Water is the worst enemy of a concrete subfloor, especially a concrete subfloor that is below grade.
These floors are very vulnerable so you should never install floor tiles of any kind to a concrete tile underlayment, until you fix the water problem.
You should always completely cover isolated, inactive cracks in a concrete tile underlayment with isolation membrane. Always fix cracks to any concrete floor before you install floor tiles.
Step ! To Preparing Your Concrete floor
Divide your concrete floor into imaginary six foot sections. Check each section of the floor for level with a four foot bubble level. Using a carpenters pencil mark the floor for cracks, high spots, or any other defects may you find.
Keep in mind that cracks could be a sign of a structure defect. Some of which you may be able to repair, while others may require professional help,
Step 2 To Preparing your Concrete Tile Underlayment
Use either a good quality hammer or sledge along with a cold chisel to open small cracks, so you can fill them. You'll want to try to create a recess that is wider at the bottom of the crack than on top.
You may be able to achieve this by angling the chisel into each side of the crack. This method helps to hold the patching cement more securely.
Step 3 Wash Then Fill Your Crack
First wash out the crack with water, and then fill the crack with hydraulic cement or thinset tile adhesive.
Use either a margin trowel or masons trowel to feather out the edges. You'll want the patch to be level with the surrounding floor.
Step 4 Fill Up Depressions
In order to fill depressions in your concrete floor pour a small amount of either thinset tile adhesive or self leveling compound into the depression. Then trowel the thinset or self leveling compound until it's level.
Step 5 Grind Down High Spots
Using a grinding tool equipped with a masonry grit abrasive wheel, grind down any of the high spots in your concrete floor that you earlier marked.
A right angle grinder will make the job go quicker. For removing the dust simply hold a vacuum hose near the grinder as you work. Once your done grinding down the highspots thoroughly vacuum and damp mop your concrete floor.
How To Tell If Your Concrete Floor Is Waxed, Sealed or has a Smooth Finish
Thinset or any other tile adhesives will not stick to a concrete tile underlayments that are waxed, sealed, painted, or finished to a gloss with a steel trowel.
The way to tell if your concrete tile underlayment is waxed or sealed is to drip a few drops of water on the surface. If the water beads up the surface has a finish on it. If the water soaks in but the surface is otherwise slick or smooth it most likely was finished with a steel trowel.
Roughening Up A Slick Concrete Floor To Install Floor Tiles
You can roughen up slick or painted finishes with a sanding block or a rented floor sander. You can even make your own sanding block by tacking a sheet of course abrasive on an eight inch 2x4. Scuff up the entire floor using light pressure. When you finish vacuum the floor.
Removing Wax And Sealers From Your Concrete SubFloor
To remove wax and sealers use a solution of four parts water to one part muriatic acid. Muriatic acid is very caustic so be sure to follow the manufacturers directions.
Wear eye protection and rubber gloves and be sure to ventilate the room. Scour the floor with the Muriatic acid wash and a scrub brush. rinse your concrete subfloor with clear water and let the floor dry though rely.
Applying Isolation Membrane Over Cracks
Apply the membrane equally on both sides of the crack or expansion joint. Use a roller to spread on a light even coat of adhesive.
Be sure to follow the manufacturers directions for applying the membrane to cured or wet adhesive.