Concrete international / september 2006 **23
**Moisture Testing
of Concrete Slabs
When 3 lbs is not 3 lbs
**Fig. 1: Problems
such as these
warped and
bubbled floor tiles
can be caused by
moisture and
high pH levels
beneath the
flooring materials
Fig. 2: Typical calcium
chloride test kit
containing a dish,
calcium chloride crystals,
and a plastic dome
**The long-awaited day has come, the project is complete,
and everyone involved is eager to show off the new
look of the facility. But wait a minute. Something is
terribly wrong. Adhesive is bleeding through joints
between the floor tiles, and there are bubbles in the sheet
goods (Fig. 1) and epoxy flooring. Worse yet, there is a
noticeable odor in the air. Sabotage? No, unfortunately
what is occurring on this project has become all too
common across the country. Such problems are being
caused by moisture and moisture-induced high pH levels
beneath flooring materials.
How can this be? The slabs were tested for moisture,
and the results appeared to comply with the requirements
of the flooring material manufacturer.
To understand what happened on this project and
many others, we must examine a test method that is
commonly used in the U.S. to quantify the moisture vapor
emission rate (MVER) of a concrete subfloor. The method,
the calcium chloride test developed by the Rubber
Manufacturers Association (RMA) in the 1950s, is now
published as ASTM F 18691 by ASTM International. MVER
testing by the calcium chloride method involves placing
an open dish containing a specified, known weight of
anhydrous calcium chloride crystals beneath a plastic
dome that is sealed to the concrete surface for 60 to
72 hours (Fig. 2). During the test, the crystals absorb
moisture vapor in the air beneath the plastic dome. At the
end of the exposure period, the difference between the
initial and final weight of the crystals is used to calculate
the MVER in pounds of water per thousand square feet
per 24 hours (commonly referred to simply as “lbs”).
By Peter Craig and George Donnelly
**24 **september 2006 / Concrete international
Today, manufacturers of most flooring materials and
coatings require that the MVER not exceed a 3-lb or, in
some cases, a 5-lb limit.
So how do floors that initially test to an acceptable
level end up testing to a much higher rate after a problem
develops? We’ve found that the following five conditions
can contribute to artificially low emission-test results,
inaccurate results, or an increase in moisture within the
slab once the flooring is installed.
Curing, sealing, or bondbreaking compounds
Curing, sealing, or bondbreaking compounds are
commonly used in concrete slab construction and inhibit
the release of free moisture from within the slab. These
materials are designed to provide moisture retention for
curing, surface densification, or the free release of concrete
tilt-up wall panels cast on the slab. Most flooring material
manufacturers and flooring standards require that these
materials be removed before the flooring is installed.
Testing conducted over a curing, sealing, or bondbreaking
compound will result in a considerably lower MVER than
will exist after the material is removed.
Adhesive residue from previous flooring
Adhesive residue is found on most remodeling projects.
Testing surfaces without fully removing these residues
can lead to artificially low test results. In all cases, we
must realize that, while each test occupies less than 1 ft2
(0.09 m2) of floor area, the results are considered to
represent the surrounding 1000 ft2 (90 m2). Preparation of
the test site must assure that the concrete surface is free
from any material or substance that could hinder the free
release of moisture from the slab. Light, dry vacuumgrinding
is the most practical and reliable means of
properly preparing a concrete slab surface for testing.
HVAC system operation
Conducting calcium chloride tests before doors and
windows are installed or before heating, ventilation, and
air conditioning (HVAC) systems have been activated can
result in MVERs that are different than those that would
be measured with the building under its normal operating
environment. Bare concrete is a hygroscopic material
that will take on or give up moisture depending upon its
surroundings. For calcium chloride test results to be as
meaningful as possible, it’s essential that the tests be
conducted with ambient conditions above the slab that
are close to the building’s normal operating environment.
Because flooring installations are frequently scheduled to
occur before HVAC systems are in operation, this is often
a difficult requirement to meet.
