Concrete surface preparation is the most important and time-consuming part of any decorative concrete flooring project. For the project to look great and hold up over time, it must begin with a surface that is clean, dry, and sound. If proper surface prep is not done, it will affect the end result, in terms of aesthetics and durability. Initially, a customer may notice imperfections in a floor that somewhat "mar" the decorative finish. These may or may not be deemed "acceptable". Later on, such imperfections can worsen, and be joined by sections of the floor where material, such as coloring or sealer, comes loose. Premier Veneers uses a well-informed, comprehensive approach to preparing a concrete slab for acid staining, including all of the methods outline below, and we use the best and latest materials and technology.
Getting a concrete floor clean means 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 surface preparation. The concrete may previously have been treated and require the removal of carpet glue, tile adhesive, paint or a gypsum underlayment. 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. This is because builders and tradesmen are on a tight time schedule and rarely take time to protect "ordinary" gray concrete.
Carpet glue, tile adhesive and paint 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 still 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, done properly, will remove all residue and staining, without compromising the integrity of the cement surface.
More aggressive methods of mechanical surface prep, 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" installed to cover up electrical or plumbing repair work. They are also required for cleaning concrete with a very rough texture or deeply embedded staining. These types of surface preparation methods do compromise the cement surface of a concrete floor and need to be followed by grinding to smooth the surface, and often a cement overlay, as well, to cover scratches and exposed aggregate.
Getting concrete clean that has not previously been treated starts with a rotary floor scrubber, black stripping pad, cleaning detergent, hot water and shop vac. A couple passes over the floor using this process will get up the majority of dirt, dust and drywall mud. Whatever is left behind will show up clearly on the wet concrete and usually has been softened enough to be easily removed with a 4" razor scraper. If not, an angle grinder fitted with a lightly abrasive sanding disc can be used for spot removal. Unfortunately, some blemishes caused by chemicals and solvents during the building process cannot be detected and only show up after acid-staining. These blemishes can only be fixed by engraving out by hand and touching up with stain or through faux finishing.
The best way for a customer or project owner to to ensure an optimal acid-staining staining result of their concrete, is to protect it during the build-out process and to hire a licensed and certified contractor. There are several durable, temporary floor covering products on the market designed for this purpose, like "Ram Board". Ram Board costs about $60 for a 3'-wide, 100'-long roll and is available online or at Home Depot, Lowe's, and numerous construction supply outlets in the Detroit and Chicago 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 concrete sound enough for acid-stained and decorative concrete flooring often means: 1) patching small holes and divots - sometimes after removing bolts or nails; 2) repairing cracks; and 3) rebuilding damaged or missing sections of concrete near joints or the periphery of a concrete slab. Patching can be effectively done with most cement grouts and epoxy repair materials. Cracks are best repaired with structural polyurethanes or semi-rigid polyureas, depending on their width and whether they are moving or stable. Areas of damaged or missing concrete need to be restored with either new cement or a structural (epoxy or polyurea) repair mortar.
Since patches, crack repairs and other fixes usually do not blend in color with the main body of a concrete floor (or react with acid stain), it is often important to mix color with the repair material beforehand, if possible. If repairs are small and/or few in number, any resulting or remaining imperfections may not stand out, or faux finishing techniques may be used to conceal them. Even if such repairs do stand out, for example after plumbing and electrical repairs or upgrades, their overall appearance may not be objectionable or can be incorporated into the overall design of the floor.
As with drywall mud, paint, insulation overspray 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.