Thanks for posting your feedback on the floor screeding project.
Screed has started to crack, and in these areas screed seems to be hollow beneath screed. What has gone wrong?
The answer is that possibly the screed was not mixed well enough. Sometimes, if not mixed thoroughly, the sand can stay in lumps within the screed. This very often happens when the sand is frozen in the winter and not allowed to thaw before mixing. The floor screed sets around the lumps which in the course of a few weeks either thaw out or dry out to become simple grains of sand within a set screed. This causes little voids within the screed and compromises the integrity of the covering. As the floor is walked in, tiny bits of the set screed around the voids, fall into the void and the screed becomes very loose and weak. If there is any discrepancy between the thicknesses of the screed over the floor area, this can introduce a crack.
The cracks may also be because the floorslab (assuming it is laid on a slab!) has cracked because of ground movement. If a crack appears in screed and the screed is being used, the sides of each crack, under the surface will fall away into the bottom of the crack making the screed hollow.
The obvious remedy, if the hollow areas are local to the cracks, is to dig up the screed about 2 inches either side of the crack to give you a 4 inch (100mm) channel which you can fill with new screed.
Is it a wet mix or dry mix?
This question has now been answered on the project page mentioned above. I apologise for it not being there before. An oversight by our trade team. For others reading the blog, a floor screed mix should be pretty dry and if you pick up a lump of the mixed screed (put gloves on first) and squeeze it in one hand, it should stay in a solid lump when you open your hand again. Very little, or no water should come out when you squeeze.Good luck with your floor screeds.