Levelling garage floor

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My garage floor is very rough, with stones and all sorts in it. On one side of the garage, where a wall was modified, the conctrete has been raised by 35mm for some reason.

I was about to do formworks and rasie the floor with conrete by 50mm (would be 15mm where the concrete is already higher). I was going to do it in four sections to make the job more manageable.

But from what I've now read on forums, putting a layer of concrete on top of an existing layer is not recommended because it doesn't stick to the layer below and because it expands at a different rate. But none of the texts I read stated how thick the new layer would need to be so that it doesn't crack.

Will I have to remove the old slab?
 
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Will garage be used to park your car or is it just storage/workshop?
If car is to be parked 50mm may be a little small, but the idea is to;
Repair any major cracks, clean the surface, if it's dust and loose a power cleaner maybe based way and sweep it clean, score the original surface with and angle grinder again removing any dust or dirt afterwards.
Then apply a bonding adhesive (following instruction on the product, as some are different)
Then do you pour, I would try to do it in one, if possible.
Also I would consider a gentle fall/slope out of the garage to prevent water coming in to it, this will be dependant on it's position on your property.
 
It will have a car, and I plan to use it as my workshop when working on the the car, so this mean trolley jacks etc. So what's the recommended thickness?
 
I did this recently for my father and I poured 75mm which was the thinnest i though i could get away with.

The big thing you have to watch out for it you haven't done much concreting before is how quickly it will dry out. The concrete underneath is bone dry for many years and so if you covered the whole lot in water the water will have soaked in by probably 15mins later.

To combat this i mixed up 3:1 water - SBR and primed the floor with this literally just as the truck arrived. Alternatively you could spread a thin sheet of polyethene over the floor not so much to stop damp as the old floor will have a dpm but just to give you more time.

Nothing worse than having your 60 year old father offer to help but then have to stop cause his back is sore after 10 mins leaving you on your own to screed tamp and trowel a 10x4 floor of rapidly hardedning concrete. :evil:

The best hardest finish will be acheived with a powerfloat or just floating by hand will give you a pretty good result. If you have never really used a float much you might consider getting a plasterer to come and give it a going over for you.
 
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I will break up the area that is 35mm higher, and I will put in a 75mm layer.
 
Might be worth looking @ Granolithic screed - or any ready mixed screed :idea: apparently you can get it like readymix concrete now.
 

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