I am in the floor prep phase of installing a floating bamboo floor in my second floor condo. Under the carpet was a type of lightweight concrete. I say type of because it does not act like the descriptions of gypcrete that I have read about in that it does not powder or scratch easily with a nail. It is not as hard as normal concrete either, the carpet tack strips were nailed into it. It is also fairly cracked and some of the pieces will rock a very small amount when you walk on it. My questions then are, do I need to know what kind of concrete this is to get the right patching compound to level the low spots, and if so how can I determine what it is? There are already some patches in low spots that I assume were done when the condo was built that are a darker grey color and are of a much softer material that bonded to the concrete well. Also is there anything that can be done about the rocking pieces or is that not to much of a concern. I doubt I can aford to rip out the concrete and replace it. I assume that the cracks are due to the substrate under the concrete not being stable and being a condo there is probably nothing that I can do about that anyway. And lastly what needs to be done to prep the concrete before applying the patching compound? All of the directions that I have read talk about cleaning off anything that could compromise the bond and alot of the floor has overspray from the wall texture on it.

Tuff call. ARDEX GS-4 is your safest bet. You say the gray compound stuck well, makes me think you can go with ARDEX K-15, after removing the loose chunks and priming it.

Both of these products that you are recomending are self leveling. I thought that it is generally not recomended for someone without experience to use self levers

Exactly. You may be in over your head, with this one.

When it is all cracked and breaking up. the only way to stabilize it, is to either bust it all out and do it again, or remove all the loose stuff and cap it all.