By definition, curing is providing adequate time, temperature, and moisture to allow hydration of cement to proceed as thoroughly as possible, thus assuring that the properties of the concrete reach the desired levels. Curing methods fall into three general categories:
- Moist curing – supplies moisture to the concrete surface using external sources, immersion or ponding, spraying or fogging, or covering the concrete with moisture saturated coverings, like burlap.
- Protection systems – provides a layer of material at the concrete surface that retards the loss of moisture from the original fresh concrete mixture such as a curing compound or impervious paper or plastic.
- Accelerated curing systems – provides external heat and moisture to increase early strength development.
Moisture sensitive materials such as vinyl composite tile (VCT), linoleum, rubber backed carpets, and wood flooring requires proper drying of the slab prior to installation of these materials. Common values used for suitability for placement of moisture sensitive floor coverings are referenced to either vapor emission as tested per ASTM F1869, Test Method for Measuring Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride or relative humidity as tested per ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. Typical values used for vapor emission testing range from 3 to 5 lb/1000 ft2/24 hr or 75 to 90 percent relative humidity depending on the floor covering material chosen.
The dilemma is how to properly cure the concrete to achieve the appropriate concrete properties, yet achieve adequately low moisture contents for application of the flooring materials at the earliest possible age. At the same time, the chosen curing method must not contaminate the concrete surface to prevent adhesives from securing the flooring material to the concrete surface.
To satisfy all of these requirements the following considerations may be appropriate. Avoid moist curing since this will add water to the slab, which must then be allowed to dry for a long period prior to the installation of floor coverings. Accelerated curing systems are impractical since they are difficult to implement over large cast-in-place applications, and elevated temperatures would not allow completion of work by other trades.
This leaves the protection systems as the final class of curing methods which may be appropriate. Of the protection systems, a spray applied curing compound will contaminate the slab surface and should be avoided, as this would require the removal of the curing compound before adhesives could be used. Finally, impervious paper or plastic may be the best alternatives for curing a concrete slab that will receive a moisture sensitive floor covering. The impervious paper or plastic will limit the moisture loss from the fresh concrete mixture providing a good curing environment for development of concrete properties, can be removed at a specific age (three to seven days, depending on the specifier’s preference) to promote early drying, and will not contaminate the concrete surface to interfere with the performance of the adhesives used to secure the floor covering to the slab.
Kosmatka, Steven H., Kerkhoff, Beatrix, Panarese, William C.,Design and Control of Concrete Mixtures, EB001.15, 15th Edition, Portland Cement Association, 2011
Kanare, Howard, Concrete Floors and Moisture, 2nd Edition, EB119, Portland Cement Association, 2008