Electro-chemical method for minimizing or preventing corrosion of reinforcement in concrete, and related apparatus

A process and system for rehabilitating mature concrete structures, which have become carbonated and/or infused with chlorides and thus represent a corrosive environment for internal reinforcing steel. The surface of the concrete is first repaired with a special mortar having resistivity and capilarity consistent with the parent concrete and the process requirements. An elongated, flat, flexible, ribbon-like electrode element is supported in spaced relation over the surface of a concrete area to be treated, being threaded back and forth and oriented in edgewise fashion to the concrete surface. Thereafter a self-adherent, cohesive mixture of delignified cellulose pulp fibers and a liquid electrolyte solution is sprayed onto the surface of the concrete, to a level to cover and embed the ribbon-like electrode element, forming an electrolytic medium associated with the electrode element. A DC voltage is impressed between the internal reinforcement and the embedded electrode element to effect electro-chemical chloride removal and/or realkalization of the concrete in a procedure of finite time duration. Preferably, the electrode strip is passed about conductive supports at one side of the treating area, so that the voltage is connected to the electrode strip at a plurality of locations. Flame and smoulder retardants are mixed with the dry cellulose fibers in advance of being applied to the concrete, by spraying through the previously mounted electrode structure.

BACKGROUND AND SUMMARY OF THE INVENTION 
Many reinforced concrete structures, which are exposed to the elements, 
will undergo a gradual change in internal chemistry in such a manner as to 
subject the internal steel reinforcing elements to corrosion. Under some 
circumstances, the concrete undergoes a slow carbonation process. As a 
result, the normally relatively high alkaline level of the concrete is 
progressively reduced. Eventually, the pH of the concrete reaches a level 
(around 9.5) at which it is no longer capable of preventing corrosion of 
the internal reinforcement. 
Concrete can also become corrosive to its internal reinforcement where it 
is exposed to chloride infusion. This is common for roadways, bridge 
decks, parking garages, and the like, where chloride salts may be employed 
for controlling ice formation in the winter time. Over time, concrete 
exposed to chloride salts can become sufficiently infused therewith, that 
the internal steel reinforcement of the structure is subject to corrosion 
by reaction with the chlorides. In some cases, chlorides may be present in 
the concrete from the beginning, being used in some instances to hasten 
the process of the setting of freshly poured concrete. 
In the above situations, if the condition is allowed to continue, serious 
damage to the concrete structure can be expected. When the internal 
reinforcement begins to corrode, it tends to expand, and the neighboring 
concrete is caused to crack and spall. 
Procedures have been developed for rehabilitating concrete structures, to 
effect realkalization thereof or to reduce the chloride content, or both, 
depending upon the particular circumstances. The Vennesland et al. U.S. 
Pat. No. 4,832,803, for example, discloses an advantageous 
electro-chemical procedure for removing chlorides from mature concrete 
structures to significantly reduce corrosive conditions in and around the 
internal reinforcement. The Miller U.S. Pat. No. 4,865,702 discloses an 
electro-chemical process for realkalizing a mature concrete structure, 
also for the purpose of minimizing the corrosive activity of the concrete 
in the region of the internal reinforcement. The Miller U.S. Pat. No. 
5,015,351 discloses certain electro-osmotic techniques which can be 
employed to the same end. Certain of the above-mentioned related pending 
applications are also directed to new and useful techniques for the 
rehabilitation of mature concrete structures by chloride removal and/or 
realkalization procedures. 
In my prior copending U.S. patent application Ser. No. 539,069, filed Jun. 
15, 1990, I disclose a rehabilitation procedure which involves application 
to an exposed surface of a concrete structure of a sprayed-on layer of 
self-adherent fibrous cellulosic pulp mixed with a liquid electrolyte. 
