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Concrete subfloors make up a large portion of the commercial substrates over which our floors are installed. It’s a fair statement that simply following the guidelines in this standard would prevent a large number of flooring failures. Flooring failures over concrete subfloors cost millions of dollars each year, and the pain of going through a replacement installation in an occupied space remains with the customer for some time.
Water to Cement Ratio
By its very nature, concrete starts as a water-saturated mass which must cure, then dry sufficiently. Concrete is a mixture of water, cement, and fine, medium and large aggregates. It can have any variety of additives depending on use and weather conditions when poured. Fly ash has been used for many years as an additive, typically comprising 15-20% of the cement mix. Higher percentages may sometimes be used, perhaps due to the desire for higher recycled content in mix designs. The reported effects of higher fly ash content are a slower initial strength gain and a slowed development of the carbonation layer of the slab surface.
The water to cement ratio (W/C) is calculated by dividing the water in the mix (in pounds) by the cement in the mix (in pounds). So if one cubic yard of the mix has 235 lbs. of water and 470 lbs. of cement, the mix has a 0.5 W/C ratio. When it comes to the quality of the concrete floor slab, W/C is a very important factor with regard to moisture migration, permeability, rate of drying and compressive strength of the slab. Water/cement ratios are practical to as low as 0.45 – 0.5 for slabs that will receive resilient flooring, as recommended by the concrete industry. Slabs with higher W/C take longer to dry, have lower compressive strength, and create problems with moisture movement through the slab, causing flooring failures. Given this information, it is troubling that we continue to see concrete mix designs with higher water/cement ratios.
Below, On, or Above-Grade Floors
Concrete subfloors can be above-grade (suspended), on-grade or below-grade. Above-grade floors are typically only affected by the mix water, although rain, spills and water leaks can create additional moisture problems. However, concrete poured on a metal deck is often produced with lightweight aggregate, which can retain excess water longer than normal-weight aggregate. Studies by the American Concrete Institute (ACI) suggest that lightweight aggregate concrete can take twice as long to dry as normal-weight concrete.
Concrete floors on- or below-grade not only require the mix water to hydrate and dry, but the ground may also produce a potentially inexhaustible source of moisture. This is why an intact vapor retarder that conforms to ASTM E-1745 is critical to the long term success of a flooring installation. It is recommended that the concrete be placed in direct contact with the vapor retarder. Seams and any penetrations through the slab should be sealed to prevent water from getting through to the flooring. The lack of a vapor retarder can lead to flooring failures, even in hot dry climates.