Note: Descriptions are shown in the official language in which they were submitted.
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FIBER-FILLED CONCRETE OVERLAY
IN CATHODIC PROTECTION
BACKGROUND OF THE INVENTION
Steel reinforced concrete structures, such as bridge
decks and parking garages, have generally performed well.
But a dramatic increase in the use of road salt, combined
with an increase in coastal construction, has resulted in
a wi~e spread deterioration problem caused by corrosion
of the reinforcing steel within the concrete.
Valve metal electrodes as typified by expanded
titanium mesh have recently gained wide acceptance for
cathodic protection of reinforcing steel in concrete.
Such electrodes, some of whlch have been detailed in PCT
Published Application No. 86/06759 can readily cover
broad surfaces. They may be rolled out on such a broad
surface as a flat bridge deck or parking deck or bridge
substructure. Such coverage has lead to the wide
acceptance of this type of cathodic protection system.
However, experience has shown that there is still need
not only to efficiently install such cathodic protection
systems, but also to efficiently and economically operate
such systems once installed.
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SUMMARY OF TH~ INVENTION
There has now been found a way for enhancing the
efficient operation of a valve metal electrod~ cathodic
protection system installed in concrete. The system can
be enhanced without deleterious change in installation
procedure. It is furthermore economical in not requiring
the need to have on hand at the work site unusual
materials. The enhancement readily lends itself to
working on a variety of surfaces, e.g., an overhead
surface, and around numerous obstructions on such
surface. The enhancement can not only provide for more
efficient operation of installed cathodic protection
systems, e.g., lower resistivity, but also can augment
the physical integrity of such systems, such as reduced
shrinkage.
In a broad consideration, the invention is directed
to a cathodically-protected steel-reinforced concrete
structure having an impressed-current anode embedded in
said concrete structure and spaced apart from steel
reinforcing members also embedded in said concrete
structure, and said an comprises an
electrocatalytically-co_~ed valve metal anode, the
improvement comprising a fiber-filled concrete overlay
for said structure, which overlay contains non-smooth,
non-conductive fiber resistant to degradation at elevated
pH, and with said fiber-filled overlay embedding said
valve metal anode.
In another aspect, the invention is directed to the
method of cathodically protec lng a metal reinforced
concrete structure by utilizii.g the above-discussed
innovation.
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DESCRIPTION OF THE PE~FERR~D E~BODIMENTS
The cathodically-protected steel-reinforced concrete
structure of the present invention can involve any of the
usual concrete structures that are steel-reinforced and
require cathodic protection with such protection
utilizing an overlay. As representative of such
structure will be a concrete bridge deck, but other such
structures include parking garages, piers, and pedestrian
walkways, as well as including the substructure or
supporting structure, e.g., support columns and the like.
Where a surface of such a concrete structure is
prepared for cathodic protection, there can then be
placed on the surface of the prepared structure, the
electrocatalytically coated valve metal anode. Suitable
preparation techniques may include the application to the
concrete structure of a polymeric separator, e.g., in
mesh form, prior to application of the anode. Such
polymeric separator application has been shown in
copending applicatior, Serial No. 376,720, the disclosure
of which is incorporated by reference.
The metals of the valve metal anode substrate which
will be useful for the cathodic protection of the steel
reinforcement will most always be any of titanium,
tantalum, zirconium and niobium. As well as the
elemental metals themselves, the suitable metals of the
anode can include alloys of these metals with themselves
and other metals as well as their intermetallic mixtures.
Of particular ir.terest for its ruggedness/ corrosion
resistance and availability is titanium and
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representatlve of such serviceable metal is Grade I
titanium.
The valve metal anode substrate may be in different
forms, e.g., a ribbon form such as discussed in copending
application Serial No. 178,422, the teachings of which
are incorporated herein by reference. ~r the anode
substrate may be in wire form, as disclosed for example
in U.K. Patent Application 2,L75,609. Although it is to
be understood that these and other shapes may be
particularly serviceable, the anode substrate will
generally be a valve metal mesh, e.g., scallop-shaped or
hexagonal shape, but most typically diamond-shaped. Such
valve metal mesh anode substrates have been more
particularly described in copending application Serial
No. 855,550 the teachings of which are herein
incorporated by reference.
