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leveling a concrete base by pouring new slab on top

I have a new brick out building about 5 by 3 metres, [about single garage size] built on an old concrete base about 3 1/2 in thick which has been in place for 30 years with minor cracking and no visible damp problem . as the slab is on a slant there is about 8 inches height difference between ends.

I want to level the slab by pouring new base or putting down a screed on top.

I could use a damp proof coarse [thin polythene membrane] or a rubberized, bitumen based brushable damp proof membrane. there is no original damp proof coarse, and there is just moist soil below.

I don’t want to use any type of high tech or mechanical fixings or epoxy resin to fix the to slabs together.

At the high end I envisage the new slab or screed to be minimum 2 in. thick [the other end will be about 8 in]. I envisage the new slab or screed simply sitting on top of the old one without cracking.

Its been suggested that I paint the existing surface with rubberized, bitumen based brushable damp proof membrane, apply a little sharp sand before it dries, to give a key, then put down a screed [4 to1 ratio using sharp sand]. My question here is: as I have 8 inches to fill at the deep end can I use some hardcore [small stones etc] to save on cement, or would it then cease to be a screed and become more of a concrete. I’m assuming that the properties/strength of screed and concrete are different.

Its also been suggested that I use a scabler the rough up the existing surface, then pour a new concrete base directly on top. My concern here is that this leaves no way to put down a damp proof coarse without reducing the adhesion

another question that pops up is, should i lay a screed that is bonded, or un bonded to the concrete floor slab, bearing in mind that the slab has cracked

The project is not subject to building regulations [building code]

Any input appreciated

Thanks for taking the time to read this

Thanks mark

use a buitumin based product to paint the floor as suggested,
where the base is thickest lay concrete on it and make sure the suface is rough so its a key for the screed
throwing sand on the wet buitumin will provide a key to bond the screed to the existing slab.
the screed 4/1 mix is correct ,if you make a fist with the mix and it holds its form in your hand, that's the consistency you need

i dont understand this part "where the base is thickest lay concrete on it " isnt the base all the same thickness
thanks

the following is news to me.
i just spoke to a concrete specialist at Tarmac , he said the if the new slab is bonded to the old one it will crack like crazy paving, because the old one will not move as its cured but the new one will move. some sort of DPM is the answer

sorry to disappoint everyone [and myself]

the buitumin is the DPM, you stated the slab is on a slant. Where the slant is, its a lot thicker, the concrete laid will reduce the thickness of the screed needed. Making the screed the same thickness if the screed is any thicker than 75mm cracking will occur