Protective
coatings often are applied to existing concrete structures that have been repaired after
corrosion-related deterioration. The coating usually is applied to minimize the moisture
that enters the concrete and to obtain a uniform appearance. However,
a protective barrier system that keeps moisture and water out of a structure also can trap
moisture inside the concrete. The effects of internal moisture can be detrimental to both
the coating system and the concrete structure. Selecting a coating system
is complicated. There are hundreds of systems on the market, and coating technology is
constantly changing. However, some considerations are common to all these systems and
their installation:
Proper surface preparation
Concrete’s internal moisture content and its variability over time and within the
structure
Atmospheric conditions during coating installation and service
Consequences of restricting moisture movement
The moisture content of concrete strongly influences the longevity of a protective
coating system. The moisture content of unprotected concrete that has been in service for
some time tends to equalize with that ofthe atmosphere. Where
humidity is high or where concrete is exposed to frequent precipitation, the internal
moisture content will remain high. Protective systems applied to moisture-laden structures
can fail prematurely if moisture movement within and through the structure is not
considered. An uncured protective coating can be damaged if moisture moves
out of the structure as water vapor during, or just after, the coating is applied.
Moisture inside concrete also can be a problem after the coating cures. Though
protective systems restrict the amount of moisture that enters concrete, they also
restrict the ability of the concrete to "breathe" or release
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moisture as needed. Various systems allow
moisture vapor transmission at different rates. Protective
systems often are installed to minimize corrosion of rebar in a chloride-contaminated
structure or freeze-thaw degradation of inadequately air-entrained concrete. Here,
restricting vapor movement can cause significant long-term problems. and defeat the
purpose of the protective system The corrosion will continue until the
concrete moisture level falls to the point where corrosion activity is no longer
supported.Research data suggest that this is about 50% to 55% internal relative humidity. |
| Encapsulating concrete can cause coating
failure because internal moisture has nowherte to go. This precast concrete slab is coated
with a nonbreathable paint. The paint is peeling from the slab bottom due to moisture
buildup in the concrete. |
If the ambient relative humidity is at least this
high, corrosion may never cease. This is the case in many concrete
structures that have been repaired. Examples are concrete bridge decks or parking garage
slabs saturated with deicer chemicals, and exposed concrete frames of high-rises built
using calcium chloride as an accelerator. Even parking structures repaired using
state-of-the-art techniques (careful removal of distressed concrete, sandblast cleaning of
concrete and exposed reinforcing steel, coating of the steel with epoxy, and installation
of sound patching materials) have developed new areas of distress and delamination beneath
a protective membrane in just 2 or 3 years.
Before applying a protective coating to parking garages and similar concrete structures, consider how the
method of concrete surface preparation may affect the internal moisture content. Wet
sandblasting, hydrodemolition, and other methods that use a lot of water soak the
structure during the repair process. This addition moisture must be considered when
selecting and installing protective coatings, and in predicting distress that may occur
beneath the coating
Avoid encapsulating moisture-laden concrete.
Encapsulation occurs when a protective system completely seals the surfaces of a concrete
member. A typical example is a concrete balcony slab on a high-rise apartment building
with the top surface covered by a protective waterproofing coating and a nonbreathable
paint applied to the underside. This situation also occurs in concrete parking garages
when extensive concrete repairs are protected with a coating on top of the slab and the
slab underside is painted to improve appearance or lighting efficiency. In these
situations, slab moisture has nowhere to go. Most available water proofing coatings have
poor moisture vapor transmission characteristics. Paints may allow greater vapor
transmission. However, it is unlikely that encapsulated concrete can dry levels that
won’t be detrimental to the concrete or the coating.
Concrete encapsulation also cap occur m a slab on grade placed over a vapor barrier if
the concrete surface is coated. These slabs can develop serious corrosion problems if the
concrete used includes an admixture containing chlorides.
Measuring moisture content
A wide variety of tests can assess the presence and amount of moisture within concrete.
The simplest of these involves taping a sheet of clear plastic to the concrete surface to
be coated (ASTM D 4263, "Standard Test Method for
Indicating Moisture in Concrete by the Plastic Sheet Method) The sheet
should be installed when the ambient conditions are about the same as expected during
system installation. Leave the sheet in place for the same period of time needed to
install and cure the coating system. If visible moisture collects on the underside of
the sheeting during this time, too much moisture is inside the concrete.
