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The BONE DRY
Moisture Meter is designed to measure the relative
humidity of the interior of a concrete slab and give results that
are compliant with ASTM F2170 which was adopted in September,
2002. By determining the relative humidity of the interior of
a concrete slab you can more accurately predict what will happen
when the concrete is covered with a floor covering or coating.
If excessive moisture is present in the slab it dramatically increases
the probability for a failure of any application that is installed
over the concrete surface.
To read the relative humidity of the interior
of any material the sensor must be placed in the interior of the
material. For concrete slabs, we drill a 7/16” diameter hole
the is 40% of the overall depth of the slab when there is a vapor
underneath it or when there is an unvented steel pan which is
commonly used in multistory concrete and steel buildings.
ASTM F2170 calls for a minimum of three
tests per site with at least one test per 1000 square feet of
surface area. These are minimum testing standards and performing
more tests will provide more data with which to evaluate the concrete
subfloor. Excessive moisture may not be uniform throughout the
concrete slab that you are testing and finding the wet spots could
eliminate expensive warranty repairs several months down the road.
- Select testing locations and mark with
a permanent marker. CAUTION: When drilling holes in
concrete always determine the location of and avoid drilling
into any mechanical or structural system components. You
should ask about electrical, plumbing, radiant heating systems,
post-tensioned cables, or anything that could be within 2”
to 3” of the surface. It’s a good idea to draw a
rough diagram of the jobsite and plot the general location of
each hole and assign each hole a number. It is also best to
select testing locations that are not affected by solar heating.
- Calculate how far into the slab you will
need to drill. The holes will need to be 40% of the overall
depth of the slab in order to get an accurate reading where
the concrete slab has a vapor barrier on the underside. Typically,
the hole will need to be approximately 2 inches deep. 5”
x .4 = 2”
- Drill 7/16” inch holes using a hammer
drill in the locations you have pre-marked. Do not use water
during drilling.
- Vacuum each hole and seal the hole with
a rubber stopper or duct tape. The hole needs to equilibrate
for at least 24 hours (ASTM F2170 requires 72 hours) so that
the drying effect of the heat from the drilling does not invalidate
the test results. NOTE: These holes should be kept sealed from
the ambient conditions in the air above the concrete until all
testing is completed by placing a rubber stopper in the hole
or by sealing the hole with an impermeable tape.
- After sufficient time has passed, remove
the duct tape and immediately place the SlabSafe into the hole.
Adjust the black rubber ring around the sleeve of the SlabSafe
so that the hole is sealed. If the rubber ring is misplaced
or damaged you can use plumber’s putty to seal the space
at the top of the hole.
* The failure to properly seal the observation holes and allow
them time to equilibrate will result in false positive results.
These test results will indicate relative humidity that is lower
than the actual conditions in the interior of the concrete and
could result in excessive moisture related product failures
after the installation is completed. The same false positive
test results can be obtained by not allowing sufficient time
for the SlabSafe sensor to reach both temperature and relative
humidity equilibrium with the concrete.
- Within several minutes you should be
able to take your first reading. NOTE: It is critically
important that the relative humidity sensor of the SlabSafe
and the concrete are at the same temperature when you record
relative humidity. Any differences in temperature will
induce error in the results. Record the jobsite address, name
of technician, date, time, hole location, temperature and relative
humidity in the hole, and temperature and relative humidity
in the air above the hole using the form provided with your
SlabSafe Start up kit.
Understanding Moisture in Concrete and Moisture Meters Used to
Measure It.
Concrete is made up by mixing an aggregate
of varying sizes (sand, gravel, or crushed stone) Portland cement
and water. The water and Portland cement mix together to form
a new chemical compound that we refer to as paste. This paste
is a liquid when it is first mixed together which flows into the
spaces that occur between all of the aggregate. A chemical reaction
also starts to occur when the Portland cement and water are combined
that causes the paste to harden. Any excess water that is originally
mixed into the concrete that is not used in the chemical reaction
remains as water droplets distributed throughout the paste portion
of the concrete. Since this chemical reaction occurs over time
some of the residual water trapped within the concrete becomes
part of the cement paste as it continues to react or cure. In
fact some of this additional water is important to the curing
process that gradually increases the strength of the concrete.
During the initial curing period (7 to 10 days), additional water
must be added to the surface to replace the water that evaporates
from the surface or a sheet of plastic is placed over the surface
to inhibit surface evaporation. Curing compounds are also sometimes
applied to the surface to inhibit surface evaporation.
