Delmhorst Instrument BD-2100 - Manual (Page 7)

Taking a Reading Below the Surface:
h Drive a pair of nails into the plaster at least 1/3 of the
total thickness of the plaster. If you are applying plaster
to a material of unknown dryness, such as brick or con-
crete, drive the nails through the plaster and into the
brick or concrete.
h Touch the heads of the nails with the contact pins ^ and read
the meter.
Testing Concrete Slabs for Flooring
Applications
Moisture meters are an effective tool to check comparative
moisture conditions in concrete slabs. They can tell you where
there may be excess moisture and help determine if you need to
conduct further testing, and identify specific areas on which
that testing should be performed. Meters do not provide
quantitave results as a basis for acceptance of a slab for
installation of moisture-sensitive flooring systems. ASTM Test
Method F2170 (RH using in situ probes), F1869 (calcium
chloride), and F2420 (RH on surface using insulated hood)
provide quantitative information for determining if moisture
levels are within specific limits.
It is important to test both the surface and mid-section of the
slab, especially if the slab is on or below grade. This will help
determine if there is continuous moisture migration toward
the surface. If this condition exists, the moisture movement
may be so slow that once it reaches the surface, moisture
evaporates and causes a “dry” reading when a surface test
is made.
However, if a sub-surface test is made, the meter may read
“wet” indicating the presence of moisture. When the slab is
covered and the upward movement of moisture continues, moisture
will move into a hygroscopic (wood) floor, or build-up pressure
under a non-breathing resilient floor, causing floor failure.
Taking a Surface Reading:
h Drive two hardened-steel masonry nails about 3/4"
apart into the finish coat of concrete slab. Drive them
about 1/8" deep so they make firm contact with the
concrete and do not move when touched.
h Touch the nails with the contact pins ^ .
h Press read key ! and read the meter. If
the meter reads in the red or “wet” area, it shows the
floor is not satisfactory for covering. If meter reads in
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the green or “dry” area, the surface is dry. However,
subsurface tests should be made to verify if the slab is
dry throughout.
Subsurface Test:
h Drill two 1/4” holes, 3/4” apart and 1/2” to 2” deep.
h Drive the masonry nails into the bottom of the holes and
make the tests as described above. Nails must not touch
sides of drilled holes.
If the meter still indicates green or “dry,” the floor should be
ready for covering. Tests should only be made in newly
drilled holes, since the inside surface of the holes can dry
out while the rest of the slab is still wet.
Even readings in the red or “wet” area can indicate
relatively low moisture content in concrete. For example,
readings in the 85-95 range on the reference scale indicate
approximately 2% to 4% moisture content.
Note: Perform minimum three tests for the first 1000 ft 2
and at least one additional test for each additional 1000
ft
2 .
Select test locations to provide information about moisture
distribution across the entire concrete slab, especially areas
of potential high moisture, or if the slab is thick (>4 in)
and air circulation is poor. For slabs on-grade and below-
grade, include a test location within 3 ft of each exterior
wall.
Key Factors to Consider:
h The age of the slab. Wait at least 30 days after the slab
is laid before checking moisture content.
h Is the slab on grade or suspended? If the slab is on
grade, is there an effective vapor barrier under the slab?
h How thick is the slab?
h What is the drainage condition of the ground?
h History of other structures in the area
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6” nails 1/8” into
concrete
Drilled holes 1/4” diameter.
Nails 1/8” into concrete at
bottom of holes.
2-3” approx.
slab
electrode
subsurface