After applying a layer of desired thickness, a mesh-like electrode is 
placed over the matte of self-adherent pulp, after which a further 
covering layer of the self-adherent pulp is sprayed over the top of the 
electrode structure so that the electrode mesh is embedded in the fibrous 
matte. By maintaining an appropriate D.C. electrical potential between the 
embedded electrode structure and the internal reinforcement of the 
concrete, while maintaining the fibrous self-adherent pulp mass in a 
moist, electrolytically effective condition, it is possible to cause a 
migration of chloride out of the concrete, or to effect an infusion of 
hydroxyl molecules into the concrete, depending on whether the process 
being carried out is one of chloride removal or one of realkalization. The 
process according to my copending application is particularly advantageous 
in that the self-adherent fibrous pulp is easily applied to the structure, 
can be frequently remoistened as necessary, and is easily removed when 
process has been completed. 
In accordance with the present invention, further significant improvements 
are made in the process and equipment disclosed in my copending U.S. 
patent application Ser. No. 539,069, enabling the procedures disclosed 
therein to be carried out even more expeditiously and economically. 
Initially, the surface of the concrete to be treated must be prepared to 
receive the treatment. Typically, concrete that has reached the stage of 
requiring rehabilitation is already suffering from cracking, spalling and 
delamination. These defects are cleared and opened up to sound concrete by 
conventional methods. Pursuant to one aspect of the invention, cracks and 
voids are filled with a specially prepared mortar mixture having 
resistivity and capillarity compatible with the parent concrete. In 
addition, the mortar mixture is suitable for extended exposure to the 
electro-chemical environment while also accommodating the electrolytic 
action or electro-osmotic action which drives the rehabilitation process. 
In accordance with known techniques, a distributed electrode structure is 
established over the face of the concrete to be treated. This electrode 
structure is embedded in an adherent electrolytic medium, advantageously a 
matte of cellulosic fibrous pulp, which is sprayed onto the surface to be 
treated as a mixture of the fibrous pulp material and a liquid 
electrolyte. A source of D.C. voltage is connected, one side to the 
distributed electrode structure embedded in the adherent coating, and the 
other terminal connected to the internal reinforcement of the concrete. 
In accordance with one of the advantageous features of the present 
invention, a novel and improved form of distributed electrode structure is 
provided for placement on the external surface of the concrete to be 
treated. The new structure includes a pair of spaced-apart electrode 
supports, which may advantageously be in the form of elongated strips of 
wood, for example, of a thickness to support electrode elements at a 
suitable distance from the surface of the concrete. The electrode supports 
in turn mount a plurality of electrode support studs, which project 
outwardly from the supports at closely-spaced intervals. The studs are 
mounted on flexible mounting strips, which can easily be conformed to 
contoured surfaces. The mounting strip for at least one of the electrode 
supports, and usually only one, constitutes a conductive bus, by which 
each of the electrically conductive support studs can be connected to a 
voltage source. The distributed electrode is formed by a continuous 
electro-conductive element, which is threaded back and forth continuously 
from one electrode support to the other, being engaged at each end by one 
of the support lugs, so that the electrode element extends sinuously back 
and forth between opposed electrode supports, along the length of each of 
them, forming the desired distributed electrode structure. 
After placing the electrode supports and installing the sinuous electrode 
element, the self-adherent electrolytic medium is sprayed onto the surface 
of the concrete, being sprayed through the installed distributed 
electrode. The sprayed medium is applied to a depth sufficient to fully 
cover and embed the distributed electrode. 
To particular advantage, the sinuously installed continuous electrode 
element is in the form of a thin, flat, ribbon-like metallic strip, 
oriented on edge relative to the surface of the concrete being treated. 
The ribbon-like electrode strip thus presents a large conductive area to 
the electrolytic fiber mass in which it is embedded, while presenting 
minimum resistance to and interference with the sprayed-on application of 
the electrolytic mass following installation of the electrode. 
The cellulosic pulp material employed in forming the self-adherent 
electrolytic mass is specially processed to facilitate its handling and 
application, and also to optimize the cosmetic aspects of the procedure, 
by minimizing staining and discoloration of the concrete. 
For a more complete understanding of the above and other features and 
advantages of the invention, reference should be made to the following 
detailed description of a preferred embodiment and practice of the 
invention, and to the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the drawing, the reference 10 (FIG. 1) illustrates 
generally a section of concrete structure to be treated according to the 
invention. The illustrated structure is a vertical wall or column. 
