Excess moisture in concrete slabs has caused many flooring failures, disrupting building operations and necessitating expensive repairs. Consequences can be particularly serious in sensitive manufacturing facilities, such as those for semiconductors and biopharmaceuticals, where tens of millions of dollars’ worth of production potential may be at stake. Moisture in Concrete
Cement particles adhere to one another because of surface forces. More surface area means more available force and stronger concrete. When water molecules react with cement molecules, the cement’s surface area increases. For example, when crushed for cement a typical pea-sized piece of cement clinker yields a surface area of about 2,000cm2. When water is added the surface area increases nearly a thousand times to 2,000,000cm2. Surface forces acting over this very large area provide the adhesion between particles that gives concrete is strength. Without sufficient water the surface area of the cement particles would be too small to ensure proper adhesion. For the same reason the surface of concrete that dries prematurely will contract and pull apart forming cracks just as mud does when it dries. If the water deficit is extreme the surface crumbles because the shrinkage forces are not balanced by strong surface attraction between particles. To avoid such damage, concrete is kept wet until sufficient adhesion develops. Problems can occur if the excess water volume is large, however. The concrete will hold the water tenaciously, slowing drying times and, in some instances, preventing on-time completion. How Much And When
Most coating and flooring manufacturers will not guarantee their products if there is more than 5% free water in the concrete. Some prohibit more than 3%. So, extra moisture amounting to 3 to 5 percent of the weight of the concrete must be removed. In other words, 115 L to 225 L (30 gal. to 60 gal.) of water must be removed from every yard of concrete in the slab before coating operations can begin or flooring can be laid. Industrysources agree that in most cases strength is sufficient at 70% hydration to allow drying to begin. How long it takes to achieve this percentage depends on the concrete mix. Rules Of Thumb
The factors affecting concrete drying rates are numerous and interact in ways that are specific to the concrete element’s shape and internal structure. Drying does not progress at a constant rate. The first water comes out relatively easily, because it is bound loosely to the other water molecules filling the concrete’s pores. The last water is more difficult to remove because it is tightly bound to the surface of the concrete; more energy and usually more time are required to break the bonds. Rules of thumb for estimating drying times have been around for years. One of these is that slabs usually dry to an acceptable moisture content for most flooring at a rate of about 25 mm (1 in.) of concrete thickness per month. Thus, a 150 mm (6 in.) slab will probably require six months to dry after 28 days of curing to 70% strength, or a total time from pouring to finishing of roughly seven months. Speeding the drying rate is possible with commercial drying equipment, but it is important to understand the drying process. | 
