Concrete mix for footings...

[James Davis]

Hi all,

I've just ordered the concrete for the floor slab of my extension and they're sending C20 for it. I asked them what they used in my footings as my builder didn't specify a type when he spoke to them, just told them it's for the footings. They checked the records and said they sent us their "standard mix" C20. Said they don't send any other type unless specifically asked for. When I've checked online, there are a number of results suggesting this isn't suitable for a building, more for light weight structures, driveways and such. Thoughts? Bit disturbing to read that and a bit late as well. That said, I'm seeing completely different opinions on different websites. Some saying two storey extension on C20/gen3 is fine.

It is a single storey extension. Footings are mostly 250mm thick, but up to 900mm in one point on solid clay ground.

 

[James Davis]

Also, floor slab going in tomorrow will also be C20 and is having a steel mesh incorporated into it. Is C20 suitable for this application? I see some mixes are considered unsuitable where metal will be incorporated.

Thanks for any advice.

 

[stevie888]

C20 will be fine for both.

 

[James Davis]

Thanks!

It's weird because I ended up speaking to several different suppliers today and asked them about the footings. Two of them said "best go with C25", one said "we'd never use less than C25! Not worth the risk and then you can go up to two storeys and don't need to worry." Another said "C20 is fine, C25 max". Another said "C10 would be alright, or C20". Then I spoke to the technical person at cemex and he said C20 is standard mix used for domestic foundations and you'll be fine. Interestingly, he said he would go for harder on the floor slab, like C30.

It seems that C25 is only a tiny bit more expensive so I would've just used that had I known for the peace of mind, but it never came up at the time. I wonder why those companies seem so keen on pushing it? Unless they're trying to sound better but then give you C20 anyway. The truck that came here was one of those ones that does the mix on request and then they barrowed it in.

 

[JohnD]

For the floor slab, to make it harder and resist wear, water it and cover it with plastic when it's a day old to keep it wet. If you stop it drying for a month it will be stronger and harder than a week.

Foundations in the ground tend to be damp but cover the top unless it's raining.

 

[stevie888]

Thanks!

It's weird because I ended up speaking to several different suppliers today and asked them about the footings. Two of them said "best go with C25", one said "we'd never use less than C25! Not worth the risk and then you can go up to two storeys and don't need to worry." Another said "C20 is fine, C25 max". Another said "C10 would be alright, or C20". Then I spoke to the technical person at cemex and he said C20 is standard mix used for domestic foundations and you'll be fine. Interestingly, he said he would go for harder on the floor slab, like C30.

It seems that C25 is only a tiny bit more expensive so I would've just used that had I known for the peace of mind, but it never came up at the time. I wonder why those companies seem so keen on pushing it? Unless they're trying to sound better but then give you C20 anyway. The truck that came here was one of those ones that does the mix on request and then they barrowed it in.

All the concrete in a normal deep strip foundation needs to be really is as strong as the soil its sitting on: in effect it's only a spacer between the wall you're building and the soil that's supporting it. We go as deep as we go to get down below moisture induced ground movement. So yes C10 would do. (Occasionally you'll encounter sites where ground water mandates some chemical resistance requirement but this isn't
common.)

It's only if you're using ground beams on piles that you need to worry about strength and even then, counterintuitivly, the actual strength of the concrete makes very little difference to the beam design.

All concrete should be used as stiff as practical: it takes little extra effort and makes barrowing much easier. Excess water makes for weaker concrete with a powdery surface. All concrere should be cured.

 

[Notch7]

Ive never seen any builder specify concrete, the only thing they ask is "when is it coming, Ive got guys on site"

And TBH when Ive ordered concrete theyve never asked what spec

Footings are mostly 250mm thick

didnt the builder just do mass filled trench footing?

I didnt know builders still used brickies to do blockwork down in a trench -Ive never seen that down here in the South.

Usually the trench is filled up to around a block and say 3 bricks below DPC -on a levelish site

 

[Ivor Windybottom]

You probably only need to worry about the floor if the bare concrete will be the final surface, e.g. a shed or garage, so you need the extra hardness to resist wear. Otherwise after a few years you may end up with a pebbly surface and some sandy dust kicking about. If you're putting any kind of surface over then you shouldn't worry.

Any concern about rebar probably relates to having enough cement to stop it rusting. This will be irrelevant for a floor, as it will be above a DPM so won't get wet again after it's set.

But I'm not sure about the foundations. But it's what you now have so don't worry about it, definitely don't spend thousands removing and replacing it. It's probably stronger than your original house.

 

[James Davis]

Thanks for the replies.

The trenches were dug for the footings, 600mm wide, and then the builder filled with what turns out to be 300mm deep of concrete although in one of the corners stepped down to 900mm. He then used concrete block etc. above.

The floor slab is now in. The the bars were already covered in rust, perhaps stored outside at the merchants? The stupid plastic feet supplied by Thorncliffe got squashed as the barrow boys were wheeling over the bars (too far for a pump), so the mesh bars have ended up nearer the bottom than the middle in places and aren't exactly even throughout. I'm hoping they'll still serve some use and won't do more harm than good (!?) The slab is 150mm thick and I asked for C25.

 

[Ivor Windybottom]

Many builders use 100mm and no rebar. You'll be fine.

The rebar is intentionally rusty, so the concrete sticks to it.

All is well!

 

[James Davis]

But what about the mesh being somewhat uneven and not in the middle of the slab's depth (plus some squashed plastic feet things)... can this do any harm or is it more a case of the metal will be of some benefit just for being in there for the concrete to set around?

 

[stevie888]

The rebar is intentionally rusty, so the concrete sticks to it.

That's a new one!

 

[stuart45]

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?

 

[James Davis]

It was surface rust in my case. Like dusty outer coating. Not flaking levels of corrosion.

 

[James Davis]

****ing cats!! Walked all over the beautiful smooth concrete. To hell with them and their owners!