Ground bearing slab curing

[Mr Punter]

Your biggest challenge will be to get the concrete in the right place, as once it is poured you won't be moving it.  If you can get the lorry near all the trenches, fantastic.  If not you would do well with a concrete pump - either boom or line type.  You will need minimum 3, preferably 4 people, plus the pump guy and concrete truck driver.

 

Work out exactly what you are doing before anything turns up.  How will the concrete lorries get in and out, traffic management etc.  6m is very wide to screed to.  Can you get some pegs in the ground as levels?

 

If there is a screed layer on top, the accuracy of the slab doesn't matter too much.

[saveasteading]

21 minutes ago, Mr Punter said:

If there is a screed layer on top, 

We don't know much about it at all!

The questions are not answered and I fear that the author doesn't know how much he doesn't know.

It is out of concern that I keep trying to help. 

@flanagaj please explain what the construction of the floor is and what stage you are planning.

[flanagaj]

32 minutes ago, saveasteading said:

We don't know much about it at all!

The questions are not answered and I fear that the author doesn't know how much he doesn't know.

It is out of concern that I keep trying to help. 

@flanagaj please explain what the construction of the floor is and what stage you are planning.

Apologies for the confusion.   I am merely talking about the ground bearing slabs and it has nothing to do with pouring the screed layer.

The slabs are basic.  150mm thick reinforced with A252 mesh and all prep work signed off by BC last Friday.  Slabs for house will eventually be covered with 150mm PIR, VCL and then screed.  Garage slab will be left as is and hoping that SCC will provide an adequate finish and negate having to use another finishing self levelling compound.  So just to reiterate and avoid confusion, this is simply about placing the concrete for the ground bearing reinforced slabs.

[saveasteading]

OK thanks. This is what I assumed. 

It is a slab not a screed.

Flattish will do but it is a shovel, raje and float (or tamp) job, not tripods and dapple bar.

I suggest you bang some steel or batten in to finished level. Not best practise but pragmatic. You'll be too busy shovelling to check with a laser.

 

How many of you?

 

Quantity is easy as you have a simple shape and presumably a consistent level.

[flanagaj]

40 minutes ago, saveasteading said:

OK thanks. This is what I assumed. 

It is a slab not a screed.

Flattish will do but it is a shovel, raje and float (or tamp) job, not tripods and dapple bar.

I suggest you bang some steel or batten in to finished level. Not best practise but pragmatic. You'll be too busy shovelling to check with a laser.

 

How many of you?

 

Quantity is easy as you have a simple shape and presumably a consistent level.

2 of us.  I think I'll be raking as SCC is too liquid to be shovelling.   

[saveasteading]

I've never used SCC so perhaps others can advise of any special measures.

 

I.m guessing here, but perhaps a length of 2x2 would be handy to persuade it all the way to the far corner.

 

Any shuttering will be prone to leakage as will gaps in plastic.

Have you a plan for the surplus and the washout location? Wherever you tell them to put it, it will keep flowing! 

 

If it is like pumped screed, it is like water until suddenly it isn't.

ripples won't matter at all but getting it level +/- 3mm is well worth while.

 

[Gus Potter]

On 20/07/2026 at 23:38, Gus Potter said:

It's time son to get your big boys hat on and wake up and smell, the coffee.

 

This is tough love

 

12 hours ago, flanagaj said:

The concrete company I was planning on using wouldn't put anything in writing, so I've cancelled them and am going to pour SCC slabs myself.  Ordered the items below

Ok, my suggestion is that you don't rush this. It's a key step as if this does not turn out the way you hope you'll store up loads of problems that you'll have to deal with later.

 

The great thing about BH is that what your are doing is quite common and folk will fesse up the mistakes they have made, so you don't repeat them. We just want to help you out! There are few if any on BH that will relish hearing that it has gone wrong for you when we could have helped. 

 

In that context. You say you have a ground bearing slab and that BC have signed off what you are going to lay your slab on. Thus it may be assumed that it is reasonably flat and level. But BC won't have checked this for you!  In general terms level means that what you are laying on does not vary too much from one end of the building to the other. Flatness is roughly a measure of call it "undulation" between often 3.0m points. A slab can be level for corner extremities but go up and down like a roller coaster in between. I suspect you don't know what tolerances you should be working to. This is your SE's fault possibly, it may be buried somewhere in you specification? 

 

You say you have a 150mm thick slab. Roughly if what you are laying on is 15mm high in places then this can start to compromise the cover to the mesh. If the top side of your slab coincides with a high spot in the sub base it really eats into the performance of the slab as you are reducing what we call the effective structural depth. Ok your sub base is down now. We can have a look at how and where you finish the top of the slab. 

 

Now as you are doing this your self I can tell you that slabs often end up high. Contractors, in  rush often leave slabs high also more often than not, bugger off and leave someone else to fix it. A over high slab impacts on everything that comes after. 

