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Zinc-Mesh
Jacket System Improves Corrosion Control
A sacrificial anode cathodic protection system has
been designed to provide corrosion control to the splash zone of steel
and steel-reinforced concrete bridges.
 The system is made up of expanded zinc mesh and a stay-in-place
fiberglass form used for rehabilitating bridges damaged by
chloride-induced corrosion. The principle behind the system is to create a
galvanic cell that is capable of providing enough electrical current to
stop corrosion of the embedded steel.
The simple installation makes the system very competitive to
conventional repairs with the advantage of providing corrosion control and
concrete restoration in one operation. The system also has the advantage
of requiring only minimal monitoring and maintenance over its extended
lifetime.
The corrosion problem
According to information provided by the U.S. Federal Highway
Administration, more than 163,158 bridges (28% of the total) were reported
deficient or substandard. This means that it would require tens of
billions of dollars to correct the growing unfunded corrosion liability.
Cathodic protection is the only rehabilitation technique proven to stop
corrosion in salt-contaminated structures, regardless of the chloride
content in the concrete. There are two forms of cathodic protection:
impressed current and sacrificial (galvanic) cathodic protection. Both
systems control corrosion by providing an electrical current to an
affected region.
The impressed-current system uses a rectifier or permanent external
power source to provide a current flow from the externally placed anode to
the corroding reinforcing steel.
The galvanic systems use dissimilar metals that are coupled together in
a common environment (battery cell) as the energy source to supply current
to the corroding steel.
Although both systems provide cathodic protection, there are major
differences in how they are maintained over the life of the system. A
galvanic system provides its own power and regulates its current output
according to the changing environmental conditions, so there is very
little need for post-installation maintenance and monitoring. With
impressed-current systems, maintaining rectifier currents and making
adjustments to changing conditions becomes an ongoing burden.
How it works
 Galvanic cathodic protection creates a current flow from the
sacrificial zinc anode to the embedded reinforcing steel. When coupled
together in a common electrolyte (chloride saturated concrete), the
sacrificial anode and the reinforcing steel become a circuit and the
current flows from the zinc anode to the steel cathode until it is
sufficiently polarized or in electrical equilibrium.
The loss of electrical energy at the anode deteriorates the metal over
a period of time in a current-to-time relationship. The life span of the
anode is then regulated by the current flow of the system and the
consumption rate of the anode being used.
Several forms of sacrificial anode cathodic protection systems have
been developed and successfully used to provide corrosion control.
 Until recently, most anode systems were externally mounted and needed
replacement within a few years of operation to maintain effective
corrosion control. In the case of cathodic protection for steel reinforced
concrete, typical installations required extensive concrete repair and
rehabilitation prior to installing the anode system. This added more cost
to the project for mobilization, remobilization, labor, supervision, and
other costs. A system was needed that could provide both cathodic
protection and concrete repair in one operation.
One operation method
 Several years ago, the Florida Department of Transportation first
tested the concept of encapsulating zinc mesh anodes within a
stay-in-place fiberglass form filled with a sand-cement mortar. The system
was made up of high-purity, specially alloyed zinc that was processed into
an expanded mesh configuration that optimized surface area and useful
mass. Based on proven anode performance from earlier trials using zinc
penny web, the new design consisted of a durable, stay-in-place fiberglass
form that positions the anode relative to the embedded steel and creates
the essential annular space for filling with an approved material. In
addition, a supplemental bulk anode was added to protect the submerged
portion of the structure that did not require concrete repair.
The system was installed on two substructure pilings of the 1949
Broward River Bridge on SR 105 in Jacksonville, Florida. The chloride
content of the water was 15,500 ppm and the water resistivity was 30
ohm-cm. The two test jackets measured 1.83 meters in length and were
fitted around 457.2-mm-square conventional reinforced concrete piles. A
50-mm annulus was provided for concrete restoration for filling with a low
resistance grout material.
Before jacket placement, Ag/AgCl reference electrodes were embedded in
the concrete to monitor the system’s performance. The electrode sites
were patched and the pile was cleaned of all marine growth and any unsound
concrete. A 21.8-kg bulk zinc anode conforming to ASTM B418-73 Type I was
fitted and placed at a depth of 610 mm below mean low tide to supplement
the jacket system and to provide protection to the submerged sections of
the pile.
After installing the bulk anode, the jacket was positioned from the
mean low tide elevation extending upward, covering the splash zone and any
affected area immediately above the high water mark.
A wired connection from the zinc mesh to the steel reinforcement was
secured in place and sealed to prevent moisture and oxygen from corroding
the connection.
The jackets were supported with a temporary bottom form and additional
wood reinforcement to maintain the jacket uniformity during concrete
placement.
The annulus was filled with a cementitious grout and allowed to set and
cure overnight before removing any of the temporary wooden forms. The
anode-to-steel connection was housed in a UV-stable PVC junction box
located immediately above the jacket and mounted on the pile face.
Before the system was energized, static voltage potentials were
recorded for both the steel and the independent anodes to establish
baseline information. In addition, anode-to-structure resistance was
measured.
The combined resistance for the entire system was measured at 1.4 ohms
and the static potential for the embedded steel was measured at -305 mV vs
an Ag/AgCl reference electrode.
Immediately after the system was energized, the voltage shifted 103 mV
to -408 mV and the average total current was measured at 52 mA.
When last tested, the system was still performing satisfactorily with
average polarization values of 208 mV and 213 mV. The average total
current for the two test systems is measuring 84 mA and 92 mA
respectively.
