Flexing floor screed - normal?

[Penelopejean]

We had an extension built on our sitting room with the floor screed laid about 6 months ago. Yesterday we lifted the temporary rug(which was put down after about 3 months) to prepare to have wood block laid and have found that some of the concrete screed(50mm thk apparently) is cracked, is flexing and has broken up. It's screeded onto that new slab insulation stuff(celotex?). The builder has repaired it today in the areas(approx 3sq ft) that have degraded but I have lost confidence in the floor itself. It flexes particularly around the walls too and sounds hollow. We think that the mix is too dry basically. What I want to know is, am I right in thinking this is not up to standard and is it a good idea to ask the builder to re-lay it and wait another 3 months for it to dry before the wood block is put down?! Also is there a particular hardness that the screed should be?
Any ideas gratefully received please.

 

[noseall]

i have stated in other posts about my faith in screeding directly over insulation.

i have none.

we always fit the insulation layer under the main floor slab, then screed upon the main slab. this way, the vulnerable screed is in contact with a compatable surface and not an alien one.

i also disagree with fitting a thin membrane between screed and concrete slab, for the same reasons. if a 1200gauge polythene membrane is already present beneath the slab, then why bother reducing the cohesion between screed and slab, with another membrane.

i suspect that it is the thickness (50mm) or composition and not the dryness of the mix, that is responsible. add fibres or wire mesh reinforcing and don't skimp on the cement.