 

But lets turn back to structural strength first and a bit of rough techy stuff. You SE will have specified a basic structural strength of the concrete. If they are doing their job correctly they should specify a level tolerance both on what you are laying on and the top surface. I do this as a matter of course,  it can go horribly wrong if you don't as you can end up with a slab that is only say 120mm thick in places.. and it's sods law that the thin bit will be under the point where there is a bit more load. 

 

Your basic might be something like a C28/35 concrete, there are lots of way we can specify concrete. It's then up to you / contractor to find a mix that can be work into place / p[properly compacted to meet the level tolerances and concrete strength.

 

Basic concrete is made by mixing cement, concrete sand and water. When the water evaporates we have voids in the concrete, like the cell structure in wood. Thus we want to design a mix that is workable enough, but does not have too much water as this reduces the concrete strength. We call this the water / cement ratio. We measure how workable the concrete is by using often called  the "slump" test. Google this and read around. It will help you lots!

 

Now we can also add things to the concrete, other chemicals and powders which also can slow down how fast the concrete cures and remains workable. One easy way is to use what we call an air entraining agent, similar to Fairy liquid (never add Fairy liquid to any concrete of mortar mix, it's dangerous) , the bubbles of air make it more workable. Modern concrete mixes use other chemicals and agents that make the concrete more workable (basically lubricate the mix) but without reducing the matrix strength. You mention SCC (self compacting concrete) this is often also referred to as self levelling concrete. 

 

But the big problem with SCC or other self levelling mixes is that they need to be handled by experts that do it day in and day out. They can very quickly "turn on the novice" within a matter of minutes. If they turn on you then they are totally unforgiving!

 

My advice is to look at a basic concrete mix with a bit more water, what we call a higher slump. Phone up a few suppliers, tell them what you are trying to do, your labour limitations, the weather you expect and the size and type of slab you are laying.

 

If you make the right approach nine times out of ten you'll find folk itching to help you out!

 

In my day job as an SE I often design quite "whacky stuff" I have a good basic knowledge but often find myself phoning the experts on a particular product to see where and when we can safely explore boundaries of design. I do check what they tell me, but I'm long past the point where I feel embarrassed to ask for expert advice, or just ask a daft question.

 

If it's ok for me to do it then it's ok for you to call up a few concrete suppliers and ask for help? You have nothing to lose but a bit of time. 

14 hours ago, flanagaj said:

Ordered the items below

You need to reflect on what you are doing. It's ok to take one step back to make two forward later. 

 

[flanagaj]

11 hours ago, Gus Potter said:

 

Ok, my suggestion is that you don't rush this. It's a key step as if this does not turn out the way you hope you'll store up loads of problems that you'll have to deal with later.

 

The great thing about BH is that what your are doing is quite common and folk will fesse up the mistakes they have made, so you don't repeat them. We just want to help you out! There are few if any on BH that will relish hearing that it has gone wrong for you when we could have helped. 

 

In that context. You say you have a ground bearing slab and that BC have signed off what you are going to lay your slab on. Thus it may be assumed that it is reasonably flat and level. But BC won't have checked this for you!  In general terms level means that what you are laying on does not vary too much from one end of the building to the other. Flatness is roughly a measure of call it "undulation" between often 3.0m points. A slab can be level for corner extremities but go up and down like a roller coaster in between. I suspect you don't know what tolerances you should be working to. This is your SE's fault possibly, it may be buried somewhere in you specification? 

 

You say you have a 150mm thick slab. Roughly if what you are laying on is 15mm high in places then this can start to compromise the cover to the mesh. If the top side of your slab coincides with a high spot in the sub base it really eats into the performance of the slab as you are reducing what we call the effective structural depth. Ok your sub base is down now. We can have a look at how and where you finish the top of the slab. 

 

Now as you are doing this your self I can tell you that slabs often end up high. Contractors, in  rush often leave slabs high also more often than not, bugger off and leave someone else to fix it. A over high slab impacts on everything that comes after. 

 

But lets turn back to structural strength first and a bit of rough techy stuff. You SE will have specified a basic structural strength of the concrete. If they are doing their job correctly they should specify a level tolerance both on what you are laying on and the top surface. I do this as a matter of course,  it can go horribly wrong if you don't as you can end up with a slab that is only say 120mm thick in places.. and it's sods law that the thin bit will be under the point where there is a bit more load. 

 

Your basic might be something like a C28/35 concrete, there are lots of way we can specify concrete. It's then up to you / contractor to find a mix that can be work into place / p[properly compacted to meet the level tolerances and concrete strength.

 

Basic concrete is made by mixing cement, concrete sand and water. When the water evaporates we have voids in the concrete, like the cell structure in wood. Thus we want to design a mix that is workable enough, but does not have too much water as this reduces the concrete strength. We call this the water / cement ratio. We measure how workable the concrete is by using often called  the "slump" test. Google this and read around. It will help you lots!

