Looking for advice on concrete for shed base Post your questions and get answers regarding all areas of drives, pathways and patios

[millsy5]

I got a builder that was supposed to pour concrete for a shed base. The specifications that the shed company specify for the shed base is a double pour.
The first pour is the initial base.
Then the steel shed is installed, with the horizontal steel bars at the base of the shed bolted into the first concrete base.
Then a second pour is done to level up the base with the height of the steel bars i.e. the height of the final base is the same height as the horizontal steel bars in the frame of the shed. I've attached 2 photos.

The builder did the bare minimum for the first pour, just enough to have something for the shed to be bolted into. And that was all he did (well he was also supposed to build a wall to the side of the shed and plaster it, but you can see he only plastered half of the wall, and badly at that).

The builder disappeared so I'm stuck with an unfinished job, having paid for most of it upfront (I know, big mistake on my part).

I'd decided to finish the job myself instead of paying someone else to do it. I've a ton bag of sand left, and about 1/3 of a bag of sand and gravel mixed (he mixed them, but I think they look more sandy than gravelly). I've 3 bags of cement, but will definitely need more. And more gravel. Because I want to use up the materials I have, I don't want to buy the ready made stuff. I'll hire a mixer for the day. I have a wheelbarrow and shovel. Maybe hire a float too. There are some bags cement that he left out in the rain and they hardened, and are unusable, so I was thinking of getting a lump hammer and breaking up to use as rubble.

Anyway I'm looking for some advice on pouring the concrete as I've never done this myself.
Someone down the pub told me I should put in some rebar to help prevent the concrete from cracking.
Any thoughts on this? The depth (from the sandy/gravelly part in the middle - see photo) to the top of the steel bars is about 120mm. As it's a shed it's only for storage, but nothing too heavy in the there (maybe stuff like bikes, lawnmower, bags of compost, etc.).

Looking for any advice and can provide any additional information. Thanks in advance.

[stoneyboy]

Hi millsys5,
Sorry to read your tale about your contractor disappearing part way through a job and well done for picking up the pieces.
Use whatever rubble you have to fill in the ground so you have 75-100mm of concrete to lay - compact the rubble.
Check the base is square - measure diagonally corner to corner.
Lay a sheet of weld mesh on the rubble and lay concrete, forget the steel bars you refer to.
Once the concrete has set either fit flat brackets bolted into the concrete and the shed base frame or bolt through the frame into the concrete edge base.
Regards S

[millsy5]

Thanks for the reply.

The shed fitters will be looking after bolting the shed into the existing concrete. I'll only be mixing and pouring the concrete (up to the height of the shed base frame (horizontal steel bars).

I googled weld mesh and it looks like it comes in a roll. I presume I can specify the amount I require and they'll cut it off the roll for me. I'll check with the local builders providers.

To compact the rubble do I need a compactor? This is likely the best way to get it really compact.

[stoneyboy]

Hi millsy5,
Have a look on line for weld or fabric mesh, it usually comes in flat sheets. See what your local builders merchants have in stock. A wicker plate is fine for compacting fine materials but will run on top of coarse rubble, better to use a club hammer and make sure compaction is even, ok it’s slower and hard work but a better result in the end.
Regards S

[millsy5]

Any thoughts on adding a plastic sheet for damp proofing? The shed fitters had mentioned it.
If so where (what layer) would this go?
I have the sub-base which has ballast (a bit more sandy but I'll add more gravel to it) and some rubble that I added. Would the plastic sheet go directly on top of this, and then the weld mesh directly on top of the plastic, which the concrete will then be poured over?

[stoneyboy]

Hi millsy5,
Due to the very irregular edge to the concrete under the shed walls a plastic sheet will not sit flat, will not support the concrete above it and will probably be pierced when you walk around laying the concrete.
Use bits of roof tiles to raise the weld mesh above the base fill and cut the weld mesh to fit inside the irregular edge concrete.
Suggest you fit a sheet of vinyl flooring once the concrete has set, this will effectively form a DPM. Because you have a metal shed this will run with condensation in cold weather so keep the edge of the vinyl away from the steel walls.
Regards S

[millsy5]

I bought some gravel today from the builders supplier. They delivered it a while ago. I took a look at it and it's very sandy (see attached pic). Is this normal. I had assumed when I ordered gravel it would have resembled small stones. But this looks like a mix of sand and various sized small stones. Maybe this is normal.

Now I have a ton of sand (not sharp) and a ton of sandy gravel. I'm wondering what sort of ratio I should use. Checking online I've seen various ratios recommened for concrete in a shed.
1:2:2 (cement : sand : gravel)
1:2:3
1:2:4

Any advise on the best ratio for me using the materials I have?

[stoneyboy]

Hi millsy5,
It looks like your gravel is actually Type 1 fill, if so, this is not suitable for a concrete mix.
If you are assured by your supplier that it is gravel I suggest you use a 2:3:4 mix.
Regards S

[millsy5]

I think it might be ballast that I bought even thought the guy at the trade counter in the builders supplier didn't mention it. I had told him I already have a ton of sand that I want to use up and that I wanted a ton of gravel. I checked their website just now and it has the following description for the gravel:

"Jumbo Bag Of Batch Gravel (Concrete Mix) is ideal for use when mixing concrete. This graded mix is ideal for builder's concrete mix for foundations or concrete for floors and footpaths. Blend with cement and water to get the strength and consistency you wish to achieve."

Not sure what it means by 'graded mix' but to me that sounds like ballast given that it doesn't mention to add any sand.

Would you still recommend 2:3:4? Ideally I probably shouldn't even use the sand but I do want to use it up.

[stoneyboy]

Hi millsy5,
That’s a bit unfortunate, suggest since you will end up with a sandy mix if you use both sand and ballast, make it 1:2:2. Depending on the amount of clay in your building sand you may find the concrete surface not very durable so some form of covering may be needed.
Regards S