Note: Descriptions are shown in the official language in which they were submitted.
2:1 ~6~2
Our Ref.: PA-23
-- 1 --
METHOD FOR PREVENTING CORROSION OF A REINFORCED CONCRETE
STRUCTURE
The pxesent inven-tion relates to a method for
preventing corrosion of a reinEorced concrete structure.
Particularly, it relates to a method for preventing
corrosion of a reinforced concrete str~cture, by means of
an impressed current method whereby the reinforcing steel
of the reinforced concrete structure can be protected
effectively from corrosion for a long period of time.
Concrete structures usually have reinforcing steels
embedded therein. Such reinforcing steels are likely to
be corroded as a result of carbonation of concrete or by
an influence of a salt content contained in the material
for concrete or by an influence of chlorine ions or
sulfuric acid ions contained in water penetrated into the
concrete. Thus, the reinforcing steels o concrete
structures had a drawback that the function as a
reinforcing material was lost in a relatively short
period of time. To prevent corrosion of reinforcing
steels, it was common to employ (a) a method of coating a
2~0~012
- 2 --
corrosion preventive paint on the surface of a concrete
structure, (b) a method fox electrolytic protection
(cathodic protection) by means of a galvanic anode
method, or (c) a method for electrolytic protection
(cathodic protection) by means of an impressed current
method.
~ owever, (a) the method of coating a corrosion
preventive paint on the surface of a concrete structure
had a drawback that the coating film formed by the
corrosion preventive paint did not have adequate physical
strength, and it was susceptible to damages. As a
consequence, corrosive factors tended -to penetrate
through the damaged portions, whereby the coating film
was inferior in the corrosion prevention for a long
period of time.
Whereas, (b) the method for electrolytic protection
by means of a galvanic anode method is a method whereby
the maintenance is easy, and when applied to e.g. a
marine concrete structure immersed in the sea water, the
2~ electrical resistance of the concrete itself is low, so
that the corrosion preventive current is easy to flow to
provide excellent corrosion prevention. However, when it
is applied to a land concrete structure in the
atmosphere, the electrical resistance of the concrete
itself is high, so that the corrosion preventive current
is difficult to flow, whereby there is a drawback that no
adequate corrosion prevention can be accomplished.
Whereas, (c) the method for electrolytic protection
by means of an impressed current method is excellent in
the corrosion prevention of the reinforcing steel for a
long period of time, since the applied voltage can be
freely adjusted even for a concrete structure in the
atmosphere, and this method has been commonly employed.
For this impressed current method, (i) a conductive
paint system (e.g. Japanese Unexamined Patent Publication
No. 52090/1989) and (ii) a reticular anode system (e.g.
WO 86/06759 and Japanese Unexamined Patent Publication
No. 25975/1981) are known as typical systems. However,
both systems have a drawback such that the practical
application is difficult, or the workability is poor.
Namely, (i) the conductive paint system is a method
wherein a kerf is formed on the surface of a concrete
structure, then a platinum-plated titanium wire is laid
in the kerf and an electrical conductive resin for its
protection is filled in the kerf, to obtain a primary
anode, and as a secondary anode, a carbon type or nickel
type conductive paint is coated on the surface of the
concrete structure. However, the application of this
method is difficult particularly to a concrete structure
having a complicated shape, and a number of steps are
required to form the kerf on the concrete surface,
whereby the workability is poor. Further, blistering or
peeling of the coating film is likely to result as time
passes, and the platinum-plated titanium wire has a
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drawback that it is expensive.
On the other hand, (ii) the reticular anode system is
an anode system wherein the secondary anode in the above
system (i) is omitted by arranging the primary anode
connected directly to the power source, in a net form on
the concrete surface to make the distribution of a
current to the reinforcing steel uniform. Namely, this
is a method wherein a titanium expanded mesh provided
with a coating film of a platinum group metal oxide or a
carbon type expanded mesh having the surface treated, is
disposed on the concrete surface, and a mortar is coated
thereon. However, the application of this method is
difficult particularly to a concrete structure having a
complicated shape, and the workability is poor. Further,
the overlaid mortar has a problem in its durability, and
blistering is likely to result as time passes, and the
anode material has a drawback that it is expensive.
