Civil Engineering Reference
In-Depth Information
Moisture Testing
crete slabs. Two British standards, BS 5325: 1996 and
BS 8203: 1996 use a hygrometer or relative humidity probe
sealed under an insulated, impermeable box to trap mois-
ture in an air pocket above the floor. The probe is allowed
to equilibrate for at least 72 hours or until two consecutive
readings at 24 hours intervals are within the precision of
the instrument (typically ± 3% RH). Acceptable relative
humidity limits for the installation of floor coverings range
from a maximum of 60% to 90%. It can require several
months of air-drying to achieve the desired relative
humidity. A method for estimating drying time to reach a
specified relative humidity based on water-cement ratio,
thickness of structure, number of exposed sides, relative
humidity, temperature and curing conditions can be found
in
Hedenblad (1997)
,
Hedenblad (1998)
, and
Farny (2001)
.
The in-place moisture content, water vapor emission rate,
and relative humidity of hardened concrete are useful
indicators in determining if concrete is dry enough for
application of floor-covering materials and coatings. The
moisture content of concrete should be low enough to
avoid spalling when exposed to temperatures above the
boiling point of water. Moisture related test methods fall
into two general categories: either qualitative or quantita-
tive. Qualitative tests provide an indication of the pres-
ence or absence of moisture while quantitative tests
measure the amount of moisture. Qualitative tests may
give a strong indication that excessive moisture is present
and the floor is not ready for floor-covering materials.
Quantitative tests are performed to assure that the floor is
dry enough for these materials.
Qualitative moisture tests include: plastic sheet, mat
bond, electrical resistance, electrical impedance, and
nuclear moisture gauge tests. The plastic sheet test (ASTM
D 4263) uses a square sheet of clear plastic film that is
taped to the slab surface and left for 24 hours to see if
moisture develops under it. The plastic sheet test is unre-
liable. In the mat bond test, a 1-m
2
(9-ft
2
) sheet of floor cov-
ering is glued to the floor with the edges taped to the
concrete for 72 hours. The force needed to remove the
flooring is an indication of the slab moisture condition.
Electrical resistance is measured using a moisture meter
through two probes placed in contact with the concrete.
Electrical impedance uses an electronic signal that is influ-
enced by the moisture in the concrete. Nuclear moisture
gauges contain high-speed neutrons that are slowed by
the hydrogen atoms in water. The affect of these encoun-
ters is a measure of the moisture content of the concrete.
Although the last three tests each yield a numeric test
result, their value is quite limited. Experience and skill are
needed to judge the trustworthiness of the devices and the
test results produced by them.
Quantitative test methods include: gravimetric mois-
ture content, moisture vapor emission rate, and relative
humidity probe tests. The most direct method for deter-
mining moisture content is to dry cut a specimen from the
concrete element in question, place it in a moisture proof
container, and transport it to a laboratory for testing. After
obtaining the specimen's initial mass, dry the specimen in
an oven at about 105°C (220°F) for 24 hours or until con-
stant mass is achieved. The difference between the two
masses divided by the dry mass, times 100, is the moisture
content in percent. The moisture vapor emission rate
(ASTM F 1869) is the most commonly used test in the
United States for measuring the readiness of concrete for
application of floor coverings. The emission rate is
expressed as kilograms (pounds) of moisture emitted
from 93 m
2
(1000 ft
2
) in 24 hours. See
Kosmatka (1985)
, and
PCA (2000)
for more information.
Relative humidity tests are used in several countries
outside the United States for measuring moisture in con-
Carbonation
The depth or degree of carbonation can be determined by
petrographic techniques (ASTM C 856) through the obser-
vation of calcium carbonate—the primary chemical prod-
uct of carbonation. In addition, a phenolphthalein color
test can be used to estimate the depth of carbonation by
testing the pH of concrete (carbonation reduces pH). Upon
application of the phenolphthalein solution to a freshly
fractured or freshly cut surface of concrete, noncarbonated
areas turn red or purple while carbonated areas remain
colorless. (Fig. 16-19). The phenolphthalein indicator
when observed against hardened paste changes color at a
pH of 9.0 to 9.5. The pH of good quality noncarbonated
concrete without admixtures is usually greater than 12.5.
For more information, see “pH Testing Methods” below,
and see
Verbeck (1958)
,
Steinour (1964)
, and
Campbell,
Sturm, and Kosmatka (1991)
.
Fig. 16-19. The depth of carbonation is determined by
spraying phenolphthalein solution on a freshly broken con-
crete surface. Noncarbonated areas turn red or purple, car-
bonated areas stay colorless. (69804)