Where the anode substrate is a valve metal mesh,
such will usually have individual strands of a thickness
that does not exceed about 0.125 centimeter and a width
across the strand which may be up to about 0.2
centimeter. The more typical "diamond-pattern" will
feature apertures havin~ 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. The mesh can be produced by expanding a
sheet or coil of metal of appropriate thickness by an
expansion 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. Further in this regard, the resulting
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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. Within this expansion factor range,
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.
The valve metal anode substrate has an
electrocatalytic coating. Usually before coating, the
valve metal substrate 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 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
mixed metal oxide coatings, which have been developed for
use as anode coatings in thé industrial electrochemical
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industry. It is particularly preferred for extended lire
protection of concrete structures that the anode coating
be a mixed metal oxide, which can be a solid solution of
a film-forming metal oxide and platinum group metal or
platinum group metal oxide.
The mixed metal oxide coating is highly catalytic
for the oxygen 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 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 metals 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.
Application of the coated valve metal anode for
corrosion protection such as to a concrete deck or
substructure can be simplistic. A roll of mesh or coil
of ribbon is simply unrolled and in so doing is applied
against the concrete. Thereafter, means of fixing the
anode to substructure can be any of those useful for
binding a metal to concrete that will not deleteriously
disrupt the anodic nature of the anode. 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
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useful. These pLastic retaining members can be inserted
into holes drilled into the concrete surface. Such
retainers may have an enlarged head engaging a strand of
mesh or wire or ribbon under the head to hold the anode
in place, or the retainers may be partially slotted to
grip a strand of the anode located directly over the hole
drilled into the concrete. Current distributor members,
e.g., metal strips, are applied to the valve metal anode
and fixed to the anode as by welding.
In such concrete corrosion retarding application,
the metal anode will be connected to current supply means
including a current distribution member, usually an
elongate member such as a metal strip laid down on top of
the valve metal anode. Such member will most always be a
valve metal and preferably is the same metal or alloy or
intermetallic mixture as the metal most predominantly
found in the valve metal anode. The current distribution
member must be firmly aî--ixed to the metal anode, as by
welding. The member in ~._rip form can be welded to a
mesh anode at every node and thereby provide uniform
distribution of current thereto. Such current
distributor member can then connect outside of the
concrete environment to a current conductor for supplying
an impressed current, e.g., at a current density of up to
200 mA/m2 of the valve metal anode surface area.
Usually when the anode is in place and while held in
close contact with the concrete substructure by means of
the retainers, an ionically conductive fiber-filled
overlay will be employed to embed the resulting mesh
structure. Such overlay will further enhance firmly
fixing the anode in place over the concrete substructure.
Useful overlays can be formulated from portland cement
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and polymer-modified concrete, i.e., latex-modified
concrete. Before application of the overlay, it may be
serviceable to apply a cement-based bonding grout to the
resuliing mesh structure.
Where the anode is resting on the concrete
substructure for example where a ribbon valve metal anode
is placed flat onto the concrete substructure, the
overlay will serve to cover the exposed upper ribbon
surface. The anode will then have a face contact the
substructure and the remainder covered by the overlay.
Where the anode is resting on a polymeric separator or
where the anode may be typically in strand form and the
strands are gripped by the heads of retainers, the anode
can be separated from or slightly above the concrete
substructure. In these instances, application of the
overlay can completely surround the anode, and will at
least substantially cover any polymeric separator.
Whether the overlay covers the anode, e.g., the flat
ribbon anode as above described, or completely surrounds
the anode such as separated from the concrete
substructure for purposes of convenience, all such
applications will typically be referred to herein as
having the anode "embedded" in the overlay.