Further, as a method for corrosion prevention of a
steel plate, a corrosion-preventing method is known
wherein an aggregate-containing primer is coated on the
surface of a steel plate to form a primer layer ha~ing a
rough surface, and a metal is metal-sprayed onto the
primer layer to form a spray coating layer, for example,
in U.S. Patent 4,971,838 or EP ~275083. This corrosion
preventing method is capable of effectively protecting
the steel plate from corrosion, since a corrosion-
preventing film is formed directly on the surface of the
2~0 ~2
steel plate. However, in the case of a reinforced
concrete structure, a reinforcing steel is embedded in
concrete, and it is impossible by the above corrosion-
preventing method to effectively protect the reinforcing
steel from corrosion, since a cor~osion-preventing film
can not directly be formed on such a reinforcing steel.
It is an object of the present invention to provide A
method for corrosion prevention of a reinforced concrete
structure, which can easily be applied even to a
reinforced concrete structure having a complex shape, and
whereby excellent corrosion prevention can be provided
efficiently at low costs for a long period of time of at
least equal to that attainable by the above-mentioned
conventional impressed current methods.
The present inventors have studied the above-
mentioned problems inherent to the impressed current
method and conducted a research to develop a method for
preventing corrosion of a reinforced concrete structure
for a long period of time, which is excellent in the
workability and can be applied at low costs, while
effectively utilizing the feature of the electrolytic
protection by the impressed current method. As a result,
the present invention has been accomplished.
Thus, the present invention provides a method for
preventing corrosion of a reinforced concrete structure
having a reinforcing steel embedded therein, which
comprises coating an aggregate-containing primer on the
- 2 ~ 2
surface of the reinforced concrete structure, to form a
primer layer having a rough surface, metal-sprayiny
aluminum, an aluminum alloy or a zinc-aluminum pseudo
alloy on the primer layer to form a metal. spray coating
layer, and applying a direct current voltage across the
metal spray coating layer as an anode and the reinforcing
steel as a cathode to conduct a corrosion preventive
current.
In the accompanying drawing, Figure 1 is a cross-
sectional side view of a part of a reinforced concretestructure to which corrosion-preventing treatment was
applied by the method according -to the present invention~
Now, the present invention will be described in
detail with reference to the preferred embodiments.
The primer to be used in the present invention is a
primer comprising an aggregate and a binder as essential
components and having a solvent (or a di.spersion medium)
a pigment or various additives incorporated as the case
requires .
The aggregate to be used in the present invention has
an average particle size of from about 10 to 200 ~m,
preferably from 30 to 100 ~m and is the one capable of
forming sharp irregularities on the surface of the primer
layer.
The aggregate may, for example, be a metal or alloy
having the same ionization tendency as the metal-spray
coating material, or various metals or alloys having
2 ~ 2
insulation treatment applied at least to their surface,
or their oxides (such as aluminum oxide or iron oxide),
nitrides or carbides. Further, silicon oxide, silicon
carbide, boron nitride or a plastic powder insoluble to a
solvent in the primer, may, for example, be mentioned.
The amount of such an aggregate to be incorporated, is
usually from about 30 to 300 volume %, preferably from 65
to 150 volume %, to the binder, and usually from about 25
to 75%, preferably from 40 to 60% as the pigment volume
concentration (PVC). By the aggregate contained in the
primer, the surface of the primer layer formed on the
concrete structure can be made to have a suitable surface
rouyhness, preferably at a level of a surface roughness
(Rz) of from about 40 to 150 ~m as prescribed in JIS B
0601. By this surface roughness, it is possible to
efficiently form a metal spray coating film excellent in
the adhesion on the surface of the reinforced concrete
structure without conducting blast treatment.
The binder to be used in the present invention is not
particularly limited so long as it is excellent in the
drying property, water resistance and adhesion.
Conventional binders for coating materials may be used
without any particular restriction. For example, one-
pack air drying type resin such as chlorinated rubber, an
alkyd resin or a vinyl resin, or a two-pack type resin
(to be used in combination with a curing agent) such as
an epoxy resin, an unsaturated polyester resin, an acryl-
2 ~ 1 2
urethane resin or a polyester-urethane resin, may be
mentioned. In the present invention, a two-pack type
epoxy resin excellent in water resistance and adhesion is
particularly preferred.
Further, the solvent (or the dispersion medium) to be
used as the case requires, may, for example, be a usual
organic solvent for a coating material, such as xylene,
toluene, butanol, methyl ethyl ketone or butyl acetate,
or water. The pigment may, for example, be a filler such
as barium sulfate, calcium carbonate or talc, or a
coloring pigment such as titanium oxide or carbon black.
The additives include a foam-preventing agent, an anti-
sagging agent and a dispersant. It is preferred to
incorporate from 0 to 50 wt~ of the solvent (or the
dispersion medium~ and from 0 to 30 wt~ of the pigment,
based on the weight of the primer.