ASTM F 1869 sets requirements for calcium chloride
testing that include the testing environment. The standard
allows testing in environments that are not under HVAC
control within certain reasonable parameters (75 ± 10 °F
[23.9 ± 5.6 °C] and 50 ± 10% relative humidity).
Temporary forced drying of the top surface of the
concrete may bring calcium chloride test results to a
desired level, only to result in flooring problems at a later
date due to moisture redistribution within the concrete
once the slab is covered.
Redistribution of moisture
Most experts in moisture testing believe that the
calcium chloride test is an indicator of moisture present
in only the top 1/2 to 3/4 in. (13 to 19 mm) of the slab. As
slab-on-ground concrete dries from the top down, it’s
common for a closed-in, uncovered slab to have a lower
moisture content in the top portion of the slab than in the
lower regions. Once the slab is covered, moisture will
redistribute within the slab, which most often leads to a
higher amount of moisture in the upper region than when
the emission tests were conducted. Redistribution of
moisture within the slab after it’s covered is a major
contributor to higher emission results being observed
when the floor is retested after a problem has developed.
Below-slab vapor retarder
Without effective, low-permeance moisture protection
directly beneath the slab, moisture migrating from
sources below the floor can, over time, lead to an increase
in the moisture content of the slab after the flooring
material is installed.2 Once the slab is covered, relative
humidity beneath the slab will often measure close to
100% regardless of the depth of the water table or soil
moisture content. The relative humidity in the slab will
also increase and can also rise close to 100%. Without an
effective vapor retarder directly beneath the slab even a
properly measured 3-lb floor will not remain 3 lb for very
long once the floor is covered.
The following suggestions for conducting calcium
chloride tests can help provide the most meaningful test
results for evaluation:
 Always completely remove curing, sealing, and bondbreaking
compounds or adhesive residue by dry,
mechanical methods such as light vacuum grinding
before conducting the tests;
 Always conduct tests when ambient conditions reflect
the normal operating environment of the facility. On
projects where HVAC systems are not active, insist on
closing in a number of rooms or spaces where conditions
can either be brought to anticipated normal conditions
or into compliance with the environmental requirements
of ASTM F 1869;
Concrete international / september 2006 **25
** Conduct testing using both the standard ASTM F 1869
method and a modified method that can give an
indication of the effects on MVER that are likely after
the floor is covered. In this modified method, curing,
sealing, and bondbreaking compounds or adhesive
residue are completely removed from a 20 x 40 in.
(0.5 x 1.0 m) area, half of which is then covered with
aluminum foil, solid vinyl, or rubber for a period of
time before the start of testing. The time period for
moisture to equalize within the slab will vary; therefore,
the longer one can leave the area covered before
the start of testing the better. In no case should the
time period be less than 1 week, and 2 to 4 weeks is
recommended when the schedule will allow. Weight
down the cover material to be sure it remains in direct
contact with the slab surface. After the cover period,
conduct two calcium chloride tests at each location.
First, remove the cover material, and immediately
place one test kit on the surface of the concrete that
has just been uncovered. Next, place a second test kit
on the prepared concrete surface adjacent to the area
that was covered. The comparative results of these
tests serve as an indicator of changes in MVER that are
likely to occur once the floor is covered;
 Use internal relative humidity testing (ASTM F 21703)
to complement and help with the interpretation of
calcium chloride test results. In-place concrete relative
humidity tests are widely used in Europe to determine
when a flooring installation can proceed safely. In 2003,
ASTM F 2170 was introduced in the U.S. for relative
humidity testing in concrete. The test procedure calls
for measuring the internal relative humidity of concrete
(Fig. 3) at a depth of 40% of the slab thickness below
the slab surface 72 hours after the drilled test holes
and sleeves have been installed. The target level for
in-place relative humidity given in ASTM F 7104 is 75%
or lower, but several floor covering manufacturers
allow a maximum of 80%. It’s important that the
sensors are left in place long enough to reach a stable
reading. This time period will vary with the type of
sensor used and can take up to 3 hours. If sensors are
not installed in the sleeves for the entire test period,
it’s recommended that they be installed the night
before the readings are taken; and
 If any moisture test result is to be relied upon, it’s
important to remember that adequate moisture
protection must be in-place directly beneath the slab
so that moisture levels within the concrete will not
increase significantly once the flooring is installed.
