Self Compacting Concrete?

[Duncan62]

Any one had any experience with Self Compacting Concrete (SCC)?

 

https://en.m.wikipedia.org/wiki/Self-consolidating_concrete

 

https://www.breedongroup.com/products-and-services/gb/our-products/ready-mixed-concrete/self-compacting-concrete

 

 

We have an insulated raft, with half the rebar in currently and are looking at options for the pour.

 

Slab is ~100m2 at 200mm thick.

 

One Suggestion is for SCC.

 

It can be made to the correct strength spec, in our case C35.

 

Apparently, it is likely to give a more level finish than traditional + power floating.

 

It will also fill underneath our threshold insulation without voids at the edge, hopefully.

 

 

[FuerteStu]

I've seen it used and because of its viscosity you need to pay extra attention to potential places where it can 'blow out' or leak.

 

The last thing you want is thermal bridges everywhere because your 5mm gap at the edge stretched to 50mm with the weight of the very wet concrete above. 

 

As with most concrete pours, the people doing it will decide whether it's actually good or not. Not the architect who is selling the spec

[Duncan62]

2 minutes ago, FuerteStu said:

As with most concrete pours, the people doing it will decide whether it's actually good or not. Not the architect who is selling the spec

 

Why is it more susceptible to blow outs?

 

Will this affect us? we only have a 200mm slab with (Jackon) insulation making the form work.

 

Thank you, in our case it's the ground work team we may use, suggesting it.

[FuerteStu]

For only 200mm only I'd use regular concrete and make sure its given a good flow into your voids with a mechanical poker. 

 

Powerfloating is just like plastering, it's an art and reliant upon timing as much as effort. Get someone very experienced to do it and the job will be much better

[FuerteStu]

Just now, Duncan62 said:

 

Why is it more susceptible to blow outs?

 

Will this affect us? we only have a 200mm slab with (Jackon) insulation making the form work.

 

Thank you, in our case it's the ground work team we may use, suggesting it.

Regular concrete isn't as runny. So it doesn't force itself into the gaps as much. Which is actually a blessing in domestic slabs which are usually only pinned at the edges with a few spikes into mud. 

"is only a thin pour, that will hold.." but it often doesn't

 

Are the edges strutted diagonally on blinding? Or just staked? 

 

[Duncan62]

22 minutes ago, FuerteStu said:

Are the edges strutted diagonally on blinding? Or just staked? 

 

 

Neither. In our case the insulation forms the formwork with an upstand.

 

Pic below. Our upstand is 200mm high and 200mm think, unlike the pic below.

[BotusBuild]

Whatever you do, DO NOT rely on the upstand itself to withhold the pressure of the concrete. BRACE, BRACE, BRACE as they say when the plane is about to ditch.

 

We used Jackon for our slab foundation, and it was very well braced come pour time.

[Nickfromwales]

14 - timber lining? I wouldn't want wood there! Why isn't that cement board also?

[saveasteading]

Perhaps I am missing something but I can't see why this stuff is necessary. 

It seems to be a concrete recipe with plasticiser and a very high 'fines' content. 

Like a pump mix but with even more sand.

You might choose it in a structure that was crammed with reinforcement so that it gets through the gaps.

Or because it stops ignorant workers adding lots of water for their convenience. 

I think it still needs skill and levelling and floating.

On the other hand it is just a thick screed so anything will do.

 

Can anyone explain "Only" 200mm? This is warehouse with 10 tonne forklift thickness, except you wouldn't put eps under it.

 

 

[JohnMo]

3 hours ago, Duncan62 said:

Slab is ~100m2 at 200mm thick.

 

One Suggestion is for SCC

Small area, normal pumped C35 will be fine, sounds like the ground worker are looking for an easy life, but doing a smallish slab isn't that difficult, with the right tool (even without them)

[Duncan62]

3 hours ago, BotusBuild said:

Whatever you do, DO NOT rely on the upstand itself to withhold the pressure of the concrete. BRACE, BRACE, BRACE as they say when the plane is about to ditch.

 

We used Jackon for our slab foundation, and it was very well braced come pour time.

 

Awesome, thank you. Do you have any pictures of how you braced it please?

[Duncan62]

1 hour ago, Nickfromwales said:

14 - timber lining? I wouldn't want wood there! Why isn't that cement board also?

Maybe a mistake.

Will be using cement board as you say.

[Duncan62]

1 hour ago, saveasteading said:

Perhaps I am missing something but I can't see why this stuff is necessary. 

 

It gives a easier level finish than traditional power floating I'm told.

 

Keen for a level floor.

Especially as it will be the final floor level + microcement at the end of the build.

[BotusBuild]

27 minutes ago, Duncan62 said:

any pictures of how you braced it please?

Not of the final bracing, but it was follows:

Scaffold boards laid sideways, then heavy concrete blocks every 2m or so, then some extra packing before driving 12mm rebar offcuts 600mm or more into the ground (so each piece of rebar was about 1200mm in length.

Our pad is partial sub basement, so where we were in a hole (like most of the build time 🙂) I also braced between the side of the hole and the scaffold boards for extra security.

Might be overkill, but worth the peace of mind, and not having to run around like a headless chicken at pour time if anything went wrong

[saveasteading]

1 hour ago, Duncan62 said:

Keen for a level floor.

It isn't easy. It is a skill.

Self levelling screed will do exactly that because it is very highly plasticised and is all fines.

