Note: Descriptions are shown in the official language in which they were submitted.
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9549-80
Cathodic Protection System
This invention relates to cathodic protection
systems and has particular reference to cathodic
protection systems used to protect iron or steel
reinforcement bars in concrete structures.
Impressed current cathodic protection systems
are well known in use to protect structures
immersed in water, particularly sea water. In such a
system the object to be protected is made a cathode
whilst the counter electrode is made an anode.
Negatively charged ion species are attracted towards
the anode where they tend to concentrate, unless
sufficient diffusion of ions occurs in the region of
the anode to disperse them. In free sea water, the
movement of the negatively charged ions occurs freely
and readily, such that there is a minimal build-up of
ions around the anode.
Reinforced concrete essentially comprises a
series of steel reinforcing bars (commonly referred to
as rebars) surrounded by a concrete mixture.
It is well known that steels are not corroded
in alkaline media. Reinforcing bars are very
frequently covered with an adherent "rust" layer when
embedded in concrete, which experience has shown
.
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improves the adhesion between concrete and steel.
With time, as may be shown by removing the concrete
cover, the rust changes chemically allowing the
formation of a dark, protective, film well adherent to
concrete. This is the very satisfactory usual
situation that exists.
Concrete, by various mechanisms, is porous to
water, albeit very slowly, and even after so called
full curing, will allow a slow uptake with some kind
of equilibrium being established with the surrounding
environment. This again is a normal situation.
However, if salt water is present on the surface, then
the salt and its contained chloride ions may penetrate
into the concrete. It is not immediately obvious why
salt contamination is more of a problem to some
concrete than others, because concrete is widely used
as a constructional material for use in seawater. The
design of the concrete structure and thickness of the
outer concrete layer may be particularly important.
In the case of chloride contaminatedfconcrete,
a risk exists that chloride ions will ~nhancc the
corrosion of the steel. The resultant corrosion
product formed by the enb~need reaction occupies a
greater volume than the space occupied by components
prior to chemical reaction, eventually creating
intense local pressure that brings about cracking of
the concrete and eventual spalling of the concrete
cover to expose rebars directly to the atmosphere.
A great deal of reinforced concrete has been
used in building and in road construction and
particularly in the fabrication of support pillars,
cross beams and road decks for bridges. Over the
years increasing amounts of common salt, sodium
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chloride, has been used in winter to prevent ice
formation on the road. The melted snow or ice and
sodium chloride solution tends to seep down into the
concrete, and it has been found that the presence of
the chloride ion penetrating to the rebars can give
rise to corrosion. In some cases, calcium chloride
has been added to the concrete as a setting agent, or
the water used to make the concrete contained
naturally high levels of chloride ions and this also
increases the rate of corrosion of the rebars.
Further, some structures are exposed to salt-laden
atmospheres, particularly in marine locations.
Electrode potential mapping of the outside
surface of concrete rebars is used as a means of
assessing the state of corrosion of embedded rebars
and by inference the depth of penetration and
concentration of salt. Around a concrete cross bar to
a motorway bridge, the variation in electrode
potential may be 0.5 volts or more. It might be
logical to expect that salt contamination from the
bridge roadway would leak onto the top surface of the
cross beams and hence lead to more rapid penetration
to rebars lying near the top surface than on the
bottom surface, and this is exactly what is found.
The problem of corrosion of the rebars in
bridges has become so significant that much effort is
being expended in an attempt to slow down or halt the
corrosion before the concrete structures in the
bridges fail.
By cathodic protection is meant the
application of an electrolytic system whereby the
electrode potential of the steel is depressed to a
cathodic (negative) potential to stop or significantly
decrease corrosion.
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The cathodic protection of ~teel in concrete
represents an especially challenging problem for
application of cathodic protection for a variety of
reasons. An obvious difference between cathodic
protection in concrete and seawater is the difference
in ionic mobility of species within the electrolyte.
Although there is an electrolytic path in concrete,
otherwise application of cathodic protection would be
impractical, nevertheless the level of diffusion
between anode and cathode is many orders of magnitude
lower than in the seawater case, and the distance
between the anode and the cathode to be protected
cannot usually exceed 15 to 30cms.