Without it, the MVER may increase over time once
flooring or coating material is installed.
Preinstallation moisture and pH testing should be
conducted by personnel qualified to conduct the tests in
accordance with ASTM standards. Interpretation or
evaluation of the moisture and pH test results should be
made only by those experienced with the testing methods,
moisture migration, conditions, influences, and the floor
covering manufacturer’s requirements. The calibration of
test equipment should be maintained and checked before
**Fig. 3: The internal relative humidity at a depth of 40% of the slab
thickness is measured with a digital meter connected to a
hygrometer sensor installed in a hole drilled after the slab has
hardened: (a) a common system uses a hand-held meter and a
probe that’s plugged into plastic sleeves installed in the slab; and
(b) a recently developed system uses a self-contained probe and
meter that remain in the slab
(a) (b)
ACI officers to be elected via
web-based baloting
The Board of Direction at its Fall 2005 meeting
agreed to transition toward web-based balloting,
instead of letter balloting, for the annual election of
ACI’s officers.
In January 2006, a letter ballot to revise the Bylaws
to allow for web-based balloting was mailed to and
overwhelmingly approved by ACI’s voting members.
Later this year, ACI members with a valid and current
e-mail address will receive an e-mail notification
when the ballot is open, along with a link to access
the ballot from the ACI website. Secure website protocols
will ensure that only eligible voters are able to access
the ballot and that only one ballot is cast per voter.
Members without Internet access but still wishing
to participate in the balloting may request a hard copy
ballot—to be completed and returned to ACI headquarters
by the due date. All requests can be directed to:
Jeri Kolodziej, Manager, Governance Support
American Concrete Institute
P.O. Box 9094
Farmington Hills, Michigan USA 48333
Telephone: (248) 848-3756
**26 **september 2006 / Concrete international
**Fig. 4: A properly installed, low-permeability vapor retarder
prevents moisture from migrating up through the slab ***Photo
courtesy of William Munyan
*the start of each project. No flooring installation should
proceed without the approval of those responsible for the
installation warranty. If ample concrete drying time can’t
be provided in the construction schedule, a topical moisture
and pH suppression system should be incorporated into
the project costs from the very beginning.
Properly conducted calcium chloride tests often result
in values higher than the 3-lb MVER required by most
flooring material manufacturers. The normal construction
timeline seldom provides sufficient time or an adequate
environment for a new slab to dry and test to a 3-lb
requirement. Knowledge of the ambient conditions before
and during calcium chloride testing can help determine if
reaching the target MVER is achievable. Rewetting a
concrete surface after a period of drying has been shown
to increase the MVER.5 Because an open slab surface will
take on moisture from the air, testing during and after
periods of high ambient relative humidity will make
achieving a 3-lb MVER difficult, if not impossible. Even if
the ambient relative humidity is subsequently reduced,
reaching a 3-lb MVER may not be possible within a short
period of time.
Because it’s measured at a depth of 40% of the slab’s
thickness, the internal relative humidity test is far less
sensitive to ambient conditions than the calcium chloride
test. Slab temperature is a consideration and is recorded
by the sensor, along with the concrete’s internal relative
humidity. Concrete internal relative humidity test results
can not only help determine the moisture-related suitability
of a concrete subfloor, but they can also help determine
whether the inability to reach a 3-lb emission rate is the
result of ambient conditions prior to the test or moisture
present at a high level deeper within the concrete.
A new type of stay-in-place relative humidity sensor
has been recently introduced that can help the project
team monitor the drying of new concrete placements.
