I am preparing myself a concrete base for a soon to be delivered shed and am amazed at the variety of advice from websites, DIY stores etc, ranging from 'just lay slabs onto grass, through to build retaining wall surround, then hardcore and concrete. I have just seen your web site and from my own research it ranks as the most comprehensive, thank you! Three questions if I may.
1) I am looking to construct 100mm high shuttering onto compacted clayish soil. I am excavating the 100mm depth (albeit into a slight R-L grade slope, approx. 1 in 15) I thought the 75mm or so soil on the lower level would be sufficient to hold. Your diagram under 'Concrete' shows a concrete base onto which a slim kerb stone is laid. Is this really necessary? Could I say sink a kerb stone into the soil (say 150 deep) as as retaining without further concreting, as I am hoping to receive a single delivery of pre-mixed and do without the hassle of mixers, bags of sand/cement etc...?
2) If I do need to construct a concrete pad onto which I mount the kerb stone, does this kerb become part of my shuttering. If not how should it work?
3) I am also looking to run an electric (PVC/SWA) cable into the shed through the floor. Do I need to tape between the membrane and plastic conduit? Are there any other special considerations (bar the trench depth, warning tape (already noted from your site) and electrical considerations, I'm fine on those!).
I hope these make sense and aren't too basic for the forum. I recognise I'll always be 'an apprentice' regarding building work but for the bit I do I'd like to get it 'right'.
Regards,
Droc
Concrete Base
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84-1093879891
Hi Droc,
Q1 - you don't have to use a kerb as a retainer. You could use a temporary timber shutter, as shown in the Formwork page, which is probably a better solution for a shed base.
Q2 - see above
Q3 - no really critical need to tape around the cable and the membrane, but it makes a better job if you do.
The purpose of the membrane is to prevent the sub-base/sub-grade sucking the water out from the concrete mix while it cures, and to protect the slab from groundwater and aggressive salts in the longer term. Although you'd not lose much mix water through the un-taped joint, it's always best to 'play safe'. :)
Have you allowed for a sub-base or a 'blinding' beneath the membrtane to protect it from puncturing?
Q1 - you don't have to use a kerb as a retainer. You could use a temporary timber shutter, as shown in the Formwork page, which is probably a better solution for a shed base.
Q2 - see above
Q3 - no really critical need to tape around the cable and the membrane, but it makes a better job if you do.
The purpose of the membrane is to prevent the sub-base/sub-grade sucking the water out from the concrete mix while it cures, and to protect the slab from groundwater and aggressive salts in the longer term. Although you'd not lose much mix water through the un-taped joint, it's always best to 'play safe'. :)
Have you allowed for a sub-base or a 'blinding' beneath the membrtane to protect it from puncturing?
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68-1093879218
Thanks for your prompt reply Tony.
The use of a kerb as a retainer was prompted by the diagram in your concrete section, which shows a concrete slab, cast adjacent an existing wall c/w deeper sand/cement channel on RHS with a kerb stone on top. It looks like (to me) some form of retaining post-concrete pour. Is this a necessary requirement to support (hold) a 4� concrete slab? If so does one dig the trench/fit the kerb stone after the concrete slab is poured or should it be dug around the formwork If the latter, I would be concerned that this trench may undermine the timber stakes supporting the shuttering. Your further valued input would be most welcome
I had not allowed for a blinding beneath the membrane, thanks for that advice.
Regarding shuttering, I am proposing to use 4� x 2� timber, with 20 ish 2� x 2� x 12� wooden stakes to support the framework (therefore stakes will be sunk circa 8� into grade). Does this sound ok, could I use 4 x 1� timber instead of 4 x 2� or will the weight of the concrete demand the more sturdy material? I am also proposing butting and fixing corners using wood screws, is this ok?
Thanks once again for your help.
Droc
The use of a kerb as a retainer was prompted by the diagram in your concrete section, which shows a concrete slab, cast adjacent an existing wall c/w deeper sand/cement channel on RHS with a kerb stone on top. It looks like (to me) some form of retaining post-concrete pour. Is this a necessary requirement to support (hold) a 4� concrete slab? If so does one dig the trench/fit the kerb stone after the concrete slab is poured or should it be dug around the formwork If the latter, I would be concerned that this trench may undermine the timber stakes supporting the shuttering. Your further valued input would be most welcome
I had not allowed for a blinding beneath the membrane, thanks for that advice.
Regarding shuttering, I am proposing to use 4� x 2� timber, with 20 ish 2� x 2� x 12� wooden stakes to support the framework (therefore stakes will be sunk circa 8� into grade). Does this sound ok, could I use 4 x 1� timber instead of 4 x 2� or will the weight of the concrete demand the more sturdy material? I am also proposing butting and fixing corners using wood screws, is this ok?
Thanks once again for your help.
Droc
-
84-1093879891
The drawing shows an edging kerb merely as a decorative option. It's not required at all - the concrete, once cured, will support itself. I drew in the kerb to illustrate the sort of thing that is possible with concrete pathways, and how a pre-cast edging kerb can be used in place of temporary timber shuttering.
You can use 4x1 (100x25) timber for the shuttering, with stakes at 500mm centres - this is plenty for a 100mm deep pour, as there's not a great deal of pressure, other than when placing and tamping the concrete. As soon as it starts to cure, the pressure reduces.
You can use 4x1 (100x25) timber for the shuttering, with stakes at 500mm centres - this is plenty for a 100mm deep pour, as there's not a great deal of pressure, other than when placing and tamping the concrete. As soon as it starts to cure, the pressure reduces.