Concrete 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 surface that is clean, dry, and sound. Unless you are starting with a brand new concrete slab, this often means removing tile adhesive, thinset mortar, carpet glue or paint, and sometimes a combination of these residues.
Clean: Yellow adhesive, shown in the first photo at right, is water-based and common with vinyl tile and carpet. Black adhesive, shown next, is solvent-based and found under asbestos tile in older homes. The most predictable and efficient method of removing these adhesives is through concrete grinding, shown in photos three and four.
Even more aggressive methods, such as shot blasting and scarifying, are sometimes required for removing thin-set mortar beneath ceramic, porcelain, and stone tile, and raised concrete "caps" that are often installed to cover up electrical and plumbing rough-ins or repair work. The down side to mechanical surface preparation is that a cement overlay is usually required to cover the gouging, pitting, swirling, or other resulting damage to the concrete.
Chemical strippers are very effective in removing thin layers of paint and sealer, as well as tacky or sticky adhesives that would clog grinders. The residue left by these strippers, though, particularly those based on natural oils, like soy, or solvents like methylene chloride, can permanently darken or stain the concrete, so a sample area should always be done first. Water-based strippers tend to be the most effective and leave the least permanent effects. If you know the manufacturer of the coating or adhesiveyou are trying to strip, they will often supply or recommend an effective product for stripping it. While most contractors hate and will not perform chemical stripping, it is one of our specialties.
Paint drips and overspray, drywall mud, and other stains or markings can usually be removed with a rotary floor scrubber and Simple Green, or a grinder fitted with an abrasive ZEC sanding disc. But there are some blemishes caused by chemicals and solvents during the building process that cannot be detected and will only show up after staining. This affects a good proportion of almost every concrete slab. These blemishes can only be fixed by engraving out by hand and touching up with stain, or through faux finishing. We provide these services at the end of every job at no additional charge. The best way for a customer to ensure an optimal staining result is to protect the floor during the build-out process with a durable covering, like Ram Board, and to hire a licensed and certified contractor.
Dry: Once the surface is clean, it should be tested for moisture content and vapor transmission. Excess water in concrete can cause problems with stain color and with the adhesion and clarity of sealers. Moisture will darken stains unevenly and cause clear sealers to turn white. This is particularly a problem with control joints in a concrete floor, especially after using large amounts of water during cleaning. Because there are a variety of testing methods for moisture in concrete, and most require special equipment and training, this is best left to a contractor or professional. If moisture is a problem, and does not have an identifiable cause that may be remedied, dehumidification equipment often helps quickly dry the concrete to acceptable levels. New concrete requires at least 28 days to cure, and usually at least 3 months for excess moisture to evaporate. We test concrete slabs for moisture before every project to ensure that they are dry enough to stain and seal. We use the Concrete Moisture Encounter from Tramex, and are certified in concrete moisture testing by the International Concrete Repair Institute (ICRI).
Sound: Concrete surface preparation usually requires some patching of holes and repairing of cracks. Patching can effectively be done with most cement grouts and mortars, although the size and nature of the repair will often dictate the use of a specific type of product, and sometimes a primer, to ensure the stability of the repair. Also, it is important to note that patches usually do not blend in color with the rest of the floor or accept concrete stain in the same manner as the surrounding concrete. Therefore, it is important to choose a cement you know has good staining properties. This is often something only a professional staining contractor can determine. If the repairs are small and few in number, these imperfections may not stand out, or faux finishing techniques may be used to conceal them.
Even if repairs do stand out, such as large patching due to plumbing and electrical repairs or upgrades, the overall appearance may not be objectionable. To remove all signs of patching, a cement overlay that covers the entire floor is usually required. Concrete overlays are also required to remove all signs of crack repairs. For this reason, cracks are usually left alone unless they are very wide or present a structural concern. Small cracks usually add to the look of acid-stained concrete, providing a more "natural" stone appearance. Some of our past customers have actually asked for cracks with stained concrete, and we have engraved them in artificially.
Unlike almost all decorative concrete contractors, we are trained in "faux finishing" and can generally make any patch or crack blend in to the existing concrete without the aid of a cement overlay.
Once the concrete is Clean, Dry, and Sound, the basis has been established for a successful decorative concrete project that will last for years to come. We use a well-informed, comprehensive approach to preparing a concrete slab for acid staining, including all the above methods, and use of the best and latest technology.