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Other methods of measuring the internal
moisture content of concrete in service require drilling small holes into the concrete
member to the depth where the relative humidity measurement is desired. A cylinder with
sealed ends and perforated sides is placed snugly into the hole. About 24 hours are
required for the relative humidity in the cylinder to equalize with that in the
surrounding concrete. Then a device is inserted into the cylinder to measure the
temperature and relative humidity inside. These probes can be purchased from electronics
equipment suppliers. This relative humidity corrected to a standard 750 E indicates the
internal relative humidity of the concrete, but does not directly indicate absolute
moisture content. Further research is needed to establish a relationship between the
measured internal relative humidity of concrete measured and the actual moisture content. |
| Several methods are available
for measuring the internal moisture content of concrete in service. Here, a dew point
hygrometer is being used to measure the relative humidi ty of a concrete slab covered with
floor tile. Problems can occur in the slab if its internal relative humidity exceeds 50%
to 55%. |
A similar method involves drilling the hole to nearly the depth where a
measurement is desired, cleaning out the hole, then drilling into and just through the
zone of interest. The concrete powder from this zone is collected and sealed in a
container. After about 24 hours, when the humidity of the air and powder equalize, a probe
is inserted into the container to measure the relative humidity and temperature. These
measurements are comparable to those made directly in the field. More sophisticated
equipment, such as a dew point hygrometer, also is available for continuously measuring
the internal relative humidity of the concrete (see photo).
When measuring internal moisture content in a concrete element or structure, don’t
expect uniform moisture conditions. The internal moisture of concrete varies widely
depending on the environment.
Classifying and selecting products
Individual manufacturer’s data should be consulted to determine the vapor
transmission rates of protective baffler coatings. Test data for the ASTM E 96,
"Moisture Vapor Rate Test," are usually cited. Generally acrylic latexes have a
relatively high vapor transmission rate, or ability to breathe. Most epoxy-based coatings
have little or no ability to breathe. The breathability of urethane-based coatings and
systems vary from low to moderate.
Consider internal moisture before applying protective coatings to corrosion-damaged or
chloride-contaminated concrete. If internal relative humidity exceeds the 50% to 55%
needed to sustain corrosion, select a protective coating with a relatively high vapor
transmission rate. Where the internal humidity exceeds about 85%, the side effects may
outweigh the benefits of coating application. In these cases other alternatives, such as
penetrating silane sealers or cathodic protection, may be appropriate. In most cases
surface-applied protective systems (typically called coatings, membranes, or sealers)
should not be applied to critically saturated non-air-entrained concrete. Experience has
shown that protective systems will fail cases where the internal humidity exceeds about
90%. For these cases, concrete removal and replacement may be the best repair technique.
How long does it take for
concrete to dry?
The Portland Cement Association (PCA) has published information on drying of concrete
in structures (Ref. 1). A 6 inch concrete wall was built using normal concrete
construction and mix design techniques. The wall was then allowed to dry at a controlled
temperature of 73 degrees F and relative humidity of 35%. The center of the wall dried to
about 76% relative humidity after 114 days, but 850 days were needed to reach a relative
humidity of 50% - roughly the moisture content that sustains corrosion activity in the
presence of sufficient chloride ions.
The 35% relative humidity in the controlled environment is well below that expected in
most of the United States. The test wall was not exposed to rain or other added
moisture. Based on this research it is reasonable to assume
that years are required for a concrete member to dry enough so that active corrosion is
suppressed. Applying a coating that has a low vapor transmission rate will make the time
even longer.
The rate of water vapor loss from a moist concrete element depends on size and shape.
Elements with large surface-to-volume ratios (such as floor slabs) dry faster than
concrete elements with low ratios (such as bridge piers and dams)
The electrical conductivity of concrete also is greatly influenced by the presence of
internal moisture. Generally, concrete with a lower moisture content is less
conductive.
References
1. M. S. Abrams and D L. Orals, Concrete Drying Methods and Their Effect on Fire
Resistance," Research Department Bulletin RX 181, Portland Cement Association (PCA),
Skokie, III., 1965.
2. Carl A. Mensel, "A Method for Determining the Moisture Content of Concrete In
Terms of Relative Humidity," Research Bulletin D4, PCA. 1955.
3 ACI 362R-85, "State-of-the-Art Report on Parking Structures," American
Concrete Institute (Ad), Detroit.
4. ACI 51 5R-79 (revised 1985), "A Guide to the Use of Waterproofing,
Dampproofing, Protective, and Decorative Barrier Systems for Concrete," Ad.
5 Robert W. Gaul, "Preparing Concrete Surfaces for
Coatings," Concrete International, July 1984, pp
17-22
Thomas L. Rewerts is senior consultant and regional manager of Madsen Kneppers
Inc., Chicago. He is a
licensed structural engineer and has been involved with numerous building investigations
or restoration projects, many involving protective coatings, over the past 18 years. |
We manufacture concrete admixtures,
penetrating concrete sealers, penetrating concrete floor sealers, flooring sealers, concrete
waterproofers, brick
sealers, block sealers, stucco sealers, plaster waterproofers, shotcrete
waterproofers,
gunite waterproofers, mortar sealers, concrete cleaners, and concrete repair products.
