Note: Descriptions are shown in the official language in which they were submitted.
TITANIUM M~SH IN CONCRETF OVERLAY
FOR CATHODIC PRO~ECTIO~ OF REIMFORCEMENT
FIELD OF THE INVENTION
This invention relates generally to cathodic
protection systems for steel-reinforced concrete
structures such as bridge decks, parking garage decks,
piers and supporting pillars therefor, as well as to
methods of installation of such systems.
BACRGROUND OF THE INVENTION
The problems associated with the corrosion of
reinforcing steel in concrete are now well understood.
Steel reinforcing has generally performed well over the
years in concrete structures such as bridge decks and
parking garages, since the alkaline environment of `
portland ce~ent causes the surface of the steel to
~passivate~ such that it does not corrode. Unfortunately,
a dramatic increase in the use of road salt in the early
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1960's together with an increase in coastal construction
resulted in a widespread deterioration problem.
This problem developed because chloride ions, whether
contained in deicing salt, in sea water, or added to fresh
concrete, destroy the ability of concrete to keep the
surface of the steel in a passive state. It has been
determined that a chloride concentration of 0.6 to 0.8 Kg
per cubic meter of concrete is the critical value above
which corrosion of steel in concrete can occur. The
resulting corrosion products occupy 2.5 times the volume
of the original steel, and ~hese introduce tensile stresses to
the surrounding concrete. When these stresses exceed the
tensile strength of the concrete, cracking and
delaminations develop. With continued corrosion, freezing
and thawing, and traffic load, further deterioration
occurs and potholes develop.
Major research and development efforts in the field
of concrete quality, construction practices, surface
sealers, waterproof membranes, coated reinforcing steel,
speciality concretes, and corrosion inhibitors have
improved the status for new deck construction. It is
generally agreed that new bridge decks constructed using
selected protection systems will exhibit a long life with
few maintenance problems. But many concrete structures
built prior to the mid 1970's are in large part salt
contaminated and continue to deteriorate at an alarming
rate. Cathodic protection is recognized as the only means
of stopping corrosion of steel in concrete without
complete removal of the salt contaminated concrete.
Cathodic protection reduces or eliminates corrosion
of a metal by making it a cathode by means of an impressed
DC current or by attachment to a sacrificial anode. In
this way external energy is supplied to the steel surface
forcing it to function as a current receiving cathode and
preventing the formation of ferrous ions. Cathodic
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protection was first applied to a reinforced concrete deck
in June 1973. Since that time, understanding and
techniques have improved, but the impressed current anodes
used to distribute current to the reinforcing steel
continue to be a major limitation. The anode should have
the following properties:
1. Ability to withstand traffic loads and
environmental conditions.
2. Design lifetime equal to or greater than the
wearing surface life.
3. Sufficient surface area such that premature
deterioration of the surrounding concrete does not occur,
and that a good distribution of current is provided to the
reinforcing steel.
4. Economically justifiable to install and maintain.
Historically, three different types of anodes have
been used for cathodic protection of steel in concrete
bridge decks: conductive overlays, slotted non-overlay,
and distributed anodes with non-conductive overlay.
The conductive overlay was the first anode to be used
and is still regarded as a useful system. In this case the
anode typically consists of a mixture of asphalt,
metallurgical coke breeze, and aggregate in conjunction
with high silicon cast iron serving as the current
contact. This system provides very uniform current
distribution over the deck surface, and because the anode
surface area is high, no evidence of acid or other
chemical attack from anodic reaction products has been
found on the underlying portland cement. The coke-asphalt
overlay has exhibited structural degradation in a number
of instances, however, and the time to replacement is
limited to a few years. Also, freeze-thaw deterioration
of improperly air-entrained concrete beneath the overlay
has limited its use to decks with proper air-removal
systems.
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Slotted non-overlay anodes were developed to extend
anode life and applicability, and to realize a system
which would not increase the dead load and height of the
bridge deck. In this system parallel slots are first cut
into the deck approximately 30-45 cm. apart. The slots are
filled with a "conductive grout" mixture of carbon and
organic resin which serves as the anode surface. Because
the conductive grout has a limited conductivity, current
is distributed to the anode by a system of platinized
metal and carbon strand conductors. This anode exhibited
adequate strength and freeze-thaw durability, but because
its surface area is small, the adjacent concrete often
experiences attack from the acid and gases which are a
product of the anodic reaction. Also, distribution of
current to the reinforcing steel is not ideal since the
slots are widely separated. Failure was also experienced
due to cracking or some other discontinuity since there is
not a redundancy of current connections. Furthermore, this
system is labor intensive and difficult to install.
Distributed anodes with ionically conductive overlays
are similar to slotted systems, but are often easier to
install. In one modification the conductive polymer grout
anode is placed directly on top of the existing deck
surface, together with platinized metal wire and carbon
strand current conductors, and the anode is overlaid with
latex-modified or conventional concrete. Rigid
non-conductive overlays are often favored because they
extend the deck life, retard additional salt penetration,
minimize freeze-thaw damage to underlying concrete, and
provide a new skid resistant riding surface. This system
still experiences the same disadvantages as the slotted
system regarding current distribution, acid or gas attack,
and lack of redundancy.
An alternative anode for use with rigid
ion-conductive overlays utilizes a flexible polymeric
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anode material which does not require a conductivebackfill. It is produced as a continuous cable and woven
into a large mesh, placed on the deck and covered with a
conventional rigid overlay. This system is less time
consuming to install, but still has the disadvantages of
current distribution, acid or gas attack, and lack of
redundancy. Such polymer anodes have been described in
U.S. Patents 4,473,450 and 4,502,929. As commercially
offered, these polymer anodes are woven into a mesh with
voids measuring about 20 cm. by 40 cm. Any breakage of
the cable at a given point will thus impair the cathodic
protection effect over a considerable area. Also the
thickness of the cable (about 8 mm) is a limitation where
only thin overlays are desirable.
A fourth type of system has more recently evolved for
use on substructures in which the anode material is
painted or sprayed directly on the concrete surface. For
example, carbon loaded paints and mastics can be applied
to the concrete. This provides a large anode area and
ideal current distribution to the reinforcing steel.
Additional platinized wire or carbon strand current
connectors are needed, however, since the resistivity is
high, and the anode material often peels off resulting in
a short lifetime.
For example, published UK Patent Application 2 140
456A describes a conductive overlay system in which a
conductive paint is applied to the surface of concrete to
form an anode film. Primary anodes of platinized titanium
or niobium are spaced apart each 10 - 50 meters for the
supply of current to the anode film and thus serve
essentially as current lead-ins.
