Issues relating to moisture (such as: delamination, peeling, blistering, staining, sweating, etc) have been known to cause problems with concrete floors employing moisture sensitive floor coverings. However, with the proper materials, procedures, and workmanship, concrete floors can be constructed to provide years of quality service. Understanding how concrete floors and floor coverings can be protected from moisture exposure can help prevent troubles from arising.
Protection begins with choosing the appropriate concrete mixture. Proper placement and curing procedures will help ensure the slab has the strength and durability performance potential. For interior floors, a vapor retarder should be used below the slab. For floors to receive moisture-sensitive floor coverings, attention should be paid to the floor’s moisture level during and after curing. More.
Concrete Floors and Moisture, 2nd Edition, EB119
This book discusses sources of moisture, drying of concrete, methods of measuring moisture, construction practices, specifications, and responsibilities for successful floor projects.
Kosmatka, S., Floor Covering Materials and Moisture in Concrete, '"PC Concrete Technology Today," September 1985, pages 4-5.
Concrete Myths, "PC Concrete Technology Today," September 1982, page 5.
Understanding Concrete Floors and Moisture Issues, CD014
This CD takes an intensive and comprehensive look at the exploding problem of moisture-related flooring failures. This computer-based training program is geared toward design professionals and covers topics such as moisture testing and a troubleshooting guide.
Relative Humidity Moisture Tests
In several countries outside the United States, standards for floor moisture were developed in the 1980s based on measuring relative humidity (RH) within, or in equilibrium with, the concrete floor slab.
This practice has several advantages over other concrete moisture measurement techniques:
- Relative humidity (RH) probes can be placed at precise depths in a concrete slab to determine the relative humidity below the surface or to determine the RH profile as a function of depth.
- Relative humidity probes placed close to mid-depth actually measure the relative humidity within the slab and are less sensitive to short-term fluctuations in ambient air humidity and temperature above the slab.
- Moisture moves through concrete in a partially adsorbed or condensed state by diffusion, not simply as unbound, free water vapor or liquid. The rate of moisture transmission depends on the degree of saturation, which is a function of the relative humidity on each side of the concrete. Therefore, the driving force for water vapor movement through a slab is the relative humidity differential through the slab’s depth, not simply the vapor pressure differential (Powers 1958 and BRABFHA 1958). RH probes are a method of directly measuring this property.
- Relative humidity is a measure of equilibrium moisture level. When a floor covering is placed on top of a slab, it restricts evaporation from the top surface of the slab; moisture within the slab then distributes itself to achieve equilibrium due to temperature and chemical interactions from the top to the bottom of the slab. In the long run, adhesive and flooring are then exposed to the equilibrium moisture level at the top of the slab. The calcium chloride kit artificially pulls moisture out of the top few centimeters of the slab and does not reflect the long-term moisture situation that will be established by equilibration. RH probes can measure the relative humidity that will exist well after the floor is covered.
- Relativity humidtiy probes can be connected to electronic data loggers to record changes in relative humidity within a slab over time. Such measurements can be very useful to determine whether a floor is getting wetter or drier, and to predict how long it might take to reach an acceptable level of moisture.
In 2002, ASTM International approved a new test method modeled on the Scandinavian Nordtest method, titled ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes.
Acceptable Relative Humidity Levels
What percentage of relative humidity is acceptable in an interior concrete floor slab? Various levels can be appropriate depending on the uses of the occupied space and applied floor finishes. Relative humidity at mid-depth in bare concrete floors—such as those found in manufacturing facilities and warehouses —can be quite high if there is no vapor retarder below the slab. Moisture vapor passes through the slab and evaporates at top the surface with no detrimental effect most of the time. However, dew point condensation can occur on or within the slab if the temperature and relative humidity of the air are right.
To minimize the opportunity for dew point condensation, relative humidity in the upper centimeter of a slab should be less than approximately 85 percent. Dense, hard troweled slabs or slabs with an applied sealer and no vapor retarder can have greater than 95 percent relative humidity in the upper centimeter. Abrading the floor, for example by shotblasting, can remove a portion of the dense surface and allow the slab to “breathe,” thus lowering the relative humidity in the upper region. However, removing a densely troweled wearing surface may reduce the wear resistance of the floor. Acceptable RH levels using in situ probes have been established and published in Finland and Sweden. These maximum permissible values are given in the following table:
Table 6-2. Maximum Value of Relative Humidity in Concrete*
Table 6-3 General Material and Workmanship Specifications for Buildings*
Accuracy and Precision of Relative Humidity Measurements
Relative humidity measurements typically are quite precise, ± 2 percent being commonly achieved in the field when attention is paid to all the measurement details. This means that repeated measurements yield similar values. However, accuracy of RH measurements (that is, how close the measurement is to the “true value” of RH in the concrete) depends on careful calibration of the sensor and on achieving thermal equilibrium before recording the measurement.
A “safety margin” of several percent should be one of the considerations in establishing RH specification limits. For example, if a flooring manufacturer believes that RH must not exceed 85 percent for the performance of a particular floor covering and adhesive system, then the maximum permissible RH measured in the field (and specified in the installation instructions) should not exceed 80 to 82 percent for the floor to be considered ready for installation.
This is an excerpt from Chapter 6 of PCA’s book Concrete Floors and Moisture, 2nd Edition.
Powers, T. C., Structure and Physical Properties of Hardened Portland Cement Paste,
Portland Cement Association, Skokie, Illinois 1958, page 2.
Building Research Advisory Board BRAB-FHA, A Study of Effectiveness of Concrete Admixtures in Controlling the Transmission of Moisture through Slabs-On-Ground,Conducted for the Federal Housing Administration, National Research Council Publication 596, Washington, National Academy of Sciences, National Research Council, 1958.