Hello "regu123". The problem you describe is actually not a problem! An inexpensive way would be to roughen the concrete surface, even if it seems relatively small, by the shot blasting process. No, sanding or brushing is not enough here, as these techniques cannot reach into the depressions of the threshold-shaped elevations. And what we need is a proper bond to a cement-bound bonded screed, which is applied over a bonding bridge. A sufficient load transfer is already achieved with an installation height of 2cm under the expected stress. If there is a step at the garage entrance, the concrete surface would have to be ground flat to the garage width and perhaps 15cm into the garage, so that a seamless transition is possible. The cured screed must then be treated with a surface protection, for example ([Haftbrücke/Grundierung nicht vergessen]) with a simple concrete paint applied by rolling. Sure, this is nothing "for eternity", but it should serve its purpose for about 5 to 8 years. A coating would of course also be conceivable, but from an economic perspective this area is simply too small (and thus uneconomical for a contractor). But as I have described it, it will work. Regards: KlaRa