Dom's workshop build

[will1983]

Also concrete has a density of 2.4T/m3 so don't underestimate its capacity for knocking over formwork or as you suggested, heaped muck.

On the positive side though it's not like water, it is a cohesive material so doesn't want to move unless it is agitated by a poker.

 

[DomD]

Ok thanks for that advice; I am aware of how heavy concrete can be after watching some formwork bending severely on youtube - I will try to avoid that with pegs every 2ft driven in with a sledgehammer.

I ideally want to use S2, but am I correct in thinking they will use a more dilute 'pump mix' if pumping? If so I suppose that would increase the risk of concrete flowing through gaps.
Do you think a vibrating poker is necessary considering only the front trench of my foundation reaches ~600mm?

Concrete pouring is certainly the bit I'm most concerned about in this build!

 

[DomD]

I sort of fixed the problem by lowering the formwork - this will mean the slab is ~100mm thick in the middle. I will then be able to use the method of filling the trenches, covering concrete at sides with earth and filling to the top.

 

[DomD]

Three sides now squared and pegged down. I have 50mm posts every two feet which I suppose should be enough (all go at least 300mm below ground).
As for level - it is as close as I can get but still not perfect (within two center lines of bubble level but not perfectly centered at all points). I think this could be down to slightly bent timber and the cleat I had to make on the long side doesn't help.
There is still loose sandy soil on the surface of the filled area - will this be problematic? Compacting with a sledgehammer doesn't help.
Thanks

 

[DomD]

Form fully layed out now, still need to add a few more pegs. Then it will be hardcore, sand, dpm and steel mesh.

The diagonals are within 1/2in, is that an acceptable tolerance?

 

[Woody2Shoes]

Ok thanks for that advice; I am aware of how heavy concrete can be after watching some formwork bending severely on youtube - I will try to avoid that with pegs every 2ft driven in with a sledgehammer.

I ideally want to use S2, but am I correct in thinking they will use a more dilute 'pump mix' if pumping? If so I suppose that would increase the risk of concrete flowing through gaps.
Do you think a vibrating poker is necessary considering only the front trench of my foundation reaches ~600mm?

Concrete pouring is certainly the bit I'm most concerned about in this build!

Hi - if pumping, the mix will be slightly wetter than it might be otherwise. If your formwork is square to the the nearest 1/2 inch that's probably just fine. If it were me doing it, I'd put down a sheet of DPM across the whole thing - wrapping the edge over your formwork and fixing it along the edge with battens. Two reasons for this:
1) the concrete will cure better, as the soil underneath won't suck out any moisture from it.
2) you'll save a bit of concrete (at £100+VAT or more per metre) from leaking out under the forms under the outer corners (I take the previous poster's word for the fact that if you follow his technique you won't 'lose' much but...especially if it's a 'pump mix').
Spending £20 on some plastic sheet and half an hour fixing it in place might save it's worth in concrete (there's a certain amount of waste with a pump anyway) and yield a better-cured slab.
Cheers, W2S

PS I'd definitely recommend a poker (a small elctric one should be fine) - I would say this is another factor in favour of the DPM sheet suggestion, as the poker does mobilise the water and fines in preference to the heavier bits.

 

[DomD]

Hi - if pumping, the mix will be slightly wetter than it might be otherwise. If your formwork is square to the the nearest 1/2 inch that's probably just fine. If it were me doing it, I'd put down a sheet of DPM across the whole thing - wrapping the edge over your formwork and fixing it along the edge with battens. Two reasons for this:
1) the concrete will cure better, as the soil underneath won't suck out any moisture from it.
2) you'll save a bit of concrete (at £100+VAT or more per metre) from leaking out under the forms under the outer corners (I take the previous poster's word for the fact that if you follow his technique you won't 'lose' much but...especially if it's a 'pump mix').
Spending £20 on some plastic sheet and half an hour fixing it in place might save it's worth in concrete (there's a certain amount of waste with a pump anyway) and yield a better-cured slab.
Cheers, W2S

PS I'd definitely recommend a poker (a small elctric one should be fine) - I would say this is another factor in favour of the DPM sheet suggestion, as the poker does mobilise the water and fines in preference to the heavier bits.

I'm putting down dpm anyway - wrapping up the edge and over the form shouldn't be an issue as I have spare: I'm guessing I would then cut it back to ground level so the slab can still dry naturally?

By fixing with battens do you mean using pieces of wood and screws to 'clamp' the dpm in place? I can always fill some earth in behind parts of the dpm below the form that are holding concrete to help with support.

Cheers

 

[MikeG.]

Dom, don't forget to have somewhere for spare concrete to go. It is inevitable that there will be a bit left over at the end of the pour, so maybe prepare a sub-base for a path, patio, a bit of hardstanding...whatever. Also, trim your 2x2 support posts down to below the level of the formwork, because they'll interfere with your tamping.