 

Now we can also add things to the concrete, other chemicals and powders which also can slow down how fast the concrete cures and remains workable. One easy way is to use what we call an air entraining agent, similar to Fairy liquid (never add Fairy liquid to any concrete of mortar mix, it's dangerous) , the bubbles of air make it more workable. Modern concrete mixes use other chemicals and agents that make the concrete more workable (basically lubricate the mix) but without reducing the matrix strength. You mention SCC (self compacting concrete) this is often also referred to as self levelling concrete. 

 

But the big problem with SCC or other self levelling mixes is that they need to be handled by experts that do it day in and day out. They can very quickly "turn on the novice" within a matter of minutes. If they turn on you then they are totally unforgiving!

 

My advice is to look at a basic concrete mix with a bit more water, what we call a higher slump. Phone up a few suppliers, tell them what you are trying to do, your labour limitations, the weather you expect and the size and type of slab you are laying.

 

If you make the right approach nine times out of ten you'll find folk itching to help you out!

 

In my day job as an SE I often design quite "whacky stuff" I have a good basic knowledge but often find myself phoning the experts on a particular product to see where and when we can safely explore boundaries of design. I do check what they tell me, but I'm long past the point where I feel embarrassed to ask for expert advice, or just ask a daft question.

 

If it's ok for me to do it then it's ok for you to call up a few concrete suppliers and ask for help? You have nothing to lose but a bit of time. 

You need to reflect on what you are doing. It's ok to take one step back to make two forward later. 

 

My main motivation for using SCC was merely due to the following.

1)  Short of labour.  It's just my wife and I
2) Due to the bricklayer rushing ahead and not being very accurate, we don't have the ability to tamp, plus tamping is hard going when only two of you.

Why do you say SCC needs to be handled by experts.  I'm very curious as the video below suggests otherwise.  I appreciate that it's a marketing video, but I wouldn't say there's anything expert going on here.
 

 

[Mr Punter]

It is self compacting, not self levelling.  No need to tamp but you need to place it to the correct level.  Do you have a boom type pump lorry?

[saveasteading]

27 minutes ago, flanagaj said:

Why do you say SCC needs to be handled by experts.

We are concerned for you, perhaps unnecessarily.

It's what you don't say.

 

Can the lorry get close?

Can its chute reach nearly all the way so that you don't have to move it?

what quantity is it?

Is the mesh already in and has the bco approved it?

Are you both physically strong in handling the chute and moving it (it's very heavy esp with concrete in it. You'll be doing this while walking in wet concrete on a layer of steel mesh.

NB the driver does not help! He turns the delivery on and off when you shout. Later he washes out.

You've probably got 30 minutes from arrival to completion or they charge a lot.

you are going to get messy.

wellies, waterproofs, gloves.

 

After the concrete is in and the lorry gone, have you asked how long it is workable?

You have not got the time or luxury of using the laser. 

there wont be any break time or rest for these 2 hours. have you a contingency if one of you has a problem?

Can you find a third person?

 

what is containing the slab? masonry or a shutter?   mark the finished level on there in advance plus some pegs in the middle.

 

Have you worked out the quantity exactly? what is it? how much have you rounded it up? the leftover is all going on your ground. 1/2m2 is a lot esp when it spreads or makes a solid lump.

 

and so on. expertise.  I have supervised maybe 300 pours from 4m3 to hundreds. I seldom handled the shovel. But I've seen it go wrong often. I've especially seen it go badly wrong on other peoples projects, esp by amateurs and poor builders, and it is a shame, a waste of money and can have knock-on effects.

 

to reiterate : It's what you don't say.

If you knew you wouldn't be asking, and you haven't asked the right questions.

 

 

 

[flanagaj]

1 hour ago, saveasteading said:

If you knew you wouldn't be asking, and you haven't asked the right questions.

My question and I wish I hadn't bothered asking was merely to understand whether SCC is a suitable choice for ground bearing slabs.  All of the other due diligence regarding shuttering / vehicle access / ensuring DPM is sealed with butyl tape and edges taped, mesh correctly installed, BCO signed off has been done.   Just for the record, I am not a complete muppet.  

[flanagaj]

1 hour ago, Mr Punter said:

It is self compacting, not self levelling.  No need to tamp but you need to place it to the correct level.  Do you have a boom type pump lorry?

The truck can reverse right up to the slab area and with the 2.4m chute will pretty much be able to get the concrete into the middle of each bay. 

[saveasteading]

2 hours ago, flanagaj said:

, I am not a complete muppet

OK. So good luck with it, and report back please, as I've never used SCC.

[flanagaj]

1 hour ago, saveasteading said:

OK. So good luck with it, and report back please, as I've never used SCC.

Yes.  I will report back, even if it goes horribly wrong.  Hopefully, it won't.  Would be nice if the weather a bit cooler, but I need to get the slabs down as I don't like leaving the membrane and still uncovered as one will degrade with UV and the other will just rust more.

[Mr Punter]

41 minutes ago, flanagaj said:

Would be nice if the weather a bit cooler

 

Wednesday looks hot as hell, so worth avoiding.

[saveasteading]

3 hours ago, flanagaj said:

will just rust more.

That doesn't matter. In fact it grips the concrete better than non-rusted, and without water it won't change.