Where the overlay is Portland cement or a mix
including Portland cement, it is contemplated that there
will be used any Portland cement which is typically
serviceable for overlay purposes. Such overlay may
additionally include a fine aggregate such as sand as
well as coarse aggregate, e.g., crushed rock or gravel,
typically having a particle size of 0.25 to 1 inch. Such
concrete overlay may be referenced to herein for
convenience simply as a "grout". When latex modified
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concrete is used, it is suitable to utilize any such
latex as may be useful in concrete such as an acrylate,
epoxy or styrene-butadiene rubber latex.
The overlay will most typical:Ly be applied to provide a
thickness of from about 1/2 inch to on the order of 2
inches thickness or more. Usually, the thinner amounts
of overlay of on the order or a 1/2 inch, e.g., 1/2 to 1
inch, will be applied to columns, pilings, parking garage
floors and the like. Thicker overlays of greater than an
inch to 2 inches or more will usually be applied to
bridge decks, pier substructures and tunnel
substructures.
For purposes of the present invention, the concrete
overlay will contain an electrically non-conductive fiber
that retains integrity at elevated pH, e.g., on the order
of pH 12. Glass fibers are representative o~ fibers that
are unsuitable since they are not resistant to
degradation in concrete as such elevated pH. Suitable
useful fibers include ceramic fibers, such as fibers of
alumina, titania and zirconia, as well as polymeric
fibers. The useful polymers can be one or more of a
great variety of polymeric fibers, both thermoplastic and
non-thermoplastic. Representative of serviceable
polymers for the fibers include polyolefins such as
polyethylene and polypropylene fibers, polyaramides such
as Kevlar~tm~ aromatic polyamide fibers, polyamides such as
nylon, polyhalocarbon fibers including
polytetrafluoroethylene fibers, polycarbonate and
polyester fibers such as polyethylene terephthalate fiber
and the like.
The fibers can be suitably used in the concrete
overlay as individual fibers or the fibers may be
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utilized as bundles, e.g., fibrillated polymer fiber
bundles. Mixing such fibrous bundles into the concrete
will serve to suitably expand the bundles into a
desirable fibrous consistency. As will be understood by
those skilled-in-the-art of utilizing fibers with
concrete, the fibers that are useful herein are not
smooth. For example, such fibers are not smooth
monofilaments, but should have a rough surface, e.g.,
fibrillated in the nature of baling wire twine, or should
be bundled or have the ability to expand to a fibrillated
bundle. Preferably for economy in use combined with
desirable roughness, there are used fibrillated
polypropylene fiber bundles.
The fibers will generally have average fiber length
at least equal to the thickness of an overlay coating
layer, and it is most useful that the fibers have an
average fiber length greater than the depth or thickness
of the overlay to be applied. Thus where an about 1/2
inch overlay is to be applied to a column, it is most
advantageous that the fibers contained in such overlay
have average length of greater than 1/2 inch, e.g., 3/4
inch to 1 inch, or more. Where thicker overlays are
applied, e.g., up to 2 inches or more on a bridge deck,
it is acceptable that the fiber length average 3/4 inch,
for example, and that several coats of the concrete
overlay, such as several 1/2 inch thick coats, be used to
provide the desired concrete overlay thickness. Most
usually, the fibers will have an average length of from
about ~/2 inch to about 1 inch or more, e.g., 1.5 inches,
and can have strand thickness of from as thin as 50
microns or less, up to a thickness for bundles of as much
as 3 millimeters or more.
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Where polymer fibers are utilized, the fiber will
most always be present in the concrete overlay in an
amount of from about 1 pound to about 20 pounds of fibers
per cubic yard of concrete overlay. Use of less than
about 1 pound of fiber may not provide sufficient fiber
for yielding desirable benefit. On the other hand,
greater than about 20 pounds of fiber per cubic yard of
concrete, can be uneconomical. Regardless of the type of
fiber, advantageously the fiber will be present in an
amount from about 2 to about 10 pounds per cubic yard of
concrete and preferably for best economy and efficiency~
the fiber is present in the concrete in an amount from
about 5 to about 8 pounds of fiber per cubic yard of
concrete overlay.
