You don't say how thick the screed is but the fact that you're concerned that it sounds hollow suggests you intended it to be bonded to the concrete beneath.
For screeds to bond to concrete, the concrete must be sound, strong, stable, with no powdery laitance on the surface. The normal methods of substrate preparation include scabbling, grinding, recylical shot blasting, or at the very least wire brushing. In measurable technical terms you need a pull off (tensile) strength > 0.8N/mm2 within the concrete in order for the concrete base to be strong enough to restrain a bonded screed.
Once that's all done, clean (e.g. vacuum), wash with clean water to 1) clean it and 2) satisfy any porosity, remove puddles but leave damp, then apply a bonding primer of Ronafix and cement, mixed in equal parts, measured by weight or by volume.
Before this bonding primer dries (you have about 10-30 minutes depending on temperature and drying breezes and porosity) mix and lay your screed.
If you want to lay a very thin screed, e.g. to a minimum depth of only 6mm, then use cement, dry medium grade sharp sand, Ronafix and water. Dry mix the cement and sand, mix proportions 1: 2.5 by weight, mix 1 part Ronafix with 1 part water, and add enough of the diluted Ronafix to the cement and sand to achieve a screed which is wet enough to lay but not too wet that’s sloppy. The test is too squeeze a ball of mortar in your hand. It should form a “cake” which you can break open into two halves. If it crumbles it’s too dry. If it squeezes through your fingers, it’s too wet.
Once mixed, lay it on to the wet/tacky primer with a float or trowel, compact it well and screed smooth. Protect it from drying out too rapidly (too avoid shrinkage cracking and curling) with either tight fitting polythene secured around the edges, or a pray applied curing membrane such as Ronacrete Monocure.
Once cured and hardened, use it.
Going back to this problem, your screed could be unbonded for a number of reasons.
Poor or no preparation.
Weak or friable concrete.
No bonding primer used.
Inadequate compaction and no contact with the substrate in the hollow area.
To rebond it is a difficult one and often a hit/miss exercise.
You can drill the area and pour in a liquid rebonder such as Ronacrete Ronabond SR79, a low viscosity acrylic resin which can rebond if all conditions are right. But if the concrete base is weak, friable, powdery, has laitance on it, or the gap between the top of the concrete and underside of the screed is too big, it won’t work.
Hope this helps.
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Mod Rupert
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