Concrete Coatings
There’s more to protect
than just concrete.
By Dean A. Frank, P.E.
Dean Frank is Director
of Technical Services at NPCA.
When specifying coatings for
use on precast concrete products, engineers
and other specifiers aim to protect the concrete.
However, many do not consider the protection
of the environment or manufacturing personnel
when deciding which coating to use. Many traditional
coatings are harmful to the environment and
toxic to humans. Therefore, when considering
more than just performance, specifiers can protect
the environment as well as the health and safety
of manufacturing personnel by making an informed
decision regarding the various concrete coatings
they specify.
Common coating uses
Precast concrete products
that most often receive the added protection
of coatings include manholes, tanks, vaults
and other below-grade products. Coatings are
usually separated into two types: exterior and
interior.
Exterior coatings are
often applied at the manufacturer’s plant
prior to being shipped to a jobsite for installation.
They may also be applied at the jobsite. Exterior
coatings are generally used for one or more
of the following reasons:
Interior
coatings are used primarily for protection
of the concrete from attack by aggressive substances
that can be found in sewer systems, such as hydrogen
sulfide gas or microbial-induced corrosion (MIC).
(see sidebar “Manhole Menace” for
more information on MIC).
Types of coatings
available
In the past, coal tar
and/or petroleum-based materials were the most
common exterior coating used for dampproofing
and waterproofing. These types of coatings are
often hazardous to the health of those applying
them. Cleanup and disposal of these coatings
also require special precautions to protect
the environment.
Today, there are better alternatives.
Water-based coatings are preferred over coal
tar or other petroleum-based materials. Water-based
coatings do not pose health hazards to personnel
applying them and they are environmentally friendly,
thus reducing and/or eliminating the necessity
for special precautions for cleanup and disposal.
They also penetrate the concrete surface well
because capillary action draws the coating into
the pores of the concrete, creating an effective
barrier.
There are a myriad of other
materials that can be used as coating materials
as well. These include epoxies, epoxy mortars,
polyesters, polyester mortars, urethanes, vinyl
esters, vinyl ester mortars, silicone, polyurea,
phenolic and specialty concrete coatings. When
considering a coating, you should contact the
manufacturer directly and request a list of
applications along with reference projects.
Most likely coal tar-based
coatings are still prevalent because specifications
that call for their use have not been updated
to reflect the availability of more user- and
earth-friendly coatings.
Alternatives to interior
coatings
Many people believe that
interior coatings cannot possibly fully protect
the concrete because small bubbles and pinholes
can form in the coating material during or after
application. These small holes offer access
to the concrete, effectively reducing the protection
that would otherwise be available. According
to a study performed by County Sanitation Districts
of Los Angeles County, the one variable that
most affects the performance of a coating is
the quality of the application. The predominant
reason for coating failure cited in the study
was the formation of pinholes.
Additionally, many are opposed
to using hazardous coatings because of the safety
and environmental hazards associated with them.
Many precast concrete manufacturers refuse to
work with hazardous coatings at their plants
altogether and subcontract the work out to specialty
companies.
One alternative is to use
a liner cast into the surface of the concrete.
These liners are composed of a material that
can resist the attack from the environment within
the sewer while utilizing the strength and durability
of precast concrete. Liners have proven to be
a very effective solution.
Another option is to use an
antimicrobial concrete additive that prevents
the harmful bacterial colonies from forming
on the concrete surface. These additives treat
the entire thickness of the concrete, they cannot
be damaged or worn off, and have been quite
effective in protecting concrete from the detrimental
effects of MIC. However, it should be noted
that such an additive only prevents bacteria
from collecting on the concrete surface, not
the other materials that might be found inside
a sewer such as manhole steps. It may be possible
for the bacteria to collect on non-concrete
surfaces and still produce sulfuric acid that
can damage the uncoated or unlined concrete,
especially if the concrete is in contact with
the non-concrete material. Nonetheless, this
technology shows great promise in the fight
against microbial-induced concrete corrosion.
A matter of ethics
The best approach for
specifiers is to determine which potentially
damaging substances are likely to be encountered
once the precast concrete product has been installed
and is in service. Once this is established,
determine the appropriate method of protection
while taking into consideration the precautions
associated with that particular material’s
use.
Engineers are ethically
and professionally responsible for the safe
use and function of structures and systems they
design, but do they have any obligation to those
who construct what they have designed? This
issue is often debated and engineers are increasingly
being held accountable for jobsite safety and
the welfare of those who actually bring their
designs into reality. Regardless of whether
engineers are legally accountable, ensuring
that the materials specified are safe not only
for the end users but also for environment and
production personnel is the right thing to do.
Thus, informed engineers and other specifiers
can take advantage of the beneficial properties
of user- and earth-friendly liners and coatings
in addition to the inherent quality, value and
permanence of precast concrete.
For more information
on concrete coatings, please use the search
engine at www.precast.org to locate a manufacturer.