An anode of flame-sprayed zinc has also been used
(see for example US Patent 4 506 485). Originally it was
thought that zinc would function as a natural galvanic
anode therefore eliminating the requirement of DC power
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supply. It has since been established that the fixed
natural voltage of zinc is too low to throw the current
for sufficient distance through the concrete, however, and
a power supply and current distribution system are still
required. This problem coupled with the problem generated
by the expansive corrosion products of zinc, have iead to
minimal use of sacrificial anode systems on bridges.
With the exception of the system using zinc anodes,
every system for cathodic protection of reinforcing steel
in concrete has to date used carbon as the
electrochemically active anode surface. Carbon was
probably first used because of its extensive use as an
anode in traditional cathodic protection. It was also used
because cathodic protection in concrete requires very low
lS current densities, which infers a very large anode surface
area. This implies that the anode must be low cost, and
carbon is relatively inexpensive.
Since pure carbon is not available in a structure
which would be suitable for use in concrete, carbon was
used as a conductive filler in organic resins,
thermoplastic polymers, paints, and mastics. This
technique put carbon into a physical form which could be
used in conjunction with concrete, but other disadvantages
of carbon remain. Carbon has a low electrical conductivity
relative to metals, requiring an elaborate system of
current conductors. Also, carbon is thermodynamically
unstable as an anode, reacting to form carbon dioxide
CO2, carbonic acid H2CO3, and carbonates HCO3
and CO3 2 , reaction products which are potentially
harmful to portland cement. These reactions are known to
be kinetically slow, but the effect of such reactions on
; anode lifetime may still be significant since, when in
contact with a solid electrolyte such as concrete, even a
small amount of oxidation will disrupt the
anode-electrolyte interEace causing a loss of electrical
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contact. Finally, carbon is a poor anode from the
standpoint of electrochemical activity. Single electrode
potentials at carbon anodes will be relatively high when
operated in chloride contaminated concrete resulting in
the release of chlorine gas C12, and hypochlorite
ClO . These reaction products will probably not be
harmful to concrete, but they are strong oxidizers which
react with the organic binders used, again causing concern
over anode lifetime.
1~ In summary, none of the anodes used to date exhibit
all of the properties desirable for cathodic protection of
steel in concrete. Although many appear to be economically
justifiable, many lack sufficient area to prevent
deterioration of the concrete adjacent to the anode, many
do not result in an ideal current distribution, and all
present serious questions about anode lifetime. Zinc
anodes are oxidized to zinc oxide which disrupts the
anode-concrete interface. All anodes containing carbon
operate at a high single electrode potential and generate
chlorine, acid, and carbon dioxide, products which are
likely to cause eventual dam3ge to the adjacent concrete
and to the organic matrix used to bind the carbon.
Electrocatalytically active anodes with valve metal
substrates are known and have been successfully used in a
number of applications, in particular chlorine, chlorate
and hypochlorite production and as oxygen-evolving anodes
in metal winning processes. Generally, the cost of such
electrodes makes them particularly advantageous in ~high~
current density applications, e.g., 6 - 10 KA/m2 for
chlorine production in a mercury cell or 3 - 5 KA/m2 in
a membrane cell. Such electrodes have also been proposed
for cathodic protection, but have found only limited
applications in this area. In one typical cathodic
- protection arrangement, a wire of platinized copper-cored
titanium is used to protect a metal structure.
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u.S. Patent 4,292,149 granted on September 29, 1981
(inventorsR.A. Lowe and M.A. Warne; assignees J. Brown
Cons. Ltd. and IMI Marston Ltd.) described such an
arrangement in which the platinized wire is wound around
an insulating rope. UK Patent Application 2 000 808A
proposed replacing the conventional platinized wires or
rods with a channel-sectioned valve metal strip having
anodically active material on the U or V-shaped spine.
Platinized valve metal meshes have also been proposed
for cathodic protection of certain structures. See for
example a paper (paper number 194) presented by ~homas H.
Lewis of Cathodic Engineering Equipment Company Inc. of
"Corrosion/79", an International Corrosion Forum held
- in Atlanta, Georgia from March 12-16, 1979, which describes
use of a rigid titanium expanded mesh measuring less that
0.05 m and coated with a layer of 1 - 15 micron of
platinum capable of carrying a current density of 2.15
A/dm2. This was used as a discrete anode in groundbeds
containing carbonaceous backfill. Rigid anode meshes of
this type having an overall area up to 0.5 m2 have been
offered as discrete anodes for the protection of remote
structures.
U.S. 4,519,886 describes a linear type of 2node
structure for the cathodic protection of metal structures
comprising a plurality of cylindrical anode segments
spaced along and connected to a power supply cable. The
cylindrical anode segments may be made of expanded
titanium bent to shape and coated with a mixed metal oxide
coating.
Obviously, none of the known coated valve metal
electrodes including those proposed for other cathodic
protection applications would be suitable for the cathodic
protection of concrete structures. In particular, the
anode designs are unsuitable for installation iD this
application and the cost of protecting an installation
would be prohibitive.
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SUMMARY OF THE INVENTION
One main aspect of the invention as set out in the
accompanying claims is a novel cathodically-protected
steel-reinforced concrete structure comprising an
impressed-current anode embedded in an ion-conductive
overlay on the concrete structure, wherein the anode
comprises at least one sheet of valve metal mesh having a
pattern of voids defined by a network of valve metal
strands. The strands of each mesh are connected at a
multiplicity of nodes providing a redundancy of
current-carrying paths through the mesh which ensures
effective current distribution throughout the mesh even in
the event of possible breakage of a number of individual
strands. The surface of the valve metal mesh carries an
electrochemically active coating. Furthermore, the anode
comprises at least one current distribution member for
supplying current to the valve metal mesh. The sheet or
sheets of the valve metal mesh extend essentially
continuously over an entire area of the structure to be
protected with no discontinuity (i.e. between two adjacent
sheets of the mesh) which is larger, in two mutually
perpendicular directions, than twice the largest dimension
of the voids of the mesh. In other words, the entire area
of the structure to be protected, excluding non-protected
openings for obstacles and the like, is covered by a
single piece of the mesh, or several pieces in close
proximity with one another.
