Shed base

[sharpie]

Hi Tony,

I'm preparing to lay a base for my new wood garage/workshop.

The primary use is for garden storage/workshop but the building could be used as an occasional vehicle storage (but never regular in-out due to situation).

I'm thinking of going for a concrete slab base vs paving slabs
to give future flexibility. Its a 5.5m square building.

Some initial questions I'd much appreciate your guidance on:
- Should I make the slab EXACTLY 5.5m square or a bit bigger?
- Is 100mm thick concrete adequate and should I consider reinforcement?
- Expansion gaps - do I need them? Even if not I would prefer
to do this in smaller steps. If so, how do you prepare the
shuttering - one 2.75m square at a time or a full size box
with inserts? I assume you lay one bit at a time, let it cure ,
remove shuttering, insert exp. gap material and lay the next bit?
- The sub grade is fairly solid clay soil. Can you advise if I need
a sub base -I'm really not sure.

Sorry so many questions but its the first time I've done this
- appreciate your help!

[Tony McC]

You need some 'spread' for the building. Assume 100mm all around, so that makes your slab 5700x5700

The usual 'rule of thumb' for concrete slabs is that the maximum 'length' of any slab should be not more than 30 times its thickness. So: a 100mm thick slab could be up to 3m in length - this would mean a 5.7 x5.7 slab would need to be split into 4, so that no section had a side longer than 3m (2.85, actually, in this example). Now, if you increased slab thickness to 150mm, you could have a slab up to 4.5m in length, still a bit short. Using this 30x rule, a thickness of 190mm would enable a slab to have a max length of 5700mm - brilliant!

Except that the second rule of slab design is to avoid any slabs with a plan area of more than 20m² - you have 32.5m², so even if we up the thickness to 190mm, you'd still need at least one movement joint to reduce the individual slab size to below 20m².

Now: you have to bear in mind that all these rules and recommendations are used for commercial applications and there is a generous factor of safety involved. As yours is a private DIY job, you could, if you wanted, decide to cast the slab in 2 bays, each being 2.85 x 5.7. It could actually be cast as a single slab and then have crack control joints cut into it the following day. For a commercial project, I'd have to construct this as a 4 slab project, each being 2.85 x 2.85: however, as a comnmercial project, it's more likely that a 150mm thickness would be specc'ed, and given the scale of the job, upping that by 40mm to 190mm would only require an additional 1.3m³ of concrete (around 120-150 quid) and so I'd be awfully tempted to do that and just create a single movement joint.

When it comes to reinforcement, I'd recommend the use of polyester fibres in all concrete slabs nowadays. they cost next to buggrall and their benefit in controlling micro-cracking on the surface is enormous. With steel mesh and/or rebar, it would be possible to design a 125-150mm thick single piece slab, but the cost of the steel would meake it more economically feasible to simply increase slab thickness.

Unofficially, I'd hazard a guess that a 150mm slab, with fibre reinforcement, cast onto a well-prepared sub-base (or sub-grade) with a single sawn crack control joint,would do the job for what you want. As you have a good clay sub-grade, I'd suggest that any sub-base would be used merely as a regulating layer (around 50-75mm thick), to level up the sub-grade and create a level and even surface for the damp proof membrane.

Has that cleared things up, or just made it more of a headache?

[sharpie]

Tony,

That's brilliant. Provoked a few more questions though!

I can understand the spread - to support any outward loading
from the building wall.

Is it crazy to entertain doing this mixing the concrete myself
(i've got a regular concrete mixer)
(access to the site is not easy for a readymix lorry (not close and
up hill - would need pumps etc)). Part of the attraction of doing
it in 4 'quarter' slabs.

What's the proceedure for constructing/supporting the joints? eg if I did this
as 4 quarters, would I construct an interior 'cross' out of shuttering timber, pour the first quarter, let it cure, remove shuttering, etc? or is the crack infill material stable enough to
support the fluid concrete. I suspect this is a stupid question
but can't just see the solution..

Can you buy polyester ad-material from builders merchants?

Cheers!

[Tony McC]

It's a lot of concrete to mix by yourself - the equivalent of a full wagon load of concrete - and the biggest problem with self-mixing is consistency. No matter how carefully you meter each batch, there is some unwritten universal law that prevents consecutive mixes being the same colour or slump.

Doing it in quarters means mixing around 1.5m³ per shift, and that means a lot of hard shovelling and barrowing to get the last mix in place before the first mix has started to go hard. If you're not used to building work, it will be rather strenuous, but not impossible. It'd be a lot easier if you had someone helping you.

The ideal way to do this would be to construct a cross-shutter within the main rectangle and to cast two diagonally adjacent corners in one shift. This would allow you to strike all of the shuttering in one go and use the pre-cast bays as 'shutters' for the last two, but if one bay is a lot of concrete mixing, doing two bays on the same day will be a real killer. It might be a better idea to, say, cast one on a Saturday and the next on the Sunday; strike the shutters on Friday night, and then repeat the process the following weekend.

I wouldn't bother using Flexcell or fibreboard between the individual slabs - there's plenty of opportunity for expansion at the edges. Some basic guidance for shuutering construction can be found on this page

Fibres - probably available via your local BM but definitely available at most Contractors' Merchants and concrete suppliers.

[sharpie]

On your initial comment:

>> You need some 'spread' for the building. Assume 100mm all around, so that makes your slab 5700x5700

Doesn't that run the risk of rain ingress under the foundation
beam of the building? I'd been planning to lay the beams on
dpm anyway but I'd previously understood that the slab should
be the exact same size as the builing to avoid rainfall on the 'spread'
finding its way into the building.

What do you think?

Cheers

[Tony McC]

??? All foundations have 'spread' - that's how they manage to carry such extensive loads. However, as this is a slab/base, you could form it as an exact fit, but I can't see any real benefit. This notion of rain ingress is a bit of nonsense - if the slab is properly constructed, any precipitation landing on the slab would drain away from the building thereon.

We've always constructed garage/shed bases to be 'oversize' and then, if it's a pre-cast structure such as a sectional garage that sits directly on the slab, the joint between building and base can be sealed should there be any concern regarding ingress of water.

[sharpie]

Tony,
Is there a section on your wonderful site about electrics
- I know its a bit off topic for paving.

At this stage I just need to allow for power into my workshop.

Does it make sense to insert a short length of flexible conduit
up through the subbase and then concrete around it?

Not much success so far on the polester fibres. B&Q min order
quantity 6 tonnes!! Will try the local BMs too. Would readymix
companies sell direct do you know?

Cheers!

[Tony McC]

I don't cover electrickery - I'm purely a paving and drainage sort of guy. However, there is a page explaining some of the ins and outs of Ducting which is probably what you need.

Polyester fibres - try PICS in Newbury (01635 202224) or Creative Impressions in Preston (01772 335435)

[Sean Buxton]

Sharpie,

If you still need advice, try posting your question on DIYnot dot com. I have found this website to be very useful and informative.

Sean