Another difference between the seawater and
concrete example relates to the change in pH
surrounding the electrodes. It is well known that
media surrounding a cathode will tend to alkalinity,
and around an anode will tend to acidity. Alkalinity
around a rebar in concrete, which is already alkaline,
is no problem. Indeed additional alkalinity could be
helpful towards the stabilisation of the steel from
corrosion.
The formation of acidity around anodes in
concrete is a major issue. Acidity cannot readily
diffuse away from the anode either by diffusion under
a concentration gradient, or by field transport to the
cathode (ie H+ to the cathode) brought about by the
applied cell voltage. Concrete is readily attacked by
acid, even at very low levels of acidity. Attack is
significant at pH 6, and while some concretes may be
more acid resisting than others, attack at pH's down
to 2 or 1 (common in some cathodic protection
situations) is extremely rapid.
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Hence in practical terms, the problem of
cathodic protection of rebars in concrete is not ~ff~
ability to arrest the corrosion of steel by depressing
the electrode potential, but the problem of acidity
surrounding anodes. Indeed the longevity of cathodic
protection systems applied to concrete may well not be
related to the durability of the electrode materials
involved, but be related to the acid attack on
concrete surrounding anodes. In this respect the
cathodic protection of rebars in concrete is very
different to cathodic protection of steel in seawater.
By the present invention there is provided a
cathodic protection system for the protection of iron
or steel reinforcement bars in concrete which includes
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a source of/electrical current connected to the
reinforcement bars and to an anode, so that, in use,
the reinforcement bars are connected as a cathode,
wherein the anode is a hydraulically porous material
bonded to the concrete so as to make~electrical
contact therewith and being exposed to the environment
over part of its surface area.
The present invention also provides a cathodic
protection system for the protection of iron or steel
reinforcement bars in concrete which includes a source
of direct current connected to the reinforcement bars
and to an anode, wherein the anode is a hydraulically
porous material bonded to the concrete so as to make
electrical contact therewith. The material may be a
ceramic material.
The hydraulically porous material may be
directly bonded to the concrete by cement.
Alternatively the porous material may be embedded in a
conducting backfill.
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A preferred material for the anode is porous
TiOX where "x" is in the range 1.67 to 1.95.
Preferably "x" is in the range of 1.75 to 1.8. The
porous TiOX material is preferably in the form of a
tile grouted to the exterior of the concrete. A
liquid mortar may be used as the grout. The porous
TiOX material may have a thickness in the range 2-3
mm. The density of the material may be in the range
2.3 to 3.5. The porous TiOX material may be in the
form of a tube passing into a hole in the concrete
structure.
The porous material may be graphite or
porous magnetite, porous high silicon iron or porous
sintered zinc, aluminium or magnesium sheet.
The porous material may be in the form of a
tile bonded to the concrete, the tile having a
projecting ear to which electrical contact can be
made. The ear may be provided with a hole or slot.
The ear may be connected to a power supply cable
having an electrically conducting core and a lead
metal exterior in electrical contact with the core,
the lead being deformed by the action of being pushed
into the slot to make electrical contact therewith.
The present invention further provides a
cathodic protection system for the cathodic protection
of steel reinforcement bars embedded in concrete, the
system comprising a plurality of anodes embedded in
spaced location in the concrete, the anodes being
formed of hydraulically porous TiOX where x is in the
range 1.67 to 1.95, the anodes being electrically
interconnected by titanium conductors, the anodes
being anodically polarised relative to the steel
reinforcement bars by the titanium conductors. The
titanium conductors may be in the form of strips.
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The titanium may be anodically passivated and
may be coloured. The anodically passivated layer may
be removed where the titanium is in contact with the
TiOX material anodes.
The titanium may be in the form of strips
having sections cut and bent out of the plane of the
strips to form tabs to which the anodes are
connected. The anodes may be connected to the
titanium strips by nuts and bolts of titanium or by a
titanium rivet. The strips may mechanically locate
the anodes on the concrete. The strips may be
provided with slots.