Preferably, the mesh consists of a sheet of expanded
valve metal, typically titanium and with a maximum
thickness of 0.125 cm, which has been expanded by a factor
of at least 10 times and preferably 15 to 30 times. This
provides a substantially diamond shaped pattern of voids
and a continuous network of valve metal strands
interconnec~ed by between about 500 to 2000 nodes per
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square meter of the mesh. Such a mesh is highly fle~ible
and can be made in sheets of large dimensions which are
conveniently coiled about an axis parallel to the long way
of the diamond pattern. Further details of the coiled,
highly e~panded valve metal mesh and its method of
production are given in concurrently filed Application
Number 508,618 (ref. E00182-01 A&~).
As an alternative to using a sheet of highly expanded
valve metal mesh, it is possible to employ a valve metal
mesh constructed of valve metal rib~ons connected
together, e.g., by welding typically in a hexagonal or
honeycomb pattern. Such a composite mesh should meet up
to the same requirements concerning its dimensions and
configuration as set out above for the expanded meshes.
Each current distribution member is preferably a
strip of valve metal coated with the same
electrochemically active coating as the mesh and is
metallurgically bonded to the mesh. In many installations
such as parking garage decks and bridge decks, the current
distributor strips may advantageously be bonded to the
mesh with a spacing of between about 10 and 50 meters,
calculated to provide an adequate current density to the
mesh. In such installations, it is also cost saving and
convenient to have a common current distributor strip
bonded to and extending across at least two sheets of the
valve metal mesh, for example across two elongated sheets
of the mesh which have been rolled side-by-side from two
rolls.
Most advantageously, the current distributor strips
are spot welded to the nodes of the mesh. This spot
welding can be achieved on the facing surfaces of the mesh
and strip which are coated with an adequately thin
electrocatalytic coating.
Points of the mesh may be fixed to the concrete
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structure by fasteners inserted in drill holes in the
structure. Alternative means of fixing the mesh to the
structure prior to applying the ion-conductive overlay are
also possible, including the use of adhesive. This will
be more fully described in connection with the
installation procedure.
At least two sheets of the mesh may overlap with one
another, either overlapping edges of two side-by-side long
sheets which may assist in reducing the number of
anchorage points during assembly, or overlapping end
sections where the overlap may be designed to provide
electrical connection. However, providing each sheet is
associated with a current distribution member, the sheets
do not have to be in touching relationship but may be
spaced apart conveniently up to a spacing corresponding to
about the maximum size (LWD) of the usually diamond shaped
apertures of the mesh.
Also, at least one sheet of the mesh may have a
cut-out section bounding an obstacle on the structure,
such as a drain in a parking garage deck or an aperture
through the deck for connection of the current
distributors to a current supply.
It is also possible, but usually not preferred, for
adjacent sheets of the mesh to be welded together directly
or by means of a connecting strip.
For most structures, the ion-conductive layer is
about 3-6 cm thick an~ consists o~ portland cement 4~
polymer-modified concrete applied in a single pass e.g. by
pouring. Usually, the overlay is preceded by the
application of a bonding grout, i.e. a separate
cement-based grout without large aggregate which is
mixed-up, poured on the surface and brushed over the mesh
immediately before overlay.
The cathodically-protected structure according to the
invention preferably also has a current supply connected
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to the current distributors and arranged to supply a
cathodic protection current at a current density of up to
100 mA/m2 of the surface area of the strands of the
mesh, either a continuous current or intermittent.
When the structure is a concrete deck covered by a
series of side-by-side elongate sheets of the mesh with a
common current distributor strip extending across the
sheets, the current distributor strip may conveniently
extend through an aperture in the deck to a current supply
disposed underneath the deck at a location where it is
readily accessible for servicing etc.
The protected structure may be an e.g. cylindrical
pillar which is encased within the mesh and ion-conductive
overlay. The current distributor may in this case be a
strip disposed vertically on the pillar and the mesh is
one or more sheets cut to size so that it is wrapped
around the pillar with little or no overlap.
The invention also pertains to a method of
cathodically protecting the aforementioned structure by
supplying a continuous or intermittent current to the
valve metal mesh at a current density, usually below 100
mA/m2 of the strand surface area, which is effective for
osygen generation on the surfaces of the coated valve
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metal mesh. This current density can be established by
taking periodic measurements of the corrosion potential of
~- the steel using suitably distributed reference electrodes
in the proximity of the reinforcing steel, and setting the
~ operative current density to maintain the steel at a
;~ desired potential for preventing corrosion.
The reference electrodes are very advantageously also
constructed of a valve metal mesh with an electrocatalytic
- ~ coating. However, these reference electrodes will be
relatively small, for esample about 1-3 cm wide by 2-10 cm
long, and are preferably made of a conventional valve
metal mesh which is quite rigid. These reference
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electrodes are placed horizontally in recesses in the
concrete structure at the same level as the steel
reinforcement and spaced horizontally by about 2-3 cm from
the steel; in this location they are favorably placed in
the electric field and are exposed to an electrolyte
composition representative of the corrosive environment
around the steel. In most structures the steel is located
about 3 to 10 cm below the concrete surface. Typically
one or two reference electrodes are arranged for each
approximately S00 m2 zone of the anode mesh. The
electrocatalytic coating on the reference electrodes may
be the same as that on the anode mesh, or it can have a
special formulation selected to produce oxygen evolution
at a precise reference potential. These coated valve
lS metal reference electrodes have considerable advantages
over the heretofore used reference electrodes. For
instance, the potential of this reference electrode is not
dependent on the concentration of an ionic species which
may vary greatly in the electrolyte, as is the case with
silver/silver chloride and copper/copper sulfate reference
electrodes. Nor is the potential subject to change due to
a reaction of the electrode surface, as is the case with a
moIybdenum/molybdenum oxide reference electrode.
The described cathodic protection system according to
the invention has the following advantages:
- use of a non-corroding valve metal (titanium). The
system involve no carbon or corrodable metals such as
copper.
- only oxygen is evolved by the coated anode mesh in
use. Active chlorine, which may itself have long term
deleterious effects, is not generated as it is with other
types of anode.
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- metallurgical bonds twelds) are used for all electrical
connections within the ion-conductive overlay. There are
no mechanical connections and no copper conductors within
the concrete.
- the fine mesh structure of the anode insures uniform
current distribution.
- the anode mesh has thousands of interconnected strands
serving as multiple current paths. These assure that the
system will continue to operate satisfactorily even if
several strands are broken due to stresses in the
structure or future coring.
- where the mesh is connected to the current distributor,
there can be several welds for each sheet of mesh even
though only one or two would suffice.
- the low cost of the highly expanded mesh, the low
catalyst loading and the ease of installation make the
system very cost effective.
