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Description  |
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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.
BACKGROUND 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 cement 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
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 this exerts tensile stresses on 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 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-void systems.
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 anode material which does not require a conductive
backfill. 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. Pat. Nos.
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 U.S.
Pat. No. 4,506,485). Originally it was thought that zinc would function as
a natural galvanic anode therefore eliminating the requirement of DC power
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 lead 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 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 CO.sub.2, carbonic acid H.sub.2
CO.sub.3, and carbonates HCO.sub.3.sup.- and CO.sub.3.sup.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 interface causing a loss of electrical
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 Cl.sub.2, and hypochlorite
ClO.sup.-. 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.
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 damage 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/m.sup.2 for chlorine production in a mercury
cell or 3-5 KA/m.sup.2 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. PCT
Application WO80/01488 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 "Corrosion/79" paper
number 194 which describes use of a rigid titanium expanded mesh measuring
less that 0.05 m.sup.2 and coated with a layer of 1-15 micron of platinum
capable of carrying a current density of 2.15 A/dm.sup.2. 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 m.sup.2 have
been offered as discrete anodes for the protection of remote structures.
U.S. Pat. No. 4,519,886 describes a linear type of anode 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 in this application and the cost
of protecting an installation would be prohibitive.
SUMMARY OF THE INVENTION
The 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 interconnected by between about 500 to 2000
nodes per square meter of the mesh. Such a mesh is highly flexible 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 expanded valve metal mesh, its method of production
and its method of installation are given in concurrently filed U.S.
applications Ser. No. 591,177, Ser. No. 855,551, now U.S. Pat. No.
4,708,888, and Ser. No. 855,550, now U.S. Pat. No. 4,900,410, the contents
of which are incorporated herein by way of reference.
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
ribbons 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 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 is more fully described
in concurrently filed U.S. application Ser. No. 855,550, now U.S. Pat. No.
4,900,410.
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 comprises about 3-6 cm thick
of portland current or 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.
In cases where a thin overlay is necessary for structural or other reasons,
the ion-conductive overlay can be applied in several thin layers by
spraying. 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 up to 0.125 cm thick is that it
can be effectively used in an overlay as thin as 0.6 cm. This cannot be
achieved effectively with any other known system.
The cathodically-protected structure according to the invention preferably
also has a current supply connected to the current distributors and
arranged to supply a cathodic protection current at a current density of
up to 100 mA/m.sup.2 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
with 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/m.sup.2
of the strand surface area, which is effective for oxygen generation on
the surfaces of the coated valve 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 example 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 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 500 m.sup.2 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 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
molybdenum/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.
metallurgical bonds (welds) 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, 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 det | | |
