As a slab dries, evaporating water can bring alkalinity to the surface and interact with some flooring adhesives, especially those which are water-based, causing scheduling delays or flooring defects. To avoid this problem, test the moisture of the slab before installing any adhesive or flooring. Contractors can use the calcium chloride test, per ASTM F1869-11, or a more comprehensive relative humidity test, per ASTM F2170-11, at any point after the slab has cured to a point to allow foot traffic. This can be as soon as seven days and as long as 60 days after pouring. Since different flooring systems have different moisture tolerances, it is best practice to compare the testing results with the parameters of the flooring being used before application.
This was verified under laboratory conditions in a 2000 study conducted by Suprenant and Malisch.4 The study asserted how, compared to normal-weight concrete slabs, lightweight concrete slabs of the same dimensions took more than four-and-a-half months longer to reach a moisture vapor emission rate (MVER) of 1.36 kg (3.0 lb) per 304.8 m2 (1000 sf), every 24 hours. However, the results of this laboratory study did not seem to accurately represent field experiences, such as changing ambient conditions and minimum slab thickness to satisfy code requirements.
To understand the differences between the laboratory results and experiences in the field, the Expanded Shale, Clay, and Slate Institute (ESCSI) sponsored a study conducted by Peter Craig, an instructor for the International Concrete Repair Institute (ICRI) moisture testing certification program, to assess more closely the drying times of lightweight concrete slabs. The study sought to accurately represent field conditions and slab thicknesses required to satisfy Underwriters Laboratories (UL) No. D916 for a two-hour fire-rated assembly.5 Slab drying was monitored using two methods: ASTM F1869 and ASTM F2170.
Based on the results reported in the ESCSI study, which conducted direct comparisons between lightweight and normal-weight concrete slabs, the answer to this question is more complicated than a simple yes or no. While the testing needs further investigation to substantiate its claims, it seems to reveal two important distinctions between lightweight and normal-weight concrete drying timelines: amount of material needed to satisfy fire ratings differs and the environment plays a larger role in drying than initial water content.
Lightweight concrete does have a higher moisture content than the normal-weight concrete, and if the two types are compared in equal dimensions, then normal-weight concrete will reach acceptable MVER levels quicker. However, to achieve a two-hour fire rating, an assembly requires 29.5 percent more normal-weight concrete thickness than lightweight concrete. The additional water-of-convenience needed for more concrete brings the total initial water levels closer together. When placed to a thickness able to satisfy fire-safety code requirements, lightweight concrete assemblies contained only 11.3 percent more water than normal-weight concrete assemblies. The difference in water quantity is not significant enough to produce a substantial change in drying times.
