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Posted: Sun Aug 29, 2004 12:36 pm
by Douglas deL
The project is a fairly large workshop, big enough to contain a 1950s milkfloat which I am restoring. I'd like the shed to be 10' x 20' (say 3x6m). So I'm assuming I shall need a reasonable sub-base of around 100mm and a 100mm concrete slab (though the shed itself does have a timber floor). For this I'll use timber shuttering, and expect the slab to be perhaps 50mm proud of the surrounding ground. Is that about right? Am I right in thinking this ought to be cast in 2 sections with an expansion/compression joint between them? I notice also that you advise arrissing the edges of slabs to prevent spalling, but I can't find advice on how I should do this either round the outer edge or between the sections.
Finally, do I put the shuttering up before laying the sub-base, or place it to the edge of the sub-base, or what?
Many thanks for all suggestions - and for this site!
Douglas deL.
Posted: Mon Aug 30, 2004 7:47 pm
by 84-1093879891
A slab of 100mm thick concrete on a 100mm sub-base should be viewed as a minimum requirement. It'll be fine for any loads under 3.5 Tonnes, assuming you have reasonable ground. Make sure you use a fibre-reinforced concrete: it's so much easier than relying on steel mesh and rebar.
50mm upstand is fine, but give some thought to falls. If this were to be a garage floor, and therefore an internal slab, it would be poured and finished to a flat level, but as this is, techincally, an exterior slab carrying a shed, then you will need to lay it to falls. 1:100 will be adequate, but don't go any flatter than that.
Splitting the slab into two 3x3m sub-slabs, makes sense. Whether you do this by using a crack control joint or cast it as two separate slabs is up to you. Perosnally, I'd go for the single slab with a CCJ, but you may feel placing, levelling and finishing two smaller slabs would be easier for someone not used to regular concrete work.
Arrissing of the edges is achieved with an Arris Float, which is basically a standard float with an arris shaped lip to one edge. There's one shown at the end of the Trowels section on the
Hand Tools page. A well-finished slab will have all edges arrissed, both to the external edges and to the edges of the CCJ/construction joint (sepending on which you opt for).
Finally, the shuttering would normally be erected on top of the completed sub-base. Once the sub-base was levelled and compacted, the shuttering or formwork would be set up and anchored into place, possibly with site pegs, before lining it with a sheet of damp proof membrane, ready to accept the concrete.
Posted: Wed Sep 01, 2004 7:11 am
by Douglas deL
Thanks, Tony!
I'll try a CCJ if you think it better, but just for information, what I was planning was simply to have put a Flexcell board across the centre before pouring the concrete. Would that work?
A neighbour of mine has suggested I rent roadform; he laid a base over a weekend and removed the roadform on day 3. The strikes me as a bit early; any comments?
Douglas deL.
Posted: Wed Sep 01, 2004 9:24 pm
by 84-1089053830
You can use a Flexcell insert if you want - this would be a 'real' movement joint, rather than a crack control joint. There's nowt wrong with that, but it may be seen by some as a bit OTT for a simple shed slab.
Roadform is ideal for shuttering this sort of project. I did think of mentioning it in my previous post, but part of me though I might be over-complicating the situation by suggesting its use, but personally, I've never understood the fascination some in the trade seem to have for using timber shuttering, knocked-up specifically and solely for this one job, when Roadform can do the job better, at not much more of a cost, and in around a quarter of the time.
Striking the Roadform after 3 days is not all that unusual, especially at this time of year. They might be left for 4-5 days in winter, but after 3 days, initial set has taken place, the concrete is well on its way to 7-day strength, and if you're on a limited time hire, you might as well get the roadform back to the hire yard.
Posted: Sat Sep 11, 2004 9:52 am
by Douglas deL
Again my thanks, Tony. I'd be interested to know whether you think a CCJ is simpler than Flexcell or just more appropriate.
"personally, I've never understood the fascination some in the trade seem to have for using timber shuttering, knocked-up specifically and solely for this one job, when Roadform can do the job better, at not much more of a cost, and in around a quarter of the time" -- well, out of the trade I was impressed at the low hire charge but appalled by a whacking £60 addition for delivery! So I've gone for shuttering which I'm hoping to re-use for something else once the concrete has set.
Yet another question: I shall want a strong anchor point in the shed to chain cycles, tools &c to. What are the pros and cons of embedding something in the wet concrete versus drilling for a rawlbolt afterwards?
Posted: Sat Sep 11, 2004 10:24 am
by 84-1093879891
A CCJ is fine for a simple slab such as this. Flexcell movement joints are more appropriate for instances where heavy structures will be placed on the slab. They are more complex to install - a CCJ is basically a saw cut, whereas a flexcell joint requires installation, shuttering and then sealing with summat like Thioflex once the concrete has cured.
The delivery charge you mention is a bit of a killer, I admit, but then most contractors would be able to collect using one of their own vehicles. Do you not have a mate with a van?
When it comes to the anchor point, I;d embed it while the concrete is being placed, as long as I knew exactly where it was to go. It can be a bit of a ball-acher if you find out you were 100mm out and there a great big ring of steel sticking out of the slab just where the door is supposed to go, so check your plans.
If it's embedded during placement, then you can be sure it has sufficient cover to make it assualt proof. However, retro-fitting means drilling, which really shouldn't be done for at leats two weeks, an dthen hoping that the Rawl-bolt bites properly, and then trying to get hold of some Lok-Set to ensure it can't be un-bolted.....all this is fine if the exact position for the ancor point can't be determined until later in the construction, but, as I said, if you know exactly where beforehand, go ferret!
Posted: Fri Oct 01, 2004 1:09 pm
by Douglas deL
Again, wonderfully helpful advice. The slab is laid and looks great. But I can't see what I should do the dpc edges. If I just trim them to ground level, will not water seep between the dpc and the slab? Is there a special means of sealing a dpc to the slab?
Posted: Sun Oct 03, 2004 4:55 pm
by 84-1094545549
DPC edges?? Do you mean the tails of dpm, the membrane used beneath the concrete? If so, these are simply trimmed off as sclose to slab level as poss.
There's no great risk of water penetrating the interface between concrete and dpm, especially not on a project such as this. On civil engineering projects, we might be required to seal the joint with, say, Thioflex, but for what you're doing, just trimming the visqueen will be more than adequate.