There may be a plurality of strips with the
strips being bolted, riveted, welded or otherwise
electrically joined together.
By way of example embodiments of the present
invention will now be described with reference to the
accompanying drawings, of which:
Figure 1 is a schematic view in section of a
road bridge;
Figure 2 is a schematic view of a support
member of a bridge wired with a series of
anodes;
Figure 3 is a cross-section of a support
member showing anodes and reinforcement
bars;
Figure 4 is a perspective view of one form of
anode;
Figure 5 is a perspective view of an
alternative form of anode;
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Figure 6 is a schematic view Of a concrete
cross beam and pillars;
Figure 7 is a schematic perspective view of an
anode connected to a portion of a strip;
Figures 8 and 9 are schematic views of tabs
formed in titanium strip;
Figure 10 is a schematic view of a connection
between two titanium strips incorporating
an anode at the connection;
Figure 11 is a plan view of a strip with an
anode embedded in concrete;
Figure 12 is a perspective view of an anode
and strip; and
Figure 13 is a sectional view of an anode and
strip.
It is necessary to understand the causes of
acidity to understand the present invention. In any
electrolytic system involving anodes and cathodes, the
overall reaction is the summation of component parts,
which includes specific electrochemical reactions at
both electrodes. In the case of an anode in concrete,
the predominating reactions, albeit at very slow rate
compared with most cathodic protection systems, is
either the oxidation of chloride ions to release
chlorine gas, or the oxidation of water to release
oxygen and leave behind H+ ions. This latter reaction
is the particularly important one, 2H20 - 4e -~ 2 +
4H+.
For every 96,540 coulombs of electricity
passed involving this reaction, 19 H+ will be
produced. Such H+ concentration (which is, of course,
acidic) will react with concrete, in a volume
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depending upon the available calcium hydroxide
accessibility within the concrete. Thus a simple
model can be considered. Hydrogen ions generated by
the reaction set out above may:
a) react with all the calcium compounds in the
volume of concrete immediately surrounding the
anode;
b) depress the pH of the surrounding concrete to
a low pH;
10 c) migrate towards the OH- near the cathode;
d) pass through the porous anode to be oxidised
by the air; or
e) pass through the porous anode and be diluted
by the moisture from the atmosphere or
rainwater.
Short term (seven day) practical tests with a
single anode passing 25mA suggests that the volume of
concrete attacked will be 12cm3, and the pH depressed
to a value of O. Assuming normal porosity, density
20 and Ca(OH)2 content of concrete, the amount of H+ ion
produced required to react with 12cm3 of concrete will
be 0.036gH+.
At a pH of O the H+ concentration will be
lg/l H+ so that the H+ requirement to depress the pH
25 of available moisture will be 0.012gH+. Therefore the
total H+ generated ~ the anode over seven days would
approximate to 0.036 + 0.012 = 0.048gH+.
The number of coulombs of electricity passed
in seven days is 15,120 coulombs. Assuming 30%
30 current efficiency for oxygen evolution, the reaction
2H2O - 4e ~ 2 + 4H+ will result in 0.047gH+ ion
generation. Hence it appears that a 25mA anode will
produce between 0.05 to 0.1gH+ per week. At lmA/anode
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the correspondiny ~igure would be 0.002 to 0.004gH+ per week. At
this rate it would take eighteen weeks to react all Ca(OH)2 in
12cm3 ~one of concrete per week, and, of course, correspondingly
longer time periods the lower the current passed or the volume of
concrete affected.
The calculations involved are approximate but are
advanced as an indication of the magnitude of the problem of acid
attack around anodes in concrete.
Some indication of the effect of porous anodes in remov-
ing acidity has been obtained from laboratory experiments in whichporous anodes were cemented to a reinforced concrete block and
were kept saturated with water artificially by means of a sur-
rounding shallow rim. With application of current to rebars with-
in the concrete, acidity develops on the outer surface of the
porous anode and after five hours of operation at 80mA at 8 volts
the pH of the distilled water fell to 2.7.
It should be noted that H+ ions generated at the anode/-
concrete interface diffused away from the cathode into water to
the outer side of the anode, presumably because of the favourable
concentration gradient.
