bottom was definitely very wrong. It needs to be encased or it will rust. Some builders go for years doing stupid things as it's the way they've always done it.

I put mine in the middle resting on concrete half-bricks, as they happen to be about half of the 150mm slab depth. Worked for me, stayed supported despite me and my barrow and it's all concrete now, without holes where the plastic cup things would have been if I'd done it properly.

Yes, concrete half bricks would've been better and my builder regretted using the plastic things. Those plastic ones have all openings in them so I'm not exorcting holes in the concrete, but they were rubbish. On laying the metal at the bottom, would it rust if there is a DPM and it is otherwise covered in thick concrete? Mine ended up more like an inch off the bottom in places where barrows went over but mostly higher. The plastic things would probably be good for pumping. Totally inappropriate for barrowing.

James Davis · Saturday at 3:11 PM

Dereekoo said:
only reason to put a single layer of mesh in the top face of a slab would be to control drying shrinkage to reduce size of cracks which should not be a problem anyway if the water cement ratio is around 0.45 and the mix is not to rich in cement hence C20 and below is preferable.

Kinnell, we come full circle.

The informational world keeps confusing.

Ivor Windybottom · Saturday at 3:31 PM

Mesh on top is for any situation where tension will occur across the top. E.g. over the top of a pillar or wall that's supporting from beneath, like a cantiliever. Or, in the case of a floor slab, where a random part of the crushed rock below ends up supporting its surrounding area due to that around it sinking.

But usually slabs and beams span a gap and are supported at their ends, so the rebar lower down is more important. In reality, the rock below the centre is where it will sink, the edges will be propped up by the wall foundation just beneath.

But in actual reality it would probably have been absolutely fine without it at all, provided the rock below was well compacted.

James Davis · Saturday at 3:37 PM

Yeah there was some "cow's belly" earth below so building inspector suggested mesh as precaution. Probably wasn't needed. Could the situation I've described do any harm then, or is it just a case of metal in there, anywhere, will do some good, but worst case, neutral?

He should've just got the concrete blocks but stupid thorncliffe didn't have the ones he usually gets and then persuaded him that they sell loads of the plastic things. Advice from a retail clerk probably best strengthening with common sense. They seem a bit shoddy at that place. Engineering bricks were all chipped and covered in black ****. Insulation boards all chewed up despite costing a fortune.

noseall · Saturday at 3:39 PM

Ivor Windybottom said:
I put mine in the middle resting on concrete half-bricks,

We usually put a layer of concrete down first...

mesh.jpg

noseall
Saturday at 3:38 PM

Ivor Windybottom · Saturday at 3:43 PM

It'll all be fine. Besides, it's done now so what you going to do? Smash it out and replace it? All designs have loads of safety margin to allow for reality.

James Davis · Saturday at 3:50 PM

I suppose not. Just very frustrating to pay for contingencies and find I'd probably have been better off not bothering at all. I'm surrounded by morons who have to be checked constantly. You cannot trust anybody in this damned country.

JohnD · Saturday at 4:14 PM

Is there still time to spray it with water and float the footprints?

Edit

No, it was yesterday.

stevie888 · Saturday at 6:32 PM

stuart45 said:
I've never heard of it being intentionally rusty. It normally has a bit of surface rust. If you didn't use rebar with a bit of rust, there'd be of lot of it dumped.
There have been tests done which say that a small amount of surface pitting and rust is acceptable, and might even help with the bond, but not all the tests say it helps bond it.
It's not acceptable to use flaky rebar, even though it does get put in.
@stevie888 , As you're an engineer, at what point in the rusting process would you stop the rebar going in?

A bit of rust is ideal because it gets rid of the mill scale that you get on new steel..

By the time the steel has been handled cut bent carted and fixed anything loose is long gone and as others have said the alkaline environment within the concrete protects the steel.

I have only once had a major issue with rust and that was where our contractor had fixed a slab and then for various reasons had left it for months and when he was finally ready to pour to steel was flaking rust and we had to refuse to pass the pour until it was cleaned. I think a pressure washer would have done the trick but he opted to employ a short length of chain rubbed up and down (it took a long, long time!!)

The right way to position mesh is to put it in place befoe concrete and yes concrete blocks (old school) are generally easuier than bits of plastic. If barrows are to be run over the mesh then a few thicker blocks to lift the boards above the mesh is all that's required. Noseall's approach is all well and good but a big slab on a drying day and you have the makings of a cold joint at the steel level.

Even more sketchy is the tread it in after pour approach loved by some cowboys.

Ivor Windybottom · Sunday at 1:09 PM

stevie888 said:
Noseall's approach is all well and good but a big slab on a drying day and you have the makings of a cold joint at the steel level.

I don't think it's a good idea at all. Definitely not how it's done in construction. The steel is unlikely to be encapsulated by the top pour, and the join means it's effectively two 75mm slabs rather than one 150mm, which would be a lot weaker.

Prop it on something, then concrete under and over in one pour.

noseall · Sunday at 2:50 PM

Ivor Windybottom said:
I don't think it's a good idea at all. Definitely not how it's done in construction. The steel is unlikely to be encapsulated by the top pour, and the join means it's effectively two 75mm slabs rather than one 150mm, which would be a lot weaker.

Prop it on something, then concrete under and over in one pour.

That whole post is nonsense. It's all done in the same pour.

You do post some crap.

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Concrete mix for footings...

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