The primer to be used for coating may be of any type
such as an organic solvent type, an aqueous type or a
liquid non-solvent type.
As the metal spray coating material to form a metal
spray coating layer to be used as an anode in the present
invention, aluminum, an aluminum alloy or a zinc-aluminum
pseudo alloy may be employed. As a metal spray coating
material, zinc is known as a typical material, but zinc
is likely to wear by e.g. white rust, and even when a
protective coating is applied thereon, blistering or the
like is likely to form. Therefore, zinc is not suitable
2 ~ 2
g
for the present invention intended to provide corrosion
prevention for a ]ong period of time.
The aluminum alloy is an alloy containing at least
50~ by weight of aluminum and having at least one other
metal such as Cr, Si, Fe, Ni, Sn, Mg or Zn incorporated.
A metal spray coating formed with aluminum or an
aluminum alloy has a merit that wear is little, since the
surface of the aluminum itself is oxidized to form a
stable and dense coating film.
The zinc-aluminum pseudo alloy is a pseudo alloy
containing zinc and aluminum in a ratio of Zn:A~ = 85;15
to 30:70 (weight ratio).
The metal spray coating formed with such a zinc-
aluminum pseudo alloy has a merit that it has a large
cohesive force and is highly dense, whereby blistering or
the like scarcely occurs.
This zinc-aluminum pseudo alloy means a state wherein
zinc and aluminum do not form an alloy tissue, and fine
zinc particles and fine aluminum particles are overlaid
on one another in a non-uniform fashion to present an
apparent appearance of a zinc-aluminum alloy. The spray
coating film of this zinc-aluminum pseudo alloy can be
formed by conducting arc metal-spraying by a low
temperature metal-spraying method such as an arc metal-
spraying method under reduced pressure, using metal spraywire materials of zinc and aluminum.
Figure 1 is a cross-sectional side vie~ of a
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characteristic part of a typical reinforced concrete
structure to which corrosion preventing treatment was
applied by the method according to the present invention.
Referring to this Figure, the method for preventing
corrosion of a reinforced concrete structure of the
present invention will be described.
The surface of a concrete structure 1 having a
reinforcing steel 2 embedded as a reinforcing material,
is cleaned to remove deposits such as dusts or oils, as
the case requires. Then, the above-mentioned primer is
coated thereon and dried to form a primer layer 3.
Coating of the primer is conducted by a conventional
coating method such as spraying, brush coating or roller
coating. The coating amount is adjusted to be usually
from about 23 to 400 g/m2, preferably from 40 to 200
g/m2 .
Heretofore, in order to improve the adhesion of the
spray coating metal film, it has been common to adopt a
rnethod wherein the surface of the substrate to be metal-
sprayed is subjected to blast treatment to make a roughsurface. However, if this blast treatment is applied to
the surface of a concrete structure, a dust will be
formed, and the working environment and surrounding
environment will be thereby polluted. Further, the
surface hardness of the concre-te structure is relatively
low as compared with e.g. steel material, and aggregate
material of concrete is likely to fall off from the
2~01 2
surface, whereby it is hardly possible to obtain such a
sharp roughened surface as is obtainable by the blast
treatment of a steel surface, and consequently it has
been impossible to form a metal spray coating film
excellent in the adhesion. According to the present
invention, this problem has been overcome by coating an
aggregate-containing primer instead of conducting such
blast treatment~
On the semi-dried or completely dried primer layer 3
thus obtained, aluminum, an aluminum alloy or a zinc-
aluminum pseudo alloy is metal-sprayed to form a spray
coating layer 4 which will serve as an anode.
As the method of metal-spraying aluminum or an
aluminum alloy, a gas flame-spraying method, an
electrical arc spraying method or a low temperature
metal-spraying method by means of a reduced pressure arc
spraying machine may be mentioned. In the present
invention, any one of these methods may be employed. In
a case where the primer layer is likely to be burned out
if the temperature of sprayed metal particles is high, it
is preferred to employ a low tempera-ture metal-spraying
method by a reduced pressure arc spraying machine as
disclosed in e.g. Japanese Examined Patent Publication
No. 2485g/1972 or Japanese Unexamined Patent Publication
No. 167472/1986.
