Concrete is manufactured by mixing cement, sedimentary elements, and water in appropriate amounts in order to create a precise consistency. The water is kept from evaporating out of this mix until the initial curing phase has passed, and the concrete is ready to set. At that point you want the excess moisture to evaporate from the slab into the air, so that it can dry and harden.
This is a process that can take days, or even weeks. The speed of the evaporation will be determined by the temperature and humidity of the surrounding air. It will also be effected by the size of the pores in the concrete. As long as the vapor pressure in the slab is greater then that in the air, water will continue to evaporate from it.
If excess moisture is present in a concrete slab when an impermeable surface treatment is installed, it will become trapped under that covering. Over time, hydrostatic pressure will force that moisture upwards, and can cause bubbles in surface treatments, and cracks in covering materials that are installed above it.
A freshly poured slab of concrete will release a lot of water into the air through evaporation. Over time the vapor pressure in the slab will decrease, in proportion to the vapor pressure in the air. If the ratio ever falls below a certain level, and the air becomes more moist than the slab, it is possible for hydration to pass back into the concrete. The ideal time to install a floor surface covering is when the vapor pressure between these two surfaces is in equilibrium.
Problems With Testing Moisture Levels In Concrete
Unfortunately different parts of the slab will contain different levels of hydration. This is true across the surface, as well as through the depths of the concrete. Environmental conditions, as well as temperature controls methods, can also change the amount of liquid in a slab. These levels and ratios will vary until the surface of the concrete is sealed.
Tests Used To Determine The Moisture Level In A Concrete Floor
Standard Test Method For Indicating Moisture in Concrete by the Plastic Sheet Method - (ASTM D 4263)
Developed by: the ASTM Committee for Protective Coatings and Lining Work for Power Generation Facilities Subcommittee on Application and Surface Preparation
This method requires you to tape a plastic sheet to the surface of the concrete. The tape has to be secure in order to create a seal around the plastic. This sheet is left in place for 72 hours. After that time a dew point hygrometer is used to test the level of moisture that is in the air under the sheet. This will tell you how much evaporation occurred over the course of the 72 hours.
Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride - (ASTM F 1896)
Developed by: Subcomittee on Practices of the Committee on Resilient Floor Coverings
This method is similar to the first in that it uses a sealed environment to determine the amount of evaporation that occurs from the concrete floor over a period of time.
This test requires you to tape down plastic sheets in three locations for every 1000 square feet of space. This will allow you to determine the moisture level across the entire surface of the floor, rather than just one specific area.
Packets of very dry calcium hydroxide are poured into a container so that the material can be weighed. These containers are then placed under the plastic sheets, which are then sealed against the concrete flooring.
After 72 hours the calcium hydroxide containers are removed and weighed again. The excess weight will tell you how much moisture the crystals absorbed from evaporation out of the concrete.
This information allows you to calculate how many pounds of water vapor are released from every 1000 square feet of space across the surface of the concrete over a 24 hour period. Generally you do not want vapor emissions to exceed 3 pounds per 1000 feet, although some floor surface coverings will be suitable for environments emitting as much as 5 pounds per 1000 feet. Make sure you check the material manufacturers recommendations before installation.
Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes (ASTM F 2170)
This method involves drilling a hole into a concrete floor and inserting an electronic meter into it, or embedding the meter in the concrete before it has dried. The relative humidity of the concrete is then tested over the course of 72 hours, and from this information the software in the meter is able to determine how much moisture is present through the core of the slab.
Which Method Should I Use?
Surface vapor tests only show you the amount of moisture being released at the surface, while embedded probes only test it within the slab. Both tests are often required to fully determine the level of moisture present in a concrete floor. In addition it may be necessary to conduct these test several times over the course of weeks as the condition of the slab can vary over time.
You also have to be aware of various elements that can effect the humidity in the air. Generally HVAC systems, both heating and cooling, will cause the air to dry out when first turned on, which can result in the slab showing a false reading. Getting an accurate measure of a concrete slab's humidity level is vital in determining whether you can proceed with surface floor covering treatments.