About Concrete Moisture & Alkalinity

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, or concrete curing, 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.  In many scenarios, concrete moisture testing will be critical.

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 concrete curing time processes are often the underlying cause of many 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.

Concrete Slab Drying

Concrete 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 32°C, and air velocity is 24 km/h (15mph). 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 Barriers

The sub-slab vapor barrier, also called a concrete moisture barrier, is part of the building envelope and is the roof upside down. It is imperative that the vapor barrier 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.

The importance of concrete vapor retarder materials has risen with the need to reduce moisture intrusion into building envelopes. Numerous companies are producing excellent products that offer measured permeability ratings below 0.1 U.S. perms. These vapor retarders are designed with tear and puncture resistant characteristics to ensure durability on a construction site. Typically they come with installation instructions that include methods of sealing around pipes and other protrusions that will necessarily penetrate the membrane.

All earth has some amount of free moisture and the construction processes often require adding moisture at a building site to achieve necessary compaction and stability. Regardless of source or causation the best means of preventing soil borne moisture from entering a concrete slab is through the employment of an effective sub-slab vapor retarding membrane.

ASTM has published performance standards for sub-slab vapor retarders (ASTM E1745) along with a standard for the installation of sub-slab moisture vapor retarders (ASTM E1643). These documents should be referenced on every construction project.

The American Concrete Institute (ACI) standards states that a vapor retarder is required for all moisture-sensitive floor finishes and to place vapor retarder directly under concrete slab with no cushion/blotter layer (ACI 302-04).

The absence of a vapor retarder will cause dew point condensation which will be higher on days with a lower evaporation rate in areas such as a warehouse.

Sub-Slab Blotters

Sub-slab blotters consist of a fill course laid below the concrete slab, over the top of the vapor retarder. Moisture in the blotter course transmits vapor into the slab, which translates into excessive concrete moisture vapor emission.

While some granular materials may be sufficiently compactable when dry, most materials used as a blotter must be wetted to achieve sufficient compaction. Field studies have shown that moisture content of blotter course material in excess of 1.5% – 2.0% will offer moisture vapor to the underside of a concrete slab. The rate at which this moisture enters and transmits through the slab is regulated by the permeability of the concrete and vapor pressure differentials that create motive force.

ASTM E1643 “Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs” contains an appendix that serves well in discussing pro and con the use of blotter courses under concrete slabs. It is known that wetted blotter materials create potential moisture source contributing to excessive concrete moisture vapor emission.

The American Concrete Institute’s ACI 302 “Guide for Concrete Floor and Slab Construction” contains a flow chart suggesting proper placement of vapor retarders. The flow chart calls for concrete to be poured/placed directly on top of a vapor retarder when moisture sensitive floor coverings or coatings are to be installed on the concrete slab surface. It is fair to say that most floor coverings and resinous coatings are moisture and/or alkali sensitive. While the concept of removing the blotter course and associated moisture source is applauded, there are pitfalls to be avoided.

Concrete slabs placed in “spec” buildings often have no idea of what future tenant needs may be. Many times in tenant spaces, saw cuts through the slab are required to run utilities (water, drains, phone, internet, etc.). When a blotter course separates the the slab from the vapor retarder, the saw blades may be held at a depth that cuts concrete but not vapor retarders. This will allow trench work to include repairs to the vapor retarder with relative ease. However, if concrete is poured directly on top of a vapor retarder the membrane will be cut along with the concrete during sawing operations and repairs may be quite difficult.

Utilizing standard construction techniques, it could be concluded that 1,000 square feet of concrete surface could easily contain 1,670 pounds, or 200 gallons of water in reserve within a 2″ thick blotter course of sand. To make this assessment, the following information is used. Dry sand weighs approximately 100 pounds per cubic foot. Wetted to achieve compaction, this sand could easily contain 10% moisture by weight, or 10 pounds of water per cubic foot of sand which will cover 6 square feet of a vapor retarder. It will take approximately 167 cubic feet of sand to cover 1,000 square feet of vapor retarder.

Osmosis example of 5% salt solution vs. pure water

Concrete Alkalinity Testing

Alkali (sodium hydroxide NaOH and potassium hydroxide KOH) is another natural component of concrete. New, wet concrete has a high alkalinity with a pH reading of 12-14 but typically drops with the carbonation of the concrete as observed through concrete moisture testing.

During the curing and drying of concrete or whenever moisture vapor is present, the moisture will dissolve the alkali salts and typically rise to the “green” concrete surface with the moisture vapor then remain as a white residue when the water evaporates. This residue may disappear as the concrete dries or can be cleaned with clean water, other times, the slab may need to be treated to solve this problem.

Some soils have a naturally high alkali level which can be a problem without an effective vapor retarder under the slab. Moisture carries the alkaline salts to the surface of concrete and these chemically react with the adhesive eventually destroying bond, causing shrinkage, and/or corroding the floor covering. The presence of alkaline concentrations also indicates elevated moisture vapor drive. Moisture can cause damage, moisture with a high pH is devastating.

The internal alkaline state of concrete is the very chemistry that prevents reinforcing steel from rusting. However, when the surface of a concrete slab has an alkalinity over 9 on a pH scale, adhesive and bonding systems may be compromised.

Alkaline water in combination with the organic constituents contained in most adhesives and flooring materials can provide an ideal environment for the growth of mold and mildew resulting in an unacceptable appearance of the flooring, offensive odors and air quality problems.

Schedules – Concrete Curing Time

One of the factors challenging everyone involved in modern construction is time. Fast track construction is becoming the norm and concrete is not being given sufficient time to naturally dry prior to the installation of floor covering materials and coatings. This issue is being exasperated by the use of curing compounds, which inhibit or prevent concrete from drying. Realize that we now attempt to adhere floor coverings utilizing water based adhesive systems to a water based material we call concrete. Excessive moisture emission from concrete that has not sufficiently dried will almost invariably interfere with the ability of an adhesive to bond or cure properly. It is recommended to test when you’ve gotten realistically close to the building’s final operating conditions.

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Thoughtful design and placement of concrete may reduce or eliminate problematic conditions, but all concrete will have a constituent vapor emission for the life of the slab. Proactively testing the concrete prior to the installation of flooring may prevent the considerable losses attributed to moisture related floor covering failure.

Using a spray applied time of placement penetrating solution like Spray-Lock SCP 327 can in many cases solve all of these issues and more.