As evidence of such concentration at least with
regard to polymer fiber for utilization in a cathodic
protection system and as it can desirably affect lowered
volumetric resistivities, such may be demonstrated with
grouts prepared from a mixture of Portland cement and
silica sand in a per cubic yard basis of 1:3. The
resistivity effect can be demonstrated with this grout
using initially a "control" containing no polymer fiber.
Additional portions contain 1.6 pounds of polymer fiber
("normal" or lx, i.e., the conventional amount that would
be utilized for this particular polymer fiber in
concrete), 3.2 pounds (2x) or 6.4 pounds (4x), per cubic
yard of concrete, of 3/4 inch long, Forta CR fibrillated
polypropylene fiber. Volumetric resistivities for cured
test samples as measured by the 4-pin technique are as
follows:
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TABLE
Concentration of
Polymer Fiber Per
Cubic Yard of Grout Volumetr c Resistivit~: Ohm-Cm.
Control (no polymer) 18,827
lx 18,702
2x 13,715
4x 14,962
It is suitable to add the fiber to the concrete at
any stage of the mixing operation. For example, the
fiber may be admixed with the cement, fine aggregate, or
fine aggregate and coarse aggregate, added to prepare the
concrete. Or the fiber can be admixed to the concrete
overlay after all other ingredients have been blended
together. It is to be understood that one or more of
additional ingredients typically used with concrete will
be serviceable for use in the concrete overlay. For
example, agents such as latex modifiers, air entraining
agents, superplasticizers, or water reducing agents may
also be present in the concrete overlay.
As mentioned hereinabove, the concrete overlay can
be applied as a single coat or as several layers. Any
application technique useful for applying a concrete
overlay to a substructure is contemplated as being useful
in the present invention. The overlay may be mixed and
placed by either the dry or wet shotcrete process. More
typically for application to vertical surfaces such as
columns and pilings, the overlay can be spray applied.
The resulting finished structure can have excellent
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mechanical properties and reduced shrinkage cracking of
the overlay providing for a longer lasting overall
system.
The following example shows a way in which the
invention has been practiced, but should not be construed
as limiting the invention.
EXAMP~E
For test purposes, concrete slabs were prepared from
Type I Portland cement, silica sand fine aggregate and 1
inch minus coarse aggregate in a weight proportion of
cement to sand to coarse aggregate, on a per cubic yard
basis, of 1:2:2.95. Each slab measured one square foot
by six inches thick and contained eight steel reinforcing
bars in double-mat construction. The concrete was cured
by spraying the surface at a rate of 200 square
feet/gallon with a water-based curing compound
(Masterkuretm) followed by maintaining the concrete under
plastic for fourteen days, lab air for seven days and
then to outdoor exposure.
Slab top surfaces were sandblasted and fitted with
an electrocatalytically coated, titanium mesh anode. The
electrocatalytic coating was a mixed metal oxide
containing oxides of iridium, titanium and platinum. The
anode mesh electrodes were more particularly anodes of
ninety-four percent void volume while having 0.09
centimeter strand thickness, with the anode mesh being
spaced two inches from the steel reinforcing bars. The
anodes were covered with an overlay. The overlay of
polymer-fiber modified concrete was 2 inches thick. The
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overlay was prepared from a m:Lxture of Portland cement,
silica sand and coarse aggregate in a per cubic yard
basis, of 1:2.56:2.03. The overlay contained 3.2 pounds
per cubic yard of concrete, of 3/4 inch long, fibrillated
polypropylene fiber. The overlay was cured one day with
wet burlap and plastic followed by six days lab air. For
cathodic protection system activation, there was used an
anodic current density of 10 and 40 milliamps per square
foot (mA/ft2). Overlaid test slabs were subjected to
outdoor exposure on above-ground racks under conditions
obtained during the months of July to November in
Northeastern Ohio.
Slabs were inspected after 83 days and none of the
slabs contained discernable cracking.
