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The Emerald Surf Condominium is a low rise, 5
story, 45 unit condominium located on the Gulf
of Mexico in Navarre Beach, Florida. It is
approximately 15 years old. The framework of the
structure consists of reinforced steel concrete
columns and beams supporting post tensioned
concrete floor slabs. The east and west concrete
sheer walls are veneered with decorative wood
facing. In 1995, numerous stalactites on the
underside of the common walkways were observed
during an engineering study investigating the
cracking and spalling of the concrete. It was
determined that these stalactites were the
result of minerals bleeding through cracks and
pores in the concrete due to a constant supply
of moisture from the environment.
The cantilevered concrete slabs, which make up
the common walkways and private balconies, are
constructed with post tensioned cables that run
completely through the slab from the front
walkways to the Gulf-side balconies, and from
end to end of the structure. The cables are on
approximate 3 foot centers with some additional
concentration in load bearing areas. The purpose
of these cables is to strengthen the concrete
the same way conventional steel reinforcing
rebars provide strength. These cables are
stressed to approximately 33,000 pounds per
square inch accordiong to the 1995 engineering
report. The critical live end anchor pockets for
the post tensoned cables occuir along the slab
edge with 2 standard reinforcing steel rebars
that act to restrain the anchors during the
stressing operation.
It was reported in the 1995 and 1996 engineering
consultant's reports that spalling and cracking
of the concrete and corrosion stains (iron
oxide) on the concrete surface were observed. It
was determined that the spalling and cracking of
the concrete were caused by corrosion of the
embedded steel members. The corrosion was
attributed to chloride contamination in the
concrete, which was enhanced by the lack of
concrete coverage of the steel. The depth of
coverage was observed to be less than 1.5 inches
in many areas. As a result of the engineering
reports, a program of concrete restoration and
post tensioned cable repair was initiated in
1997. It was determined from the engineering
study that a minimum of 20 post tensioned cables
would require reconditioning. and approximately
75 percent of the pull anchor ports would
require repairs. The building had lost 4 of the
post tensioned cables due to corrosion, and
there was concern for additional falures if the
corrosion process was not stopped. In addition,
a solution to solve the ongoing corrosion
problem was needed to ensure a continued long
life for the existing reinforcing steel and post
tensioned cables.
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After investigating numerous
corrosion mitigation systems
available for the reinforcing steel
and post tensioned cables, it was
determined that cathodic protection
offered the most economical and
technically reliable method
available. Both impressed current
and galvanic cathodic protection
systems were investigated to
determine which type of system would
offer the optimum performance for
this application. The impressed
current system was eliminated due to
the concern of hydrogen
embrittlement of the post tensioned
cables, higher maintenance costs,
and the requirement for continuous
AC power supply. In addition the
galvanic system selected had been
installed on a similar post
tensioned structure on the Gulf
Coast, and there was engineering
data indicating the systemn was
effective in mitigating corrosion.
The concrete restoration and cable
rebar repairs were completed prior
to the cathodic protection
installation, and electrical
continuity was determined for the
reinforcing steel and all the post
tensioned cables. At those
locations, where discontinuities
existed, metal to metal connections
were made by the cathodic protection
contractor using a proprietary
system. After all structures were
bonded together electrically, the
complete structure was checked again
for electrical continuity to ensure
that all reinforcing steel and post
tensioned cables were electrically
continuous.
The design of the cathodic
protection system called for
installation of a zinc galvanic
anode system on the slab edge of the
balconies on the south and west ends
of the building. The zinc anode
system used consisted of a sheet of
zinc foil coated with an ionically
conductive hydrogel pressure
sensitive adhesive (electrolyte).
The anode system was electrically
connected to the reinforcing steel
and the post tensioned cables with
the proprietary connection system.
The installation of the cathodic
protection system was completed in
April 1997. The cathodic protection
system was commissioned by a
National Association of Corrosion
Engineers International (NACE)
corrosion specialist in April 1997.
At that time, tests were performed
to determine the effectiveness of
the cathodic protection system.
These tests and measurements were
performed on each slab edge on every
floor of the condominium. The
criteria used to determine the
effectiveness of the cathodic
protection system was a minimum of
100 millivolt depolarization, which
is recommended by NACE. This
criteria was achieved at 100 percent
of the test locations, indicating
the system was operating
effectively.
As a result of this installation,
the Emerald Surf Condominium should
have many years of corrosion free
life for the post tensioned cables.
In addition, this corrosion
abatement system offers an economic
solution for the mitigation of
corrosion of post tensioned type
construction which did not exist
before. Of even more importance is
the fact that the effectiveness of
the corrosion mitigation system can
be determined in real time
measurements, and the user no longer
has to wait for cracking and
spalling of concrete to occur before
realizing that the corrosion is
taking place. |
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