Also, the electrocatalytic coating used in the
present invention is such that the anode operates at a
very low single electrode potential, and may have a life
expectancy of greater than 20 years in a cathodic
protection application. Unlike other anodes used
heretofore for the cathodic protection of steel in
concrete, it is completely stable dimensionally and
produces no carbon dioxide or chlorine from chloride
contaminated concrete. It furthermore has sufficient
surface area such that the acid generated from the anodic
reaction will not be detrimental to the surrounding
concrete.
Another main aspect of the invention as set out in
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the accompanying claims is a novel method of installing a
coated valve metal electrode as impressed-current anode in
a cathodic protection system for a steel-reinforced
concrete structure.
In this novel method, first there is provided a roll
of a flexible sheet of valve metal mesh. This mesh
consists of a network of valve metal strands connected at
a multiplicity of nodes providing a redundancy of
current-carrying paths through the mesh which provide
effective current distribution throughout the mesh even in
the event of possible breakage of a number of individual
strands. It is greatly preferred that this valve metal
mesh consists of a sheet of highly expanded valve metal
with a pattern of voids having on its surface an
electrochemically active coating, as set out above.
Optionally, the coiled mesh may have at least one valve
metal current distribution member metallurgically bonded
thereto and extending generally parallel to the axis of
the roll. Alternatively current distribution members may
be bonded to the mesh on-site, i.e. after unrolling.
The basic principle of the installation method
according to the invention is that one or more rolls of
the coated valve metal mesh are unrolled and installed in
conformity with the concrete structure to be protected,
the valve metal mesh is fixed to the structure and the
fixed valve metal mesh is embedded in an ion-conductive
overlay.
As mentioned above, a current-distribution member may
be metallurgically bonded to the rolled mesh, i.e. prior
to installation. This is particularly suitable for
relatively small concrete structures such as supporting
pillars. Such a current distribution member may be welded
to the mesh prior to coating the mesh, or welding could
take place on-site after unrolling the coated mesh and
cutting it to size and before installing the mesh on the
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structure.
However, for most large structures it has been found
highly advantageous to metallurgically bond strips of
pre-coated valve metal to the mesh after unrolling it onto
the structure, e.g. by on-site spot welding of the strips
to the unrolled mesh. These on-site welded strips are then
used as current distributors for supplying electrical
current to the mesh.
For large structures such as bridge decks and parking
garage decks, a preferred installation procedure involves
unrolling two or more rolls of the mesh side-by-side and
then connecting the meshes electrically by a common
transverse current distributor strip which extends across
the side-by-side meshes. Advantageously, this is achieved
by laying current distributor strips on a generally flat
structure to be protected. The strips of coated valve
metal are spaced apart by a suitable distance calculated
according to the desired current-carrying capacity of the
system typically within the range from 15 tc 50 meters.
These strips are laid transverse to the direction of
unrolling of the rolls. Then the first roll is unrolled
and the mesh is spot welded at its nodes to the transverse
strips prior to or, preferably, after fixing the mesh to
the structure. Next, the second roll of mesh is unrolled
and welded to the transverse strips, and so on, until the
entire structure is covered.
Alternatively, and particularly for vertical
surfaces, for structures of odd shapes and for
downwardly-facing surfaces, the mesh can first be unrolled
onto the ground. Then current distributor strips are
welded and the sheets of mesh are cut to size and, if
appropriate, joined by welding. Next, the mesh with
current distributor can be applied to the surface of the
structure and conformed to this structure. This operation
may include wrapping around curved surfaces, bending
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around corners, bending the mesh in its own plane, e.g. to
fit a spiral surface, and stretching of the mesh as needed
prior to fixing it by adequate means.
Various methods of fixing the mesh to the structure
have been implemented successfully for different
structures. One method involves drilling holes in the
concrete and inserting fasteners of suitable shape which
firmly hold the mesh down. In another method, the mesh is
secured by an adhesive e.g. by applying a hot melt
adhesive to nodes of the mesh and holding the mesh to the
surface for example using a PTFE-coated steel heat sink.
Hot melt adhesive cured in this manner sets in about 10
seconds. Alternatively, a snap-clip is secured by epoxy to
the surface underneath a node of the mesh. After the epoxy
has set, the hinged top of the clip is snapped down to fix
the mesh. It is also possible to combine these methods, by
drilling holes and inserting fasteners at some locations
and by using adhesives at other locations.
Prior to fixing, the unrolled mesh is preferably
stretched longitudinally and/or laterally in order to
improve its flatness and in particular to avoid any
bulges. Generally a longitudinal stretching of up to about
10% increase in its nominal SWD dimension will be quite
adequate.
Because of the presence of the transverse current
distributor strips, it is not necessary for the sides of
adjacent meshes to contact one another or be welded
together. In fact, a spacing of up to about 1 LWD
dimension is quite satisfactory. Nevertheless, for
structures requiring a large number of fastening points of
the mesh e.g. if the surface is uneven or if for
structural reasons the ion-conductive overlay must be very
thin, it can be expedient to have the sides of the
adjacent meshes slightly overlapping. This reduces the
number of necessary fixing points.
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On large deck surfaces several rolls of mesh are
rolled out and laid down side-by-side. The width of these
surfaces will usually be such that the entire width can be
fitted with a given number of rolls by appropriately
spacing the rolls apart (usually the maximum desirable
clear spacing will correspond to 1 LWD dimension of the
mesh) or by bringing them close together or by making them
overlap by the appropriate amount. This avoids a costly
operation of fitting a special edge strip of mesh.
At locations of the structure where there are
obstacles such as drains in a parking garage deck, it is a
relatively simple matter to cut out sections of the mesh
in order to fit around the obstacles. This can be done
on-site using simple wire-cutters.
For long structures exceeding the length of a single
roll of the mesh, the end of one unrolled mesh may overlap
with the next adjacent section to provide electrical
connection, or the sections could be welded together or
connected by a weld strip for this purpose. This is only
necessary when the end section is of insufficient length
to be connected to its own transverse current distribution
strip. When electrical connection is provided by overlap,
firm fi~ing of the overlapping sections to the underlying
surface is advantageous.
At extremities of a structure where the roll of mesh
comes up against a wall or the like, it is a simple matter
to cut the mesh to a length such that the end of the mesh
can be bent up against the wall and then trimmed as
required.