However, it is to be understood that the procedures of an apparatus of the 
invention are applicable to surfaces of concrete structures which are 
horizontal, vertical, upwardly facing, downwardly facing, etc. For 
convenience, a structure of large area typically is processed in 
individual segments of predetermined area, which may be less than the 
entire structure. 
As a preliminary step in the rehabilitation process, the surface area of 
the concrete to be processed is examined for faults, such as cracks, 
spalls and delaminations. Any such faults are opened up by power chisels 
or other means to expose sound concrete. The surfaces of the "wounds" are 
then cleaned of dust, rust and loose particles, as by brushing, vacuuming, 
water jets, sandblasting or other suitable procedures, depending on the 
circumstances, so that a solid, sound surface is exposed for treatment. 
After cleaning, as above described, any cracks or other deep wounds in the 
concrete are filled with a suitable mortar having characteristics 
compatible with the subsequent procedures to be carried out. Among other 
things, the mortar should have an electrical resistivity compatible with 
that of the parent concrete and the requirements of the electro chemical 
process, typically between about 200 ohm-cm and about 3,000 ohm-cm. It 
must also have a high permeability to chloride and hydroxyl ions. The 
mortar must be sufficiently self-adherent to the basic concrete substrate 
to bond adequately without the use of so-called bonding bridges, since the 
materials used for such bonding bridges often are relatively impervious to 
the dissolved ions and may be relatively poor conductors as well. The 
mortar material also should be capable of setting in a relatively short 
time, for example, 30 minutes, under relatively alkaline conditions, and 
must have a long term resistance to highly alkaline (e.g., pH 13) 
conditions. The mortar also must be highly resistant to shrinkage and 
cracking, even if applied in relatively thick layers, for example, several 
centimeters. 
Pursuant to one aspect of the invention, a suitable mortar, meeting the 
requirements indicated above, is formulated using a special mortar base 
which is made available, as of the filing date of this application, by 
Liquid Plastics, of Preston, England, under the trade designation NCT 2000 
MB. The indicated mortar base incorporates, in addition to the usual 
cementitious materials, polymeric fibers, usually polypropylene, 
polyamide, and/or polyester to control shrinkage. The mixture may contain 
other polymers to make the mortar resistant to a sodium carbonate 
environment, as may be required particularly during realkalization 
procedures. 
In order to achieve an acceptable degree of uniformity in the 
rehabilitation process, particularly if the initial repair with mortar is 
rather extensive, the resistivity and capillarity of the cured mortar must 
be compatible both with the concrete and with the process itself. 
Otherwise, there may be a preferential migration of ions or molecules in 
certain portions of the overall area under treatment. I have found that, 
by controllably mixing microsilica into the mortar mix, otherwise 
comprised of the mortar base and a desired amount of sand or fine 
aggregate, the resistivity of the cured mortar may be quite accurately 
controlled to give the best compromise between the characteristics of the 
parent concrete and the requirements of the process. 
Before preparing the mortar mix, measurements are made on the existing 
parent concrete to establish its approximate resistivity. Reference can 
then be made to the graphs shown in FIGS. 5-10 to select a suitable 
mixture. For example, if the process to be performed is chloride removal, 
the process environment calls for the electrolytic medium to be 
principally a water-pulp mixture, with minor additions of sodium carbonate 
to provide adequate conductivity. Reference would then be made to FIGS. 
5-7, which indicate resistivity of various mixtures of mortar base and 
sand, with various amounts of added microsilica, over a period of time. In 
each Figure, the curves labeled "0", "1", "2", and "3" refer to the weight 
of sand per unit weight of the mortar base. FIG. 5 shows resistivity with 
no microsilica additions, FIG. 6 with 5% microsilica by weight of the 
mortar base, and FIG. 7 with 10% microsilica addition. The proper amount 
of sand to be used typically is selected in accordance with the strength 
and other physical characteristics desired in the mortar. 