In a further experiment, hydraulically porous Ti407
material was used to divide the volume of a glass beaker into two
approximately equal volumes. In one volume was placed sodium
chloride solution and a titanium metal strip cathode. In the
other (representing the atmosphere side in the bridge deck) was
put distilled water. With the hydraulically porous Ti407 separa-
tor connected as an anode and
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anodically polarised with respect to the titanium cathode,
acidity developed with time in the initially distilled water
compartment. Such acidity develops in spite of the good ai-
ffusivity of H ions in the sodium chloride side, i.e. H ions
diffused in the "wrong" direction away from the cathode.
Referring now to the drawings, as illustrated
schematically in Figure 1, many road bridges are based on a
series of upstanding pillars 1, 2 supporting a cross member 3.
Members 1, 2 and 3 are formed of reinforced concrete. The cross
members 3 carry plurality of substantially rectangular section
steel girders 4, 5 which carry the actual road bed 6.
As shown in Figures 2 and 3 there is provided a
mechanism for cathodically protecting the rebars in the structure
3. The rebars 7 (more clearly shown in Figure 3) are connected
to a source of electrical current 8 as cathodes. A series of
hydraulically porous TiOX tiles 9, 10 are grouted to the
exterior of the concrete structure 3 and an electrical connect-
ion is made to the tiles in a suitable manner. The preferred
value for x is 1.75, but tiles where x is predominantly in the
range 1.75 to 1.8 are acceptable. Electrodes of this material
are described in United States Patent 4 422 917.
As the TiOX material will conduct electricity as an
anode it may be used to pass an electrical current through the
moisture in the concrete into the rebars 7. During operation
anions such as Cl are attracted to the anodes and by using
porous TiOX material the anions can diffuse through the TiOX
to be oxidised by the air in the atmosphere or to be washed
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12
away by the irrigation of the anodes which will occur
from rain water washing over the surface of the
support structures, or by applied water irrigation.
It will be appreciated that tubular anodes can
be used. If necessary water could be directed through
the anodes from gulleys on the road deck.
By irrigating the anodes, excessive anion
build-up in the concrete can be reduced.
One method of connecting the anodes to the
electrical conductor line is shown with reference to
Figure 4. In Figure 4 the anode plate 11 is provided
with an upstanding ear 12 integral with the plate. A
hole 13 exposed through the ear 12. Electrical
contact is made by bolting or clamping a suitable wire
through the aperture 13.
An alternative method of making a connection
is illustrated in Figure 5. In the design of Figure 5
a tile 14 of circular or eliptical shape is provided
in the centre with three up-standing ears 15, 16 and
17. The ears are provided with slots 18, 19 and 20.
It will be seen with the slots 18 and 20 face in the
opposite direction the the slot 19.
A lead coated wire having a copper core is
threaded around the ears 15, 16 and 17 and the slots
are so positioned that tightening of the lead covered
wire causes the wire to bite into the slots 18, 19 and
20 to make an electrical contact.
An alternative mode of operation may be used
containing TiOX as the electrical conducting
constituent formed by packing powder into grooves in
the concrete back-filled into grooves in the concrete.
The current density which needs to be applied
to the anodes is very low. Typically a current
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density of the order of 20 milliamps/m2 will protect
the steel rebars. Thus for concrete structure l~m2by
14m long, a total current number across being 1-21
amps would be sufficient. Operating 15cm x l5cm tiles
at 5 amps/m2 would permit each tile to pass 0.1 amp so
that 15-20 tiles per cross-beam would be sufficient.
It will be appreciated that the number of
tiles used would be determined by the required
cathodic protection throwing power. Each anode would
be electrically connected to the power source used for
the impressed current cathodic protection system.
The hydraulically porous TiOX material,
particularly material having a density in the range
2.3 to 3.5g/cc is readily bonded by cement to concrete
and has ,the distinctly advantageous property of not
being cffccte~ by water freezing within the pores of
the material. As material is hydraulically porous
water as a liquid (rather than merely as a vapour) can
pass through it; for example material of 3mm thickness
with a head of water of 30cm will pass one litre of
water per 5cm2 of exposed surface area per day.