 

[DomD]

Dom, don't forget to have somewhere for spare concrete to go. It is inevitable that there will be a bit left over at the end of the pour, so maybe prepare a sub-base for a path, patio, a bit of hardstanding...whatever. Also, trim your 2x2 support posts down to below the level of the formwork, because they'll interfere with your tamping.

Will be trimming down the posts; just going to use a 2x4 for the screed board so hopefully they send some that are not too warped.

As for extra if I am correct I can order volumetric concrete with an estimate of volume and they will only pump as much as I need? This seems to be what it suggests online and I'd much rather do no more digging.

Dom

 

[DomD]

I now have a quote for £95/m^3 of C25 (+vat) volumetric (mixed at site concrete). The pump is then £350 (+vat).
With ~6m^3 of concrete that's quite expensive but I would rather do the base correctly.
Fibres are an extra £10/m^3, worth it to prevent cracking?
Dom

 

[Woody2Shoes]

Dom, don't forget to have somewhere for spare concrete to go. It is inevitable that there will be a bit left over at the end of the pour, so maybe prepare a sub-base for a path, patio, a bit of hardstanding...whatever. Also, trim your 2x2 support posts down to below the level of the formwork, because they'll interfere with your tamping.

Will be trimming down the posts; just going to use a 2x4 for the screed board so hopefully they send some that are not too warped.

As for extra if I am correct I can order volumetric concrete with an estimate of volume and they will only pump as much as I need? This seems to be what it suggests online and I'd much rather do no more digging.

Dom

The waste comes from the fact that - depending on the size and design of the pump - there could be something in the region of a m3 "in the works" which has already been delivered from the mixer (and you will thus buy this, along with all the other concrete that came out of the mixer too) but is somewhere between the input hopper and business end of the output pipe. This needs to be dumped/washed out of the pump - what I do is build a small "paddling pool" with scaff boards and dpm to allow the pump operator to clean out into that (this will need space off road). You'd be well advised to have a few spare bits of DPM around anyway to catch any small spills between mixer and pump (or at least make them easier to tidy up).
Cheers, W2S

 

[Woody2Shoes]

Hi - if pumping, the mix will be slightly wetter than it might be otherwise. If your formwork is square to the the nearest 1/2 inch that's probably just fine. If it were me doing it, I'd put down a sheet of DPM across the whole thing - wrapping the edge over your formwork and fixing it along the edge with battens. Two reasons for this:
1) the concrete will cure better, as the soil underneath won't suck out any moisture from it.
2) you'll save a bit of concrete (at £100+VAT or more per metre) from leaking out under the forms under the outer corners (I take the previous poster's word for the fact that if you follow his technique you won't 'lose' much but...especially if it's a 'pump mix').
Spending £20 on some plastic sheet and half an hour fixing it in place might save it's worth in concrete (there's a certain amount of waste with a pump anyway) and yield a better-cured slab.
Cheers, W2S

PS I'd definitely recommend a poker (a small elctric one should be fine) - I would say this is another factor in favour of the DPM sheet suggestion, as the poker does mobilise the water and fines in preference to the heavier bits.

I'm putting down dpm anyway - wrapping up the edge and over the form shouldn't be an issue as I have spare: I'm guessing I would then cut it back to ground level so the slab can still dry naturally?

By fixing with battens do you mean using pieces of wood and screws to 'clamp' the dpm in place? I can always fill some earth in behind parts of the dpm below the form that are holding concrete to help with support.

Cheers

Concrete ideally needs to be kept covered so it doesn't dry out too quickly - for the first two or three weeks anyway - water is important to the curing process.

Yes, the battens would be horizontal along the outside of the top of the form "clamp" the edge of the DPM.

 

[Woody2Shoes]

I now have a quote for £95/m^3 of C25 (+vat) volumetric (mixed at site concrete). The pump is then £350 (+vat).
With ~6m^3 of concrete that's quite expensive but I would rather do the base correctly.
Fibres are an extra £10/m^3, worth it to prevent cracking?
Dom

I like fibres - I don't know why they don't come as standard! But you would no doubt be fine without them (although if it were screed, I'd definitely say they're worth it).

 

[DomD]

Hi - if pumping, the mix will be slightly wetter than it might be otherwise. If your formwork is square to the the nearest 1/2 inch that's probably just fine. If it were me doing it, I'd put down a sheet of DPM across the whole thing - wrapping the edge over your formwork and fixing it along the edge with battens. Two reasons for this:
1) the concrete will cure better, as the soil underneath won't suck out any moisture from it.
2) you'll save a bit of concrete (at £100+VAT or more per metre) from leaking out under the forms under the outer corners (I take the previous poster's word for the fact that if you follow his technique you won't 'lose' much but...especially if it's a 'pump mix').
Spending £20 on some plastic sheet and half an hour fixing it in place might save it's worth in concrete (there's a certain amount of waste with a pump anyway) and yield a better-cured slab.
Cheers, W2S

PS I'd definitely recommend a poker (a small elctric one should be fine) - I would say this is another factor in favour of the DPM sheet suggestion, as the poker does mobilise the water and fines in preference to the heavier bits.