This low temperature metal-spraying method by means
of a reduced pressure arc spraying machine is a method
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wherein a metal wire material i5 continuously
electrically arc-melted under an environment where the
central portion is depressurized than the peripheral
portion by means of a low temperature air stream jetted
in a cylindrical shape, and at the same t:ime, the melted
metal is suctioned into a forward jet stream, pulverized
and quenched, whereupon the metal particles in a super
cooled liquid state are sprayed on the primer layer. In
the case of the zinc-aluminum pseudo alloy, metal
spraying is conducted by this low temperature metal
spraying method.
The thickness of the metal spray coating layer 4
formed on the primer layer 3 is preferably from about 20
to 200 ~m, more preferably from 30 to 150 ~m. However,
the thickness may be as thick as e.g. 1000 ~m.
The metal spray coating layer 4 thus formed and the
reinforcing steel 2 will then be connected by an
electrically conductive material 5 having the surface
coated with an insulating material, via a power source 6.
According to the present invention, a metal spray
coating layer constituting an anode is thus formed on the
reinforced concrete surface with the primer layer
interposed, and using the reinforcing steel as a cathode,
a direct current voltage is applied by a power source
across the reinforcing steel and the metal spray coating
layer to conduct a corrosion preventive current, thereby
to prevent corrosion of the reinforcing steel embedded in
2~g~2
the concrete structure. The direct current voltage is
applied so that the potential of the reinforcing steel
will be from -l,000 mV to -550 mV (based on a saturated
Ag/AgCe electrode), preferably from -900 mV to -600 mV.
In Figure l, reference numeral 7 is an electrode such
as a saturated calomel electrode or a Ag/AgCe electrode,
and numeral 8 indicates a voltmeter, and they were
provided to measure the potential of the reinforcing
steel.
The method for preventing corrosion of a reinforced
concrete structure of the present invention is as
described in the foregoing. However, in order to prevent
wear by rusting of the metal spray coating film, it is
advisable to coat a corrosion preventive paint or polymer
cement on the metal spray coating layer to provide a
protective coating film.
The method of the present invention is applicable not
only to newly built or existing marine reinforced
concrete structures but also to various reinforced
concrete structures such as bridges or tunnels on land.
According to the method of the present invention, the
operation will be easy even to a reinforced concrete
structure having a complicated shape, and electrolytic
corrosion prevention by an impressed current method can
be efficiently conducted at low costs, whereby it is
possible to obtain a reinforced concrete structure
excellent in the corrosion prevention for a long period
2 ~ Q 1 2
of time of at least equal to that at-tainable by
electrolytic corrosion prevention by the conventional
impressed current methods.
Further, according to the present invention, a rough
surface is formed by the primer coating on the surface of
the reinforced concrete structure, whereby the adhesion
of the metal spray coating layer is excellent, and it is
unnecessary to roughen the surface of the reinforced
concrete structure by blast treatment which used to be
conducted prior to metal-spraying, whereby environmental
pollution by a dust generated by such blast treatment can
be avoided and the operational time required for such
treatment can be saved.
Now, the present invention will be described in
further detail with reference to Examples. However, it
should be understood that the present invention is by no
means restricted by such specific Examples.
Primer
275 g tvolume of the solid resin content: 100 cm3) of
an epoxy-polyamide resin having 40% nonvolatile, which
was prepared by dissolving 100 g of an epoxy resin
(Epichlon 4051, trade name, manufactured by Dainippon Ink
and Chemicals, Inc.; epoxy equivalent: 950) in 80 g of
xylene, 60 g of methyl ethyl ketone and 25 g of butanol
and adding 10 g of a polyamide resin (Epicure 892, trade
name, manufactured by Ceranese; active hydrogen
equivalent: 133) thereto, and 221 g (volume of particles:
2 ~ 1 2
70 cm3, PVC: 41%) of silicon carbide having an average
particles size of 48 ~m (green silicon carbide CG320,
trade name, manufactured by Nagoya Kenmakizai Kogyo K.K.;
specific gravity: 3.16) were thoroughly stirred to obtain
a primer.
Reinforced concrete test sPeCimen
-
A reinforced concrete test specimen (height x length
x width = 900 mm x 900 mm x 100 mm) was used which was
prepared by embedding a total of 12 deformed reinforcing
steel bars so-called D13 as prescribed by JIS G3112, i.e.
12 bars in a covering depth of 20 mm in concrete,
attaching a lead wire to the end of each steel bar and
embedding a Ag/AgC~ electrode, as shown in Figure 1.
The concrete was prepared by using normal Portland
cement at a ratio oE water/cement = 60/40 (weight ratio)
at a ratio of sand/concrete aggregate = 54/46 (weight
ratio) and in a unit amount of cement of 320 kg/m3. To
avoid an influence of the effects of the end portions,
the four side surfaces other than the surface on which a
metal spray coating film was to be applied, were sealed
by coating a solventless epoxy resin coating material
thereon. However, the rear side surface opposite to the
surface on which the metal spray coating was applied, was
non-treated.
