Concrete moisture vapor emissions are natural components of any concrete slab regardless age of grade level. Moisture vapor is the mixture of air and water which can travel in places that water in a liquid state can not. Concrete drying creates an emission from the slab regardless of whether the concrete slab is on, above, or below grade. The temperature and humidity differential between the building interior and the moisture source will cause moisture vapor to be drawn out of the slab into the building.

Moisture vapor emission from suspended concrete is often overlooked as a potential cause of floor covering failure and this specific problem may be exasperated by the use of light-weight aggregates and higher water-cement ratio to make the concrete easier to pump.

Beyond the drying process, moisture vapor emission may be the result of moisture vapor transmission from below the slab. The moisture source can be water trapped in a blotter course over a vapor retarder or moisture from the earth passing through the slab system.

The major concerns surrounding this issue have been driven by changes in floor covering adhesives and coatings, which are more sensitive to moisture and alkali attack than previous materials. More important than floor covering system failure is the concept that "sick building syndrome" and other indoor air quality (IAQ) issues often start at the floor surface and are fed by the high sustained humidity levels created by concrete moisture vapor emission.

Some flooring materials that are less breathable, such as resilient flooring and carpet with waterproof backings, can make the moisture problem worse. They can act as a surface moisture retarder and trap moisture vapor between the floor covering and the concrete slab.

Vapor Pressure:

Water molecules will migrate from areas of high water vapor pressure (high relative humidity) towards areas of lower water vapor pressure (lower relative humidity). At each gradient of temperature and humidity, a subtle pressure measured in "pounds per square inch" exists.

This pressure has been studied, quantified, and has for years been considered in the design of wall systems when moisture vapor movement through a wall is of concern. This same concern must now be recognized in the design of concrete floor slab systems.

Inside a building envelope the static vapor pressure is often only half that of the pressure inside of or below a concrete floor slab. Therefore, available moisture is drawn into the envelope and often trapped beneath floor covering materials.

Slab Porosity:

When excess water evaporates, it leaves voids inside the slab creating capillaries which is directly related to the slab porosity and permeability. The volume of moisture which may pass through a slab is contingent on the permeability of that slab. Permeability is governed by porosity, which in turn is a direct consequence of the water/cement ratio of the original concrete mix design. As the water/cement ratio increases in linear form, the porosity and permeability of the finished concrete product increases exponentially.

Concrete contractors may add an excess amount of water to the concrete mix to make it more workable which can contribute to longer-term moisture related problems. Too much water will take much longer to dry and decrease the compression strength. Proper water/cement ratios have lower porosity, a tighter cement particle structure, and create a stronger, more durable slab.

Concrete Curing:

Improper curing is the underlying cause of may slab problems that result in high moisture emission levels and create other problems with subfloor preparation problems for floor coverings. Curing compounds can leave residues and cause adhesive bond failures. They can also be the cause of false concrete surface moisture test readings when not proper removed. If a curing compound has been used, it should be mechanically removed as soon as possible after the concrete has set.

The concept of curing concrete is often confused with the process of drying concrete. Curing is best described as the chemical reaction which turns the raw ingredients of a concrete mix into a man made agglomerate rock.

Curing of modern concrete takes numerous forms and may use one of many methods. However, studies have shown that wet curing the slab, preferably through the use of curing blankets, results in a slab with increased strength and density when compared to other methods. The use of curing blankets allows a contractor the direct control of both the curing and drying processes. The Portland Cement Association has performed testing which demonstrated that concrete wet cured for seven day is four times less permeable than concrete cured using compounds.

Slab Drying:

Slab drying is the process of evacuating all of the excess water in the mix not used to hydrate and set the cement. Inconsistent factors that affect drying time include if the slab is on, above, or below grade, total slab thickness, water/cement ratio, temperature of the concrete and temperature of the excess water in the slab, presence of a vapor retarder under slab, types of aggregates used, how the slab was cured, and if steel deck construction is utilized. Environmental conditions that also impact the drying time can include seasonal conditions, ambient temperature and humidity, air flow velocity from the wind or air blowers, and efficient HVAC systems.

Through capillary action, water can evaporate prior to reaching the surface of the slab making the concrete appear to be dry. Rain often times rewets a slab before the building is enclosed or from improper perimeter drainage preventing a slab from properly drying or significantly slowing down the drying process. Once a building is enclosed, the HVAC systems should be made operational as soon as possible to acclimatize the interior. Industrial dehumidification systems can also be used to accelerate the drying process.

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.

There is an old rule-of-thumb principal that suggests it takes a month of drying time per inch of slab thickness in addition to the curing time under ideal conditions. The ideal ambient conditions are a minimum temperature of 70°F, maximum 30% relative humidity and constant air movement at 15mph. This principal does not account for mix design water/cement ratio that will dramatically impact the required dry time along with other variables.

In H.W. Brewer's 1965 study, "Moisture Migration – Concrete Slab-On-Ground Construction," he tracked moisture outflow of concrete as it dried. His study shows that high water/cement ratio concrete takes longer to achieve low level outflow than drier mix designs. This study alone justifies specifying concrete with a maximum water/cement ratio of between 0.45 and 0.50 on all projects that will require floor covering installation at slab ages of 6 months or less.

According to a monograph published by the American Concrete Institute, “typical “ concrete will lose water at a rate of 0.23 lb/ft²/hour (1.1 kg/m²/hour), provided that air temperature is 90°F (32°C), RH is 60% (74°F dew point), concrete temperature is 90°F (32°C), and air velocity is 15mph (24 km/h). If there is no air movement (zero velocity), the drying rate may be lowered by a factor of 10 to 0.025 lb/ft²/hr (0.1 kg/m2/hour). If the air is dehumidified to 10% (26°F dew point) the drying rate should by essentially double, to 0.45 lb/ft²/hour (2.2 kg/m²/hour). 

Employing typical construction specifications, a typical 4000 psi, 4-inch thick slab contains 1697 pounds of free water per 1000 square feet based on a 0.50 water-to concrete ratio. Other data used for this conclusion is that 33 gallons of water per cubic yard equals 275 lb/cubic yard. A water/cement ratio of 0.25 is need to hydrate concrete which uses 137.5 pounds of water per yard leaving and excess of 137.5 pounds of free water per yard which equals 1697 lbs. H₂O/1000ft².

Sub-Slab Vapor Retarders:

The sub-slab vapor retarder is part of the building envelope and is the roof upside down. It is imperative that the vapor retarder is installed properly and protected from punctures and other damage often caused by rebar or worker/equipment traffic. Concrete finishers will sometimes create holes in the vapor retarder to speed up the initial curing and their finishing process. Damage often occurs when utility trenches are cut/removed resulting in moisture problems when not properly repaired.