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I have an area that used to be the floor of a massive sun house. 18 foot by 15 foot in a half octagon shape. Perfectly flat except for a strip along the back against a 4 foot wall, about 1 to 2 inch deep, about 18 inches wide running the width of the whole area.
I was going to fill it with concrete to level it with the rest of the concrete pad so I could then put down a largish shed (about 14 x 10)
Any suggestions on what I would need to do mix wise? First idea was to get a couple of bags of postcrete to put down as a test to see how it took, if it worked the way I hoped then continue with that. Failing that I'm not sure whether I need a cement/sand/gravel mix or something else.
I am clearly not a concrete expert
Its quite thin so I'd go with trying to "stick" the new concrete to the old. Get some SBR. Wash the area with a broom and water to get rid of any dirt, then apply a liberal slurry mix of SBR and cement (about 50/50) with the broom, while that is is still wet pour your concrete. Standard mix with a good splash of SBR in it. Should stick OK and will certainly be fine for a shed base. Another option is not to level it and just add a little extra timber strip to your shed base to sit in the step.
Move the shed 18" away from the wall, always handy to get all the way around.
What is the bottom of the strip you want to fill in? If it part of the original slab but a bit lower, then do as nickjb suggests. If it's just earth, I'd move the shed away. Otherwise, dig it out to 20cm and fill with concrete, but you really need to tie that concrete to the rest of the slab, which means drilling holes in the side of the slab or cutting it back at an angle - can give more advice on that if you think it's something you'd want to try.