Drying begins when water is no longer available at the exposed surface. If concrete is moist cured by sealing in the original mix water with wet burlap or plastic sheets, drying will begin when these covers are removed. Spray-applied curing membranes are somewhat breathable, and therefore, drying begins shortly after the membrane is applied. However, curing compounds can drastically reduce the drying rate and significantly extend the drying period.

A method of testing the moisture condition of a concrete slab is to measure the relative humidity of the air in the concrete pore system. This is done by placing a relative humidity probe into a hole drilled in the concrete. The relative humidity achieved within a concrete slab depends on a combination of factors including the initial water-to-cement ratio, drying history, pore structure, and concentration of soluble ions in the pore water solution.

Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete’s temperature stays within a suitable range. During the curing period-from five to seven days after placement for conventional concrete-the concrete surface needs to be kept moist to permit the hydration process. New concrete can be wet cured using soaking hoses, sprinklers or covered with wet burlap, or as we see today, can be coated with commercially available curing compounds, which seal in moisture (can result in high rH and lower Calcium Chloride readings) and must be removed prior to patching the floor or installation of the floor covering. Any type of sealer or curing compound on the concrete surface can interfere with all of the non-destructive moisture tests. These surface treatments can also interfere with the adhesion of patching compounds and adhesives. Manufacturers will recommend that the surface treatments be removed before the flooring application.

Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 50 Fahrenheit during the curing period.

Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time. It happens rapidly at first and slows down as time goes by. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected by specification writing authorities as the age that all concrete should be tested for strength (28 days does not mean it is dry). At this age, under the right conditions, a substantial percentage of the hydration has taken place.

If the drying period of concrete is critical then it should be protected from re‑wetting. Rainfall on the concrete slab, infiltration into joints and wetting of subgrades will extend the drying period. For floors, ideally the concrete should not be placed until the building has been enclosed.

The speed of the drying process is a function of many things, such as the following.

from the concrete.

If shorter concrete drying times are necessary then reducing the water-cement ratio is recommended. The use of admixtures can assist in reducing the water-cement ratio and maintaining the workability required. Reducing the water-cement ratio below 0.4, as no further reduction in the drying time was obtained. A water-cement ratio of 0.5 will generally allow drying within three months, and slabs with water-cement ratios greater than 0.6 will take an very long time to dry and cause adhesives or floor coverings, or both, to fail due to high moisture permeability.

The drying environment will also affect the time required for the concrete to dry. As lower relative humidity or higher temperature environment will allow faster drying. An air-conditioned space or one which is heated should reduce the drying time. Caution must be exercised if the surface is dried rapidly by the use of equipment such as heaters and blowers due to the increased risk of shrinkage cracking at an early concrete age. Also, while the surface or upper layer of concrete may indicate moisture levels acceptable for the installation of finishes, the moisture present at greater depths will re-distribute following covering of the surface. Depending on the drying time allowed, this may result in unacceptable moisture levels beneath the flooring and cause future problems. Measuring moisture content at the concrete surface (Calcium Chloride) will generally not give an accurate indication of the final moisture content, and whether the concrete is dry enough for the application of a floor finish or coating.

In new construction the excess water in the concrete needs time and conditions which allow it to dry out before

installing flooring. Placing high solids curing compounds on the slab to hold the water in to cure the concrete. This then prevents the slab from drying properly to flooring installation. An untreated slab with no moisture from the top or from below will take 60 to 90 days to adequately dry at 72 F and 50 % relative humidity. Curing compounds or cold weather can slow this process. If the moisture level in the air is high, water in the concrete evaporates very slowly, or not at all. This slows construction and increases the potential for mold.

An industry rule of thumb for estimating the drying time necessary for concrete floors to reach acceptable moisture content is 1 month of drying for each inch of concrete thickness. So a 6 inch thick slab, may take 6 months to dry sufficiently for flooring. Optimum indoor drying conditions occur at 30% relative humidity and 65to 70 F with constant airflow over the slab surface.

Special Thanks to JJ Haines for the information