Concrete floor slab design

[John37]

I'm going to lay a concrete floor inside a large house. It will be approx 200m2 in two sections of 100m2 joined by two interconnecting doorways. My idea is DPM, 150mm concrete slab with 6 or 8mm diameter mesh mid-level, 120mm celotex or similar, vapour barrier, 50mm liquid screed enclosing CH water pipes. There's a lot of digging out first because I want the finished level to be at the same level as the oversite concrete that lay under the original suspended timber floor -- we are lowering the original floor level by about 450mm.

Question: does this construction sound suitable? Some people are saying we need crushed aggregate/hardcore under the 150mm slab. Other people are saying that if the concrete slab is reinforced with mesh (we might get those special fibres in the mix too to strengthen it further), there is no need for any aggregate under the slab. Any advice please?

 

[noseall]

If the substrate is not suitable then you should lay minimum of 100-150mm compacted stone layer.

Fibres in a slab can not be substituted for hard core layer.

 

[John37]

How do we know whether or not the substrate is suitable, please? What characteristics are we looking for?

 

[noseall]

If it is as hard as compacted mot then leave it.

 

[Deluks]

Less concrete, more screed, I also think 120mm of celotex is overkill. Try 100 conc with a142 mesh, 100mm celotex, min 65mm screed.
Will save a fair few quid over you proposed spec. As Nose knows, the hardcore is the most important bit.

 

[John37]

Thanks for your kind advice.
What is the function of the hardcore?
What problem arises if it is omitted?

 

[noseall]

Vegetative ground or fertile ground should be removed and replaced with good bearing and draining ground i.e. a compacted granular hard core layer.