For most structures, the ion-conductive layer is
about 3-6 cm thick and consists of portland cement or
polymer-modified concrete applied in a single pass eg by
spraying. Usually, the overlay is preceded by the
application of a bonding grout, i.e. a separate
cement-based grout without large aggregate which is
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mired-up, poured on the surface and brushed over the mesh
immediately before overlay.
In cases where a thin overlay is necessary for
structural or other reasons, the ion-conductive overlay
can be applied in several thin layers. The mesh may be
substantially embedded by the first layer: for example
more than 90% of the mesh may be covered. At this point,
it is possible to identify protruding sections of the mesh
and flatten and/or trim these before applying the next
layer or layers. An advantage of the invention, which
typically employs a mesh having strands up to 0.125 cm
thick is that it can be effectively used in an overlay as
thin as 6 mm. This cannot be achieved effectively with
any other known system.
As set out above, the described method of
installation of a cathodic protection system according to
the invention has many advantages. The installation
; method is easy to perform, is not labor intensive and can
be adapted easily to structures of different shapes and
dimensions. By employing a valve metal anode mesh in
convenient coiled form, large areas of structure to be
protected can be fitted rapidly. The fine anode mesh
structure provides thousands of interconnected strands
which serve as multiple current paths. This insures that
the system will continue to operate even if any strands
are broken due to stresses in the structure or coring or the
concrete. On-site weldin~ of the current distributors is
simple and convenient and several welds can easily be
provided for each sheet of the mesh even though only one
or two would suffice. Finally, the ease of installation
of the mesh combined with the low cost of the highly
espanded mesh with a low catalyst loading makes the system
; very cost effective.
. .
,~,
.r.U
-
2~ O
-- 20 --
BRIEF DESCRIPTION OF DRAWINGS
FIGURE 1 shows a diamond-shaped unit of a greatly
expanded valve metal mesh employed in this invention.
FIGURE 2 is a section of the valve metal mesh having
a current distributor welded along the LWD and welded to
mesh nodes.
FIGURE 3 is an enlarged view of a mesh node showing
the node double.
FIGURE 4 is a perspective view illustrating the
installation procedure on a steel-reinforced concrete deck.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The Highly ExPanded Valve Metal Mesh Anode
The metals of the valve metal mesh will most always
be any of titanium, tantalum, zirconium and niobium. As
well as the elemental metals themselves, the suitable
metals of the mesh can include alloys of these metals with
themselves and other metals as well as their intermetallic
mixtures. Of particular interest for its ruggedness,
corrosion resistance and availability is titanium. Where
the mesh will be expanded from a metal sheet, the useful
metal of the sheet will most always be an annealed metal.
As representative of such serviceable annealed metals is
; Grade I titanium, an annealed titanium of low
embrittlement. Such feature of low embrittlement is
necessary where the mesh is to be prepared by expansion of
a metal sheet, since such sheet should have an elongation
of greater than 20 percent. This would be an elongation
~;~ as determined at normal temperature, e.g., 20C., and is
the percentage elongation as determined in a two-inch ~5
.
.
1.2~
- 21 -
cm.) sheet of greater than 0.025 inch ~.0635 cm.)
thickness. Metals for expansion having an elongation of
less than 20 percent will be too brittle to insure
suitable expansion to useful mesh without deleterious
strand breakage.
Advantageously for enhanced freedom from strand
breakage, the metal used in expansion will have an
elongation of at least about 24 percent and will virtually
always have an elongation of not greater than about 40
percent. Thus metals such as aluminum are neither
contemplated, nor are they useful, for the mesh in the
present invention, aluminum being particularly unsuitable
because of its lack of corrosion resistance. Also with
regard to the useful metals, annealing may be critical as
for example with the metal tantalum where an annealed
sheet can be expected to have an elongation on the order
of 37 to 40 percent, which metal in unannealed form may be
completely useless for preparing the metal mesh by having
an elongation on the order of only 3 to 5 percent.
Moreover, alloying may add to the embrittlement of an
elemental metal and thus suitable alloys may have to be
carefully selected. For example, a titanium-palladium
alloy, commercially available as Grade 7 alloy and
containing on the order of 0.2 weight percent palladium,
will have an elongation at normal temperature of above
about 20 percent and is expensive but could be
serviceable, particularly in annealed form. Moreover,
where alloys are contemplated, the expected corrosion
resistance of a particular alloy that might be selected
may also be a consideration. For example, in Grade I
titanium, such is usually available containing 0.2 weight
percent iron. However, for superior corrosion resistance,
Grade I titanium is also available containing less than
about 0.05 weight percent iron. Generally, this metal of
lower iron content will be preferable for many
~.Z~ O
- 22 -
applications owing to its enhanced corrosion resistance.
The metal mesh may then be prepared directly from the
selected metal. For best ruggedness in extended metal
mesh life, it is preferred that the mesh be expanded from
a sheet or coil of the valve metal. It is however
contemplated that alternative meshes to expanded metal
meshes may be serviceable. For such alternatives, thin
metal ribbons can be corrugated and individual cells, such
as honeycomb shaped cells can be resistance welded
together from the ribbons. Slitters or corrugating
apparatus could be useful in preparing the metal ribbons
and automatic resistance welding could be utilized to
prepare the large void fraction mesh. By the preferred
expansion technique, a mesh of interconnected metal
strands can directly result. Typically where care has
been chosen in selecting a metal of appropriate
elongation, a highly serviceable mesh will be prepared
using such expansion technique with no broken strands
being present. Moreover with the highly serviceable
annealed valve metals having desirable ruggedness coupled
with the requisite elongation characteristic, some
stretching of the expanded mesh can be accommodated during
installation of the mesh. This can be of particular
assistance where uneven substrate surface or shape will be
most readily protected by applying a mesh with such
stretching ability. Generally a stretching ability of up
to about 10 percent can be accommodated from a roll of
Grade I titanium mesh. Moreover the mesh obtained can be
expected to be bendable in the general plane of the mesh
about a bending radius in the range of from 5 to 25 times
the width of the mesh.
Where the mesh is expanded from the metal sheet, the
interconnected metal strands will have a thickness
dimension corresponding to the thickness of the initial
planar sheet or coil. Usually this thickness will be
- 23 -
within the range of from about 0.05 centimeter to about
0.125 centimeter. Use of a sheet having a thickness of
less than about 0.05 centimeter, in an expansion
operation, can not only lead to a deleterious number of
broken strands, but also can produce a too flexible
material that is difficult to handle. For economy, sheets
of greater than about 0.125 centimeter are avoided. As a
result of the expansion operation, the strands will
interconnect at nodes providing a double strand thickness
of the nodes. Thus the node thickness will be within the
range of from about 0.1 centimeter to about 0.25
centimeter. Moreover, after expansion the nodes for the
special mesh will be completely, to virtually completely,
non-angulated. By that it is meant that the plane of the
nodes through their thickness will be completely, to
virtually completely, vertical in reference to the
horizontal plane of an uncoiled roll of the mesh.
