On Sunday, 27 January 2019 08:25:32 UTC, harry wrote:

I could never see the point of unbonded screeds.
Maybe too thin or the wrong mix.
https://www.ardexbuildingproducts.ie/types-of-screed/

As your link says, for a 50m+ thickness like mine it needs to be unbonded. I suspect it is because the extra thickness results in a greater differential in drying time throughout the depth hence greater potential for movement (as in this case!).