Any water that remains after curing is substantially
completed will continue to evaporate through the exposed surfaces
of the concrete. In the case of a floor slab that is poured on
an unvented steel pan or a 6 mil polyethylene vapor barrier, only
the exposed upper surface can accommodate the evaporation of this
excessive moisture. During the period concrete is exposed to an
exterior environment this drying is retarded by high exterior
relative humidity; rain and snow. It is not unusual to have very
high interior moisture levels in a concrete floor slab that is
many months or possibly several years old. The water to cement
ratio that occurs during the initial mixing is the primary factor
that influences the amount of residual moisture contained in the
concrete. The amount of water required to chemically react all
of the Portland cement is fixed by the quantity of the Portland
cement in the mix. As more excess water is trapped in the concrete,
the resultant concrete structure is more porous due to the spaces
that remain after the water has dried out. It is common practice
to add extra water making the concrete “soupy” so
that it flows and levels more easily and makes it easier to place
the freshly mixed concrete when pouring a floor. Or in the case
of high rise construction extra water is added to make it easier
to pump the concrete to the upper floors.
Curing vs. drying: Although
the terms curing and drying are often used interchangeably, they
are quite different. As discussed earlier, curing describes a
chemical reaction between H20 and Portland cement which results
in a hardened cement paste that surrounds and binds the aggregate
together. Curing begins during the initial combining of the ingredients
that make up concrete and continues for a great deal of time.
The drying process also begins immediately as water evaporates
from the exposed surfaces of the concrete after it is poured.
Information on the water-to-cement ratio of concrete when it is
initially placed is not within the control of those who work in
the building after it is closed in and weather tight. This is
important information to try to obtain so that we can understand
the microstructure of the concrete, but in the real world the
records concerning the quantities of materials contained in each
batch of concrete are probably not available or reliable. Even
if the records are accurate, where is the exact physical location
of each batch?
Curing of concrete is a topic of interest
for those who are involved in the strength and structural properties
of concrete. It is architects and structural engineers who deal
with strength and stiffness of concrete during the design and
erection of the structure. Back in the real world when you are
supplying and installing interior finishes which require that
the interior environment conditions are the same as they will
be when the building is occupied, the strength and stiffness of
the structural concrete is not an area of responsibility or concern.
But the amount of moisture contained in the concrete is of vital
importance. We have to be able to accurately measure the moisture
and then predict its effects.
Measuring moisture content:
The traditional method of quantifying the moisture content of
materials is as a ratio or percentage which compares the weight
of the water contained within the material with its oven-dry weight.
The most accurate method of determining moisture content is to
take a sample, accurately weigh it and then place it in an oven
to remove all of the water without using too much heat that would
damage the sample. Next weigh the sample again to determine its
oven-dry weight (at 0% moisture content). By subtracting the oven-dry
weight from the initial weight we can determine the weight of
water that was originally contained in the sample. By dividing
the weight of the water by the oven-dry weight we compute a ratio
which is the moisture content. It usually takes about 24 to 48
hours at 210 degrees F to properly oven dry a sample. Although
the process is fairly straightforward, it does require the destruction
of the sample which must be removed from the site for testing
to be performed. Moisture content is not as directly usable as
relative humidity because we have to figure out how much moisture
it takes to affect the floor covering materials we are using.
In Situ Relative Humidity ASTM F2170: Another
method of determining the amount of moisture contained within
a material is to measure the relative humidity that exists in
the interior of the material. This is done by inserting a relative
humidity (RH) probe into the material and allowing the probe to
reach a temperature and RH equilibrium with the material. If the
material RH is greater than normal ambient air RH, we know that
the material contains some excessive moisture. When we are using
wood materials over a concrete subfloor we can very accurately
predict the moisture content of the wood placed in contact with
the concrete by using a table that shows the direct relationship
between moisture content in wood and the equilibrium it reaches
when exposed to specific relative humidity and temperature conditions.
Table 3-4
Moisture content of wood in equilibrium with stated temperature
and relative humidity (Wood Handbook)
The recommended relative humidity levels
in concrete for wood floor installations are 1) 75% or less for
Engineered wood flooring, and 2) 60% or less for Solid wood flooring
which are adhered directly to a concrete surface. You should note
that when solid wood is installed over concrete, wood flooring
industry standards require that a very low permeability vapor
barrier should be installed over the surface of the concrete.
Although adhesives are often designed to be “water proof”
this does not necessarily mean that the adhesive is a proper vapor
barrier. In fact most adhesives do not effectively function as
proper vapor barriers. The exception to this is a heavy application
(at a spread rate of no more than 30 square feet per gallon) of
Asphalt adhesive combined with 6 mil thick polyethylene.