As will be observed in FIGS. 5-7, the levels of resistivity of the 
solidified and curing mortar generally increase with increasing additions 
of microsilica. Accordingly, after determining the resistivity of the 
concrete, in the area under treatment, a mortar mixture is formulated by 
resort to a series of curves, of which those of FIGS. 5-7 are 
representative, to achieve the necessary resistivity, usually in the range 
of from about 200 ohm-cm to about 10,000 ohm-cm. 
Inasmuch as mortar resistivity increases with time, it is generally 
preferred to select a resistivity value which is slightly lower than that 
of the concrete, at a time appropriate to the process being carried out. 
For a chloride removal procedure, the relevant time period for resistivity 
determination may be about 14 days, i.e., after the mortar mix has cured 
for about 14 days in the process environment (principally a water 
electrolyte). 
For realkalization procedures, the process environment preferably involves 
a sodium carbonate electrolyte, for which FIGS. 8-10 are illustrative of 
resistivity values over a limited range. Realkalization procedures 
typically are of shorter duration than chloride removal, and it may be 
appropriate to select resistivity values at about 4 days, i.e., after the 
mortar mix has cured for about 4 days in an environment of an 
approximately 1 molar solution of Na.sub.2 CO.sub.3. As with the prior 
example, mortar resistivity preferably is selected to have a 4 day value 
slightly below that of the concrete to which it is applied. 
A second factor in the preparation of the mortar mix is its capillarity in 
a cured or partially cured state, which desirably should be at least as 
great as that of the parent concrete so as not to act as a barrier or 
partial barrier to the passage of ions or molecules as required by the 
electrolytic or electro-osmotic procedures. This is accomplished by 
varying the amount and type of sand which is added to the cementitious 
mortar base. Desirably the sand (or fine aggregate) is screened to have a 
relatively consistent particle size, rather than a full spectrum of 
particle sizes, which tends to result in a mixture which is too dense for 
the purposes of the procedures to be carried out. The proper amount and 
type of sand can be empirically determined. 
As illustrated in FIG. 1, an area 11 to be rehabilitated is bounded at the 
top and at the bottom by upper and lower strip-like electrode supports 12, 
13. The electrode supports 12, 13 are comprised of elongated strips 14, of 
wood or other suitable insulating material. Each strip mounts on its outer 
surface a plurality of spaced-apart electrode support studs 15, spaced a 
short distance apart over the length of the electrode supports. Typically, 
the studs 15 may be spaced apart a distance of approximately 50 mm. The 
studs desirably are secured to flexible strips, which are in turn mounted 
on insulating elements, such as the wood strips 14. This allows the 
electrode support structure to be conformed to non-flat contours where 
necessary. At least one of the electrode supports, and this would be the 
upper support 12 in the arrangement illustrated in FIG. 1, incorporates a 
flexible conductive metal bus strip 16, preferably copper, brass or 
stainless steel, which extends along the working length of the electrode 
support and is electrically connected to each of the electrode support 
studs 15, substantially in the manner indicated in FIG. 2 The studs 15 and 
conductive strip 16 are secured to the insulating strip 14 by any suitable 
means, not specifically shown. The upper and lower electrode supports 12, 
13 are secured to the surface of the concrete area 11, by any suitable 
means, such as bolts anchored in the concrete structure. 
A continuous electrode element 17 is installed on the electrode support 
studs 15 in a vertical zig-zag pattern, as particularly shown in FIG. 1. 
The electrode element is anchored at one end, as for example, at the stud 
18, shown in FIG. 1. It is then passed vertically downward to a 
corresponding stud 19 mounted on the lower electrode support 13. 
Advantageously, the electrode element is then extended horizontally about 
the adjacent lower stud 20 and then vertically upward to an opposed stud 
21 carried by the upper electrode 12. The electrode element 17 is thus 
threaded back and forth between the respective upper and lower electrode 
supports 12, 13 to form an electrode structure consisting of a large 
number of parallel strands of the electrode element, spaced apart a 
suitable distance, typically about 50 millimeters. At the opposite end of 
the structure, the electrode element 17 is suitably anchored to the final 
electrode support stud 22. 