In considering the preferred valve metal titanium,
the weight of the mesh will usually be within the range of
from about 0.05 kilogram per square meter to about 0.5
kilogram per square meter of the mesh. Although this
range is based upon the exemplary metal titanium, such can
nevertheless serve as a useful range for the valve metals
generally. Titanium is the valve metal of lowest specific
gravity. On this basis, the range can be calculated for a
differing valve metal based upon its specific gravity
relationship with titanium. Referring again to titanium,
a weight of less than about 0.05 kilogram per square meter
of mesh will be insufficient for proper current
distribution in enhanced cathodic protection. On the
other hand, a weight of greater than about 0.5 kilogram
per square meter will most always be uneconomical for the
intended service of the mesh.
The mesh can then be produced by expanding a sheet or
coil of metal of appropriate thickness by an expansion
~z~
- 24 -
factor of at least 10 times, and preferably at least 15
times. Useful mesh can also be prepared where a metal
sheet has been expanded by a factor up to 30 times its
original area. Even for an annealed valve metal of
elongation greater than 20 percent, an expansion factor of
greater than 30:1 may lead to the preparation of a mesh
exhibiting strand breakage. On the other hand, an
expansion factor of less than about 10:1 may leave
additional metal without augmenting cathodic protection.
Further in this regard, the resulting expanded mesh should
have an at least 80 percent void fraction for efficiency
and economy of cathodic protection. Most preferably, the
expanded metal mesh will have a void fraction of at least
about 90 percent, and may be as great as 92 to 96 percent
or more, while still supplying sufficient metal and
economical current distribution. With such void fraction,
the metal strands can be connected at a multiplicity of
nodes providing a redundancy of current-carrying paths
through the mesh which insures effective current
distribution throughout the mesh even in the event of
possible breakage of a number of individual strands, e.g.,
any breakage which might occur during installation or
use. Within the expansion factor range as discussed
hereinbefore, such suitable redundancy for the metal
strands will be provided in a network of strands most
always interconnected by from about 500 to about 2000
nodes per square meter of the mesh. Greater than about
2000 nodes per square meter of the mesh is uneconomical.
On the other hand, less than about 500 of the
interconnecting nodes per square meter of the mesh may
provide for insufficient redundancy in the mesh.
Within the above-discussed weight range for the mesh,
and referring to a sheet thickness of between about
0.05-0.125 centimeter, it can be expected that strands
~ 35 within such thickness range will have width dimensions of
..:,
, ' .
-'
' - -' ' ~ ~
- ,
~.2~9~0
\
- 25 -
from about 0.05 centimeter to about 0.20 centimeter. For
the special application to cathodic protection in
concrete, it is expected that the total surface area of
interconnected metal, i.e., including the total surface
area of strands plus nodes, will provide between about 10
percent up to about 50 percent of the area covered by the
metal mesh. Since this surface area is the total area, as
for example contributed by all four faces of a strand of
square cross-section, it will be appreciated that even at
a 90 percent void fraction such mesh can have a much
greater than 10 percent mesh surface area. This area will
usually be referred to herein as the "surface area of the
metal" or the "metal surface area". If the total surface
area of the metal is less than about 10 percent, the
resulting mesh can be sufficiently fragile to lead to
deleterious strand breakage. On the other hand, greater
than about 50 percent surface area of metal will supply
additional metal without a commensurate enhancement in
protection.
After expansion the resulting mesh can be readily
rolled into coiled configuration, such as for storage or
transport or further operation. With the representative
valve metal titanium, rolls having a hollow inner diameter
of greater than 20 centimeters and an outer diameter of up
25 to 150 centimeters, preferably 100 centimeters, can be
prepared. These rolls can be suitably coiled from the
mesh when such is prepared in lengths within the range of
from about 40 to about 200, and preferably up to 100,
meters. For the metal titanium, such rolls will have
weight on the order of about 10-50 kilograms, but usually
~ below 30 kilograms to be serviceable for handling,
;~ especially following coating, and particularly handling in
the field during installation for cathodic protection.
In such greatly expanded valve metal mesh it is most
~ 35 typical that the gap patterns in the mesh will be formed
,~
~:
.2,R~9
-- 26 -
as diamond-shaped apertures. Such "diamond-pattern~ will
feature apertures having a long way of design (LWD) from
about 4, and preferably from about 6, centimeters up to
about 9 centimeters, although a longer LWD is
contemplated, and a short way of design (SWD) of from
about 2, and preferably from about 2.5, up to about 4
centimeters. In the specific application of cathodic
protection in concrete, diamond dimensions having an LWD
exceeding about 9 centimeters may lead to undue strand
breakage and undesirable voltage loss. An SWD of less
than about 2 centimeters, or an LWD of less than about 4
centimeters, in this application, can be uneconomical in
supplying an unneeded amount of metal for desirable
cathodic protection.
Referring now more particularly to Fig. 1 an
individual diamond shape, from a sheet containing many
such shapes is shown generally at 2. The shape is formed
from strands 3 joining at connections (nodes) 4. As shown
in the Figure, the strands 3 and connections 4 form a
diamond aperture having a long way of design in a
horizontal direction. The short way of design is in the
opposite, vertical direction. When referring to the
surface area of the interconnected metal strands 3, e.g.,
~ where such surface area will supply not less than about 10
-~ 25 percent of the overall measured area of the expanded metal
as discussed hereinabove, such surface area is the total
area around a strand 3 and the connections 4. For
example, in a strand 3 of square cross-section, the
surface area of the strand 3 will be four times the
depicted, one-side-only, area as seen in the Fiqure. Thus
in Fig. 1, although the strands 3 and their connections 4
appear thin, they may readily contribute 20 to 30 percent
surface area to the overall measured area of the e~panded
metal. In the Fiq. 1, the ~area of the mesh~, e.q., the
square meters of the mesh, as such terms are used herein,
,
,~,.
,~ .
, ., , . ~ . , -
,, ' .'` ' '
. .
~. .
1 2R~
- 27 -
is the area encompassed within an imaginary line drawn
around the periphery of the Figure.
