Concrete Floor Slab - how to lay

[LeanTwo]

I'm at the point where the Building Control Officer has inspected the foundations to a small single story extension and I'm starting the preparation for the slab pour.  I have my compacted rubble/sand/insulation/dpm going in and want to check the next steps so the pour goes right.  The slab is only 2.5m X 4.5m and I'm having a mini-mix chute delivery (with retarder and fibre added). I've a small posse of helpers with wheelbarrows, rakes etc. at the ready.  

 

It would help to be able to get some tips on getting this right.  Are here any techniques that I should know about?  Maybe you have found a really good YouTube video? 

 

Any thoughts or guidance would be great.

[Onoff]

My 3x3m bathroom: I added an admix that has deaerator in to my mix, less air bubbles in it. I use it all the time now. I screwed screed rails either side and made a tamping board. Wanted it pretty accurate as I want to tile straight on top without having to screed.

 

 

Smooth and level as I think. Had to fettle things with a trowel by the end wall as it pushes the excess that way.

 

 

If you're screeding later and your footing bricks are pretty level then just tamp using that as a gauge.

Edited June 23, 2017 by Onoff

[Russell griffiths]

I'm sort off not understanding what stage you are at. 

You said foundations where in, have you built up your brickwork to dampcourse level? 

Or are you casting a slab that a frame and brickwork sits on? 

 

Maybe be a bit more of an explanation of what you have done so far, with pics, will get a few more answers. 

[LeanTwo]

Many thanks for the replies.  The screed rails look like a great idea and I'll certainly follow that one.  Do the rails just pull out after the concrete has cured and do the indents need to be filled?

 

What I have done on-site so far....

1. Excavated to get levels and trenches for foundations.
2. Constructed form-work from 12mm shuttering plywood.
3. Foundation concrete has been delivered and poured.  It has now been in-situ for five days.
4. Shuttering ply has been removed.
5. I am starting to fill the excavated area inside the foundations with hardcore and levelling it with scalpings, using a vibrating compactor.
6. Today, we have installed the sewer and sink waste connections and made provision for the rising water main.
7. I will finish with a layer of sand ahead of laying the dpm.
8. A layer of insulation (Kingspan or similar) will then be installed.
9. The dpm will extend over the inner first course of Thermalite blocks, which will also define the slab concrete, which will be poured over the insulation layer.  The cement mix will have a fibre and retarder additive.

The Building Control Officer has visited and approved all work to-date.

 

Any experience, tips or techniques for the slab pour would be very helpful.

[Onoff]

Erm...I SCREWED Unistrut to the stud walls either side then just unscrewed them when the slab had gone off. The tamping board I notched. These show it better:

 

 

 

[jack]

Nice use of unistrut @Onoff! 

[Onoff]

3 minutes ago, jack said:

Nice use of unistrut @Onoff! 

 

Those were heavy duty back to back 3m lengths. SPOT welds about every 100mm you need to drill out to separate. Also used as extension bars on occasion atop the car roof rack . Since recycled into shed shelving.

[jack]

Yeah, it's good stuff.  I still have a bit left over from the build.

[LeanTwo]

Thanks for the additional pictures Onoff.  They illustrate the technique very clearly.  The rails, I can see will give a very accurate levelling of the concrete.  I too was hoping to do away with a screed coat. I will be laying an engineered wooden floor on underlay on top of the slab. I would need to attach the rails to my first course of blockwork but the methodology seems very similar. I will have the protruding DPM to contend with which will get in the way a bit!  I guess that you chose the metal rails for strength and accuracy but would wooden rails do the job as well?

 

I notice that in your original set of pictures, you have divided up the slab into sections.  Is this the way to go forward so that you are only levelling small sections at a time?

 

The extension that I'm building is to enlarge our current, cramped kitchen diner.

[Nickfromwales]

The divided sections in Onoffs pics are for dedicated spaces, for the slightly sunken bath and the shower wetroom corner . 

Laying the slab is indeed in one section in that pic and all slabs, unless huge and requiring expansion breaks, should be poured in one go wherever possible. 

Timber for the guide rails will work just fine as long as they're relatively straight lengths. You could screw the timbers ( say 7x2" if that's what you'll retain and reuse later for the rafters ) to the outside of the block work with the top of the timber proud by an inch or so. That'll allow you to get it all perfectly flat without worrying about hitting the top of the block work. Simply use another 7x2" as the tamping board with timber 'lugs' added each end to drop the 7x2 to the required depth for the slab. The added be benefit of this method is you can simply lift the tamping board off after laying and smooth out the corners / end with a trowel. 

Check your levels a good few times before stating the pour and you'll be fine. . 

[LeanTwo]

Thanks for the additional explanation Nickfromwales.  I've not got a huge slab to cast but I want to get it as near the desired finish as possible.  The rafters idea is a good one as these should be sufficiently robust to keep their shape during the tamping process.  The roof that I'm constructing will be approximately 3.25m X 4.5 meters, with Double Roman clay tiles.  Not that I'm experienced in this but is a timber as big as 7"x2" needed?  ..but I'm happy to buy whatever is needed to get the slab pour done!

