This is a question I get all the time. I hear it from flooring contractors, general contractors, rookies and veterans. It happened on all types of job sites in all types of weather conditions. They tell me they believe they have done everything right. They mixed and poured the concrete to exact specifications, the climate is not too humid and (they believe) adequate moisture barriers are in place.  Yet, days go by and the slab just won’t dry. When they ask for my input I respond with a few questions.

Usually through their responses it becomes evident why the problem is occurring.  I encourage you to consider these questions when you ponder a concrete slab that stubbornly refuses to dry. 

Is the slab an open or closed slab? In other words: is the slab protected from outside moisture diffusion? If it is open, outside moisture diffusion can be brought into the slab by many sources; runoff, high water tables, soil with high fines, sprinkler systems too close to the structure, poor drainage around then structure, etc. You can never trust an open slab against moisture problems.

If a slab has an effective moisture retarder/barrier, it is considered a closed slab (because their protected from outside moisture). But not just any barrier will do. An effective vapor retarder/barrier should be a plastic sheet at least 10-mil thick positioned directly beneath the slab (this conforms to ASTM E-1745.) Many slabs feature a value-engineered membrane made using recycled plastic. They are usually have a thicknesses of  4 to 8 mils, not enough to meet the requirements for a vapor retarder.

What is the water-cement ratio of the concrete? The chart included on the following page shows the approximate drying time, in days, at 73°F, at 50% Relative Humidity, 15 mph wind velocity, 24 hours per day, 7 days per week for a 4” thick slab to reach 3 lbs./1,000 sq. ft./24 hours. According to ASTM F-710 – Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring, “Moderate to moderately low water-cement ratios (0.40 to 0.45) can be used to produce floor slabs that can easily be placed, finished, and dried, and which will have acceptable permeability to moisture. Floor slabs to receive a resilient floor material is to be 0.40 – 0.45”.  The resilient industry allows up to a 0.50 water-cement ratio.

A typical 0.50 water-cement ratio concrete (4000 psi) contains 33 gallons of water per cubic yard or 275 lbs. cu/yd. The water needed for hydration is about 16.5 gallons or 137.5 lbs.  This leaves 16.5 gallons or 137.5 lbs. of free water per cubic yard to evaporate.  There is 204.5 gallons or 1697 lbs. free-water per 1000 sq. ft. in a 4-inch slab. 

ASTM F-710 – X1.2 states “Floor slabs with water-cement ratios above 0.60 take an exceedingly long time to dry and cause adhesives or floor coverings, or both, to fail due to high moisture permeability”.

Is there any curing, sealing, or parting compounds on the surface of the concrete? If there is that may be the source of your trouble. Any topical treatment slows moisture migration from the slab. determine the nature of the treatment as each has difference considerations:

Curing Compoundsare designed to hold the moisture in for a period of time to allow the concrete to cure and then, after a period of time, degradate to allow the moisture vapor to escape.

Sealersare used on the surface of concrete to help prevent a dusty walking surface. They certainly are not designed as an underlayment and will hold water in the concrete for extended periods of time.

Parting Compoundsare used for tilt-up construction as a bond-breaker. Its very difficult to get an adhesive to stick to a bond breaker and must be removed.

When did the concrete start to dry? If this question seems puzzling, the answer is simple: The last day the slab was allowed to get wet.

Is the concrete normal weight or light weight? While they may look similar when in place, this is a huge consideration. Light weight concrete has a much higher rate of  absorbency (this is due to the lighter weight aggregate as well as the greater porosity) As a result, lightweight concrete can take twice as long to dry as normal weight concrete.(See Chart)

How old is the slab? Why does age matter? Because an old slab (over one year) will take longer to dry than a new slab.