Prevention of Failures of Floor Coverings Over Concrete Slabs-on-Ground

The frequency and magnitude of floor covering failures has today reached unprecedented levels. In all too many cases, owners are being faced with costly and aggravating situations, often ending in finger pointing between designers, contractors, installers, and even their lawyers.

To avoid floor covering problems over concrete and to meet construction schedules in a timely manner, there are many factors that need to be considered and incorporated into the construction process. A winning strategy for successful flooring installations can be formulated if it includes a thorough understanding of all related factors. Some of the more important factors are presented in this booklet.

We at Construction Investigation & Analysis have been helping clients solve concrete related problems for over 40 years. Currently much of our work involves the investigation of existing floor covering failures; -- however, our comprehension and knowledge of the conditions influencing floor coverings over concrete are best used to avoid problems before they occur. We invite you to call upon us to discuss your particular floor covering situation.

Drying Time

The drying time of concrete is a function of several variables. Concrete slabs-on-ground (slabs-on-ground) with a direct-contact vapor barrier beneath the slab generally take the longest to dry (as opposed to elevated slabs with bottom forms removed). Lightweight concrete may take at least twice as long to dry, due to the high water content within the cellular lightweight aggregate.

The only way to verify that the concrete is in fact dry enough to receive floor converings is to perform vapor emission testing in accordance with the ASTM standard test methods utilizing the calcium chloride method.

The variables influencing drying time are:

1.The thickness of the slab. The thicker the slab, the longer the drying time.

2.Rating of the floor covering material. The less the rating (e.g., 3 lbs./1,000 sq. ft./24 hours vs. 5 lbs.), the longer the drying time. The rating is established by the manufacturer of the covering material.

3.The ambient temperature of the drying environment. The higher the temperature, the shorter the drying time.

4.The ambient relative humidity of the drying environment. The lower the relative humidity, the shorter the drying time. (For temperature and relative humidity, a dry, arid condition is most conducive to drying. In addition, air movement within and air exhaust from the environment is helpful in removing the moisture being evaporated from the concrete.)

5.The strength classification of the concrete. The higher the strength of the concrete, the lower the water-to-cement ratio, and thus the less uncombined water to evaporate, and thus the shorter the drying time. However, caution is noted with the use of high strength concrete as to cement richness and the effect on other properties such as the working life of the concrete (cement rich mixtures are problematic as to rapid slump loss and rapid setting time, particularly during warm to hot weather conditions).

© Copyright 1999, Herman G. Protze, III, Herman G. Protze, Construction Investigation & Analysis