self mix or ready mix

[LSB]

Hello

We have some underpinning to do, current barn foundations are about 40-50 cm but we need 100cm

 

SE says we have to do 1m at a time to reduce the risk of any collapse, so 1m, leave 2m, do 1m, leave 2m etc.

 

So, as it's going to be a manual dig (we've cleared the surface junk with digger), are we okay to use the mixer and do our own for this.

 

BC will need to inspect lots as there will be 4 runs, but getting a readymix delivery for such a small amount is rather pricy compared with the amount per cubic metre for the trench.

 

TIA

 

 

 

[crispy_wafer]

Reckon you've probably got enough manual work to do, get a volumetric mixer in and have a cuppa whilst they mix and pour, then just pay for what you use.

[PeterW]

2 hours ago, crispy_wafer said:

Reckon you've probably got enough manual work to do, get a volumetric mixer in and have a cuppa whilst they mix and pour, then just pay for what you use.

+1 to this ..! Volumetric mixer, bring him back every time and you’ll get a consistent mix and no backache.. SE may also insist on a “known” ratio which unless you make some gauging boxes you can’t do with a mixer and a labourer 

[JohnMo]

We used a volumetric a couple of times on our build. Not cheap, but we'll handy, basically mixes as much or little as you need when you need it to any ratio and slump you want or need.

[bassanclan]

Work out your volume.

If you are 1m x 0.5m and probably 0.5m wide, each hole will be 0.25 cubic m of concrete.

 

If you are doing 24 holes at a time you have 6 cubic m, so a full ready mix load.

[markc]

Underpinning is donkey work, the digging alone will push you to breaking point. Micro digger with bucket turned wrong way will make life a lot easier scooping under the existing, then get volumetric or mini ready mix but as previous posts the volumetric will be your friend, go down well with BCO and you will be using more than you expect.

Edited February 7, 2023 by markc

[LSB]

Right, thanks all, I haven't made this clear, this is only for 6m, so 2 x 1m three times.

For the normal trenches (24m) we are having ready mix with pump, we can time this for the same time for one lot of 2m, but still have the others two to do.

But, can you get readymix for a slump of about 100 which is what was specified.

The other nuisance is we have to reinforce each time so that it's tied into the next bit of underpinning.

If we had lots to do in one go then we would definitely get volumetric.

But, it sounds like we may have to bite the bullet due to the SE and BCO.

 

[markc]

How wide are the existing footings? Let’s say 500mm as mentioned above, you need space to do the digging, at 1m deep you will need around 500mm to work in order to dig under the existing so  each pour will be 1/2 cube (500x500x1000) under the existing plus the hole you need to dig for access. Yes you can shutter and save a bit of concrete but it’s so much easier to fill the hole to above the bottom of the existing. After doing the digging you knock rebar starters into the existing soil either side, tie these in with your cage then when you excavate the next section your starter bars are already there.

[George]

What's the reason for underpinning?

[Iceverge]

It can be done, I did 8m3 in my tractor mixer for the floor of the garage as I was on my own and had time to burn during covid. 

 

Price the cement carefully however, for a strong structural mix it'll shock you how much goes in compared to commoner garden "patch a hole" concrete. It cost more in cement alone than ready-mix did for me. 

 

C35 concrete needs 410kg/M3. This translates to about 30% by volume. The smaller the mix the harder it seems to be to get a consistent mixture. 

 

If you have a good car trailer and van and a large container you could collect 1/2 a cube at a time from the batching plant with plenty of retarder in it to save on lorry charges. 

 

Having done it both ways though, I'd get a truck for your 1.5m³. 

 

 

 

[Mike]

Also consider whether you can concrete something else at the same time to boost the load size. Foundations for kerb stones, garden path, duck pond?

[LSB]

On 07/02/2023 at 20:32, George said:

What's the reason for underpinning?

we are converting a barn which means we have to keep walls and the SE has specified underpinnining of the 6m in question, and more, but that will be tackled separately

 

[LSB]

On 07/02/2023 at 23:35, Mike said:

Also consider whether you can concrete something else at the same time to boost the load size. Foundations for kerb stones, garden path, duck pond?

Wish we could, no road side, not allowed paths or driveway (in the countryside), but we do have 24m to do in 1 hit so will use this for 2m, so only 4m in question and we are thinking about doing other walls as well.
The problem is that we have to keep the walls and if we dig too much they might 'fall' down, not that I would mind, but the LPA might.

 

[HughF]

Volumetric, always…. Unless you live in a volumetric desert (Frome in Somerset for example)

 

It’s the same price as buying the gear and mixing it yourself. At least it is down in Dorset.

[LSB]

On 24/02/2023 at 18:49, HughF said:

Volumetric, always…. Unless you live in a volumetric desert (Frome in Somerset for example)

 

It’s the same price as buying the gear and mixing it yourself. At least it is down in Dorset.

but we already have the gear apparently.

[PeterW]

13 minutes ago, LSB said:

but we already have the gear apparently.

as in a mixer ..?? If you’re underpinning or anything structural then the SEng or BCO may want evidence of mix strength - you can’t do that in reality with a 1/2 bag mixer 

[LSB]

26 minutes ago, PeterW said:

as in a mixer ..?? If you’re underpinning or anything structural then the SEng or BCO may want evidence of mix strength - you can’t do that in reality with a 1/2 bag mixer 

I totally agree, but will leave for HID (building) to discuss with BCO as that's beyond my expertise.

Got slightly more complicated now as digging down yesterday to expose some of the existing foundations we have found a DPC across the whole floor of this 'room' which, as it's nominally a conversion, we may be required to keep which will lead to multiple levels in the dwelling.  We were planning on underpinning then when the walls were stable digging down to start from scratch.

There is no insulation so that will need adding which will raise that particular level from the rest quite a bit.  None of the other floors had DPC when we dug down.

 

Edited February 27, 2023 by LSB