Other methods that have been used
to measure moisture content in concrete have not proven to be
as accurate and reliable as interior relative humidity and the
oven test. Some of these devices measure properties that do not
accurately correlate with moisture in concrete; some of the tests
methods don’t provide quantitative data; and some of the
tests only measure moisture that is within 1” or less of
the exposed upper surface of the concrete. This means that a positive
test result will not accurately indicate whether or not excessive
moisture is lurking deep in the interior of the concrete. Research
has shown that this hidden excessive moisture equilibrates through
the entire depth of the concrete slab once the expose surface
is covered with an impermeable coating or floor covering.
Rubber Mat/Polyethylene: A
rubber mat or sheet of 6 mil polyethylene approximately 12”
x 18” is placed on the concrete surface for 24 to 48 hours
to see if any condensation or discoloration develops beneath it.
This test is not quantitative in that it doesn’t tell us
how much moisture might be present. It can also give a false positive
because dryness at the surface can be affected by the ambient
atmospheric temperature and relative humidity conditions when
the testing occurs.
Pin type/Electrical Resistance:
Steel pins are driven into the concrete, and the electrical resistance
between the pins is measured with an ohm meter which is then displayed
as moisture content on the meter scale. Although this is a very
effective method when used with wood, scientific research to establish
the correlation between electrical resistance and moisture content
in concrete has been shown to be inaccurate. Even if it were accurate
to some degree the test would only be useful to the depth of penetration.
The most common use of these meters is to simply touch the surface
of the concrete with the pins which provides no information on
excessive moisture that is present deep in the interior of the
concrete.
Non-destructive/Surface meter/Capacitance/Power
Loss: This meter is placed on the surface of the material
and measures an impedance or electromagnetic field which correlates
to the density or specific gravity of the material. This measured
density is then compared to the material’s predicted oven-dry
density, and the difference is considered to be the amount of
water present. Inaccuracy develops when the predicted oven-dry
density of the material does not match its actual oven-dry density
and any error is directly related to the magnitude of this difference
in density. This turns out to be a not very accurate method of
determining moisture content because of the natural variations
in density that exist with wood (+ or – 10%). Concrete can
also vary in density based on variability of aggregates, water-to-cement
ratio, and any air entrainment that is incorporated into the design
of the batch mixture. Here again even if they were accurate to
some degree the test would only be useful to the depth of penetration.
These meters are generally designed to measure to a depth of ¾”
to 1” from the surface of the concrete which provides no
information on excessive moisture that is present deep in the
interior of the concrete. These non-destructive meters have been
very popular because they are simple and easy to use. Unfortunately,
easy does mean accurate.
Calcium Chloride (CaCl) crystals:
This test is performed by placing a small dish of Calcium Chloride
crystals under a plastic dome that is fastened to the bare concrete
surface. The crystals are weighed at the beginning of the test
then placed under the dome for 48 to 72 hours then weighed a second
time at the end of the test. The increase in weight is directly
related to the amount of water vapor absorbed during the time
the crystals are exposed under the dome. Then calculations are
made based on the time of exposure and the change in weight which
yields a water vapor emission rate which is stated as “pounds
of water per 1,000 square feet per 24 hour period.” The
results of this test procedure are directly related to the temperature
and relative humidity in the area above the concrete during the
time of the testing. And here again even if they were accurate
to some degree the test is generally only useful in providing
information to a depth of ½” to 1” from the
surface of the concrete depending on the porosity of the concrete
and provides no information on excessive moisture that is present
deep in the interior of the concrete. Therefore it can also give
a false positive because dryness at the surface is dependent on
the temperature and relative humidity conditions when the testing
occurs. The awkward aspect concerning CaCl testing is that most
flooring and adhesive manufacturers have come to recognize this
as the definitive test. The Portland Cement Association (PCA)
has published new information concerning CaCl testing versus In
Situ Relative Humidity. You should check with individual manufacturers
for their recommended concrete testing procedures. But you should
be aware that a lot of concrete with acceptable water vapor emission
rates before the floor is installed results in catastrophic excessive
moisture failures two to four months after the floor is installed.
We do not recommend this procedure, and do not sell test kits
for CaCl.
Calcium Carbide: This test
is used in Europe but not very frequently in the U.S. A small
area of the concrete slab is broken out with a chisel and pulverized
into a fine dust which is carefully weighed then placed in a pressure
vessel with Calcium Carbide crystals. The concrete dust is then
mixed with the Calcium Carbide crystals and when the water vapor
that is contained in the concrete reacts with the Calcium Carbide,
acetylene gas is created which generates pressure within the container
which is measured by a pressure gauge. The moisture content of
the concrete is then determined by the pressure created. Moisture
content is not as directly usable as relative humidity because
we have to figure out how much moisture it takes to affect the
materials we are using.
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