Concrete surface preparation is the most important and time-consuming part of any stained concrete flooring or other decorative concrete project. For the project to look great and hold up over time, it must begin with a surface that is clean and sound. The most efficient way to get concrete clean, without damaging it, is diamond cup concrete grinding. The key to ensuring that concrete is sound is making the appropriate repairs.
Getting a concrete floor clean may mean using a variety of manual and mechanical methods, ranging in aggressiveness, depending on the age and condition of the floor. Almost all concrete slabs, unless they are brand new and have been protected with a temporary floor covering, require a great deal of cleaning. The concrete may previously have been treated and require the removal of carpet glue, tile adhesive, paint or a gypsum underlayment. Even if the concrete has not been treated, there is still always a great deal of drywall mud, paint or primer spatter, insulation overspray, solvent stains, glue and other construction adhesives to be found on the surface, because builders and tradesmen are on a tight time schedule and rarely take time to protect "ordinary" gray concrete.
Concrete grinding, done properly, shaves off whatever is on the surface of the concrete, while affecting the concrete, itself, as minimally as possible. Materials like carpet glue, tile adhesive and paint generally need to be ground off. Chemical strippers are effective in removing tacky or sticky adhesives that may clog grinders, and can be helpful at the outset of a project, but they will not get all residue off the floor and need to be followed by light grinding to get a concrete floor clean enough to stain. Also, while strippers may be effective in removing thin layers of paint and sealer, and small, isolated areas of adhesive, the oils and solvents in these strippers can permanently stain or darken the concrete, as can the carpet glue or tile adhesive itself. Grinding is the best way to remove all traces of a substance from a cement surface without compromising its integrity.
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 and porcelain tile, and raised concrete "caps" that are often installed to cover up electrical or plumbing repair work. Shot-blasting or scarifying may also be required for cleaning concrete with a very rough texture or deeply embedded staining. These types of concrete cleaning methods do compromise the cement surface of a concrete floor, and as such, need to be followed by grinding to smooth the surface. These methods may require a concrete overlay, as well, to cover scratches, furrows and exposed aggregate.
Getting concrete clean does not always require diamond cup grinding. If the concrete has not previously been treated, scrubbing with a rotary floor machine, black stripping pad, cleaning detergent, hot water and shop vac is also effective. A couple passes over the floor using this process will usually get up the majority of dirt, dust and drywall mud. Whatever is left behind will then show up clearly enough on the wet concrete, and be sufficiently softened, to be removed with a 4" wallpaper or tile scraper. If not, an angle grinder fitted with a lightly abrasive sanding disc can also be used for spot removal. Unfortunately, there are some blemishes caused by chemicals and solvents during the building process that cannot be detected during cleaning, and only show up after staining a concrete floor. These blemishes can only be fixed by engraving them out, and touching up with stain, or through "faux finishing".
The best way for a customer or project owner to ensure an optimal result to their stained concrete flooring project is to protect the concrete during the build-out process, and to hire a licensed and certified decorative concrete contractor. There are several durable, temporary concrete floor protection products on the market designed for this purpose. "Ram Board" costs about $60 for a 3'-wide, 100'-long roll and is available online or at Home Depot, Lowe's, and construction supply outlets in the Detroit and Chicago metro areas. It is waterproof and easy to install and remove.
Determining whether concrete is dry - or at least dry enough to install a decorative finish - can only be done by testing for moisture content and moisture vapor transmission. Excess moisture in concrete will cause problems with stain color, the curing of coatings, and the adhesion and clarity of sealers. For example, wet spots darken acid stains unevenly (see photos below) and cause clear sealers to turn white. This is particularly a problem with cracks or joints in a concrete floor, after large amounts of water have been used during cleaning. There are a variety of testing methods for moisture in concrete. Most require special equipment and training, so this is always best left to a contractor or professional. Read more about how Premier Veneers verifies concrete is dry enough for decorative concrete flooring installations in the Moisture Testing services section of this web site.
Making a concrete floor sound enough for acid-stained and other types of decorative concrete flooring, like polished concrete, metallic epoxy and concrete overlays often means using one of more of three applications: 1) the patching of small holes and divots - sometimes after removing bolts or nails; 2) the repairing of cracks; and 3) the rebuilding of damaged or missing sections of concrete near joints or the periphery of a concrete slab.
The patching of concrete can be effectively done with most cement grouts and epoxy repair materials. However, these materials often require at least several hours to dry, and in some cases may need to sit overnight. This may not present a problem if there is a great deal of patching to be done. But if it's only a small amount, and the next step of a flooring project needs to proceed quickly, there are also specialized, structural urethane and polyurea repair products than can be used, which dry in a matter of minutes.
Cracks may also be repaired with cement grouts or epoxies, but since crack repair is a two-step process, urethane or polyurea repair materials are better to use since they cure so quickly, and the second step of the process - removing excess material from the surface - may begin almost immediately. There are "structural" urethanes and polyureas, or "semi-rigid", depending on the width of the cracks, and whether they are moving or stable. The excess material of structural crack materials must be ground off, while the excess material from semi-rigid materials has to be shaved flush with a long-handled razor scraper.
The rebuilding of damaged or missing sections of concrete may be done either with new cement or a repair mortar based on cement, epoxy or polyurea. Cement and cement mortars are inexpensive but can take a long time to cure. Epoxy and polyurea repair mortars are more expensive but dry faster.
Since patches, crack repairs and rebuilt sections of concrete usually do not blend in color with the main body of a concrete floor, it is often important to mix color with the repair material beforehand, if the concrete floor is going to be stained. That way, any resulting or remaining imperfections may not stand out, or faux finishing techniques may be used to conceal them. Even, if the repairs do stand out, for example after plumbing and electrical repairs or upgrades, the overall appearance may not be objectionable, if the repairs are few in number, or they be incorporated into the overall design of the floor. As with drywall mud, paint and other construction-related contaminants found on a concrete floor, much damage to a concrete floor happens during the build-out process and can be prevented by using a temporary, protective floor cover covering.