I suspect this product will flow with less shovelling than a normal mix, but will still need tamping and floating.

Even a powerfloat finish has a 3mm flatness  tolerance.

But I know nothing about microcement, except that is a hand-trowel applied screed. so can't advise on what accuracy or finish  you need below it. 

 

100m2 will require crack control joints.

[Chanmenie]

I had self compacting fibre reinforced concrete in my slab.

The advantages were. I didn’t need rebar which saved quite a bit, it didn’t need floating which saved time, there’s no laitance which saves time and money, and not sure if it’s true but was told it would transmit the heat from the UFH pipes better as the fine aggregate make it more like a screed 

[saveasteading]

35 minutes ago, Chanmenie said:

self compacting fibre reinforced concrete

Tell me more @Chanmenie

Do you still have the spec for it?

 

In my experience the fibres make it very heavy and sticky to handle. Isn't the additive just to reduce  that a bit?

I agree re the rebar.

But then you say it didn't need floating. This sounds like cemfloor which self levels because it is incredibly fluid. I wonder how much fibre content could go in that before it became sticky and stuck in the pump hose.

 

I'm still interested in this self-compacting claim. There are things I haven't heard of , of course, but on structural concrete you would still be using vibrating pokers to get any trapped air out of it,  to move it along, and to compact it. I wonder if it is a fancy name for  'runny', and a bit of air in the concrete doesn't really matter in a domestic slab.

[Duncan62]

1 hour ago, Chanmenie said:

I had self compacting fibre reinforced concrete in my slab.

The advantages were. I didn’t need rebar which saved quite a bit, it didn’t need floating which saved time, there’s no laitance which saves time and money, and not sure if it’s true but was told it would transmit the heat from the UFH pipes better as the fine aggregate make it more like a screed 

 

Wish I'd known about fibre reinforced. The amount of mesh in my slab = considerable

[Duncan62]

57 minutes ago, saveasteading said:

I'm still interested in this self-compacting claim. There are things I haven't heard of , of course, but on structural concrete you would still be using vibrating pokers to get any trapped air out of it,  to move it along, and to compact it. I wonder if it is a fancy name for  'runny', and a bit of air in the concrete doesn't really matter in a domestic slab.

 

Take a look: https://www.concretecentre.com/Specification/Special-Concrete/self-compacting-concrete-(SCC).aspx

 

Short video with very basic explanation.

[Mr Punter]

The water reducing additives - superplasticisers - allow the concrete to flow easily without addition of excessive water, keeping the concrete strength high and the water content low.

 

If you use this in formwork and there are any "issues", it pisses out pretty fast.

 

I wouldn't bother on a slab if the concrete crew are good.

[SteamyTea]

I would like to know more about different concrete mixes.

Having been involved in composite plastics for decades, the holy grail was to choose the reinforcement with the right characteristics, then mould it with the least amount of resin.

This was often misunderstood by many people, including engineers from different fields, but more usually supervisors on bonuses, this caused higher rejection rates and mechanical failures.

Got composites a bad name as well.

I blame Reliant Cars for much of this. Clever people at the development companies created fantastic systems, then let c***ish twats from Tamworth decide they knew better.

[saveasteading]

1 hour ago, SteamyTea said:

reinforcement with the right characteristics, then mould it with the least amount of resin.

I think it is a very different purpose.

Fibres in concrete are only there to resist visible cracking, by causing millions of tiny cracks and slightly helping to spread the load.

Steel mesh in the middle of a slab does the same.

For strength though (beams and suspended slabs) the steel is working in tension or compression with the concrete also working in compression. Keeping them apart makes the steel work more like an I beam, with the middle concrete keeping them apart. 

That's why I am surprised to see all those domestic floor slabs on BH with heavy steel mesh top and bottom. Nobody is going to fall through the floor, even with zero steel.

Perhaps the worry is dropping a piano from some height when there is eps under the slab.

 

On our Highland project I argued back to the compulsory external SE that it was overdesigned and they (reluctantly) agreed. Saved us £20k or so by shifting from 2 layers of heavy mesh , to fibre anti-crack.

 

Does that begin to answer your question or is it off track?

[SteamyTea]

6 minutes ago, saveasteading said:

Fibres in concrete are only there to resist visible cracking

 

7 minutes ago, saveasteading said:

Does that begin to answer your question or is it off track

Kind of, but not really.

 

Composites are complex structures and different 'mixes' are often used to get different characteristics, I suspect concrete mixes are similar.

One thing that additives can do is change the curing rates, and who the curing takes place i.e. cure from the middle outwards or cure from outward to the middle.  It is very hard to get an even cure though the whole material at the same time.

But yes, we are going off topic a bit, but it does highlight that 'concrete' is not a simple product that can be just used without regard to seeing how it fits into the whole.

[Duncan62]

1 hour ago, saveasteading said:

Saved us £20k or so by shifting from 2 layers of heavy mesh , to fibre anti-crack.

 

That's a big number, congratulations, fantastic result.

 

Our two layers of mesh cost less than 3k (Inc detached garage) so not a massive amount for us (in the grand scheme, 3k is obvs 3k tho!)

[Duncan62]

Mesh spacing for our garage 150mm slab.

 

Bottom cover 75mm

Top cover 25mm

2x layers of mesh is 20mm + 20mm = 40mm.

 

So 75+25+40= 140mm = 10mm space between the layers... 

 

Doesn't seem worth having two layers?!