To particular advantage, the electrode element 17 is a flat conductive 
metal strip, which is suitably flexible to be guided back and forth 
between the opposed sets of electrode support studs. A particularly 
preferred form of such metallic strip is made available by Eltech Systems 
Corporation, under the trade designation Elgard Anode Ribbon. This is a 
titanium strip, available in widths of approximately 6.3 or 12.7 mm and 
having a thickness of approximately 0.63 mm. Usually, the wider (12.7 mm) 
strip is preferred for the purposes of this invention. The titanium ribbon 
material is preferably coated with an electro-catalytic deposit of 
platinum metal oxides. As shown in FIG. 2, the conductive electrode 
support studs 15 preferably are of a bobbin-like configuration, with inner 
and outer flanges 23, 24 defining a central flat-bottom recess 25 of a 
width suitable to receive the flat strip-like electrode element 17, 
substantially as shown in FIG. 2. This arrangement allows the electrode 
strip to be oriented on edge, relative to the flat surface of the concrete 
area 11 to be treated, which is a particularly advantageous orientation as 
will subsequently appear. 
Desirably, the configuration of the insulating strip 14, the conductive bus 
strip 16 and the bobbin-like studs 15 is such that the center line of the 
electrode strip 17 is spaced from the surface 26 of the concrete by a 
distance of at least about 10 mm. 
As shown in FIG. 2, the internal reinforcement 27 of the concrete is 
connected as a cathode to a source of DC voltage 28, and the bus stud 16 
is connected as an anode to the same voltage source. Electrical contact 
with the internal reinforcement is made in any convenient manner, 
sometimes to a section exposed in an existing crack in the structure, and 
otherwise accessed by drilling or chiselling into the structure to expose 
a section of the reinforcement. The number and location of the connections 
to the internal reinforcement may vary widely. Desirably, however, there 
should not be less than two such connections, and if the area to be 
treated is greater than 50 square meters, there should be at least one 
connection per every 50 m.sup.2 of the area under treatment. 
As is evident in FIG. 1, the conductive bus 16, extending along the full 
length of the upper electrode support 12 serves to make an electrical 
connection with the electrode strip 17 at each point where the electrode 
passes over a support stud 15. Accordingly, theoretically, at least, it is 
not necessary for the strip to be of great length. It is convenient, 
however, to utilize a single strip to form the electrode structure for a 
given area under treatment. If necessary, strips may be spliced together 
by spot welding or otherwise. Likewise, the strips may be anchored to the 
bobbin-like electrode supports 15 by forming end loops in the strip, by 
spot welding, clamping or the like. 
Following the mounting of the anode structure, comprised of the 
spaced-apart electrode supports 12, 13 and the sinuously arranged 
electrode strip 17, the surface of the concrete, in the area to be 
treated, is sprayed with an electrolytic composition, comprised of a 
fibrous cellulosic pulp mixed with a liquid electrolyte, to form a 
self-adherent, cohesive electrolytic medium. The pulp material 
advantageously is formed of 100 percent natural cellulose fibers, with 
additions of mineral fire and smoulder retardants, such as borax, 
magnesium and/or aluminum oxides, and certain silicates, for example. 
Desirably, the dry fiber is premixed with the fire retardants. At the job 
site, the fibrous electrolyte is applied to the surface to be treated by 
way of a suitable mixing spray nozzle, which simultaneously mixes and 
discharges a spray comprised of the fibrous material together with a 
desired liquid electrolyte. 
Preferably, the cellulose fiber has been processed to remove as much as 
practical of its lignin content, which is subject to being leached into 
the concrete in the alkaline environment to which it is exposed. Ideal 
cellulosic pulp materials for the purposes of the invention are made 
available commercially as of the filing date hereof by Excel Industries, 
Ltd. Ebbw Vale, Gwent, England, under the trade designations NCT 2000 FG 
and NCT 2000 FW. The material typically has a pH of about 6.5-7.5, a loose 
bulk density of approximately 25 kg/m.sup.3. The material is formulated to 
provide for water absorption of approximately 1500 per cent of the weight 
of the dry fiber, to wet out rapidly and have slow drainage 
characteristics. 
