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CONCRETE FLOORS AND MOISTURE
 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 mix. 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.
Resources
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NEW!
Concrete Floors and Moisture
This book discusses sources of moisture, drying of concrete,
methods of measuring moisture, construction practices, specifications,
and responsibilities for successful floor projects. For more
information or to purchase, click
here. |
Floor Covering
Materials and Moisture in Concrete (PL853)
Concrete Myths
(PL823)
Moisture in Concrete
Slabs Video (VC127)
Understanding
Concrete Floors and Moisture Issues (CD014)
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:
-
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.
- RH 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.
- RH 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 F 2170, Standard
Test Method for Determining Relative Humidity in Concrete Floor
Slabs Using in situ Probes.
Acceptable RH 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%. Dense, hard troweled slabs or slabs with an applied sealer
and no vapor retarder can have greater than 95% RH 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 tables:
| Table 6-2. Maximum Value of Relative Humidity
in Concrete* |
Max. %RH |
Cover Material |
85% |
Plastic carpet with felt or cellular plastic
base
Rubberized carpet
Cork tile with plastic film barrier
Textile carpet with rubber, PVC or rubber-latex coated
Textile carpet made of natural fibers |
90% |
Plastic tiles
Plastic carpet with no felt or cellular plastic base
Linoleum |
60% |
Parquet board with no plastic film between
wood and
concrete |
80% |
Mosaic parquet on concrete |
| * The Finnish
SisaRYL 2000 Code of Building Practice. |
| Table 6-3. General Material and Workmanship Specifications
for Buildings* |
Max. %RH |
Cover Material |
80% |
Wood and wood-based materials |
80% |
Vinyl floor coverings with a backing which
may provide nutrients for mycological growth |
90%
85%
|
Bonded floor coverings which do not tolerate
degradation of floor adhesive by alkali in the concrete
Layered products
Homogeneous vinyl materials |
80%
85%
|
Cork tiles
Without vinyl layer on the underside
With a vinyl layer on the underside |
| * Swedish HusAMA83. |
Accuracy and Precision of RH Measurements
RH measurements typically are quite precise, ± 2% 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% 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% for the floor to be considered ready for installation.
This is an excerpt from Chapter 6 of PCA’s new book Concrete
Floors and Moisture.
References
Powers, T. C., Structure
and Physical Properties of Hardened Portland Cement Paste,
RX094, 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.
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