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I am building a 6ft x 4ft garden shed which will have a concrete base.

I've levelled some ground, built a frame, and made a floor out of old bricks.

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The ground is fairly clay-heavy. I've been stamping down on the bricks and they seem stable. Cement will be about 6-9cm from the top of the bricks to the top of the frame. There is a few millimetres of gap between the bricks, I imagine the cement will not flow between them and hence it will be kept clear of pooling water. The bricks are partially above and partially below the level of the surrounding soil.

I haven't worked with cement before but based on what I've read I believe this will work.

Can you see any problems? Is there anything missing? Do I still need grit? Rebar? Or is it okay to just go ahead and pour?

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    The trouble with that sort of base is that it creates specific stress lines. If your slab cracks, it'll almost certainly crack along those joints. You're better off with something more fluid, like gravel.
    – isherwood
    Commented Mar 16, 2021 at 19:37
  • That said, I don't have experience pouring concrete on rigid bases.
    – isherwood
    Commented Mar 16, 2021 at 19:42
  • How cold is in the winter? You might find your slab has settled unevenly and cracked in the next weather cycle. If digging is too much for you, you shall at least add rebars, or wire mesh, to make the concrete slab stronger, and less prone to crack.
    – r13
    Commented Mar 16, 2021 at 21:59
  • I just put brick on level ground and that has been the floor for 20 years. Insulated shed so no freeze/ thaw. Water spills drain very well. Commented Aug 13, 2021 at 23:17

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Concrete slabs perform best when it has a uniform thickness, installed on uniform compacted soil, square (or near square) in shape (not “L” shaped), and has temperature reinforcing steel if the slab will expand or contract too much due to changing weather, roof loads transferred to perimeter.

Your building use will probably require large loads of fertilizers, water bins, etc. that will impose significant loads that will cause differential settlement. Using a brick base will add to the possibility of cracking by making the slab an uneven thickness, non-uniform depth at various locations, etc.

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I don't understand why you've taken all that time to lay out those bricks like this

In my experience. Being the son of a self employed builder . I can tell you there's no substitute for compacted hardcore for a concrete slab.

Just break up all those bricks into approx 4 pieces and the old technique is to get a wood pole with a square block or round dont matter. And use it to compact the hardcore (broken bricks). By compacting the hardcore you will effectively prevent the concrete slab from subsiding and breaking up in areas which you would get with the method as those bricks will sink into the clay/soil bed underneath. Once you have broken up and compacted your hardcore . I'd go the extra mile and put 5 or 6 inches of gravel and sand onto the hardcore and use a whackerplate to ensure you fill in any voids and compact the base sufficiently enough to prevent any cracking and subsiding .

If you are doing an extra large slab . It pays to have a joint down the middle so you effectively have 2 or more slabs . As giant slabs of concrete will inevitably subside causing the slab to break up.

Once you have a good bed of hardcore and a we'll compacted and leveled gravel and sand bed.

Then you can get mixing up concrete .

Don't forget to build a wood form around the perimeter. A simple frame to the desired thickness of your slab

And also the strength of the concrete .

So many people think they need to use vast amounts of cement in there concrete .

But that's just crazy.

A ratio of 8-1 8 parts sand and gravel to one part cement . Or four shovels of gravel four sand and one cement .

Tho ensure you use equal amounts on the shovel

Not a small shovel of cement then huge shovels of sand etc.

Common mistake .

Don't hold me on the ratio I'd def look that up

But my point is. It doesn't need to be that stronger mix

A cement mixer would save you a lot of effort but failing that mix it in a wheelbarrow.

Using the technique you can find on YouTube .

Even as a kid I would mix up enough concrete for a 60/30ft barn . AHH the good old days . Where id work fory dad for £5 a week lol. If that..

Anyway .good luck with your project .but def have a rethink mate. As yourll be throwing your money away

Concrete slabs are incredibly heavy. So those bricks will sink over time.

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I’m no expert but it looks like you’ve made a good job for your shed base and will be more than substantial for a 6x4 shed. When you talk about ‘cement’ you mean concrete? So that’s cement with aggregate in for strength. If the wooden surround is level from side to side, once filled you’ll be able to strickle across to produce a smooth and level surface (or sloping fractionally for drainage?). Regardless of the underling surface the shed should go onto a slight wooden frame (use tanilised wood) to raise it slightly to avoid the base of the shed from rotting. Once your concrete base is finished and the surrounding ground levelled (or whatever you intend doing with it), see how the water drains and this will determine the height the shed needs to be raised. Usually this is only 2-3cm. Good Luck!

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  • Missing appears to be a much deeper hole layered with gravel and sand. That's my understanding of how to form the base. This is, however, not my forte.
    – DAS
    Commented Apr 16, 2021 at 8:15

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