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What's New: December/January 2009
Detecting & Avoiding Carbonation Before Staining a Concrete Floor
Carbonation is a natural chemical process in concrete that is exposed to air, and it continues throughout the lifetime of a concrete slab or structure. Most often, this process occurs slowly and does not affect the surface properties of a concrete floor. But if accelerated carbonation occurs during the placement of the concrete, it can completely ruin any potential for a concrete floor to be acid stained. Unfortunately, this phenomenon happens all too often, and our company sometimes has to break the bad news to a potential new customer that their concrete floor cannot be acid stained.
Acid stain that was easily removed from carbonated concrete floor during the post-stain rinsing process. This floor had to eventually be covered with carpet.
This extreme type of carbonation is most commonly found with basement floors poured and placed during the winter with a poor to average grade concrete mix and by an inexperienced or unskilled contractor. Cold winter temperatures often force the use of fossil fuel burning heaters to keep fresh concrete from freezing during placement and the initial curing period afterward. However, these heaters emit carbon dioxide, which if not vented outside, settles on the surface of the concrete, prevents it from hardening properly, and creates a soft, chalky surface. If a contractor or homeowner unknowingly stains such a floor, there may not be a problem initially, and the results may even appear magnificent. But during the rinsing process, or shortly after sealing, the color of the stain or sealer will almost inevitably come off the floor (see photo at right) or scratch very easily.
For a floor that has been carbonated during placement, there is really no remedy that will allow acid staining. The carbonation normally goes too deep in the concrete to be effectively removed. Concrete “densifiers” can be applied to harden the weak surface, and sometimes works, but even if it does, this process generally uses up valuable cement needed for the staining process, and makes the floor too smooth and dense to be coated with a clear sealer afterward. This is one major reason that concrete should always be tested, and color samples done, before embarking on a floor staining project. If the pH of the floor is between 7 and 9 (the pH of "good" concrete is normally between 10 and 12), and the stained color samples (underneath the leftover stain residue) can be partially or wholly rinsed off the floor with a wet sponge, there is a high likelihood of carbonation. A good staining contractor will always test the pH of a concrete floor and do color samples during the pre-project consultation process to avoid this potential pitfall and ensure that the concrete can be successfully stained.
If your basement floor hasn’t been
placed yet, and must be poured in a northern climate during the winter, your
concrete contractor can prevent carbonation by using a high cement content
concrete mix (e.g., “six sacks” and no fly ash), with a low water-cement
ratio (.45 or below), using electric or vented heaters and a wet curing
method. These preconditions
are not an imposition for knowledgeable and skilled concrete contractors,
and they should not drastically increase the cost of the pour, although the services of
a good contractor may be a little more expensive upfront. Premier Veneers
works closely with builders and concrete contractors in the Chicago area to
specify concrete mix designs and site conditions that will minimize the risk
of carbonation during winter pours. We also do stained color samples and
carefully test the pH of every concrete floor with an electronic pH meter
and phenolpthalein solution before a staining project begins.
Safely Sealing Acid-Stained
Concrete Basement Floors
Solvent-based acrylic "lacquer" sealer being applied to residential basement acid-stained concrete floor with a 9" roller frame and cover.
Residential basement floors are one of the most popular applications today for acid stained concrete. The finished basement industry has taken off in recent years, there is an abundance of new building contractors that specialize in this area, and people are looking for a unique flooring option to accentuate this new area of their home. Staining a basement floor, however, poses a unique set of challenges. Not only must the work be done with an artistic touch, but it must be protected with a well-chosen brand and type of clear sealer that is durable, resistant to moisture, and relatively easy to fix if need be. The problem is that the type of sealer that fits these requirements best, a solvent-based acrylic (or “lacquer”) usually has significant safety concerns associated with it during the application process, and the project and owner can encounter big problems if the proper safety precautions are not taken.
There are two primary concerns in sealing an acid-stained basement floor: odor and flammability. The solvent used to manufacture most acrylic lacquers is based largely on xylene, a hydrocarbon that exhibits a strong odor as it is evaporating. This odor is relatively sweet-smelling, but it is very heavy and can produce headaches and be somewhat dizzying or intoxicating. If you haven’t been exposed to it before, you wouldn’t want to be around it for very long. Also, xylene fumes or vapors are highly flammable, especially when they build up in a confined area, such as a basement. This safety risk is compounded by the fact that there are several sources of ignition in most basements: the pilot lights of water heaters, the electric heating plates of furnaces, and the active electric motors of sump pumps. If these sources are not turned off during sealer application, the sealer fumes can ignite and potentially lead to an explosion.
Basement utility room with water heater pilot light put out, furnace switch turned off, and sump pump deactivated during sealing of acid-stained concrete.
On the positive side, however, these risks can be well-managed and minimized, if not completely eliminated. In a home, the odor of an acrylic lacquer is largely confined to the basement. It is noticeable on the main floor but is generally tolerable, creates no serious or lasting health risk, and dissipates almost completely within 36 hours, especially if basement windows are opened, and proper ventilation is established. In our company's experience, even if a customer cannot bear the odor, they usually have no trouble staying with a relative or at a hotel over night, until the odor has passed. The odor level can be reduced even further by using one of the new VOC-compliant acrylic lacquers on the market today that substitutes a safer and less smelly solvent for xylene. The flammability issue can be effectively addressed by taking a few minutes to temporarily put out all pilot lights and turn off all electrical equipment. Detailed, easy-to-follow instructions are usually printed on the side of all water heaters and furnaces. As an added precaution, plugs connected to electrical outlets may be removed or wrapped in duct tape. If there is no source of ignition, and the vapors have an open means of escape, there is little risk of flammability. Electric fans may be used to aid in ventilation but should be kept at the top of the basement stairs, not in the basement, and they should be positioned so as to draw air outward.
Premier Veneers carefully discusses sealer safety issues with the homeowner before every basement staining project begins. We take every safety precaution possible, including using one of the new, VOC-compliant acrylic lacquers mentioned above. Fortunately, we haven’t had one objection, complaint, or concern raised with us to date.