In Fig. 1, the area within the diamond, i.e., within
the strands 3 and connections 4, may be referred to herein
as the "diamond aperture ". It is the area having the LWD
and SWD dimensions. For convenience, it may also be
referred to herein as the "void", or referred to herein as
the "void ~raction", when based upon such area plus the
area of the metal around the void. As noted in Fig. 1 and
as discussed hereinbefore, the metal mesh as used herein
has extremely great void fraction. Although the shape
depicted in the figure is diamond-shaped, it is to be
understood that many other shapes can be serviceable to
achieve the extremely great void fraction, e.g.,
scallop-shaped or hexagonal.
Referring now to Fig. 2, several individual diamonds
21 are formed of individual strands 22 and their
interconnections 25 thereby providing diamond-shaped
apertures. A row of the diamonds 21 is bonded to a metal
20 strip 23 at the intersections 25 of strands 22 with the
metal strip 23 running along the LWD of the diamond
pattern. The assembly is brought together by spotwelds
24, with each individual strand connection (node) 25
located under the strip 23 being welded by a spotweld 24.
Generally the welding employed will be electrical
resistance welding and this will most always simply be
spot welding, for economy, although other, similar welding
techniques, e.g., roller weldinq, are contemplated. This
provides a firm interconnection for good
electroconductivity between the strip 23 and the strands
22. As can be appreciated by reference particularly to
Fig. 2, the strands 22 and connections 25 can form a
substantially planar configuration. As such term is used
herein it is meant that particularly larger dimensional
sheets of the mesh may be generally in coiled or rolled
2 ~
- 28 -
condition, as for storage or handling, but are capable of
being unrolled into a "substantially planar~ condition or
configuration, i.e., substantially flat form, for use.
Moreover, the connections 25 will have double strand
thickness, whereby even when rolled flat, the
substantially planar or flat configuration may
nevertheless have ridged connections.
Referring then to the enlarged view in FI~. 3, it can
be seen that the nodes have double strand thickness (2T).
Thus, the individual strands have a lateral depth or
thickness (T) not to exceed about 0.125 centimeter, as
discussed hereinabove, and a facing width (W) which may be
up to about 0.20 centimeter.
The expanded metal mesh can be coated before or after
it is in mesh form with a catalytic active material,
thereby forming a catalytic anode structure. Usually
before any of this, the valve metal mesh will be subjected
to a cleaning operation, e.g., a degreasing operation,
which can include cleaning plus etching, as is well known
in the art of preparing a valve metal to receive an
electrochemically active coating. It is also well known
that a valve metal, which may also be referred to herein
as a ~film-forming~ metal, will not function as an anode
without an electrochemically active coating which prevents
~; ~ 25 passivation of the valve metal surface. This
electrochemically active coating may be provided from
platinum or other platinum group metal, or it may be any
of a number of active oxide coatings such as the platinum
group metal oxides, magnetite, ferrite, cobalt spinel, or
. 30 mixed metal oxide coatings, which have been developed for
use as anode coatings in the industrial electrochemical
; industry. It is particularly preferred for extended life
protection of concrete structures that the anode coating
be a mixed metal oxide, which can be a solid solution of
. ~ ~
~ 35 a film-forming metal oxide and a platinum group metal
, ~ :
:,
...... .
: ~ " ' ,. ~ .,;
~. ' ' -
~ .
-
~' ' .
, ~
- 29 -
oxide.
For this extended protection application, the coating
should be present in an amount of from about 0.05 to about
0.5 gram of platinum group metal per square meter of
expanded valve metal mesh. Less than about 0.05 gram of
platinum group metal will provide insufficient
electrochemically active coating to serve for preventing
passivation of the valve metal substrate over extended
time, or to economically function at a sufficiently low
single electrode potential to promote selectivity of the
anodic reaction. On the other hand, the presence of
greater than about 0.5 gram of platinum group metal per
square meter of the expanded valve metal mesh can
contribute an expense without commensurate improvement in
anode lifetime. In this particular embodiment of the
mesh, the mixed metal oxide coating is highly catalytic
for the cxygen evolution reaction, and in a chloride
contaminated concrete environment, will evolve no chlorine
or hypochlorite. The platinum group metal or mixed metal
oxides for the coating are such as have been generally
been described in one or more of U.S. Patents 3,265,526,
3,632,498, 3,711,385 and 4,528,084. More particularly,
such platinum group metal~ include platinum, palladium,
rhodium, iridium and ruthenium or alloys of themselves and
with other metals. Mixed metal oxides include at least
one of the oxides of these platinum group metals in
combination with at least one oxide of a valve metal or
another non-precious metal. It is preferred for economy
that the coating be such as have been disclosed in the
U.S. Patent No. 4,528,084.
In such concrete corrosion retarding application, the
metal mesh will be connected to a current distribution
member, e.g., the metal strip 23 of Fig. 2. Such member
will most always be a valve metal and preferably is the
same metal alloy or intermetallic mixture as the metal
~,
'~
.
~ 2~
- 30 -
most predominantly found in the expanded valve metal
mesh. This current distribution member must be firmly
affixed to the metal mesh. Such a manner of firmly fixing
the member to the mesh can be by welding as has been
discussed hereinabove. Moreover, the welding can proceed
through the coating. Thus, a coated strip can be laid on
a coated mesh, with coated faces in contact, and yet the
welding can readily proceed. The strip can be welded to
the mesh at every node and thereby provide uniform
distribution of current thereto. Such a member positioned
along a piece of mesh about every 30 meters will usually
be sufficient to serve as a current distributor for such
piece.
In the application of the cathodic protection for
concrete, it is important that the embedded portion of the
current distribution member be also coated, such as with
the same electrochemically active coating of the mesh.
Like considerations for the coating weight, such as for
the mesh, are also important for the current distributor
member. The member may be attached to the mesh before or
after the member is coated. Such current distributor
member can then connect outside of the concrete
environment to a current conductor, which current
conductor being external to the concrete need not be so
coated. For example in the case of a concrete bridge
deck, the current distribution member may be a bar
extending through a hole to the underside of the deck
surface where a current conductor is located. In this way
all mechanical current connections are made external to
the finished concrete structure, and are thereby readily
available for access and service if necessary.
Connections to the current distribution bar external to
the concrete may be of conventional mechanical means such
as a bolted spade-lug connector.
Meshes produced according to the following
,,~
,
. . ~ ~ . .
- 31 -
specifications were used in the example of the method of
installation described below.