Instead of using lead coated wires as
connectors titanium strips may be used. The titanium
strips may themselves have a coating to restrict
corrosion of the strips in which case an oxide film
formed by anodising is preferred. This arrangement is
shown more clearly in Figures 6 to 13.
Referring to Figure 6 this shows the concrete
cross beam 3 supported on the pillars 1, 2. The beam
is a conventional steel rebar reinforced concrete
structure. The pillars 1 and 2 are also conventional
concrete with steel rebars. Extending along the
length of the beam 3 is a titanium strip 21 and
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14
extending vertically from the strip is a plurality of
strips, two of which are shown at 22, 23. The entire
length of the concrete beam 1 will be covered by
strips 22, 23. The strips 5, 6 are spot welded,
riveted, bolted or otherwise connected to the strip 21
and a vertical strip 24 is connected to the strip 21.
Strip 24 is connected to a suitable source of
electrical current as an anode and a suitable
connection is made to the steel reinforcement within
the beam 3 as a cathode. Thus electricity can be
conducted along strips 24 and 21 to the plurality of
vertically extending titanium strips 22, 23. A
plurality of TiOl.7s anodes are bolted to the strips
22, 23 as shown in Figure 7. The anodes 25 are of a
ceramic-like material and are bolted to tabs formed
integrally in the strip 26. The tabs are shown in
more detail as 27 and 28 in the strips 29 and 30 shown
in Figures 8 and 9.
As can be seen in Figure 10 the tabs can be
formed as connectors for adjacent strips 31, 32 and
also to connect in an anode 33. The anodes are
secured by means of titanium nuts and bolts 34.
The anodes may be embedded into holes drilled
into the concrete and are grouted into position as
shown in Figure 11. The anode 34 is surrounded by
grout 36 located in the concrete structure 37. The
anode is bolted to titanium strip 38.
By this system the permanently connected
anodes can be distributed over the surface of the beam
3 and of course, if required, over the surface of the
upright pillars 1, 2. Any other structure can
simultaneously be protected.
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Because the concrete grout securing the anodes may well
become weak during operation, even if it is one chosen for its
acid resistance such as a high alumina cement, it is probably
desirable to use mechanical means to hold the anodes in situ in
addition to the grout. The titanium strips can carry out both
funetions of supplying eurrent to the anodes and holding the
anodes in situ. When such a system is used, slotted strips 39
(Figures 12 and 13) can be used. To install such a system,
initially the position of the shear (longitudinal) rebars in the
material is located, marked out with chalk, and thus enabling the
location of the hydraulically porous tile anodes to be marked in
approximate position. The hydraulieally porous tile used measured
50mm x 50mm and have a hole drilled in one loeation to take a
titanium metal nut 40 and bolt 41. The connector strip 39 of
slotted titanium had a width of 20mm and was 0.5mm thickness. By
means of a self tapping screw, the strip is located at its upper
end, leaving slot locations for the anode and defining positions
for the self tapping holding screws 42. Because of large aggre-
gate in the conerete eover, it is diffieult to drill holes exaetly
in the eonerete, thus the use of the slots faeilitated installa-
tion. Then the anode tiles are grouted at loeations down the
strip and the titanium nuts 40 screwed loosely in position and the
self tapping screws 42 serewed into position while the acid re-
sisting cement is still soft. This sequence is progressed along
the side of the eonerete eross beam 3 (Figures 1, 2, 3 and 6).
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16
When the electrically conducting and
chemically resistant grout has hardened, the nuts to
the anodes are tightened, and a horizontal linking
titanium strip connector applied.
The horizontal connector will also be attached
to the concrete structure, but only after all other
positioning had been completed. The system can then
be used to protect the rebars.
Obviously the cathodic protection system could
be used to protect rebars in concrete in any
situation, for example car parks, foundations, marine
structures etc. In the case of bridges the system can
be installed on the underside of the bridge deck
itself to protect the bridge deck. This installation
can be done without interfering with the traffic flow.