[Onoff]

As @Nickfromwales said my bathroom slab is not perhaps atypical of a slab like in your sense where there aren't any walls yet.

 

The divisions were just "shuttering" level with the top of the finished slab. The corner bit is for the wet room and will be laid to falls to the drain. The bath bit is slightly sunken so that the floor of the bath is level with the tiled floor in the bathroom. Figured it would lose some of the bath within the room as the bath is pretty massive and make for a "step free" / "level" experience getting in and out. A LOT of work just to lower the bath a bit but well worth it imo, it just feels "right". Drains are completely hidden too (50mm again thanks to @Nickfromwales and it empties super quick and is near silent).

 

You want a heavy tamping board tbh so it does its job compacting the mix. Two person job makes it easier, one each end.

 

Here's my shed slab, being filled. You can see the tamping board over the back by the fence. Here I used the shuttering to tamp against / for level

 

 

 

You can see the fan effect tamp lines one end as the board didn't fit where it was cut into the shallow slope. I didn't bother floating this afterwards btw...it's a SHED! Bang on though if you put a 6' level on it in any direction:

 

 

[Russell griffiths]

I would think you need to look at how you are going to get a good finish 

If you want this as you finish concrete before the final floor covering goes down, tamping with a board will not be good enough, you will have to trowel the surface up when it has started to set, you will need to make up a board system so you can reach the whole way across to the centre, you will only know when it is the right time to trowel up by having a go and looking at the finish, if it's to wet have a cuppa and try it again in ten minutes. 

[Onoff]

5 minutes ago, Russell griffiths said:

I would think you need to look at how you are going to get a good finish 

If you want this as you finish concrete before the final floor covering goes down, tamping with a board will not be good enough, you will have to trowel the surface up when it has started to set, you will need to make up a board system so you can reach the whole way across to the centre, you will only know when it is the right time to trowel up by having a go and looking at the finish, if it's to wet have a cuppa and try it again in ten minutes. 

 

I'm honestly more than happy with the levelness of my bathroom floor for tiling straight onto. I've dry laid some tiles and they sit fine one to the other. The adhesive bed I feel will take anything up.

[Russell griffiths]

2 hours ago, Onoff said:

 

I'm honestly more than happy with the levelness of my bathroom floor for tiling straight onto. I've dry laid some tiles and they sit fine one to the other. The adhesive bed I feel will take anything up.

I'm sure you are, you obviously put a great deal of effort in to get it right. 

But @LeanTwo has asked for ideas to get a good finish, I would hate for him to think that giving it a good tamping with a piece of timber is going to get him the finish he needs. 

And having some prior insight into how to trowel it up could help him out on the day. 

[Nickfromwales]

2 hours ago, Russell griffiths said:

I would think you need to look at how you are going to get a good finish 

If you want this as you finish concrete before the final floor covering goes down, tamping with a board will not be good enough, you will have to trowel the surface up when it has started to set, you will need to make up a board system so you can reach the whole way across to the centre, you will only know when it is the right time to trowel up by having a go and looking at the finish, if it's to wet have a cuppa and try it again in ten minutes. 

To be perfectly honest a trowel in the hands of a novice will likely make the surface worse, not better . Once you start catching and digging, rather than feathering and flatting it'll soon go Pete Tong. 

If you get the mix spot on and not too much water you'll be able to get the aggregate to stay submerged, as it's the aggregate surfacing and poking out which is the real pita. Those bits are like, erm, granite, and require chiselling or grinding flat so any investment for a newbie slab layer should be in the mix imo. 

Id recommend leaving an end of the lay slightly void of mix so where the tamping board stops, you can draw it clear from the slab without it 'sticking'.  Just back fill that little defecit with self leveller prior to final floor laying. Alternative, if you can, is to tamp towards a knock through ( door way ) and drag the excess mix back to the cavity void. Bag the void first so any spill can be easily removed when part cured. 

 

[LeanTwo]

Many thanks for the additional advice and experience.  I

 

I've got some sawn planks (40 mm x 90 mm x 3600mm) that were actually used for making a ramp to get a mini digger onto the plot, which I'm thinking of using to make tamping rails. I'm planning to run the rails along the shorter sides of the slab so allowing me to tamp more than once if required.  An alternative would be to run them along the longer side but I would have to fill and tamp as I go (so just the once).  This orientation would make a board system for trowelling possible (shorter boards between the rails).  I have a builder friend working with me who is very competent at troweling!

 

The mix is being made by a local company and I can vary the mix to my requirements so a dryer mix would be possible if this helps to get a good surface.  It has a 10mm aggregate in it, plus fibre and a retarder.

 

I've (hopefully!) attached some pictures from today.  They don't show the first block course, unfortunately as this is the next task after the hardcore/scalpings/sand layering.  The back wall of the ground floor plus window and door will come out and the small kitchen/diner behind will be incorporated into one room.