A question we get asked a lot is whether you can screed over smooth concrete and whether or not the screed will bond.
Our typical answer to this question is that if you are concerned enough about the smoothness of the concrete slab to ask the question, then you have probably answered your own question.
In seriousness, a smooth concrete slab, especially one that has been finished by the concreters with a polishing disc or machine always going to be risky for a screed to bond to. Whilst a good quality adhesive slurry (never just cement powder and water) will definitely help, the slurry needs a coarse slab surface to create a mechanical key to bond to. Also, the surface of the slab should be "open" so that the slurry can penetrate into the slab to form a quality bond.
Unfortunately, we from time-to-time see projects where the slab was smooth and the screed fails to bond (either entirely or in patches - which often grow) indicating a low bond strength between the slurry and the slab.
These circumstances can be difficult for projects to deal with resulting in either the removal of the screed following by grinding of the slab to create the correct surface profile before rescreeding, or accepting that the screed has delaminated and the consequences of this (increased risk of cracking, hollow sound, potentially shortened life of the screed and flooring product).
As a rule of thumb, the concrete slab should be CSP level 3 but definitely not polished smooth.
A common misconception is that the delamination of the screed has something to with the screed. It actually has nothing to do with the screed and is entirely to do with a failure of the slurry to bond to the slab. It is a failure at the interface between the slurry and the slab.
A good quality slurry can definitely reduce the risk of the bond failure and we always recommend the use of a high quality slurry from a reputable supplier, but if the slab is smooth, in many cases, even the best slurries will struggle to bond to the slab as they have nothing to form a mechanical bond to and can't penetrate into the slab.
If in doubt, grind the slab. Its much cheaper to grind the slab first, rather than remove the screed and grind the slab later before having to replace the screed.
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