Surface preparation is the most important part of any decorative concrete flooring project. For the project to hold up over time, you must begin with a concrete that is clean, dry, and sound. Unless it's a brand new slab, this often means removing tile adhesive, thinset mortar, carpet glue or paint - sometimes a combination of these residues. Even if the concrete has not been treated, there is still always a great deal of drywall mud, paint, primer, insulation overspray, hydraulic fluid, glue and other construction adhesives found on the surface. Builders and tradesmen on a tight time schedule rarely take time to protect "ordinary", gray concrete.
Getting a concrete floor clean may mean using a variety of methods, ranging in aggressiveness, depending on the age and condition of the floor. Concrete diamond grinding is the most common method method. Done properly, it shaves off whatever is on the surface while affecting the concrete as little as possible. Materials like carpet glue, tile adhesive and paint generally need to be ground off. Chemical strippers are sometimes effective in removing tacky or sticky adhesives that clog grinders, so they may be helpful at the outset of a project, but they will not get all residue off the concrete ,and need to be followed by light grinding. Also, while chemical strippers may be effective in removing thin layers of paint and sealer, and isolated areas of adhesive, the oils and solvents in these strippers often permanently stain or darken the concrete, as does carpet glue and tile adhesive itself. Diamond grinding is the best way to remove all traces of a substance from the cement surface without compromising its integrity for concrete staining.
OTHER CONCRETE CLEANING METHODS
More aggressive methods to clean concrete, such as shot-blasting and scarifying, are required for removing thicker layers of material, like epoxies, gypsum-based underlayments, thinset mortar under ceramic tile, and raised concrete "caps" covering up electrical or plumbing repairs. Shot-blasting or scarifying may also be necessary to clean concrete with a very rough texture or deeply embedded staining. These cleaning methods do compromise the cement surface of a concrete floor, leaving ridges or "row" marks. Therefore, they need to be followed by grinding to smooth the surface. These methods may also require a thin cement overlay to resurface the concrete and cover scratches, furrows and exposed aggregate.
Getting concrete clean does not always require an aggressive method grinding, shot-blasting or scarifying. If concrete has not previously been treated, scrubbing with a rotary floor machine, using a black stripping pad, cleaning detergent, and warm water can also be effective. A couple passes in crisscrossing directions will usually get up the majority of dirt, dust and drywall mud. Anything left behind then shows up clearly enough on the wet concrete, and has been sufficiently softened, to be scraped off with a 4" wallpaper razor scraper. An angle grinder fitted with a lightly abrasive sanding disc can also be used for spot removal. Some blemishes caused by chemicals and solvents during the build-out process cannot be detected during cleaning, and only show up during the concrete staining process. Such blemishes can only be removed by engraving them out with a Dremel-type tool, and then touching up with stain; or through "faux finishing" using artist tints.
Making concrete sound enough for the installation of acid-stained or other types of decorative concrete flooring, like polished concrete, metallic epoxy and cement overlays, will generally mean using one or more of three repair applications: 1) patching small holes, indentations and divots; 2) repairing cracks; and 3) rebuilding damaged or missing sections of concrete.
Patching is necessary on almost every concrete floor prior to staining. Small chips or missing pieces of concrete are commonplace, and may look "natural" enough, or so inconspicuous, as to not need to be addressed. But larger areas, like sections of "spalling", and half-dollar-sized holes left by nails from carpet tack strips, or the removal of bolts, do stand out. Patching this type of damage can effectively be done with cement- or epoxy-based grout repair materials. But these materials require at least several hours to dry; and in some cases need to sit overnight. This may not present a problem, if there is a great deal of patching to be done that takes a good part of the day. But if it's only a small amount, and the next step of a flooring project needs to proceed quickly, there are specialized, two-part urethane- and polyurea-based repair products than can be used, which are typically mixed with sand and dry in a matter of minutes.
Cement- and epoxy-based grout repair materials can also be used to repair cracks, but since crack repair is almost always a two-step process, urethane- or polyurea-based repair materials are preferred. They set up much more quickly, and the second step of the process - removing excess material from the surface - can begin almost immediately. There are two categories of urethanes and polyurea crack repair materials: 1) "structural", for stable cracks; and 2) "semi-rigid", for cracks that may continue to move. The excess material from structural repairs cures so hard that it must be ground off, while the excess material from semi-rigid, which remains flexible, must be shaved flush with a long-handled razor scraper.
Damaged or missing sections of concrete are most commonly encountered on indoor decorative concrete flooring projects along, or at the intersection of joints. On outdoor decorative concrete projects, like a patios, driveways or pool decks, they are often also found around the periphery of a concrete slab. Large repairs like this may be done with either straight cement, or specially engineered repair mortars that are based on cement, epoxy or polyurea. Cement, and cement-based mortars, are inexpensive, but they can take a long time to cure. Epoxy- and polyurea-based repair mortars dry much faster, and have much better long term durability, but they are more expensive and difficult to use.
Lastly, since patches, crack repairs and rebuilt sections of concrete usually do not blend in color with the main body of a traditional, gray concrete floor, it is often important to mix color, such as color hardener, powdered pigment, or liquid colorant, with the repair material beforehand, if the concrete floor is going to be decoratively stained. That way, remaining imperfections from the repairs may not stand out, or faux finishing techniques may easily be used to conceal them. Even if the repairs do stand out, for example after large, saw-cut plumbing and electrical repairs or upgrades, the overall appearance may not be objectionable, if the repairs are few in number. Or, they can be incorporated into the overall design of the floor. Much of the structural damage that happens to a concrete floor occurs during the build-out process and may be prevented by using a temporary, protective floor cover covering.