To provide adequate conductivity, the liquid electrolyte solution must 
contain sufficient ions. These must be associated with hydroxyl ions in 
order to insure that the treated concrete retains sufficiently high 
alkalinity to passivate the internal reinforcement, and to maintain the 
steel reinforcement in a passivated state. This is achieved by insuring 
that alkali metal ions are present in the electrolyte. To particular 
advantage and in accordance with the invention, the liquid electrolyte 
contains sodium carbonate in concentrations varying from 1/100 molar for 
procedures designed strictly for chloride extraction, to one molar, for 
procedures designed purely for electro-osmotic realkalization of the 
concrete. Sodium carbonate is the material of choice, as it absorbs carbon 
dioxide from the air only to a limited extent before an equilibrium is 
attained. This assures that the pH of the resulting solution does not get 
significantly below about 10.6, which is adequately high to cause and 
maintain passivity of the reinforcing steel. Moreover, cathodic reactions 
with the steel reinforcement tend to cause production of hydroxyl ions, 
resulting in the formation of sodium hydroxide, eventually changing to 
sodium carbonate. Sodium carbonate is also desireable because of its 
relative cheapness, ready availability, non-toxic nature, and its low 
ionic dissociation. Sodium carbonate is thus a preferred electrolyte 
material from a number of viewpoints. 
Sodium carbonate, in the form of light soda ash, available from Solvay 
Chemical and others, is preferably utilized in the preparation of the 
sodium carbonate electrolyte. The preference for light soda ash is due to 
its ease of solution as compared to dense soda ash, which is granulated, 
and to crystalline soda, which both can be considerably more difficult to 
dissolve on site. Light soda ash is easily soluble in water at ordinary 
tap temperatures by simple stirring, as opposed to the other grades which 
often require preheating of the water of solution, and/or vigorous 
agitation. 
In the process of the invention, the mixture of pulp fiber and liquid 
electrolyte is sprayed over the area to be treated, to a depth of at least 
about 30 mm, measured from the surface of the concrete, forming a layer 
30, as shown in FIG. 2, which completely embeds the electrode strip 17. In 
this respect, by utilizing a ribbon-like strip material for the electrode 
17, and orienting the material on edge, adequate conductive electrode 
surface area is presented to the electrolytic mass 30, while at the same 
time the adjacent sections of the electrode strip can be spaced far enough 
apart that the surface of the concrete is fully exposed and accessible to 
the spray equipment. This arrangement enables the electrode structure to 
be installed over the bare concrete, and the electrolytic fiber mass to be 
applied in a single spray operation. This has proven to be more 
expeditious and economical than utilizing a mesh form of electrode, for 
example, which generally requires that the electrode structure and fiber 
mass be installed as a three-part operation, with a first layer of fiber 
being sprayed on the open face of the concrete, after which the electrode 
mesh is installed over that layer, following which a second layer of 
fibrous electrolyte is sprayed over the top of the electrode mesh. 
Additionally, although a titanium based mesh electrode may be theoretically 
reused a large number of times, the practical aspects of installing and 
reinstalling mesh electrode are such that it may no longer be a useful 
material after two or three installations, because of mechanical 
deformations through handling, shaping, etc. By comparison, the strip 
electrode, used in the manner disclosed herein, can be rereeled and used 
over and over again. If it is cut or broken, it can be readily spliced by 
spot welding or otherwise joining overlapping ends of the material. 
As reflected in FIG. 2 of the drawing, the sprayed-on fibrous electrolytic 
mass 30 is supplied in such a way as to be kept effectively spaced from 
the conductive bus strip 16, as otherwise the bus could corrode heavily 
during the process. Thus, in an installation as shown in FIG. 1, where 
upper and lower spaced-apart electrode supports 12,13 are employed, it is 
advantageous that only the upper electrode support be provided with a 
conductive bus 16. The lower electrode support 13 usually is subjected to 
gravity flow and dripping during the spraying-on procedures, and also 
during subsequent re-wetting of the adherent mass, which typically is 
required on a regular basis. If desired, the two spaced-apart electrode 
supports 12 and 13 could be oriented vertically, in which case it might be 
practical to employ bus strips 16 along both electrode sup ports, as long 
as the spray application of the adherent fiber mass is conducted in such 
manner as to keep the strips on both sides free of the electrolyte. 