System improvement
 Since the initial test installations, the system has been modified to
optimize internal components, materials, and pre-assembly functions to
give a simple installation in the field. The zinc mesh anode and
fiberglass forms are commercially produced in a wide variety of shapes,
sizes, design life, and to varying levels of protection as might be
required by job specifics.
For normal repairs (non-structural), the annular space is 50 mm. For
structural repairs where new steel reinforcement may be required, the
annular space may be 100 to 150 mm to allow for proper concrete cover.
This repair usually requires more zinc mesh and a stronger fiberglass
composite to provide additional protection to the new steel reinforcement
and to withstand the greater pressure exerted by a greater mass of
concrete when filling the cavity.
 Instead of using a cementitious grout, as is the case with
non-structural jacket repairs, the structural jacket system is filled with
a micro-concrete which has a much greater compressive strength, usually 50
to 60 N/mm squared.
In both cases, the jacket system is equipped with non-conductive
standoffs for positioning the jacket and affixing the anode mesh in place.
An accompanying photo is typical of the damage that occurs after
extensive exposure to chloride attack and total disbondment of the
concrete at the steel interface. If left unattended, this structure would
most likely need replacement or at least a new crutch bent. Pile
conditions of this severity require the consultation of a structural
engineer to properly survey the load conditions and the options for
repair.
A combination of both structural and non-structural jackets may be
needed to rehabilitate a structure to functional value.
Using the system
Since the initial installation in 1994, this system has become a
standard FDOT method to rehabilitate marine pilings and has been installed
on 26 state bridges, protecting more than 700 piles. In addition to state
bridges, several counties and other authorities have adopted the system as
their primary substructure repair/rehabilitation strategy. Installations
have been completed in six states and in eight countries totaling more
than 4,000 successful installations.
Since each piling has a slightly different set of conditions (sun
exposure, concrete resistivity, chloride saturation level, and so on), the
self-regulating nature of the zinc mesh system will only provide the
necessary level of protection in each case. Mainly the anode current
output and the volume of anode material available for consumption limit
the life expectancy of jacketed zinc mesh systems. If the current output
of the zinc mesh anode of any given weight is known, its approximate
useful life can be calculated. This calculation is based on the
theoretical ampere-hours per kilogram of anode material, its current
efficiency, and a utilization factor. The depletion rate of zinc in
coastal environments is approximately 10.7 kg per amp-year while operating
at 90% efficiency. The utilization factor for zinc is 85%, meaning that
when 85% of the anode is consumed, it will need replacement. The zinc mesh
system is typically designed for a 25- to 30-year service life. The
systems can be engineered to satisfy much longer life expectancies.
According to FDOT, typical polarization produced by this system ranges
from around 0.5 volts on the submerged portion of the piles to 0.1 volts
on the areas above the water level with a steady-state current density
10.7 mA/m squared. Based on these average values, it is estimated that the
effective service life of the zinc mesh system is 45 years.
 The system further preserves its useful life by mitigating the amount
of oxygen and chlorides from reaching the affected area by the inert,
stay-in-place fiberglass form. The table below shows typical data that can
be collected and analyzed for determining the effectiveness of cathodic
protection and estimating service life. Measurements can easily be taken
by shunting the connection located in the junction box above each jacket
and compared with the static condition of the steel.
Zinc mesh anodes in simple configurations can supply long-term cathodic
protection to steel and steel-reinforced concrete structures in marine
environments. High purity zinc mesh anodes are commercially available and
can be designed, manufactured, and easily installed to protect a wide
variety of structures in varying degrees of deterioration. The zinc mesh
jacket systems offer the advantage of accomplishing concrete repair and
cathodic protection for both structural and non-structural rehabilitation.
In many cases the jacket system is also being considered for cathodic
prevention as an economical alternative to other more frequent and routine
maintenance measures.
The cost of sacrificial anode systems using zinc mesh compares
favorably with the cost of standard pile jacketing and becomes more
economical with time, compared to high-maintenance impressed-current
systems. The simple installation makes the system very competitive to
standard repairs such as patching and guniting with the advantage of
providing full corrosion control. The system also has the advantage of
requiring only minimal monitoring and maintenance.
Since external power supplies are not required, rectifier monitoring
and adjustments are not necessary. It also eliminates costly wiring and
complex conduit systems for routing current to the source.
Since tidal changes affect concrete resistivity and ultimately anode
current output, it becomes essential in many cases to supplement the zinc
mesh systems with a submerged bulk anode. The bulk anode polarizes the
submerged portion of the bridge, preventing excess consumption of the mesh
anode during high tide. The dual-anode configuration provides long-term
polarization (in excess of 200 mV) on the steel reinforcement in the
splash zone and submerged portion of the substructure.
The expanded zinc mesh anode cast into concrete pile jackets is an
efficient method for providing sacrificial cathodic protection to marine
bridge pilings. The system is capable of providing and maintaining
cathodic protection to the reinforced steel as demonstrated using the
100-mV polarization criterion (NACE standard RP0290-90).
The design of the mesh anode is sufficient to provide cathodic
protection at a steady-state current density of 10.7 mA/m squared for a
period estimated at 45 years.
Douglas L. Leng is the primary investor of the Lifejacket Cathodic
Protection System as manufactured by Alltrista
Zinc Products.
Reprinted from Better Roads Magazine
November 2002 |