I'm putting down dpm anyway - wrapping up the edge and over the form shouldn't be an issue as I have spare: I'm guessing I would then cut it back to ground level so the slab can still dry naturally?

By fixing with battens do you mean using pieces of wood and screws to 'clamp' the dpm in place? I can always fill some earth in behind parts of the dpm below the form that are holding concrete to help with support.

Cheers

Concrete ideally needs to be kept covered so it doesn't dry out too quickly - for the first two or three weeks anyway - water is important to the curing process.

Yes, the battens would be horizontal along the outside of the top of the form "clamp" the edge of the DPM.

Ideally I was going to start bricklaying 2days after the pour - I know it doesn't reach full strength till a month but I have read bricks after a couple days are fine and a wooden structure after a week is fine too, but heavy machinery should be kept off it?
I can always cover the site overnight and when not in use with a tarpaulin.

 

[TopCat 32]

i am a batcher for 1 of the major ready mix concrete suppliers, you will be lucky to get a pump operator to pump a S2 (70 slump) mix, pump mixes are S3 (130 slump), as for fibers i take it by fibers you mean the glass fiber type, these only help with surface cracking from shrinkage they have no structural benefit the larger Duros or Nova fibers are for structural work, as i have not followed whole post i guess you are laying a course or 2 of bricks on this footing? in which case save your money and dont add fibers i produce in the region of 30k M3 a year and have never put fibers in footings, if you are worried about the structural integrity order a stronger mix, but as most 2 story houses are built on C10 mixes in our area (clay) i wouldn't think you would need anything to strong, but to pump concrete it is suggested a minimum cement content of 280kg pr M3.
, hope this helps

Regards Tim

PS shouldn't be any over as a good pump operator should gauge when you are getting near the end of your pour and how much he has in his line and run pump empty to top up footing

 

[Woody2Shoes]

PS shouldn't be any over as a good pump operator should gauge when you are getting near the end of your pour and how much he has in his line and run pump empty to top up footing

I think this does depend on the type/design of pump - and the amount of additional pipework attached. My local supplier asks for - and uses - a "washout box" 2mx2mX200mm which usually gets about two-thirds-full of surplus concrete material and washing water from the pump. I also need to have on hand extra bags of dust (one per 20m of pipe) for grout to prime the pump. Cheers, W2S

PS I like the idea of fibres in reinforced concrete to potentially increase the life of the steel (because cracks happen and fibres are a cheapish way to reduce them - I quite agree they'd be pointless in this case, although very useful for pretty much any screed).

 

[DomD]

PS shouldn't be any over as a good pump operator should gauge when you are getting near the end of your pour and how much he has in his line and run pump empty to top up footing

I think this does depend on the type/design of pump - and the amount of additional pipework attached. My local supplier asks for - and uses - a "washout box" 2mx2mX200mm which usually gets about two-thirds-full of surplus concrete material and washing water from the pump. I also need to have on hand extra bags of dust (one per 20m of pipe) for grout to prime the pump. Cheers

Is a suitable 'washout box' just a ditch in the ground? The concrete supplier has said it will be 2-3 barrels from the augers and perhaps a little extra from too much concrete getting blown through.

I am also looking into hiring some tools for preparation and the concrete pour:
- Concrete poker I can get for £32(+vat)
- Power float I can get for £80(+vat)
- 'Easy float' for £36(+vat)

I am wondering if you need to first use a boom float then a power float or I could just hire the latter.
Thanks

 

[adidat]

I had this bad boy to do my base 17m boom!!! liebherr know how to make good gear!

Adidat

 

[adidat]

i might be putting the cat amongst the pigeons here but i had my almost 90 sqm base 120mm thick base bored in three sections on top of 200mm compacted hardcore. without a scrap of iron, fibres or membrane and two years on its still fine.... I did put my grandad (a 92 year old @rse kicking ww2 RAF vet engineer) and my beloved labrador's ashes in the concerete so that probably made a big difference!! :lol: :lol: :lol:

My only regret was not having it professionally power floated, but having all the work done including garden clearing for about 4k I was fairly strapped for cash....

adidat

 

[adidat]

And i didn't require the washout area as the driver just dumped it in the area for the next weeks pour. this proved problematic when we ran out of gear on the last pour and caused 2 very panicked trips to bradfords for a dumpy bag of ballast and cement. Which was hand mixed on site, all on a very hot july day

But all worth while in the end

Adidat