After application of the adherent electrolytic mass, a DC voltage is 
applied by the voltage source 28 at a level to provide for a current flow 
through the area of treatment at a rate of not more than about five 
amps/m.sup.2, with approximately one amp/m.sup.2 as a preferred value. 
Generally, the voltage source 28 does not exceed 40 volts DC, for safety 
purposes. 
The application of electrical current to the external electrode structure 
and internal rebar structure is continued as long as necessary (usually a 
period of several days or a few weeks) until the desired level of 
realkalization and/or chloride extraction has been achieved. This is 
determined by analysis of concrete samples extracted at various time 
intervals. 
Depending on conditions, it may be advantageous to interrupt and/or 
temporarily reverse current flow to prevent polarization of the internal 
reinforcement. 
Once the desired level of treatment has been achieved, the adherent 
electrolytic pulp mass is removed by scraping and/or washing. The 
ribbon-like electrode strip 17 is removed and rewound onto a suitable reel 
for reuse, and the studded electrode supports 12, 13 are removed from the 
surface of the concrete, washed, and readied for reuse. Holes formed for 
mounting of the electrode supports and/or testing of the concrete are 
filled with mortar. In this respect, the procedure of the invention 
usually enables the rehabilitation to be carried out using fewer electrode 
supports than are required when using a mesh type of electrode structure. 
Accordingly, there is less patching to be performed upon completion. The 
electrode strip is sufficiently stiff edgewise to enable an opposed pair 
of electrode supports 12,13 to be spaced considerably farther apart than, 
for example, with a mesh electrode, without requiring intermediate 
supports. The electrode element should not, of course, contact the 
concrete directly, but only through the electrolyte medium. 
In FIG. 4 of the drawing, there is shown one possible modification of 
electrode support stud, which may be employed in place of the stud 15 
shown in FIG. 2. The stud 40 of FIG. 4 includes a post or stem 41 mounted 
on a flexible support 46 and projecting outward therefrom. At its outer 
end, the stem 41 joins with an arm 43 extending at right angles thereto 
and oriented to point toward one side edge 44 of the mounting strip 42. 
The modified support studs 40 are spaced apart in the same manner as shown 
in FIG. 3, for example. All of the arm portions 43 are oriented in the 
same direction, toward a side of the insulating supports 42. The supports 
are used in spaced-apart pairs, in the same manner as shown in FIG. 1, 
with the upper and lower strips being oriented so that the arms 43 are 
directed away from each other. The continuous electrode element 45 is 
threaded back and forth between studs at opposite sides, in the same 
manner as indicated in FIG. 1. As will be understood, a wide variety of 
support studs and conductive busses may be employed. Desirably, all of the 
studs carried by at least one of the supports 42 are formed of conductive 
material and are mounted on a conductive bus strip 46, so that voltage may 
be applied to the continuous electrode element 45 at various points along 
its length (i.e., at each of the conductive lugs 40 connected to a bus 
trip 46). 
The procedure and apparatus of the present invention add significant 
simplification and economy to the practice of the processes, known from 
our above-mentioned patents, of rehabilitating mature concrete structures 
by chloride removal and/or realkalization. Moreover, the utilization of 
mortar mixtures specifically formulated to have compatible resistivity and 
capillarity, with respect to the parent concrete as well as the process 
requirements, assures a high level of uniformity in the processing of the 
concrete, especially where considerable advance mortar repair is required. 
It should be understood that the specific forms of the invention herein 
illustrated and described are intended to be representative only, as 
certain changes may be made therein without departing from the clear 
teachings of the disclosure. Accordingly, reference should be made to the 
appended claims in determining the full scope of the invention.