EXAMPLE 1
The surface of the reinforced concrete test specimen
was cleaned by high pressure water washing. Then, the
~ ~6~ ~
- 16 -
primer was coated thereon by an air spray in an amount of
50 g/m2 and dried in air for 2 hours to form a primer
layer having a surface roughness (Rz) of 60 ~m.
Then, an aluminum wire material was metal-sprayed
onto the primer layer by a reduced pxessure arc spraying
machine (PA-100, manufactured by Pan Art Craft Co.) to
form a metal spray coating layer having a thickness of 80
~m. The metal spray coating layer as an anode was
connected to the lead wires attached to the ends of steel
bars as a cathode via a power source as shown in Figure
1.
The metal-spraying was conducted by low temperature
metal-spraying by using an aluminum wire having a
diameter of 1.1 mm at a wire conveying speed of 5 m/min
at a voltage of 17 V at a current of 120 A under an air
pressure of 6 kg/cm2 at an air flow rate of 1 m3/min at a
spray distance of 20 cm.
EXAMPLE 2
A test specimen was prepared in the same manner as in
Example 1 except that after forming a primer layer in the
same manner as in Example 1, a metal spray coating of a
zinc-aluminum pseudo alloy [Zn/A~ = 72/28 ~weight ratio)]
was formed on the primer layer by means of a reduced
pressure arc ~praying machine (PA-100, manufactured by
Pan Art Craft Co.), and an acrylic resin polymer cement
paint was further coated thereon to form a protective
coating film of 100 ~m. The metal-spraying was conducted
%~06U~L~
- 17 -
by low temperature metal-spraying using a zinc wire and
an aluminum wire each having a diameter of 1.1 mm at a
wire conveying speed of 6 m/min at a voltage of 15 V at a
current of 120 A under an air pressure of 6 kg/cm2 at an
air flow rate of 1 m3/min at a spray distance of 20 cm.
COMPA~ATIVE EXAMPLE 1
In the same manner as in Exarnple 1, an aluminum spray
coating layer was formed, and the spray coating layer as
an anode was connected to the lead wires attached to the
ends of steel bars as a cathode via a power source,
except that the sur~ace was roughened by sand blast
treatment instead of forming a primer layer on the
surface of the reinforced concrete test specimen.
COMPARATIVE EXAMPLE 2
A titanium mesh (aperture : 35 x 70 mm) having a
platinum-type metal oxide coating applied thereto, was
put on the surface of a reinforced concrete test
specimen, and an acryl resin type polymer cement mortar
was coated thereon to form a protective layer of 20 mm.
The mesh as an anode was connected to lead wires attached
to the ends of steel bars as a cathode via a power
source.
With respect to the test specimens obtained in
Examples 1 and 2 and Comparative Examples 1 and 2, the
potentials of steel bars were monitored by Ag/AgCe
electrodes preliminarily embedded at the positions of the
steel bars, and an electric current was conducted while
,
, ,,
;
2~01~
- 18 -
adjusting the current so that the monitored value would
al~ays be -800 mV, and the following cycle test was
conducted.
Cycle test method
A specimen was left to stand in a constant
temperature chamber at a temperature of 20C under a
relative humidity of 60% for 24 hours and then immersed
in a 3% sodium chloride aqueous solution at 50C for 24
hours. This process was regarded as one cycle and
repeated 100 cycles.
Measurement of the natural potential after the 100
cycle test, measurement of the current density (using a
small current ampere meter) and inspection of the outer
appearance after each cycle and inspection of the rusted
area (%) of the steel bars after the 100 cycle test, were
conducted. The results are shown in Table 1.
2~ 012
- 19 -
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- 21 -
It is evident from Table 1 that in Examples 1 and 2
wherein corrosion prevention was conducted by the method
of the present invention, the operation was efficient and
inexpensive, and corrosion prevention was substantially
equal, as compared with Comparative Example 2 wherein
corrosion prevention was conducted by a conventional mesh
anode system.
In Comparative Example 1 wherein blast treatment was
conducted, the metal spray coating film had poor adhesion
and peeled off, and the specimen was poor in corrosion
prevention for a long period of time.
Further, the non-treated test specimen showed
craclcing in the concrete, and rust formed substantially
over the entire surface of the steel bars.
,
:........ ~ ' . ,
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.
