Moisture in concrete and moisture-sensitivity of current floor adhesives and coatings are an inherent difficulty in floor finish installations today. However, there are several key items that can be considered to improve the chances of a successful adhered finish floor installation. Concrete slabs on-grade or onground must be placed over an effective vapor retarder (or waterproofing membrane if required by the site conditions). The vapor retarder or waterproofing membrane will protect the concrete and floor finish from elevated levels of moisture present in the underlying soils. The authors agree with the requirements of American Concrete Institute (ACI) 302.2R (Guide for Concrete Slabs that Receive Moisture-Sensitive Floor Materials) regarding installation of a sand blotter layer between the concrete slab and vapor retarder , as was the case in Case Study No. 2. ACI 302.2R cautions against the use of a sand blotter layer when moisture-sensitive finishes are to be applied onto the slab. The intent of the blotter layer is to reduce the tendency of the slab to curl as it dries, but moisture that is entrapped in the blotter layer will likely diffuse through the concrete and cause the softening of the floor finish adhesive, producing similar deterioration to a floor slab without a vapor retarder.
Concrete slabs with an effective vapor retarder or waterproofing membrane could contain enough moisture at the time of flooring placement to cause adhesive deterioration. As discussed in the previous text, moisture is a main ingredient in concrete and the moisture must be allowed to dry to manufacturer-accepted levels before floor finish installation. Otherwise, moisture levels could be high as a result of ordinary water trapped in the concrete slabs. It is imperative to
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perform moisture testing (ASTM F1869 and F2170) of the concrete slabs prior to flooring installation. The moisture tests should be performed by a certified tester once the building is enclosed and at operational temperature and humidity. As in Case Study No. 1, moisture testing of the existing slab must also recognize the time of year the floors are tested. When the slab system does not contain a vapor retarder, periodic testing over a longer period of time will be needed to determine the highest vapor drives as the moisture content changes with the seasons, level of the ground water, water vapor flow, and project location.
When moisture levels exceed flooring manufacturer’s limits, installation of a topical moisture mitigation coating to protect the sensitive adhesives is recommended. As illustrated in Case Study No. 2, not all moisture mitigation coatings have proven successful. Selecting the appropriate moisture mitigation coating for a particular application and understanding the performance track record of selected products are important to the long-term durability of a flooring system. Given the increased sensitivity of these new low VOC adhesives to moisture, all concrete slabs-on-grade without an underlying vapor retarder should be covered with a moisture mitigation coating to account for the potential seasonal variability of concrete slabs in contact with soil when using these moisture-sensitive finishes. Installation of moisture mitigation coatings may also require the installation of a leveling underlayment (between the mitigation coating and the flooring adhesive). The underlayment will level the surface of the floor, which will likely require a rough profile by many reputable moisture mitigation coating manufacturers. The leveling underlayment will also serve as a blotter layer for the water-based acrylic floor adhesives. Without an absorptive material below the adhesive, the solvent water cannot evaporate to allow the adhesive to cure.
Many manufacturers have developed more moisture-tolerant adhesives that comply with current VOC limits. When moisture levels are borderline and a vapor retarder is present, upgrading to one of these adhesives may allow flooring installation without the need for application of a moisture mitigation coating. Confirming the compatibility of the adhesive with the rest of the floor system materials is critical to long-term performance. Evaluating the durability of the adhesive based on its track record in similar applications will help minimize the chances of moisture-related flooring failures.