Anode Mesh Specifications
TYPe 1 Mesh
Composition Titanium Grade 1
Width of Roll 45 inches (112.5 cm)
Lenqth 250 to 500 ft. (75 to 150 m)
Weight 26 lbs./1000 ft.2 ~11.7 kg/lOOm2)
Diamond Dimension 3" LWD x 1 1/3u SWD
(7.6 cm LWD x 3.3cm SWD)
Resistance Lengthwise
~45 inch/112.5 cm wide) .026 ohm/ft. (0.086 ohm/m)
Resistance Widthwise with
Current Distribùtor .007 ohm/ft. (0.02 ohm/m)
Bending Radius 3/32 inches (0.24 cm)
Bending Radius in Mesh Plane 50 ft. (15 m)
:
.,
TYpe 2 Nesh
Composition Titanium Grade 1
Width of Roll 4 ft. (122 cm)
Length 250 to 500 ft. (75 to 150 m)
Weight 45 lbs./1000 ft.2 (20.2 kg/100 m2)
Diamond Dimension 3" LWD x 1 1/3" SWD
(7.6 cm LWD x 3.3 cm SWD)
Resistance Lengthwise
(4 ft., 122 cm wide) .014 ohm/ft.
Resistance Widthwise with
: i~
~ ~ Current Distributor .005 ohm/ft. (0.016 ohm~m~
; ~ 8ending Radius 3/32 inches (0.24 cm)
~ Bending Radius in Mesh Plane 50 ft. (15 m)
i ,~'11,0
- 32 -
These meshes are coated typically with a mixed metal
oxide catalytic coating providing good oxygen specificity
at a maximum recommended anode-concrete interface current
density (i.e. the current density on the strands of the
mesh) of 10 mA/ft2 ~about 100 mA/m2). The precious
metal loading of the catalyst is between about 0.05 and
0~5g/m2 of the mesh. The same thin catalytic coating is
applied to current distributors typically made from strips
of the same titanium having a width of about 0-5 inch
(1.25 cm), and a thickness of about 0.04 inch (0.1 cm).
INSTALLATION PROCEDURE
Application of the coated mesh for corrosion
protection such as to a concrete deck or substructure can
be simplistic. A roll of the greatly expanded valve metal
mesh with a suitable electrochemically active coating,
sometimes referred to hereinafter simply as the nanoden,
can be unrolled onto the surface of such deck or
substructure. Thereafter, means of fixing mesh to
substructure can be any of those useful for binding a
metal mesh to concrete that will not deleteriously disrupt
the anodic nature of the mesh. Usually, non-conductive
retaining members will be useful. Such retaining members
for economy are advantageously plastic and in a form such
as pegs or studs. For example, plastics such as polyvinyl
halides or polyolefins can be useful. These plastic
retaining members can be inserted into holes drilled into
the concrete. Such retainers may have an enlarged head
engaging a strand of the mesh under the head to hold the
anode in place, or the retainers may be partially slotted
to grip a strand of the mesh located directly over the
hole drilled into the concrete.
When the anode is in place and while held in close
ZR~tO
-- 33 --
contact with the concrete substructure by means of the
retainers, an ion~ically conductive overlay will be
employed to completely cover the anode structure. Such
overlay may further enhance firm contact between the anode
and the concrete substructure. Serviceable ionically
conductive overlays include portland cement and
polymer-modified concrete.
In typical operation, the anode can be o~erlaid with
from about 2 to about 6 centimeters of a portland cement
or a latex modified concrete. In the case where a thin
overlay is particularly desirable, the anode may be
generally covered by from about 0.5 to about 2 centimeters
of polymer modified concrete. The expanded valve metal
mesh substrate of the anode provides the additional
advantage of acting as a metal reinforcing means, thereby
improving the mechanical properties and useful life of the
overlay. It is contemplated that the metal mesh anode
structure will be used with any such materials and in any
such techniques as are well known in the art of repairing
underlying concrete structures such as bridge decks and
support columns and the like.
FIG. 4 illustrates the installation of a mesh of
highly expanded titanium as specified above on a
steel-reinforced concrete deck designated generally by
40. Before proceeding, the steel reinforcement of the
deck is tested for its degree of corrosion and its
suitability for preservation by cathodic protection, using
known techniques including suitable potential measurements.
Prior to laying the rolls 32 of mesh, catalytically
coated titanium current distributor strips 23 are laid
across the deck 40 with a suitable spacing. In
installations with the type 1 mesh, the current
distributors 23 are typically spaced lengthwise by about
60 feet (18 meters). For the type 2 mesh, this spacing is
35 about 100 feet (30 meters). At given locations, not
~ . "
~: '
1 2~
-- 34 --
shown, the strips 32 extend through holes in the deck 40
for connection to a current supply; for the type 1 mesh
the spacing of these power feed locations is about 24 feet
~7.2 meters) widthwise of the meshes. For the type 2 mesh
this widthwise spacing is about 32 feet (9.8 meters).
FIG. 4 shows a first anode mesh 30 which has already
been laid by unrolling from its roll, stretched
longitudinally by about 5-10% and fixed to the deck 40 by
inserting plastic clips 31 in holes drilled in the deck.
After this fixing, the mesh 30 is spot welded to the
transverse current distributor strips 23 at nodes 25 of
the mesh (as shown in FIG. 2). For this welding
operation, a copper bar 35 is inserted under the mesh 30
and strip 23; this enables a sufficient welding current to
be passed through the weld. After welding all or a
selected number of the nodes across the width of the mesh
30 to the strip 23, the bar 35 is withdrawn from under the
mesh and placed under the strip 23 in position to receive
the next roll of mesh 30, as shown in FIG. 3.
As illustrated, the adjacent unrolled sheets of mesh
30 are spaced by a distance D. Clear spacings of up to
about 1 LWD dimension are possible while producing an even
cathodic protection effect on the underlying steel.
Alternatively the edges could overlap, e.g. by about i LWD
of the mesh or more, if necessary to conform to the width
of the deck 40.
After laying all rolls of the mesh in this way, and
fitting any odd shapes at corners, edges, etc., the deck
40 with mesh 30 is embedded in a thin layer of cement
based grout. Then an ion-conductive layer of about 4-6 cm
portland cement or polymer modified concrete is applied,
by pouring or spraying.
It is to be noted that during installation, i.e.,
after laying and fixing the mesh 30 it is possible to work
on the surlace, dlive vehicles over it etc. with little or
:
,
.
o
- 35 -
no risk of damaging the mesh and further with the
assurance that any accidental breakage of several strands
will not adversely affect the cathodic protection effect,
due to the enhanced redundancy of the mesh.