Several of these sensors are installed in each slab placement
once the roof is on, the building closed in and watertight,
and any curing compound is removed. When the weekly
readings reach a relative humidity less than 80%, the
testing firm can be contacted to conduct the full ASTMcompliant,
preinstallation moisture study with reasonable
confidence of favorable results. This approach can save
thousands of dollars in repetitive moisture testing costs.
Most problems with floor coverings over concrete
slabs-on-ground can be avoided by using a properly
designed slab system (Fig. 4) and providing ample drying
time or using commercial drying services. A low-permeance
vapor-retarding material, properly installed and in direct
contact with the underside of the slab, greatly reduces
the likelihood of moisture moving from the ground into
the concrete and shortens the concrete drying time.2
Concrete to receive floor coverings or coatings can be
cured with dry, wet-strength curing paper or lay-flat
polyethylene for up to 7 days. These methods don’t retard
the loss of moisture from the slab beyond the curing
period. Slabs cast in the open, however, are subject to
rewetting until the building is enclosed and watertight.
If a curing compound is used for initial curing or
rewetting protection for slabs placed in the open, the
material must be completely removed from the slab
surface as soon as the building is considered watertight.
Otherwise, a letter of compatibility must be obtained from
both the floor covering and adhesive manufacturers. Our
experience has been that most manufacturers of flooring
materials will not issue such a letter, making removal of
the curing compound a requirement. Removal should not
be delayed beyond enclosure of the building to provide
as much drying time as possible before moisture testing.
Finally, the slab must be reliably tested and the data
presented to the flooring manufacturer, flooring contractor,
general contractor, architect, owner, and any other party
associated with the flooring installation.
We’re fully aware that these recommendations go well
beyond what is presently being done or required on most
projects. Given the very common, serious, and costly
nature of flooring failures, however, it’s believed that
these measures are necessary to reduce the risk of
flooring problems and the significant cost associated with
failure of the flooring system.

  1. ASTM F 1869-04, “Standard Test Method for Measuring
    Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous
    Calcium Chloride,” ASTM International, West Conshohocken, PA,
    2004, 3 pp.
  2. Brewer, H.W., “Moisture Migration—Concrete Slab-On-Ground
    Construction,” Bulletin D89, Portland Cement Association, Skokie, IL,
    May 1965.
    Concrete international / september 2006 **27
    **For more than
    33 years, ACI member
    **Peter Craig **has
    provided consulting
    and quality assurance
    services on many
    aspects of concrete
    construction, repair,
    maintenance, and
    protection. He is a past President of the
    International Concrete Repair Institute, a
    member of the ASTM Task Group responsible
    for three moisture-related standards, and
    former Co-Chair of the Moisture Task
    Group of ACI Committees 302, Construction
    of Concrete Floors, and 360, Design of
    Slabs on Ground.
    ACI member **George
    Donnelly **is the owner
    of George Donnelly
    Testing and Inspections
    and has more than
    25 years of experience
    in the floor covering
    industry with positions
    in sales, management,
    and as a Director of Technical Services. He
    is a member of ASTM International and the
    World Floor Covering Association. Along
    with testing, analysis, and consulting, he
    offers seminars on concrete moisture
    vapor emission, covering moisture
    sources, design characteristics of
    intrusion prevention, and approaches to
    topical remediation for existing slabs.
  3. ASTM F 2170-02, “Standard Test
    Method for Determining Relative Humidity
    in Concrete Floor Slabs Using In-Situ Probes,”
    ASTM International, West Conshohocken, PA,
    2002, 5 pp.
  4. ASTM F 710-05, “Standard Practice for
    Preparing Concrete Floors to Receive
    Resilient Flooring,” ASTM International,
    West Conshohocken, PA, 2005, 6 pp.
  5. Suprenant, B.A., and Malisch, W.R.,
    “Are Your Slabs Dry Enough for Floor
    Coverings?” Concrete Construction, V. 43,
    No. 8, Aug. 1998, pp. 671-677.
    Selected for reader interest by the editors.

Thanks Marcel good information needs to be read more then once . Thanks
… Cookie